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GEOLOGY by FRANK H. T. RHODES
Illustrated by
RAYMOND PERLMAN
FOREWORD "THE EARTH - l ove it or l eave i t , " is a pop u l a r s l og a n evol v i ng from o u r recog n i t i o n of the peril s of a n e x p l o d i ng p o p u l a t i o n , a po l l uted envi ronment, a nd the l i m i ted natura l resources of an a l ready p l u ndered p l a net . E ffec tive s o l u t i o n s to our societa l p r o b l e m s d e m a n d an effec tive knowledg e of the e a r t h on w h i c h w e l ive . The ob ject of t h i s book i s to i ntroduce the e a r t h: its re l a t i o n to the rest of the u n iverse , the rocks a n d m i nera l s o f w h i c h i t i s made u p , t h e forces that s h a pe i t , a n d t h e 5 b il l i o n year s of h i story that have g iven it its present for m . Many t o p i c s that a re d i scussed i n the book have a very practi ca l and u rgent s ig n ificance for o ur society, th i ng s such a s water s u p p l y, i nd u s t r i a l m i nera l s , o re deposits, a n d fue l s . Others a re of i m portance i n l o ng er p l a n n i ng a n d deve l o pment, such as earthquake effects , m a r i n e eros i o n , l a n d s l id e s , a r i d reg i o n s , a nd so o n . The e a r t h provides the u l t i mate b a s i s of o u r present society: the a i r w e breath e , t h e water w e d r i n k , t h e food w e eat, the mate r i a l s we use. All ar e the products of o u r p l a ne t . The s t u d y of the earth o p e n s o u r eyes to a v a s t sca l e of t i m e that provides u s with n e w d i me n s i o n s , m ea n i ng s , a n d perspectives . And it revea l s the order of the ear th , i t s dyna m i c i nterdependence a n d i t s structu red beauty. I a m most gr atefu l to Sharon Sa nford , who typed the m a n u sc r i pt of the revised editi o n . F . H . T. R . Revised Edition, 1991. Copyright© 1991, 1972 Golden Books Publishing Company, Inc., New York, New York 1 01 06. All rights reserved. Produced in the U.S.A. No part of this book may be copied or reproduced without written permission from the copyright owner. Library of Congress Catalog Number. 72-150741. ISBN D-307-24349-4.
A Golden Field Guide'", A Golden Guide", G Design'", and Golden Books" are trademarks of Golden Books Publishing Company, Inc.
CONTENTS
GEOLOGY AND OURSELVES 4 THE EARTH I N SPACE . . . . . . . . . . . . . . 10 THE EARTH'S CRUST: COMPOSITION . 21 THE CRUST: EROSION AND DEPOSITION . . . 32 WEAT H E R ING . . . 34 GLAC I E RS A N D G LAC IATION . 56 T H E OC EANS . 63 WINDS . .. . 74 P R O D UCTS OF D E POSITION . . . 78 THE CRUST: SUBSURFACE CHANGES 82 VOLCANO E S . . . . . . . . . . . . . . . 83 C LASSIF ICAT I ON OF IGNEOUS ROC KS . . . . . . . . . 89 METAMO R P H ISM . . . . . . . 94 MINE RALS AND C I V I L I ZATION . . 96 T H E C H ANGING EARTH . . . . . . 104 ROC K D E F O RMATION . . 106 ROC K F RACTUR E S . . . . . . . . . . . . 110 MOUNTAIN B U I L D ING . . . . . . . . . . 115 THE ARCHITECTURE OF THE EARTH. 122 EARTHQUA K E S . . . . . . . . . . . . . . . . 124 THE EARTH'S INT E R I O R . . . . . . . . 126 THE OCEAN FLOOR . . . . . . . . . . . 136 P LATE T ECTONIC S . . . . . . . . . . . . . 144 HOW THE EARTH WORKS . . 146 THE EARTH'S HISTORY . . . . . . . . . . . . 148 OTHER INFORMATION . . . . . . . . 1 56 INDEX . . . . . . . . . . . . . . . . . . . . . . . . 157 . . . . . . • . . .
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Volca n i c eru ption in Kapoho, Hawa i i . Many i s l a n d s in the Pacific a re volca n i c in orig i n .
GEOLOGY AND OURSELVES Geol ogy is the study of the earth . As a science, it is a newcomer in comparison with , say, astronomy. Whereas geology i s only about 200 yea rs old, astronomy was actively studied by the Egyptians as long a s 4, 000 yea rs ago. Yet specu lation a bout the earth and its activities must be as old as the human race . Surely, primitive people were fa m i l i a r with such natural disasters as earthqua kes and volcanic eruption s . Grad u a l ly, human society beca me m o r e dependent upon the earth i n i ncrea singly complex ways . Today, beh ind the insulation of our modern l iving conditions, civi l i zation rema i n s basica lly dependent upon our knowledge of the earth . All our m i nera l s come from the earth's crust . Water supply, agricu lture, and land use a l so depend upon sound geo logic informati o n . Geology sti m u l ates t h e m i nd . It ma kes u s e o f a l most a l l other sciences a n d g ives much to them i n return . I t i s the basis of modern society. 4
T H E BRA N C H OF G E O LOGY
emphasized here is physical geol ogy. Other Golden Guides of this series, Rocks and Minerals and Fossils, dea l with the branches of m i nera l ogy, petro logy, and paleonto logy.
PH YSIC AL GEOLO G Y
is t h e overa l l study of the earth, embracing most other branches of g e o l o g y but stress i n g the dyna mic a n d structura l aspects . It i n c l udes a study of landscape development, the earth's i n te r ior, the nature of mounta i n s , and t h e composition of r o c k s a n d m i nera l s .
HISTORIC AL GEOLOGY
i s the study of the h i story of earth and its inhabitants. It traces ancient geog raphies and the evo lution of l ife .
ECONOMIC GEOLOGY
is geol ogy applied to the search for and exploitation of m i neral resources such as meta l l i c ores, fuel s , and water.
STRUCTUR AL GEOLOGY
(tec tonics) i s the study of earth struc tures and their relationship to the forces that produce the structures .
GEOPH YSICS
is the study of the e a r t h ' s p h y s i c a l p r o p e r t i e s . It inc l udes the study of earthquakes (seismology) and methods of m i n e r a l a n d o i l explorat i o n .
PH YSIC AL OCE ANOGRAPH Y
is c losely related to geology and i s concerned w i t h t h e seas, major ocean basins, seafl oors, and the crust beneath them .
T H E S I ZE A N D S H A P E OF T H E EARTH were not always calculated accurately. Most ancient peoples thought the ea rth was flat, but there are many sim ple proofs that the earth i s a sphere . For instance, as a s h i p a pproaches from over the horizon, masts or funnels a re visi ble . As the s h i p comes closer, more of i t s lower pa rts c o m e i nto view. F i nal proof, of course, was provided by c i rcumnavigating the g l obe a nd by photographs taken from spacecraft . The Greek geographer a nd astronomer E ratosthenes was proba bly the first (about 225 B . C . ) to measure suc cessfully the ci rcumference of the earth . The basis for his calculations was the measurement of the elevation of the sun from two different poi nts on the globe . Two s i multane ous observations were made, one from Alexa ndria , Egypt (Point B , p. 7), and the other from a site on the N ile near the present Aswan Dam (Point A) . At the latter point, a good vertical sighting could be made, as the sun was known to shine d i rectly down a well at noon on the longest day (June 23) of the yea r. E ratosthenes reasoned that if the earth were round, the noonday sun could not a ppear in the same position in the sky a s seen by two widely separated observers. H e com pared the ang ular displacement of the sun (Y ) with the distance between the two g round sites, A and B .
6
RECENT D ATA
from orbiting earth sate l l ites have confirmed that the earth i s actua lly s l i g htly flattened at the poles. I t i s a n oblate spheroid, t h e p o l a r c i r cumference being 27 mi les less
than at the equator. The fol low i ng measurements are currently accepted: Avg . diameter 7,91 8 mi. Avg . radius 3 , 959 m i . Avg . c i rcumference 24, 900 m i .
LA R G E AS T H E E A R T H I S , it i s m i nute i n comparison with the u n iverse, where distances a re measured i n light yea rs the d i stance light, movi ng at 1 80, 000 m iles per second , travels i n a year. This is a bout 6 billion m i les or 1 0 m i lli o n , m i llio n kilometers. Using these u nits o f measurements, t h e m o o n i s 1 . 25 lig h t seconds from t h e earth , t h e sun i s Slight minutes from the earth , and the nearest star i s 4 light years from the earth . Our galaxy is 80, 000 light years i n d i a m eter. T h e m o s t d i stant galaxies a re 8 billio n lig h t yea rs from earth . It i s esti mated that there a re at least 400 m illion galaxies "visible" from earth using rad i o telescopes a nd s i m ilar means of detectio n . Galaxies a re either ellip tical or spi ral i n shape .
ERATOSTHE NES
measured the distance (X) between Points A and B a s 5 , 000 stad ia (about 575 m i l e s ) . Although the observer at Point A saw the sun d irectly over head a t noon, the observer at B found the sun was i n c l ined at a n angle of 7° 1 2' (Y) to the vertica l . S i nce a rea d i ng o f 7 ° 1 2' corre sponds to one-fiftieth of a full c i r c l e ( 3 6 0 ° ) , Er a t o s t h e n e s reasoned that t h e measured ground d istance of 5 , 000 stadi a m u s t represent one-fiftieth of t h e earth's c i rcumference . He calcu lated the entire c i rcumference to be about 2 8 , 750 m i les.
T H E EART H'S S U R FAC E ,
for the purpose of measure ments, i s commonly assumed to be un iform , for mounta i n s , va l leys, a nd ocea n deeps, g reat as some are, a re relatively insignificant features i n comparison with the diameter of the ea rth . But mounta i ns are not insignificant to humans. They p l ay a major role i n contro l l i ng the c l i mate of the continents; they have profoundly i nfl uenced the patterns of human m i g ration and settlement . Mounta i n ranges, with very few exceptions, a re narrow, a rcuate belts, thousands of m i les in length , genera lly developed on the marg ins of the a ncient cores or shields of the continents . They consist of g reat thicknesses of sed i mentary a nd volcanic rocks, many of them of marine ori gin. Thei r i ntense fol d i ng and faulting a re evidence of enormous compressive forces. Mounta ins a re not l i m ited to the land . The ocean floor has even more relief than the continents . Most of the continenta l margins extend a s conti nenta l shelves to a depth of a bout 600 feet below the l eve l of the sea , beyond 8
7 6 5 4 3 2 I ocean
which the seafloor (conti nenta l slope) p l u nges a bruptly down (see p . 66) . The ocea n floor adjacent to some isla nds a nd continents has long, deep trenches, the deepest of which, off the P h i l i ppi nes, is about 61/2 m i les deep (p. 1 3 8 ) . A worldwide network of mid-oceanic ridges, of mountainous propor tions, encircles the earth . This network has geophysical and geologic characteristics that suggest it occupies a unique role i n earth dyna mics - that a l ong these ridges new seafloor i s constantly being created . T H E E A R T H 'S C R U ST,
derived from the dense r, underlying mantle (pp. 1 28- 1 29), consists of two kinds of roc k . The continenta l crust differs from the ocea n i c crust i n bei ng l i g hter (2 . 7 g m . /cc. compared with 3 . 0) , thicker (35 k m 7 0 km versus 6 km), o l d e r (up t o 3 . 5 b i l l i o n yea rs versus a maximum of 200 m i l l ion years), chemica l ly d ifferent, and much more complex i n structure . These d i fferences reflect the d i fferent modes of formation of the two kinds of crust ( p p . 1 40- 1 45 ) . 9
e Pluto
THE EARTH RELATIVE DISTANCES OF PLANETS FROM THE SUN
EARTH is one of n i ne pla nets revolving i n · 1 ar ( e11·1pt1ca · I) or b"tts aroun d our near 1 y c1rcu star, the sun . Ea rth is the third pla net out from the sun and the fifth l a rgest p l a net in our solar
PLANETS
vary i n size, composition, and orbit. Mer cury, with a diameter of 3 , 1 1 2 miles, is the planet nearest to the sun. It orbits the sun in just three earth months. Jupiter, about ten times the diameter of Earth (88 , 000 miles), is the largest planet and fifth in distance from the sun, taking about 1 1 3/• earth years to orbit the sun. Pluto, the most distant planet, tokes about 2473/• earth years to orbit around the sun. The inner planets hove densities, and probably compositions similar to Earth's; outer planets ore gaseous, liquid, or frozen hydrogen and other gases.
Neptune
THE SU N,
on overage-sized star, makes up about 99 percent of the moss of the solar system. Its size may be illustrated by visualizing it as a marble. At this scale, the earth would be the size of a small groin of sand one yard away. Pluto would be a rather smaller groin 40 yards away.
Uranus
SATELLITES
revolve around seven planets. Including the earth's moon, there ore 6 1 satellites altogether; Mars has 2, Jupiter 1 6, Saturn 1 8 , Uranus 1 5 , Neptune 8 , and Pluto 1 .
Saturn
COMETS
Jupiter
ore among the oldest members of the solar system. They orbit the sun in extremely long, elliptical orbits. As comets approach the sun, their toils begin to glow from friction with the solar wind.
e Mars Earth Mercury
Cl
e
e
Venus
IN SPACE syste m , having a diameter of about 7, 9 1 8 m i l e s . It completes one orbit a round the sun i n about 365V4 days, t h e length o f time that g ives us our unit of time cal led a year.
THE MOON,
earth's natural satellite, has about 'I• the diameter, 1/a 1 the weight, and 3/s the density of our planet. The moon completes one orbit around the earth every 2 7113 days. It takes about the same length of time, 291/2 days, to rotate on its own axis; hence, the same side, with an 1 8% variation, always faces us. The moon's surface, cratered by meteorite impact, consists of dark areas (maria) which are separated by lighter mountainous areas (terrae). Terrae are part of the orig inal crust, formed about 4 . 5 billion years ago; maria are basins, excavated by meteorite falls, filled by ba saltic lavas formed from 3 to 4 billon years ago.
ASTEROIDS,
the so-called minor planets, are rocky, airless, barren, irregularly shaped objects that range from less than a mile to about 480 miles in diameter. Most of the asteroids that have been charted travel in elongated orbits between Mars and Jupiter. The great width of this zone suggests that the asteroids may be remnants of a disintegrated planet formerly having occupied this space.
METEORS,
loosely called shooting stars, are smaller than asteroids, some being the size of grains of dust. Millions daily race into the earth's atmosphere, where friction heats them to incandescence. Most meteors dis integrate to dust, but fragments of larger meteors some times reach the earth's surface as "meteorites." About 30 elements, closely matching those of the earth, have been identified in meteorites.
COMPARATIVE SIZES OF THE PLANETS Pluto
e
Mercury
Q
Mars
O
Venus
Earth
WINTER
SPRING AND F A LL
----�, I I
\
'
w ,.
s
N
I I I I \
- .... ,,
s'
\
..
..
"SLLM MER
..
w
..
w N
..
..
s
Ti lted a x i s determ i nes d iffere nt positions of sun at s u nri se, noon, a n d s u n set at d ifferent seaso n s i n m i d d l e north latitude. THE E A R T H 'S MOT I O N S determ i ne the daily phenomenon of day and night and the yea rly phenomenon of seasona l changes. The earth revolves a round the sun i n a slig htly e l l i ptica l orbit and a l so rotates on its own axis. Si nce the earth's axis is tilted about 231/2° with respect to the p l a ne of the orbit, each hemisphere receives more light a nd heat from the sun during one half of the yea r than d u r i ng the other half. The season in which a hemisphere is most d i rectly tilted towa rd the sun is summer. Where the tilt is away from the sun, the season is wi nter.
RELATIVE MOTIONS OF THE EARTH REVOLUTION is earth's motion PRECESSION is a motion at the
about the sun i n a 600- m i l l ion mile orbit, a s it completes one orbit a bout every 3651/• days trave l i n g at 66, 000 m p h .
ROTATION
i s a w h i r l i ng motion of the earth on its own axis once i n about every 24 hours at a speed of about 1 , 000 m ph at the equator.
NUTATION
i s a daily circular motion at each of the earth's poles a bout 40 f t . in diameter.
12
poles describing one complete circle every 26, 000 years due to axis tilt, caused by gravitational action of the sun and moon.
OUR SOLAR SYSTEM
revolves around the center of our M i l ky Way Galaxy. Our portion of the Milky Way makes one revolution each 200 m i l lion yea r s .
GALAXIES
seem to be reced ing from the earth at speeds propor tional to their d i stance s .
N
THE S U N is the source of a l most a l l energy on earth . Solar heat creates most wind and a l so causes eva poration from the ocea n s and other bod ies of water, resu lting i n prec i p i tatio n . Ra i n fi l l s rivers and reservoirs, and ma kes hydro electric power possible. Coa l and petroleum are fossi l rema ins of pla nts and a n i ma l s that, when living , requ i red sunlight. In one hour the earth receives solar energy equiv a lent to the energy conta i ned i n more than 20 b i l l ion tons of coa l , and this is only half of one b i l l i onth of the sun's tota l radiation . J ust a sta r o f average size, the sun i s yet s o vast that it could conta i n over a m i l l i o n ea rth s . Its d i a m eter, 864 , 000 m i les, is over 1 00 times that of the earth . It is a gaseous mass with such high temperatures ( 1 1 , 000° F at the sur face, perhaps 3 25 , 000, 000°F at the center) that the gases a re inca ndescent. As a huge nuclear furnace, the sun con verts hyd rogen to hel i u m , simulta neously chang i ng four m i l lion tons of matter i nto energy each second .
Solar pro m i nences com pared with the size of the earth
THE MILKY WAY,
l i ke many other galaxies, i s a wh i r l i n g spiral w i t h a centra l lens-shaped d i sc that stretches into spiral arms. Most of its 1 00 b i l l i o n stars a r e l ocated i n t h e disc. The Milky Way's di ameter is GALAX I E S
about 80, 000 l i g h t yea r s ; i t s thickness, about 6 , 500 l i g ht years. (A l i g h t yea r is the distance light trave l s i n one year at a velocity of 1 86, 000 m i . per sec . , o r a total o f a bout 6 t r i l l i o n m i l es . )
are huge concentrations o f stars. With i n the universe, there are i n n u merable g a l axies, many rese m b l i ng our own M i l ky Way. Sometimes ca l l ed extraga lactic nebu lae or i s l a nd u niverses, these star systems a re mostly visible only by telescope. Only the g reat spira l nebula Andro meda and the two i rreg u l a r nebu lae known a s the Magel lanic C l ouds can be seen with the naked eye. Telescopic i nspection revea l s galaxies at the furthermost l i m its of the observable u niverse. A l l of these g igantic spira l systems seem to be of comparable size and rotating rapidly. N early 5 0 percent appea r to be isolated i n space, but many g a l axies belong to multiple system s conta in i ng two 14
or more extraga lactic nebu lae. Our g a l axy is a member of the loca l G roup, which conta i n s about a dozen o ther galaxies. Some a re e l l i ptica l i n shape, others irreg u l a r . G a laxies may conta i n up to hund reds of b i l l ions of sta rs and have d i a meters of up to 1 60, 000 l ight years . G a l axies a re separated from one a nother by g reat spaces, usua l l y o f about 3 m i l lion l i g h t yea rs. Many g a la xies rotate on their own axes, but all g a l axies move bod i l y through space at speeds of u p to 1 00 m i les a second . I n addition to this, the whole un iverse seems to be expa n d i n g , movi ng away from us at g reat speed s . Our nearest g a l a xy, i n Andromed a , is 2 . 2 m i l l i on light years away. About 1 00 m i l l i o n ga laxies are known, each conta i n ing many billions of sta r s . Others undoubted ly lie beyond the reach of our telescopes . It seems very probable that many of the stars the ga laxies conta i n have p l anetary system s s i m i l a r to o u r own . It has been estimated t h a t there may b e a s m a ny a s 1 0 1 9 of these . Chances o f l ife occuring on other pla nets wou l d , therefore, seem very h i g h , a lthough it may not bea r a n exact resembla nce to l ife on earth .
WHIRLPOOL NEBULA in Canes Venatici , showing the relatively close packing of stars i n the cen tra l part
GREAT SPIRAL NEBULA M31
in Andromeda i s s i m i l a r i n form but twice the s i ze of our awn g a l a xy, the Mi lky Way.
T H E C H E M I CAL E L E M E NTS
a re the s i m plest components of the universe and cannot be broken down by chem ica l mea n s . N i nety-two occur natura l ly on earth , 70 i n the sun . They deve lop from thermonuclear fusion within the sta rs, i n which the e lementary partic les of the l i g htest elements (hydrogen a nd helium) are transformed i nto heavier e lements . /
T H E O R I G I N OF T H E U N I V E R S E
is u nknown , but a l l the bodies in the universe seem to be retreating from a common point, thei r speeds becoming g reater a s they get farther away. This gave rise to the expanding-un iverse theory, which holds that a l l matter was once concentrated in a very sma l l a rea . O n l y neutrons could exist in such a compact core . Accord ing to this theory, at some moment i n time at least 5 b i l l ion years ago - expa nsion beg a n , the chem i ca l e l ements were formed , and turbu lent cel l s of h o t gases proba bly origi nated . The latter separated i nto galaxies, withi n which other turbulent clouds formed , a nd these u ltimately condensed to g ive sta rs. Proponents refer to this as the " B i g Bang" theory, a term descri ptive of the i n itia l event, perhaps as long as 1 0- 15 bi l lion yea rs a g o . T H E O R I G I N O F T H E S O LA R SYSTEM
is n o t fu l ly under stood , but the s i m i l a r ages of it s components (Moon , meteorites, Earth at about 4 .5 -4 . 6 b i l l ion years) a n d the s i m i l a r orbits, rotation, a nd d i rection of m ovement a round the sun, all suggest a single orig i n . The theory currently most popu l a r suggests that it formed from a c loud of cold gas, ice, and a little dust, which beg a n s l owly to rotate and contract. Conti nuing rotation and contraction of this d i sc-shaped cloud led to condensation and thermonuclear fusion - perhaps triggered by a nearby supernova , from 16
which sta rs such as the sun were formed . C o l l i sion of scattered materia l s in the d i sc gradua l l y led to the forma tion of bodies - pl a netisma l s - which beca me proto p l a nets . T h e g rowi ng heat o f t h e sun probably eva porated off the light e lements fr om the i n ner pla nets (now represented by the d e n s e , r o c k y " te r restr i a l" p l a n e t s - M e r c u r y , Venus, Earth, M a r s , a nd t h e Moon) . The outer p l a nets, beca use of their g reater d i stance from the s u n , were less affected and reta i ned their l i ghter hyd rog e n , heli u m , and water compositio n . Perhaps they formed from m i n i-solar pla net system s withi n the l a rger disc. This composition may wel l reflect that of the parent gas cloud . Each pla net seems to have had a d i sti nct "geologic" history. Some, l i ke Ea rth a nd l o, a moon of J upiter, a re sti l l active . Others, l i ke Mercury, Mars, and our moon, had an ea rlier a ctive h i story, but are now "dead . " This theory, i n a n earlier version, has a long h i story, going back to I m m a nuel K a nt, the p h i l osopher, in 1 75 5 , and t h e French mathematician P ierre-Simon de La p l ace ( 1 796). Arti st's i nterpretation of t h e d u st·cloud theory
THE EARTH'S ATMOSPHERE is a gaseous enve l op e sur rounding the earth to a height of 5 00 m i les a nd i s held in p lace by the earth's g ravity. Denser gases l i e withi n three miles of the earth's surface . Here the atmosp here p rovides the gases essentia l to l ife: oxygen, carbon d ioxide, water vap or, a nd n itrogen . Differences i n atmosp heric moisture, temp erature, and p ressure combi ned with the earth's rotation a nd geo grap hic features p roduce varying movements of the atmo sp here across the face of the p la net, and conditions we exp erience a s weather. C limatic conditions ( ra i n , ice, wind , etc . ) a re i mp ortant i n rock weathering; the atmo sp here a lso i nfluences chemica l weathering . G ases in the atmosp here act not only as a g i gantic insulator for the earth by fi ltering out most of the u l travio l et and cosmi c radiation but a l so burn up m i l l ions of meteors before they reach the earth . The atmosp here insulates the earth agai nst l a rge tem p erature changes and makes long- di stance rad i o commu nications p ossible by reflecti ng rad i o waves from the earth . It a lso p robably reflects much interste l l a r "noise" i nto space, which would make rad i o and tel evision as we know them i mp ossib l e .
THE RATIO OF GASES
Composition of a i r at altitudes u p to about 45 m i les
Composition of air at a ltitudes a bove 500 m i les helium 50%
n itrogen 78% hydrogen 50% oxygen 2 1% argon0 .93% carbon d ioxide 0 . 03% other gases o.o.! %.____ ___.
in the atmosphere is shown i n the chart at left . C louds form i n the tropo s p h e r e ; the o ve r l y i n g s t r a t o sphere, extending 50 m i les above the earth, is clear. The iono sphere (50-20 0 m i les) contains layers of charged particles (ions) that reflect rad i o waves, perm i t ting messages to be transmitted over long d i stances . Faint traces of atmosphere exist i n the exo sphere to about 500 m i les from the earth's surface .
300
250
ultraviolet rays fmmsun
200
150
aurora borealis 100
90
80
70
--900
-67°
60 vJ/>.ll/l OZONE
50
unmanned balloon 27 miles
40
�
TROPOSPHERE
30
20
-10
0
Atmospheric circulation i nvolves the cont i n uo u s recirculation of various su bsta nces . O U R P R E S E N T ATM O S P H E R E and oceans were probably derived by degassing of the sem i - molten earth a nd conti n uing later additions from volcanoes and h o t spri ng s . These gases - such as hydrogen , nitrogen, hydrogen c h l o rides, carbon monoxide, carbon dioxide, and water va por probably formed the atmosphere of ear l ier geologic times. The lig hter gases, such as hyd rogen , proba bly esca ped . The later devel opment of l iving organisms capa b l e of pho tosynthesis slowly added oxygen to the atmosphere, ulti mately a l lowing the colon ization of the land by provi d i ng free oxygen for respiration and a l so forming the ozone layer, which shields the earth from ultravio let radiation of the sun . Some evidence for this seq uence in the deve lopment of the atmosphere is conta i ned in the sequence of P recam b r i a n rocks and foss i l s , which suggests a transition from a non-oxygen to free-oxygen envi ronment.
20
THE EARTH'S CRUST: COMPOS IT ION We have so fa r been able to penetrate to only very sha l l ow depths beneath the surface of the earth. The deepest m i ne is only a bout 2 mi les deep, and the deepest wel l a bout 5 mi les deep. But by using geophysical methods we can "x-ray" the earth. Ca refu l tracing of earthquake waves shows that the earth has a d i sti nctly layered structure. Studies of rock density and compositio n , heat flow, and mag netic and g ravitational fields a l so aid i n constructing a n earth model of three layers: crust, ma ntle, and core. Estimates of the thickness of these l ayers, and suggested physica l and chemical cha racteristics form an i m portant part of modern theories of the earth {p. 1 26). The crust of the earth i s formed of many different kinds of rocks {p. 92), each of which is a n agg regate of m i nera l s , descri bed on pp. 22-3 1.
Gra nd Canyon of Colorado River, Arizona, i s 1 m i l e deep, but exposes only a s ma l l port of u pper portion of earth's crust.
M I N E RALS
a re natura l l y occurring substa nces with a char acteri stic atomic structure and characteristic chemica l and p hy si ca l properties . Some m i nera l s have a fixed chemical composi ti on; others va ry wi thi n certai n li m i ts . It is their atomi c structure that d i stingui shes mi nera l s from one a nother. Some m i nera ls consist of a single element, but most m i nera l s a re composed of two or more el ements . A dia mond , for i nstance, consi sts only of carbon atoms, but quartz i s a compound of sili ca and oxygen . Of the 1 05 elements presently known , n i ne make u p more than 99 percent of the mi nera ls and rocks. 2.0 2.59 2.83 3.63 5.00 "'
� "'
Aluminum
8. 1 3
�
0 "' " -= c:
"'
""'''"''"''.,..··"-'"'
�E liii�.ik��:r;::!;.', :..
" -.;
c: D ..., c: " ...a D "' 0
.00
:::ii:
Tota l
22
98 .59
OXYGEN AND SILICON are the
two most abundant elements i n the earth's crust. Their presence, i n such enormous quantities, i n d i cates t h a t m o s t of the m i nera l s a r e s i l i cates (compounds of met als with s i l i con and oxygen) or a l u m i nosi l i cates . Their presence i n rocks i s also a n i ndication of the a bundance of quartz (SiO, , s i l icon dioxide) i n sandstones and granites, a s well a s i n quartz veins and geodes .
The most strik i ng feature of m i nera l s is the i r crysta l form , and this is a reflection of the i r atomic structure. The s i m plest example of this i s rock sa lt, or h a lite ( N a C I , sod i u m c hloride), i n which t h e positive i o n s (charged atoms) of sod i u m a re l i nked with negative l y charged c h lor ine ions by their u n l i ke electrica l charges. We can imagine these ions a s spheres, with the spheres of sod i u m having a bout half the rad i u s of the chlorine ions ( . 98 A a s aga i n st 1 . 8 A; A i s a n A ngstrom U nit, which is equiva lent to one hundred m i l lionth of a centimeter, written numerica l l y a s 0. 0000000 1 em or l 0-8 c m ) . T h e u n i t i s na med for Anders A ngstrom , a Swedish physicist.
X-RAY STU DIES
show that the internal arrangement of halite is a defi nite cubic pattern, i n which i o n s o f s o d i u m a l te r n a t e w i t h those o f c h l o r i n e . Each sod i u m ion i s t h u s held i n t h e center o f and at equ a l d i stance f r o m six symmetrica l l y a rranged chlorine ions, and vice versa . It i s t h i s b a s i c a to m i c arrangement or crysta l l i n e structure t h a t g ives hal i te its d i stinctive cubic crysta l form and its characteristic physi cal properties .
H a l ite crysta l
SODIUM ATOM joi n s CHLORI N E ATOM
to form ionic crysta l sod i u m c h l oride
T H E ATOM I C S T R U CT U R E
of each m i nera l is d i sti nctive but most m i nera l s a re more com plicated than h a l ite, some beca use they comprise more elements, others beca use the ions a re l i n ked together in more complex ways . A good exa m p l e of this is the difference between d iamond and graph ite. Both have a n identica l chemica l composition (they are both pure carbon) but they have very d i fferent physical p roperties. Diam ond is the hardest m i neral known , and graph ite is one of the softest. Their different atomic structures reflect their different geolog ic modes of orig i n .
DIAMOND,
t h e hardest natura l substance known, cons i sts of pure carbon atoms. Each carbon atom i s l inked with four others by e l ec tron-sharing. The four electrons in the outer she l l are shared with four ne i g h b o r ing a t o m s. E a c h atom of carbon then h a s eight electrons i n its outer shell. This provides a very strong bond. Its crysta l form i s a reflection of its structure and of the cond i tions under which i t was formed. Dia mond i s usua l l y pale yel low or colorless, but i s found a l so in shades of red, orange, green, blue, brown, or black. Pure white or blue-white are best for gems.
24
GRAPHITE,
quite different from diamond , i s soft and g reasy, and widely used as an industrial lubri cant. I n graphite, carbon atoms are a rranged in layers, g iving the m inera l its flaky form. The atoms w i t h i n each layer h ave very strong bonds , but those that hold s u c c e s s i v e l a y e r s t o g e t h e r are very weak. Some atoms between layers are h e l d together so poorly that they m ove freely, g iv ing the graphite its soft, s l i ppery, lubricating properties. Because of its poor bonding , g raphite i s a good conductor of electricity. Its b e s t - k n o w n u s e is in " l e a d " penc i l s.
CR YSTAL FORM of m i nera ls is an i m portant factor in their i d e n t i fi c a t i o n . G r o w n w i t h o u t o b s t r u c t i o n , m i n e r a l s develop a cha racteristic crysta l for m . The outer a rrange ment of p l a ne surfaces reflects their i nterna l structure . Perfect crysta l s a re ra re . Most m i nera l s occur i n i rreg u l a r masses o f sma l l crysta ls because o f restricted g rowth . Si nce a l l crysta l s a re three-di mensiona l, they may be c l a ssified on the basis of the i ntersection of the i r axes . Axes are i m a g i nary l i nes passing through the geome tric center of a crysta l from the middle of its faces and i ntercepti ng i n a single poi nt.
CUBIC CRYSTALS
have three a x e s of equa l length meeting a t right a n g l e s , as i n g a l e n a , gar net, pyrite, and halite.
TETRAGONAL CRYSTALS
have three axes a t right angles, two of equal length, as i n z ircon , rut i l e , and scapo l i t e .
Galena
Zircon
HEXAGONAL CRYSTALS
have three equa l h ori zontal axes with 60° angles and one shorter or l o n g e r at r i g h t a n g l e s , as i n quartz and tourma l i n e .
ORTHORHOMBIC CRYSTALS
-, Quartz
Staurol ite
MONOCLINIC CRYSTALS
have three unequa l axes, two formi n g an obl ique a n g l e and one perpen dicu lar, as i n augite, orthoclase, and epidote.
have three a x e s at r i g h t a n g l e s , b u t e a c h i s of d i fferent l e n g t h , a s i n barite a n d stauro l i t e .
Epi dote
TRICLINIC CRYSTALS have three
axes of unequal lengths, none form ing a right a n g l e with others, as i n plagioclase feldspars .
25
MINERAL IDENTIFICATION i nvolves the use of various chemi cal and physical tests to determine what m i neral s a re present i n rock . There are over 2 , 000 m i nera l s known , and ela borate l a boratory tests (such a s X -ray d iffraction) a re requ i red to identify some of them . But many of the common m i nera l s can be recogni zed after a few simple tests . Six i m portant physical properties of m i nera ls (hardness, l us ter, color, specific g ravity, c l eavage, and fracture) a re easily determi ned . A ba la nce is needed to fi nd s pecific gravity of crysta ls or m i nera l frag ments . For the other tests, a hand lens, steel fi le, knife , and a few other common items a re h e l pfu l . Specimens can sometimes be recogn ized by taste, tenacity, tarnish, transpa rency, i ridescence, odor, or the color of the i r powder streak, espec i a l l y when these observations a re combi ned with tests for the other physica l properties. diamond
MOHS' SCALE OF HARDNESS
HARDNESS
i s the resistance af a m inera l surface Ia scratc h i ng . Ten wel l -known m i nera l s have been arranged i n a sca l e af i ncreasing h a r d n e s s ( M o h s ' sca l e ) . O t h e r m i nera l s a r e assig ned compara ble numbers from l to l 0 to rep resent relative hardnes s . A m i neral that scratches orthoclase (6) but is scratched by quartz (7) would be assig ned a hardness va lue of 6 . 5 .
LUSTER
is the a ppearance of a m i neral when l i g h t i s reflected from its surface . Quartz i s usua l l y g lassy; g a l e n a , meta l l i c .
Ga lena crysta l s
SPECIFIC GRAVITY
is the rela tive weight of a m i neral com pared with the weight of an equal vo lume of water. A balance i s n o r m a l l y u s e d to determine t h e two weights. S a m e m i nera l s a r e s i m i l a r superfi c i a l l y b u t differ i n density. Barite may resemb l e quartz, but quartz has a specific g ravity of 2 . 7; barite, 4 . 5 .
COLOR varies i n some m i nera l s .
Pigments or i m purities m a y b e the cause. Quartz occurs i n many hues but i s sometimes colorless. Among minerals with a consta nt color are galena ( lead g ray). sul fur (ye l low), azurite (bl ue). and malachite (g reen) . A fresh sur face i s used for identification, a s wea t h e r i n g c h a n g e s the t r u e color.
CLEAVAGE
is the tendency of some m i nerals tq split a long cer tain planes that are para l l e l to their crysta l faces . A hammer blow or pressure with a knife blade can cleave a m i nera l . G a l ena and hal ite have c u b i c cleav age. Mica can be separated so easily that it i s said to have per fect b a s a l c l eava g e . M i n e ra l s without an orderly interna l arrangement of atoms have no cleavage.
Rhombohedra l cleavage: ca l c i te
FRACTURE
is the way a m i neral breaks other than by c leavage . Minera l s with little o r no cleavage are apt to show good fracture surfaces when shattered by a hammer blow. Quartz has a she l l l i ke fracture surface . Copper h a s a rough, hackly surface; clay, a n earthy fracture .
Conchoidal fracture: obsidian Uneven fractu re: arsenopyrite
27
COMM O N R O C K - F O RM I N G MI N E RALS
incl ude ca rbon ates , sulfates , and other compounds . Many mi nera l s crys ta l l i ze from molten rock materia l . A few form i n hot springs a nd geysers, and some during metamorphism . Others a re formed by preci pitation , by the secretions of orga nisms, by eva poration of sa l i n e waters , and by the action of ground water.
MINERAL CARBONATES, SULFATES, AND OXIDES LIMONITE is o group nome for GYPSUM i s o hydrated calcium
hydrated ferric oxide m i nera ls, Fe,O,. H,O. I t i s o n amorphous m i nera l that occurs i n compact, smooth, rounded mosses or i n soft, earthy m o s s e s . No cleav age. Earthy fracture . Hardness (H) 5 to 5 . 5 ; S p . Gr. 3 . 5 to 4 . 0 . Rusty or blackish color. D u l l , e a r t h y l u ste r g i v e s o ye l l ow brown strea k . Common weather ing product of iron m i nera l s .
CALCITE i s o c a l c i u m carbonate,
CoCO,. It has dogtooth or flat hexagonal crysta l s with exce l lent cleavage. H. 3 ; Sp. Gr. 2. 7 2 . Colorless or white. I m purities show colors o f ye l l ow, orange,
sulphate, CoS0,. 2H,O. Tabular or fibrous monoc l i n i c crysta l s , or massive. Good c l eavage . H. 2 . S p . Gr. 2 . 3 . Colorless o r white. Vitreous to pea rly luster. Streaks ore white. F lexible but n o elastic flakes. Sometimes fibro u s . Found i n sed i m entary eva porites and as single crysta l s i n block sha les. The compact, massive form is known as alabaster.
brown , and gree n . Transparent to opaque. Vitreous or dull lus ter. Ma jor constituent of l i me stone. Common cave and vein deposi t . Reacts stron g l y i n d i lute hydroch loric acid.
Calcite
F i brous Gypsum
QUARTZ is s i l icon dioxide, SiO, .
Quartz crysta l
Massive or prismatic . No cleav age. Conchoidal fracture. H. 7; Sp. Gr. 2 . 6 5 . Commonly color less or white. Vitreous to greasy luster. Tra nsparent to opaque . Common in acid ig neous, meta morphic, and c lastic rocks, vei ns, and geode s . The most common of all m i nera l s .
ROCK-FORMING SILICATE MINERALS FELDSPARS
a re a l um i na-sil icates af e i t h e r p o ta s s i u m (KA I S i308 orthoclase, m icroc l ine, etc . ) ar sod ium and calcium (plagioclase fe l d s p a r s N a A I S i ,08 , C a A I , Si,08}. We l l -farmed monoc l i n i c ar triclinic crysta ls, with good cleavag e . H. 6 to 6 . 5 ; Sp. Gr. 2 . 5 to 2. 7 Orthoclase feldspars are white, g ray, or pink, vitreous to pea r l y l uster, and lack surface striations . P l ag ioclase feldspars are white or g ray, have two good cleavages, which produce fi ne para l l e l striations on cleavage surfaces . Common i n igneous and metamorphic rocks, and arkosic sandstones .
MICAS
a r e s i l i c a t e m i n e ra l s . W h i t e m i c a ( m u scovi te} i s a potassium a l u m i no - s i l i cate. Black mica (biotite} i s a potassi u m , iron, magnesi u m a l u m i no-si l i cate . Both occur i n thin, mono clinic, pseudo-hexagona l , sca l e l i ke crysta l s . Superb cleav age g ives t h i n , flexible flakes . Pea r l y to vitreous l uster. Micas are common i n ig neous, meta morphi c , and sed i mentary rocks.
B i otite (black mica)
PY ROXENES
i n c l ude a l a rg e g r o u p of si licates of c a l c i u m , magnes i u m , and iron . Augite, ( C a Mgfe A I ),•( AIS i ), 06, a n d h y p e r s t h e n e , ( FeMg ) S i03, a r e t h e m o s t com m o n . Stubby, eight sided prismatic, orthorhombic or monoc l i n i c crysta l s , or massive. Two cleavages meet at 90° (com pare a m phi boles), but these a re not a l ways developed . Gray or green, grading into black. Vitre ous to d u l l l uster. H. 5 to 6. Sp. Gr. 3 . 2 to 3 . 6 . Common i n nearly all basic ig neous and meta mor phic rocks . Sometimes found in meteorites.
AMP H I BO L E S
are complex hydrated sil i c a t e s o f c a l c i u m , magnesium, iron, a n d a l u m i n u m . Hornblende, a c o m m o n a m p h i b o l e , h a s l o n g , s lender, pris matic, six-sided orthorhombic or monoc l i n i c crysta l s ; s o m e t i m e s fi b r o u s . T w o g o o d c l e a v a g e s meeting a t 56°. H . 5 t o 6; S p . Gr. 2. 9 to 3 . 2 . Black or dark g reen . Opaque with a vitreous luster. Common i n basic i gneous and metamorphic rocks. Asbestos is an amphibo l e .
OLIVINE
i s a magnesium-iron s i l i c a t e , ( Fe M g ), S i O, . S m a l l , g lassy gra i n s . Often found i n large, granular masses. C rysta l s a r e relatively r a r e . P o o r cleav age. Conchoida l fracture. H. 6 . 5 t o 7 ; S p . Gr. 3 . 2 t o 3 . 6 . Various shades of green; sometimes ye l lowi s h . Transparent or translu cent . Vitreous l u ster. Common i n b a s i c i g n e o u s and metamorphic rocks. Olivine a l ters to a brown color.
30
C O M M O N OR E MIN E R A L S GALENA i s a lead sulphide, P b S .
Heavy, b r i t t l e , g r a n u l a r masses of cubic crysta l s . Perfect cubic cleavage, H. 2 . 5 ; S p . Gr. 7 . 3 to 7 . 6 S i lver-gray. Meta l l i c l uster. Streaks o re lead-gray. I m portant lead ore. Common vei n m i nera l . Occurs with zinc, copper, and si lver.
SPHALERITE is a zinc s u l phide,
ZnS. Cubic crysta l s or granu lar, compact . Six perfect cleavages at 60° . H. 3 . 5 to 4; Sp. Gr. 3 . 9 t o 4 . 2 . Usua l l y brownish; some times yel l ow or black. Transl ucent to opaqu e . Resinous l uster. Some s p e c i m e n s a r e fl u o r e s c e n t . I m portant zinc ore. Common vein m i neral with g a l e n a .
PYRITE
i s a n iron s u l p h i d e , FeS, . Cubic, brassy crystals with striated face s . May b e granular. No cleavage. Uneven fracture. H . 6 to 6.5 ; S p . Gr. 4 . 9 to 5 . 2 . Brassy yellow color. Meta l l i c lus ter. Opaque and brittle. A l so c a l l e d foo l 's g o l d . C o m m o n source o f sulfur.
THE CRUST: EROSION AND DEPOSITION The earth's crust is i nfluenced by three g reat processes which act together: G ra d a t i o n incl udes the va rious surface agencies ( i n contrast t o t h e two i nterna l p rocesses below), w h i c h break down the crust (degradation) or build it u p (aggradati o n ) . Gradation is brought a b9 ut b y r u n n i n g water, winds, ice and the ocea n s . Most sed i ments a re finally deposited in the sea s . D i a s t ro p h i s m i s the name given t o a l l movements o f the sol i d crust with respect to other parts (p. 1 06) . Someti mes this i nvolves the gentle upl ift of the crust . Ma ny rocks that were formed a s marine sed i ments gradu a l l y rose until they now stand thousands of feet above sea leve l . Other dias trophic m ovements may i nvolve intensive fol d i ng a nd frac ture of rocks.
COASTLIN ES t h e w o r l d ove r
provide evi dence of changes i n the earth's unstable crust . The photograph shows c l i ffs made of rocks that were depo s i ted under the seas that covered the area about 130 m i l l ion years ago . These were u p l ifted and folded , so the i r original layers now stand a l most vertica l . At present they are undergoing erosion by the sea, typified by the form of the arch. Eroded material i s being re d e p o s i t e d os a b e a c h . T h e r o c k s of w h i c h coa s t l i n e s a r e formed are themselves t h e resu l t o f earlier gradationa l events .
NATURAL ARCH
reflects process of erosion, w h i l e deposition has p r o d u c ed the be a c h . D o r s e t , England.
Eruption of Kapoho, Hawa i i , show i n g paths of molten lava V U LCAN I S M
i nc l udes a l l the processes associated with the movement of molten rock materia l . This i n c l udes not o n l y volca nic eruptions b u t a l so t h e deep-seated intrusion o f granites a nd other rocks ( p . 83). These three processes act so that at any time the form and position of the crust is the resu lt of a dyna m i c equ i l i b r i u m between them , a lways reflecting the c l i mate, season, a ltitude, a n d geologic environment of parti c u l a r area s . As a n end product of deg radatio n , the conti nents would be reduced to flat p l a i ns, but the ba lance i s restored a nd the processes of erosion counteracted by other forces that tend to elevate parts of the earth's crust . These cha nges reflect changes i n the earth's i nterior (p. 1 46 ) . 33
Yosem ite va l ley i s the res u l t of i nteraction of vari o u s types of eros ional processes.
EROSION i nvolves the brea king down and remova l of materi a l by var ious processes or deg radation .
YOSEMITE VALLEY in California
i s a good example of the complex interplay of gradational proc esses . A narrow canyon was first carved by the River Merced . This was later deepened and widened by glaciation. Running water i s n o w m o d i fy i n g t h e r e s u l t a n t hang ing and U-shoped vo l l eys ( p .
5 8 ) , so characteristic of glacial topography. The level of the main vo l ley floor lies 3 , 000 feet below the upland surface of the Sierras . D ifferences in topography ore portly the result of differences in j o i nting and resistance of u nder lying granites. H a lf Dome and El C a p i t a n ore r e s i s t a n t g r a n i t i c monoliths l a i d bore b y t h i s d iffer e n t i a l weathe r i n g . The r e g i o n thus shows t h e effect of many degrodotionol processes . But the 300-ft . -thick sed iment on the vo l l e y floor revea l s t h e continuing aggradational effects that ore a lso a t wor k .
WEAT H E R I N G
Weathering is t h e genera l n a m e f o r a l l t h e ways i n w h i c h a rock may be broken down. It takes p lace because m i nera l s formed i n a particular w a y (say at a h i g h temperature i n t h e c a s e of a n ig neous rock) a re often unsta b l e when exposed to the various conditions affecting the crust of the earth . Because weathering i nvolves i nteraction of the litho sphere with the atmosphere and hyd rosphere , it va ries with the c l i mate. But all k i nds of weathering ulti mately produce broken m i neral a nd rock fragments and other prod ucts of decompositio n . Some of these rema i n in one place (clay or laterite, for exam ple) while others are d i ssolved a n d removed by r u n n i n g water. The earth's surface, above the level of the water ta b l e ( p . 5 0 ) , i s everywhere subject t o weathering . T h e weath ered cover of loose rock debris (as opposed to solid bed r o c k ) i s k n o w n a s t h e r e g o lith . T h e t h i c k n e s s a n d d i stribution of the reg o lith depend upon both the rate of weathering and the rate of remova l and tra nsportation of weathered materia l .
THE EFFECTS o f weathering are
most str i k i n g l y seen i n arid and s e m i a r i d e n v i r o n m e n t s , where bare rocks are exposed without a cover of vegeta t i o n . Bryce Can· yon, Utah , shows the effects of the bedding and d iffering resis· lance of rocks i n producing d i s· t i n c t i v e e r o s i o n a l l a n d fo r m s . Weathering is of g reat i m por· lance to humanki nd . Soils are the resu lt of weathering processes, and are enriched by the activities of a n i m a l s and plants . Some i m portant e c o n o m i c resources, such as our ores of iron and alu· minum, are the result of residual weathering processe s .
MECHANICAL WEATHERING
FROST SHATTERING
i s often produced by a lternate freezing and thawing of water in rock pores and fissures . Expansion of water during freezing causes the rock to fracture .
SPHEROIDAL WEATHERING
occurs i n we l l - j ointed rocks, because weathering tokes place m o r e r a p i d l y at c o r ne r s a n d edges (3 and 2 sides) than o n sin gle faces .
involves the d i sintegration of a rock by mechanical processes . These include freezing and thaw ing of water in rock crevices, d i s r u p t i o n by p l a n t r o o t s o r burrowing ani m a l s , and the changes in volume t h a t resu l t from chem ical weathering within the rock. T h i s weathering i s espe cially common in high latitudes and a l titudes, which hove d o i l y f r e e z i ng and t h a w i n g , a n d i n d e s e r t s , w h e r e t h e re is l i t t l e water or vegetation. Rother ang u l a r r o c k f o r m s o r e p r o duced, and little chemical change in the rock is involved . It was once thought that extreme do i l y tem perature changes caused mechani cal weathering, but this now seems unce rtain.
C H E MIC A L W E AT H E RING
invo lves t h e d e c o m p o s i t i on o f r o c k by chem ical changes or s o l u t i o n . The c h i e f processes ore o x i dati on, car bona tion and hydration, and solution i n water above and below the surface . Many i ron minera l s , for example, ore rapidly oxidi zed ("rusted") and l i m e s t one is d i s s o l ved by water conta ining carbon dioxide. Such decomposition i s encour aged by worm, wet c l i matic con d i t i o n s a n d i s m o s t a c t i ve i n tropical and temperate c l i mates . Blankets of s o i l or other mater i a l o r e produced which ore so thick and extensive that solid rock i s rarely seen in the t r o p i c s . Chem ical weathering i s more wide spread and c o m m on than mechani cal weathering , a l though usua l l y both oct together.
S O I L is the most obvious result of weathering . It is the weathered part of the crust capable of supporting p l a nt l ife . The thickness and character of soil depend upon rock type, rel ief, c l i mate, and the "age" of a soi l , as we l l as the effect of living orga n i s m s . I mmatu re soils a r e l i t t l e more t h a n broken r o c k frag ments, g r a d i n g down into so lid rock . Mature s o i l s i n c l ude qua ntities of humus, formed from decayed pla nts, so that the upper surface (topsoi l ) becomes d a r k . Orga nic acids and carbon d i oxide released during vegetative decay d i s solve l i m e , i ron, and other compounds and carry them down i nto the l i g hter subsoi l . Residua l soils, formed i n place from the weathering of underlying rock, incl ude laterites , prod uced by tropica l leaching and oxidizing conditions which consist of iron and a l u m i n u m oxides with a l most no humus. Tra nsported soi ls have been carried from the parent rocks from which they formed ond deposited el sewhere. Wind-blown l oess ( p . 77), a l l uvia l deposits ( p . 44) , a n d g l a c i a l t i l l ( p . 5 9 ) a re common exa m p l es of transported soi l s .
LAT E RITIC SOI L P R O F I L E
Fr iable clay Concretions rich i n iron and manganese oxides Iron-rich clays
MAT U R E SO I L P RO F I L E
} clay H u m u s-rich
l
C l a y with l i mestone fra g ments Fres h l i mestone
zone serpentine
37
',
\\ rock f a l l �
I '
I I
ROCK FALLS
forming talus s lopes ore o n example of m o s s wa sting . T h e finer materi a l tends t o b e concentrated nea r t h e bose o f t h e slope. S u c h fa l l s m a y b e either small a n d irregular or massive a n d sudden, causing a rock ava lanche.
ion
MASS WAST I N G
i s the name given to all downslope move ments of regol ith under the predominant i nfl uence of g rav ity. Weathered materi a l is transported from its p lace of origin by g ravity, streams, winds, g l aciers, and ocean currents. Each of these agencies is a depositi ng , a s wel l as a tra n sporting agent and, though they ra rely act i nde pendently, each produces rather different results. The prevention of mass wasting of soil is of g reat i m por tance in a l l pa rts of the world . E n g i neering and m i n i ng activities usua l ly req u i re geolog ica l advice on s l ide and subsidence dangers . Several trag ic dam fa i l u res have resu lted from slides. The Va iont reservo i r slide i n Ita ly in 1 963 clai med 2 , 600 l ives . Ca refu l geologica l site surveys can prevent such d i sasters .
EARTH FLOWS
may be s l ow or very rapid . Slow movement (soli fl u c t i o n ) tokes p l a c e i n a r e a s where port o f t h e ground i s per manently frozen . These areas cover about 20 percent of the earth . On slopes, the thawed upper Ioyer slides on the frozen g round below it. On flat ground, latera l movement g ives stone polygons. Sudden flows may fol low heavy rains.
SOIL CREEP
is the gentle, down h i l l movement of the regolith that occurs even o n rather moderate, g r a s s - c o v e r e d s l o p e s . It c a n often b e seen i n road cuts.
SLUMPS
are s l i des i n soft, uncon s o l i d a ted s ed i m e n t . Some a r e submarine i n o r ig i n . Fossil slumps are recog nizable i n some strata . Usua l ly, s l u m ps are on a rather small sca le, a s when sod breaks off o stream b a n k .
H i l l creep affects fences a n d trees, a s wel l a s bend i n g of vertical strata .
S U BSIDE N C E
is d o w n w a r d movement o f the earth's surface caused by natural means (che m i cal w e a t h e r i n g in l i m e s t o n e a r e a s ) o r a r t i fi c i a l m e t h o d s (excessive m i n i ng or brine pump ing). Heavy loading by buildings o r e n g i n e e r i n g structures may a l so cause subsidence .
LANDSLIDES
are mass move ments of e a r t h or r o c k a l o n g a d e fi n i t e p l a n e . T h e y o c c u r i n areas o f h i g h relief where weak planes-beddi n g , joints, or faults ( p . 1 1 0 - 1 1 1 )- a r e stee p l y inclined; where weak rocks u n d e r l i e m a s s i ve o n e s ; w h e r e large rocks a r e undercut; and where water has lubricated slide planes .
Typical l a n d s l ide topogra phy i n Wyo m i ng shows debris o f huge bou l ders.
Rock g l a c i ers i n v o l v e s l o w downs lope movement of "river" of rock .
RUNNING WAT E R Running water i s the most powerfu l agent of erosion . Wind, g l aciers, and ocean waves are a l l confined to rela tively l i m ited l a nd a rea s, but running water acts a l most everywhere , even i n deserts . One fourth of the 35, 000 cubic mi les of water fa l l i ng on the conti nents each year runs off into rivers, carrying away rock fragments with i t . I n the U n i ted States, this erosion of the land surface takes p lace at a n average rate of a bout one i nch i n 750 yea rs. Running water breaks down the crust by the i m pact of rock debris it carries .
THE HYDROLOGIC CYCLE is a
continuous change in the state of water as it passes through a cycle of eva pora t i o n , condensat i o n , a n d precipitati o n . O f t h e water that fa l l s on the land, up to 90 percent i s evapo rated . Some is absorbed by plants and subse quently transpired to the atmo sphere , some runs off i n streams and rivers, and some soaks into the g round . The relative amounts
of water fol lowing these paths vary considera b l y and depend upon the slope of the g round; the character of the soil a nd rocks; the amount, rate , and distribu tion of rainfa l l ; the amount and type of plant cover ; and the tem p e r a t u r e . The h y d r o s p h e r e i s esti mated t o i n c l ude over 300 m i l l i o n c u b i c m i l e s of w a t e r , about 97 percent o f which i s i n the ocea n s .
moves to continent > moist air mass
E<--..f
condensation pr c i p i tation eva poration i n fa l l i ng
eva poration from
runoff
-,
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.
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OCEAN ... ....
... ... _ _ _ _ _ _ _ _ _ _
_
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I
I I
......-'. -"/II ...,.
loss by stream ru noff
water table
,("--
A single shower may i nvolve the downpour of more than a billion tons of water. Each ra i ndrop i n the shower becomes im portant in erosion, espec i a l l y in a reas with sparse vegetation and unconsolidated sed i ments . Runoff water rarely trave l s far as a conti nuous sheet, for it is broken u p i nto rivu lets a nd strea ms by surface i rreg u l a r i t i e s i n r o c k type and re l i ef. Running water carries its load of rock debris partly in suspension, partly by rol l i ng and bouncing it a l ong the botto m , and partly i n sol ution . The carrying power of a strea m is proportional to the square of its velocity, and so is enormously increased i n time of flood .
THE FORM OF RIVERS depends
upon mony facto r s . Water runs down h i l l under the i nfluence of gravity, the flow of the water being cha racte r i s t i c a l l y turbu lent, with swi r l s and eddies. The overa l l long profi l e of a river val ley i s concave upwards, however much its g radient (its slope or f a l l i n a g iven di stance) may v a r y . T h e ve locity of the stream i ncreases with the g radient, but a l so depends on other factors, includ ing the position within the river channel, the deg ree of turbu lence, the shape and course of the channe l , and the stream load (transported materia l s ) .
DENDRITIC DRAINAGE PAT T E R N of D i a m a nt i n a R i ver,
Queensland, Austr a l i a , i s typical of river development i n areas where the underlying rocks are relative l y uniform i n their resis tance to erosio n . T h i s pattern may be modified by continued downcutting of the river.
Diagrammatic model of a river syste m , showing cross sections of channel at t hree points
Niagara F a l l s are formed by res i sta nt bed of dolom ite. R I V E R P R O F I L E S reflect the varying devel opment of drain age systems. During early development, changes a re reflected by d i rection and shape of stream courses . U lti mately, erosion produces an equi l ibrium between the slope and vol u m e of the r iver and its erosional and depositional power. T h i s resu lts i n a g raded profi le, or profi l e of equ i l i b r i u m . F i n a l eq u i l ibrium is never reached because of sea son a l and geologic changes . The downwa rd l i m it of erosion by a river i s the base leve l , below which the river cannot downcut a ppreciably beca use it has reached the level of the body of water i nto which it flows . Sea level is the ulti mate base level for a l l rivers . Larger rivers and l a kes i nto which rivers flow constitute loca l base leve l s . Stages i n t h e cycle of river erosion were once l a beled as "youth, " "maturity, " a nd "old age . " Although these stages describe certai n characteristics, they i m ply no par ticu l a r age i n years, only chang i ng phases i n development. Rivers often show a condition of old age near thei r mouths, but a re mature or youthful i n their h i g her rea ches . For that rea son , the terms are rather mislead i n g , and a re now rarely used .
42
RIVER PROFILES ore influenced by geology and structure of their drainage area and tend to assume a graded, concave profile as they approach equilibrium. IRREGULAR PROFILE-common in upstream portions of rivers shows high gradients, waterfalls, rapids, steep-sided va l leys, irregular courses, few tributar ies, and erosion. The Gunnison River in Colorado is an example. longitud i n a l profi l e ....
SMOOTHER STEEP PROFILE is steep, with high relief, but fewer irregularities. Erosion and trans portation of underlying rocks gives wider valleys and smoother topography. Tributaries are well established. The White River of southwestern Missouri shows these features. long itud i n a l profil e ....
GRADED PROFILE reflects dep osition of transported load. The river flows sluggishly over a wide, flat flood plain, with meandering pattern, ox-bow lakes, and sand bars. This represents a baseline equilibrium between erosion by the river and deposition in the sea or lake into which it flows. The lower Mississippi and Amazon ore examples. lo ng itud i n a l profil e ....
43
REJUVENATION
or uplift may i nterrupt the cycle of erosion at any stage to provide new energy for downcutting . The character of the stream i s then often a combi nation of recently cut , steep gorges in a n older meander course . Ancient flood-plains are often left "stranded" as terraces. U p l ift and warping may be rela tively sudden or s low, frequent or rare, local or reg ional i n extent. Te rraces ar e cut a s river swings from one side of va l ley to oth e r .
WAT E R FALLS A N D RAPI DS
are loca l i ncreases in g radient i n the long profi le of a river. Most are due to unequal erosion of the stream bed .
THE FALLS
of the Yel l owstone River (the lower of which i s twice as high as N iagara F a l l s ) result from resistant lava flows . N iag ara Falls i s held u p by an SO-foot thick bed of dolom ite, which is more resistant to erosion than the underlying shales and thin l i mestones . Other fa l l s resu lt from over deepening of a m a i n va l l ey, often by ice, as i n the Bridal Vei l F a l l s i n Yosemite, so t h a t t h e tri butar ies are left "hanging" (p. 34) . Conti nued erosion by a stream leads to upstream m i g ration and ultimate smoothing out of waterfa l l s .
U N E Q U A L E R O SIO N
has a l ready cut N iagara F a l l s back about 7 m i les from the N iagara esca rpment, since it began cut ting about 9 , 000 years ago. The weaker shales below the Lockport Dolomite are undercut by the tur bulent water i n the plunge poo l , s o that t h e overlapping dolomite i s u n d e r m i n e d , a n d eventu a l l y c o l l a ps e s . The N i agara R i v e r flows i nto Lake Ontario from Lake E r i e , w h i c h w i l l u l t i m a t e l y be drai ned by upstream m i g ration of the fa l l s .
C l i nton ..;. Li mestone ...__ and Sha l e ---. �
Thoro l d Sa ndstone Whirl pool Sa n d st one
� /
. .. •.
STREAM PIRACY t a k e s p l a c e
when the t r i b u t a r i e s of o n e strea m (A) e r o d e faster t h a n those o f another (B) i n the same area . The headwaters of the less active stream a re u l t i mately diverted or cut off . When such " b e h e a d e d " strea m s a b a n d o n their path through a ridge, a wind gap res u l t s .
DELTAS
are masses of sed iment deposi ted where rivers lose their ve locity a s they enter lakes or sea s . Thick, relatively uniform layers of sediment accumulate on the steep outward s l o pe . Deltas of the Mississ i p p i , Ganges, and Po are thousands of square m i les i n area . Deltas have a character i s t i c s t r a t i fi c a t i o n i n t h e i r deposits . T
POTHOLES are circular hol lows .
in a stream bed , d r i l led out by swi r l i ng currents of water carry ing gravel and pebbles. This "hydra u l i c d r i l l i n g " i s a n i m por tant m e t h o d of d o w n - c u t t i n g , even i n hard rock .
DRAI NAGE
CONSEQUENT STREAMS
flow in directions determined l a rgely by the orig i n a l s lope and shape of the ground . When their courses are modified by features of the geology, such a s va l ley cut ting i n soft strata , the adjusted tributaries a re known as subse quent stream s .
DENDRITIC
d r a i n a g e patter n s are t h o s e that show tree l i ke branching because the bedrock has a u niform resistance to ero sion and does not i nfluence the d i rection of stream flow.
SUPERIMPOSED
dra i nage gen era l l y has stream courses that are i ndependent of rock structure . A s t r e a m ' s e ro s i o n a l power m a y have been strong enough t o main tain its a ntecedent course during
TRELLIS patterns a r e character
i s t i c of u n i f o r m l y d i p p i n g o r strongly fol ded rocks . I n this rec tangular pattern , the tributaries are nearly perpend i c u l a r to the main strea m .
upl ift and deve l opment o f a new geologica l structure . Or it may keep its same course after it cuts through younger, overlying , flat, sedimentary rock to a n older, irregular rock mass.
PAT TERNS
RADIAL
patterns deve l o p o n young mounta i n s , such a s volcan oes where stream s radiate from the h i g h centra l area .
Piracy m a y c au s e river d i vers ion . Ancestra l Shenandoah River ca ptured headwaters of Beaver D a m , leavi ng a n abandoned water g a p.
MODIFIED DRAINAGE may be
caused by p i racy ( p . 45) as wel l as by glacial o r volcanic blocking of stream courses. Glacial diver sian results from overdeepening of basins and river vo l leys by ice,
b l o c k i ng of d r a i nage by ice, mora i n i c deposits, and meltwa ter, which may produce g l a c i a l l a k e s and n e w outlets . Changes i n sea level may a l s o modify drainage .
Great lakes basins were carved by ice from soft strata . Original drainage was blocked by ice, producing local crustal depressi o n s .
E R O S I O N A L LA N D FO RMS are produced by running water and other erosiona l agents . Mesas a re flat-topped rock mounta i ns, which sta nd as remna nts of a once conti n u o u s p latea u . Buttes are sma l ler exa m p l es of t h e sa me thi n g . Monuments describe any iso lated rock pinnacle.
hogback escarpment
HOGBACKS
are l o n g r i d g e s formed b y steeply dipping resis tant strata ; cuestas are gently sloping ridges formed i n gently dipping strata .
NATURAL BRIDGES
are formed of resistant strata , usua l l y sand stone or l i mestone . U nderground erosion has taken place below the original stream bed .
PINNACLES
at B ryce Canyon, Utah, show d i fferentia l weather i n g . Erosion has removed the soft er, more soluble rocks. Rocks here are of Tertiary ( E ocene) age.
G R O U N DWAT E R is found a l most everywhere below the ea rth's surface . Most origi nates from ra i n and snow, but sma l l quantities come from water trapped i n sed i ments during their deposition (connate water) or from igneous magmas ( j uve n i l e water) .
SEPTARIAN NODULE
is formed from m i n e r a l s deposi ted by groundwater i n c l a y l i ke rocks. G r o u n d w a t e r is on i m p o r t a n t agent i n both deposition o n d ero sion of surface rock s . It can dis solve original cementing materi a l s and deposit new one s . F o r example, it produces caves and caverns by solution in car bonate rocks.
POROSITY,
the percentage of p o r e s p a c e to tota l v o l u m e of a rock, depends upon the grain size, shape, packi n g , and cement of rock particles. The permea b i l i t y of a r o c k , or its capacity t o transmit or y i e l d water, depends upon the size of pores, rather than the i r tota l vol u m e . Pores smaller than 1 /20 of a m i l l i meter w i l l n o t a l l o w w a t e r t o fl o w through them .
-c c 0
� .:00 ->"
·-
E" �0
a.. a..
0
Gra i n d i a m eter
WELL-SORTED SAND
(enlarged view) with high porosity due to sorti n g , which has removed fine grai ned particles
POO R LY SORTED SAND h a s
lower porosity than wel l - sorted sand, because the pore spaces are fi l l ed by fine particles.
LIMESTO N ES
genera l l y h o l d water i n e n larged j o i nts formed by solution; they lack the "pore spoce11 of sandstones.
49
T H E WAT E R TA B L E depends on the d i stribution of ground water. The open spaces i n the rocks of the upper part of the cr ust a re fi l led m a i n l y with a i r. This is the zone of aeration . Water moves downward through this zone i nto the zone of saturation , where openings ar e fil led with water. The upper surface of this saturated zone i s the water ta ble. I n m ost a reas, the water table i s only a few tens of feet below the sur face, but in arid reg ions, i t is much deep er. Water-bea r i ng rocks are rarely found below 2 , 000 feet . Rock pores are c losed by pressure at depth , and this determi nes the lower l i m it for g roundwater. Most rocks wi l l g ive off water whenever they i ntersect the water ta ble. But the level of the water table fa l l s after a dry seaso n , so rock formations must be deep enough to pene trate the water ta ble a l l year round . Someti mes a per ched water ta ble resu lts when a p ocket of water i s held a bove the normal water table by a saucer of i m pervious roc k . Any water-pr oducing rock formation is cal led a n aqu ifer.
ARTESIAN WELLS
are those where water i s confi ned t o a permeable aquifer by i m pervious bed s , and where the catchment or i ntake area (and thus the water
level i n the aquifer) is higher than the wel l head . This a l l ows the water to flow toward the surface under its own i nternal pressure.
t;'l 'l ra '/n fa ll
,
,
i
gran ite (after H o l m e s ) S P R I N G S are sources of running water produced by the water table intersecting the g round surface . A few of the many ways they can be formed a re shown i n the d i a g ra m s above . S o m e s p r i n g s a re dry at sea sons w h e n t h e water table is depressed ; others flow without i nterruptio n .
HOT SPRINGS
a re g e n e r a l l y confined t o oreos o f recent vu l canism where groundwater is heated at depth by contact with igneous magmas. Such springs ore wel l devel oped i n Yel l owstone Notional Pork and North Island, New Zea land. Terrace deposits may be produced when hot spring water deposits d i ssolve m i neral matter. Mammoth Springs of Yel lowstone Notional Pork ore formed of c a l c i u m c a r bonate (travertine) . Geysers, intermit tent f o u n t o i n l i k e hot s p r i n g s , often b u i l d cones of s i l iceous geyserite.
FUMAROLES
ore gentle geysers located i n volcanic reg ions that emit fumes, usua l l y i n the form of stea m .
Hot Spri ngs, Thermopo l i s , Wyo m i n g Norris Geyser B a s i n , Wyom i n g
GEYSERS
are thermal springs that period i ca l ly d i scharge their water with explosive violence. All geysers have a long na rrow p i pe extend ing down from their vents into their reservo i r s . A build-up and sudden release of steam bub bles probably relieves the pres sure on the heated water below g round so that it bo i l s and surges upward . T h e p e r i o d between erupti ons varies from m i nutes to m o n t h s i n d i ff e r e n t g e y s e r s , depending upon the structure of the geyser, its water supply, and its heat source.
OLD FAITHFUL
i n Yel l owstone o col umn of water and steam up to 1 70 feet high approximately every 65 m i nutes .
<1111 National Park d i scharges
Pudding B a s i n Geyser, New Zea land shows typical eruption. ..
Typical geyser structure shows c o m p l e x s y s t e m of fi s s u r e s extend i n g down to reg ions where g ro u n d w a t e r b e c o m e s s u p e r heated and fi n a l l y erupts .
Geyser ite ( S i nter) Tem perature 2 l 2°F
I
Depth 0' 33 '
295'
CAV E S
r e fl e c t t h e w o r k o f g roundwater. Li mestone i s dis so lved by c i rculating water i n s u b s u r f a c e j o i n t s a n d fi s s u r e s . T h e enlargement gradu a l l y pro duces a cove . Inside a cove, drip p i n g w a t e r , r i c h in c a l c i u m bicarbonate and CO,, often pro duces precip itates that form sta lagmites and sta lactites . G E O LO G I CAL WO R K OF G R O U N DWAT E R
is i m portant i n solution and deposition i n the rocks through which it passes . It d i sso lves l i mestone and other carbonate rocks to form caves and sink holes . Li mestone a reas are often marked by a karst topog raphy of s i n k s , few surface strea ms (they flow underground), and l a rge spri ngs. I n caves, the deposition o f ca lcite d i ssolved i n d r i p p i n g g rou ndwater produces sta lactites, icicle-shaped forma tions hanging from the cave cei lings, and sta l a g m i tes, formations that build up from the cave floor. The ca rryi ng away of m i nera ls i n solution usu a l ly occurs a bove the water ta b l e . Below that leve l , deposition, replacement, a n d cementation a re i m porta n t . Ba l l - l i ke masses (concreti ons), h o l l ow, globular bodies (geodes) , and the cement i n many sed i mentary rocks a re the result of the action of ground water at depth . geode
concretion
KARST TOPOGRAPHY
Cotter Dam suppl ies water for Austra l i a n cap ita l c i ty of Can berra .
T H E E A R T H 'S WAT E R S U P PLY
is one of its m ost precious natura l resources . Although nea rly three qua rters of the g lobe is covered by water, over 97 percent of the 326 m i l l io n cubic mi les of earth's water is locked up i n the ocea ns, too salty for drink ing water or for agriculture . Another 2 percent is frozen in g laciers and ice sheets . The tiny fraction that is ava i l a b l e for water supply is very unevenly d istri buted . One third of the earth's l a nd surface is desert or sem iarid . Even in humid a reas, water supply and conservation present major problems. The location and deve lopment of new industries depend upon adequate water supplies. Wor l d demand for water i s expected to double in the next twenty yea rs. It requ i res 600, 000 g a l lons o f water t o produce o n e t o n o f synthetic rubber. The d a i l y consu m ption of the average househo l d i n the U nited States is 400 g a l lons. 54
About three quarters of the water used in m ost humid i ndustr i a l a reas comes from surface waters (rivers, l a kes, artificia l reservoirs, etc . ) . The rest comes from g roundwa ter. P o l l ution and waste sti l l prevent m axim um use of sur face waters, on which we depend . Over one fourth of the earth's land surface is desert, and dam construction can be vita l i n these a rea s . The Aswa n Dam i n E gypt brought a l most 2 . 5 m i l l ion acres of new land i nto cultivation and generates 1 0 billion k i l owatt hours of electricity each year. Desa l i n i zation of sea water, a lthough u sed i n some arid areas, is sti l l too expensive for genera l use . Water conservation is a pressing world need si nce sup p l ies, a l though they a re never exhausted but are rep len ished i n the water cycle, can never be increased . They can , however, b e more efficiently used and d i stributed . P o l l u t i o n o f water supplies b y domestic and i ndustr i a l wastes ca n upset the delicate ecologica l ba lance, a nd has very serious biologic, econo m i c , and recreational effects .
Arid and sem iarid reg ions cover a bout 1 /3 of the earth's s urface. T h i s water hole i n P a k i stan i s typical of l o c a l water s u p p l i e s .
Lake Sol itude, Wyom i n g , a typical area of recent g laciation
G LACIERS AND G LACIATION We l ive today i n the twi l i g ht of a g reat epi sode of refrig eration, when much of the Northern Hemisphere was cov ered by conti nenta l ice sheets, l i ke those that sti l l cover Antarctica and G reenland. Althoug h the ice itsel f has now retreated from most of E u rope, Asi a , and N orth America , it has left traces of its i nfl uence across the who le face of the landscape in jagged mounta i n pea ks, gouged-out u p l a nd va l leys, swamps, changed river courses, and bou l der-strewn , table-flat prai ries i n the lowlands. G l aciers a re thick masses of s l ow-movi ng ice. I n the higher l a nds a nd polar reg ions, the annual wi nter snowfa l l usua l l y exceeds the summer loss b y melti n g . Permanent snow fie l d s build u p , and thei r l owest boundary i s the snow l i n e , the actu a l height of which va ries with latitude and c l i mate. Buried snow recrysta l l izes to form ice, which moves s l owly under its own weight. It moves most ra pidly i n the m i d d l e of the g l acier. 56
G LAC I A L E R O S I O N
has a powerful effect upon l a nd that has been buried by ice and has done much to shape the mounta i n ranges of our present world . Both va l l ey and conti nenta l g l aciers acq u i re many thousands of bou lders and rock frag ments, which, frozen i nto the sole of the g lacier, gouge a nd rasp the rocks over which the g l aciers pass . The rocks are s l owly abraded down to a smooth , fl uted , g rooved surface . Glacial meltwater, from periods of daylight or summer thaw, seeps i nto rock fissures and joints. When it freezes a g a i n , it helps to shatter the rocks, some of which may become frozen i nto the body of the g lacier and be carried away as the g l acier moves down slope. Ava la nches and undercutti ng of va l ley sides add to the rock debr i s .
ROCK SURFACES
d i splay flut ing, striation, and polishing effects o f g l acial erosion . The form and d i rection of these g rooves c a n b e used t o show the d i rection i n which the ice moved .
TYPICAL LONGITUDINAL SEC TION of a v a l l e y g lacier shows i t s structure and its profi le of bed roc k . Surface ice i s brittle, but underlying ice crysta l s bend , shear, and g l ide, causing ice to flow by deformation . •• ' •
·
·
Z O N E OF WASTAGE
•Z O N E O f. ACC UMU (A'F I O N .
•
. ·
.
·
·
'
: :·
.s·� ��fQ;t.<.'
·.
·
THREE STAGES OF GLACIAL HORNS, ARETES, AND CIRQUES EROSION are i l l u strated above, are a l l products of g l a c i a l ero
showing mountain country before, during, and after glacia t i o n . G laciers cut U -shaped va l l e y s , modifying and deepening the i nterlocking pattern of earlier meandering river erosion . The va l l eys a r e stra i g htened and truncated b y ice flow. Tributary hang ing va l leys develop where the rate of erosion by tributary g laciers i s l ower than that of the g lacier i n the main vo l ley. As the ice retreats, waterfa l l s flow out of them i nto the main vol ley.
sion. Horns ore sharp, pyramidal m o u n t a i n peaks f o r m e d w h e n headword erosion o f several g la ciers i ntersect . Aretes ore sharp ridges formed by headword g la c i a l ero s i o n . Continental g laciers tend to produce a smoothed-out effect o n the landscape, such as that of the laurentian Shi.e ld in C o n od o . C i rq u e s o r e b ow l shaped vo l l eys formed a t heads of g laciers and below oretes and horned mounta i n s ; often contai n a sma l l lake, cal led a tor n .
U -sho ped g laciated volley, C l i n t o n Canyon, N e w Zea land
Horns a n d oretes i n glaciated area, Switzerland
Retreating ice s heet
I D EALIZE D G LAC I A L LA N DSCAPES show typical depo sitional features . They are collectively ca l led till deposits . Glacial deposits of rock frag ments a re carried by the g lacier on its surface withi n the ice and at its base . This materi a l is deposited either beneath or at the foot of the ice fiel d , form ing unsorted and unbedded "ti l l . " Me ltwater strea ms flowing from the g l acier form sorted , stratified g l a ciofluvi a l or outwa sh deposits. These and other g lacial deposits are often descri bed as "drift . " Deposits a l so occur during retreat of ice.
MORAINES ore deposits of g l a DRUMLI NS ore l ow ,
c i a l t i l l formed either as arcuate mounds at the snout of the g lacier (terminal moraines) or a s sheets of t i l l over considerable areas (boulder c l o y ) . Successive term i n a l moraines often mark retreat stages of g l aciers (recessional moraines). Moraines ore mode u p of a variety of unsorted rock frag ments i n unbedded cloy matri x .
ERRATICS ore boulders of "for
eign" rock carried by g laciers . Some ore up to 1 00 feet across, and most ore found many m i les
rounded h i l l s , sometimes reaching a m i l e i n length, found in g l aci ated area s . Aligned i n the d i rection of ice flow, the i r steeper, b l u nter ends point toward the d i rection from which the ice come. They are f o r m ed by p l a st e r i n g of t i l l around some resistant rock m o s s . They p r o d u c e a c h a ra c t er i s t i c "basket-of-eggs" topography.
from their points of orig i n . They often hove b l unted edges and rather smooth faces, but most lock glacial striations .
59
KAMES
a re isolated h i l l s af strat ified materi a l farmed from debris that fe l l i nto openings i n retreat ing or stag nant ice. Kame ter-
races are benches of stratified mater i a l deposited between the edge of o val ley g lacier and the wa l l of the val ley.
GLAC I O F LU V I A L D E PO S I T S
a re a l l sorted and bed ded strea m deposits. Outwash depos its, formed by meltwater strea m s , a re flat, i nterlocking a l l uvia l fa n s .
ESKERS a r e l o n g ,
narrow, and often branching sinuous ridges of poorly sorted g ravel and sand formed by deposition from for mer g l a c i a l stream s .
KETTLE HOLES
are depressions (sometimes fi l led by lakes) due to melting of large blocks of stag nant ice, found in any typica l g la c i a l depos i t .
G LAC I A L LAKE D E PO S I TS are formed ei ther by meltwater or the b l ocking of river cou rses . Deltas and beaches mark the levels of many such lakes.
VARVE CLAYS are lake deposits
of fi ne-grai ned silt, showing reg u l a r s e a s o n a l a l t e r n a t i o n s of l i g ht-colored, thicker bands deposi ted during wet, summer months and thinner, dark bands r e p r e s e n t i n g the fi n e r , o f t e n orga n i c , mate r i a l of wi nter deposits that settle below t h e fro zen lake surface.
Max i m u m extent of P l e i stocene ice sheets and g l a c iers
A N C I E N T P E R I O D S O F G LACIAT I O N produced l a nd fea tures sti l l i n evidence today. The features a l ready descri bed can be seen in connection with existing g l aciers, but older g lacial deposits and erosiona l features prove the occur rence of earlier g l acial episodes . The most recent of these i s the Pleistocene g l aciation which began a bout two m i l l io n yea rs ago . It i nvolved four m a j o r episodes o f g laciati o n , when continenta l i c e sheets covered a b o u t o n e qua rter of the earth's surface, including parts of North America , northern E u rope, and northern Asi a . G l a c i a l adva nces were sepa rated by warmer, i nterg lacia l period s . I n the areas outside those covered by g laciers, espec i a l l y i n the Southern Hemisphere, corresponding p l uvia l periods of abnorma l l y heavy rainfa l l marked Pleistocene times, prob ably ca used by changes i n the genera l pattern of wind circulation produced by conti nenta l g laciers. Pleistocene g l aciation molded such fa m i l i a r featu res of our present la ndsca pes as the jagged pea ks of the Rockies and the Alps, the rich fa rm soi l s of the northern midwestern states, and the Great Lakes .
61
G LAC I AT I O N
is i m portant because it has m o lded the topography of much of the N o rthern Hem isphere . It a lso poses a number of basic geologic problems.
CHANGES IN SEA LEVEL of up CAUSES OF ICE AGES are sti l l
to 300 feet occu rred when much of the ocean's water was l ocked in c o n t i n e n t a l g l a c i e r s . E v e n today, if present g laci ers a n d ice sheets that cover 1 0 percent of the earth's surface were to melt, sea level would r i se by some 300 f e e t . The c o n t i n e n t a l m a r g i n s wou ld b e flooded, and many of the world's major ports would be submerged . T h i s may happen agai n . If it does not, a n d w e are i nstead living i n an i nterg lacial rather than postg lacial episode, then c o n t i n e n t a l g l a c i e r s m a y a g a i n spread across m u c h of the earth .
unknown . Possible causes m a y be changes i n the broad pattern of the circulation of the oceans, changes i n the relative position of the earth and s u n , changes i n solar radiation, a nd the presence of some blanket such a s volcanic dust to reduce solar radiation reaching the earth . I t has also been suggested that the P leisto cene g laciation of the N orthern Hemi sphere could have been the result of surges i n the Antarctic ice sheet prod u c i n g wide ice shelves around the Southern Ocea n , which cooled the North ern Hemisphere.
LOADING THE CRUST
PRE-PLEISTOCENE GLACIA TIONS are much less easy to
by ice caused sags to develop which reached about 1 , 500 ft. under the thickest ice. With the melting of the g laciers, the crust began to rise a g a i n , and the h i story of this rise can be traced i n Scand ina via, N orth America, and Europe . The crust rises a bout 9 i nches per century. U PU ", IN METERS
'
Since 6800 B .
detect than the very recent Pleis tocene. One major epi sode of g l a c i a t i o n t o o k p l a c e in t h e Southern Hemisphere i n Permo Carboniferous times, about 230 million years a g o . T i l l i tes (indur ated g l a c i a l t i l ls) and striated rock pavements show large areas of Austra l i a , South A m e r i c a , India, a n d South Africa t o have been g l a c iated . The cha racter of these g l a c i a l deposits suggests that these areas, now remote, formed a s i n g l e massive continent (Gondwanaland) at that t i m e . There i s a l so evidence of a n Ordovician (about 4 5 0 m i l l i o n years a g o ) and a late Pre-Cam brian g laciation (about 600 m i l l i o n yea rs ago).
T H E OCEANS
Oceans play a major role in the earth's natura l processes beca use of their production a nd control of c l imate, supply ing moisture to the atmosphere and provid i ng a vast c l i matic regu l a tor. They form the u l ti mate site o f deposition of a l most all sedi ment a n d are the home of m a ny living species of a n i m a l s and p l a nts . The ocea n s cover over 70 percent of the earth's surface . The continents a re sur rounded by sha l l ow, gently sloping conti nenta l shelves . The average ocean depth is a l most three m i les, but trenches, u p to 36, 000 feet deep, a re found i n places . Although much of the deep ocean floor is a flat p l a i n , some parts a re more mounta i nous than the mounta i n regions of dry land . A worldwide system of midocea n i c ridges includes submarine mounta i n cha i ns , marked by i ntense vulcanism and earthquake a ctivity, a nd offset by transform fa ults . They a re the sites of the formation of new crusta l rocks ( p . 1 4 1 ) . There are a lso many volcanic islands, including many submerged below sea level .
C a l i forn ia Coast s hows force of brea k i n g waves erod i n g s hore l i ne .
OCEANIC CURRENTS AND DRIFTS
warm
...
cool
�
1 . N . Equatorial 2 . S . Equatorial 3 . E q . countercurrent 4. N. Atlantic drift 5. N. Pacific
CURRENTS
1 0.
have a major influ ence on world weather patterns . Differences in the density of sea water of varying sa l i nity a nd dif ferences i n temperature produce water c i rculation i n the ocea n s . T h e colder, m o r e saline, denser water sinks downward to produce deep ocean currents. Nearer the surface of the sea, the combi ned influence of wi nds and the rota tion of the earth produce the more fam i liar surface currents, including the Gulf Strea m . These s u r f a c e c u r re n t s f a l l o w g r e a t , swi r l i ng routes around t h e ocean bas i n s and the equator. Some currents move at speeds of aver 1 00 m i l e s a day.
64
6. 7. 8. 9.
Humboldt Kuroshio A l a ska Labrador Canaries
11. 1 2. 1 3. 1 4. 15.
Falkland Benguela West W i n d Drift F l orida C a l ifornia
SEA WATER i n c l udes about 3 . 5
percent o f d i ssolved chemica l s by wei g h t . Salt ( N aC 1 ) i s the most common solute, with smaller quantities o f magnesium chlo ride, magnesium a n d calcium sul fates, and traces of about 40 other e lements . Salinity i s the number of grams of these dis so lved salts i n 1 , 000 grams of sea water. Although the proportions of these salts to one another are very s i m i l a r throughout the oceans o f the worl d , the tota l sa linity of the oceans varies from place to place and with depth . It i s l ow near river mouths, for example, and h i g h i n areas of h i g h evaporati o n .
TIDES are twice-dai l y movements
of b i l l ions of tons of oceon woter i nfluenced by mony factors o n the surfoce of the eorth as wel l as from spoce . Moinly, the g ravita tiona l pull of the moon upon the earth causes the waters to bulge toward it twice a day, creating what we ca l l h i g h tides . The correspond i n g bulge or h i g h
WAVES
are produced chiefly b y t h e drag of winds on the surface of water. The water i s driven into a c i rcular motion, but only the wave form, not the water itself, moves across the ocean surface . Waves genera l l y affect o n l y the uppermost part of the ocea n s . Wave b a s e i s h a l f t h e wave length of any particular wave syste m . When they run into shal low water, waves d rag botto m , and the topmost water particles break against the shore. Waves play an i m portant part in the shaping of coastlines, both in sed i ment transport and in ero sion . Some large waves (tsuna m i ) a r e caused by earthquakes .
tide on the d i stant side of the earth from the moon i s caused by the cor respo n d i n g lower attroction o f the moon at t h i s g reater d istance, a l lowing t h e o c e a n s to " swi n g " outwa r d . Tides r i s e o n l y two o r three feet o n open coast l i nes, but i n restricted channels can reach fifty feet .
T H E E D G E S OF T H E C O N T I N E NTS
are commonly marked by margins of broa d , flat shelves which slope gently (at a bout 1 : 1 000) to a depth of a bout 450 feet. At this depth , they merge i nto the steeper continenta l slope. The width of shelves varies from a few m i les to 200 or m o re m i l es. Commonly, shelves a re a bout 30 m i les wide. The shelves seem to be formed by the deposition a nd erosion of fa irly young sedi ments, many of them of Pleistocene age. Changes in sea level of some 500 feet have be�n i nvolved during this period .
T H E C ON TI N E N TAL S H E LF AND SLOPE r i m the conti nents,
t h e w i d e s h e l f d r o p p i n g off steeply at t h e slope t o t h e depths of the seafloor. At the base of the continental slope , there is often a convex rise, formed from s l u m pe d s e d i m e n t s . T h i s a rea
G; .!
0 .. -c c:
:;:
::> 0 ..c: ....
0
littora l : l Sha l low l Zone T : Water : 1 Neritic Zone 1
4 8 12
CONTI N E N TAL PLATFORM
ranges from a few m i les to about 1 00 m i les i n width. The g reat vertical exaggera tion of the diagram suggests a much steeper profi le than rea l l y e x i s t s . Even so, the s l o p e i s a l most a hundred ti mes steeper than the shelf.
Deep water
/ Effective Sun l ight- P l a n kton Zone Twi l i g ht Zon e Com p letely clark Aloyua l Zane OCEANIC PLATFORM BASIN
SUBMA R I N E CANYO N S
cut through the continental shelves ond slopes and are widely dis tri buted a long the edges of con t i n e n t s . S o m e s e e m to b e continuations o f rivers o n the land, but others show no such relation to drainage and do not extend across the conti nental she lves . A l l canyons tend t o have a V-shaped profi l e and to have tri butary systems much l i ke those of terrestrial rivers . Their deeper mouths are marked by g reat del tal ike fa ns of sed i ment, which g radua l l y build u p to form the continenta l rise.
Submarine canyons are thought to be formed by the erosion of turbidity currents, which some times attain considerable ve loc ity. Heavy with s i l t , they have a s t r o n g sco u r i n g a n d e r o s i ve power.
Experimenta l turbid ity current i n a la boratory ta n k
T U R BIDITY C U R R E N TS
are dense, flowing masses o f sed i ment-ca rrying water flowing at speeds of up to 50 m i les per hour. Many are probably trig g e red by e a r t h q u a k e d i st u r bances o f unconsol idated sed i ment on the continental shelves and slopes. The coincidence of some submarine canyons with
river courses, such as those of the Hudson and Congo, has been thought t o b e the resu lt of river erosion at earlier periods o f l o w e r sea l e v e l . P r o b a b l y i t i s the resu lt o f e ither the pres ence of thicker, unstable masses of sedi ment near river mouths or t u r b i d i t y fl o w f r o m r i v e r mouths i n times o f flood . 8000 6000 4000 2000
O feet
Coa sta l view of H a rgrove's lookout, New South Wa les, A u stra l ia COASTLI N E S mark the boundaries of l a nd and sea . Although the g reat variety of rock types, structu res , cur rents, tides, c l i mate, and fluctuating sea l eve l s produce many different types of coast l i nes, each can be understood as the product of three simple processes: erosi o n , deposi tio n , and changing sea level . Coasta l erosion is the resu lt of the twice-daily pound ing by the sea , wea ring down the marg i ns of the l a n d , creating coasta l features, and cutting back the shore l i n e at a rate of severa l feet a yea r.
CLIFFS AND WAVE-CUT PLAT CAVES are formed b y erosion FORMS a r e c h a r a c t e r i s t i c o f a long a conspicuous l i ne of wea k
shore l i nes u n d e r g o i n g eros i o n . Waves undercut the rocks near sea leve l .
ness i n a c l i ff, such a s along joints and fa u l t s . Conti n u i ng erosion may form an arch .
BAYS AND H EADLANDS
are produced b y relative differences i n the resistance to erosion of coasta l rocks. The more resistant standout as head lands, but u l t i m a t e l y, the c o n c e n t r a t i o n of wave erosion on the headlands and deposition i n the bays have a tendency to produce a straight coast l i n e .
Drowned Va l l ey, v iew from MI. Wel l i ngto n , Ta s m a n i a A C H A N G I N G S E A L E V E L is represented by many fea tures a round coast l i nes. With i n historic times, esta b l i shed towns have been submerged. Ra i sed beaches a nd wave cut platforms are common in many a reas, rising many feet above present sea leve l . Far i n land a nd high on the sl opes of mounta i n s , foss i l s of marine a n i m a l s g ive further proof of older and more profound changes in sea leve l , reflecti ng major changes i n the geog raphy of the past. Submergence and emergence of coastli nes mod ify the genera l features of erosion and depositi o n .
EMERGENT COASTLINES
are less common thon the sub merged . They ore marked by
rai sed beaches and c l i ffs, and often by an a l most flat coasta l p l a i n , sloping gently seaward.
S U BMERGED COASTLI NES,
d u e to postg l a c i a l r i s i n g sea leve l , a r e i n d e n ted c o a st l i n e s w i t h deep i n lets a n d su bmerged glacial va l leys. The "grain" of the coastl i ne depends upon the character and structure of the roc k s . In At l a n t i c - type c o a s t l i nes, t h e structural trends are more or less perpend i c u l a r to the coast . I n Pacific type, they are para l l e l to the coast.
69
MAR I N E D E PO S I T I O N
may be recogn i zed as the domi nant process where shoreli nes ore ma rked by a number of fa m i l ia r features . U ltimately, the ba lance of coasta l ero sion and deposition tends to produce a coastline i n tem pora ry equi l i b ri u m . Although this is more quickly formed i n soft strata , it req u i res thousa nds of yea rs in resistant rocks .
BEACHES
c o n s i s t of sed i m e n t sorted o n d transported b y waves and currents . Most of the sed i m e n t i s the s i z e of s a n d or o f g rave l , but l o c a l c o b b l e a n d boulder deposits are a l s o com m o n . Beaches vary g reatly, de pend ing upon the sed iment supply, the form of the coast line, wave and current cond i -
l i o n s , and seasonal c h a n g e s . O n i rregular coast l i nes, t h e y t e n d t o b e confined to t h e bays . Oblique waves a n d l o n g s h o r e c u r re n t s often produce constant latera I m o ve m e n t of b e a c h m a t e r i a l . Beaches thus exist i n a state of dynamic eq u i l ibrium, a s a moving body of wave-washed and sorted sediment.
I nteraction of river a n d marine deposition a n d ero s i o n , Wa l e s
OFFSHOR E BARS
formed of sand a n d pebbles a r e genera l l y separated f r o m the m a i n shore line by narrow lagoons . They are ancient beach deposits that are characte r i s t i c of submerged coasts . I n N orth America, off shore bars are common a l ong the Atlantic and G u l f coasts. They are genera l l y para l l e l to exist i ng coast l i n e s .
SANDBARS
a re formed o n indented coast l i nes by longshore drift of sed i ments para l l e l with the coast. When sedi ment i s car ried i nto deeper water, as i n a bay, the energy of the waves or currents i s reduced , and the sed iment i s deposited a s a bar. The s l ower the current, the more rap idly the sed iment i s deposited . The bar is an e l o ngation of the adjacent bea c h , partly or com pletely cutting off the bay. Spits are bars that extend i nto open water rather than i nto a bay. The free ends of many spits are curved l a n d w a r d by wave refra c t i o n . Spit growth may lead to blocking of harbors and d i version of rivers that flow i nto the sea , sometimes req u i r i ng d redg i n g .
Typical c l iff a n d beac h scenery s h o w s ba l a n c e of e r o s i o n a n d deposition . ( Dorset, E n g l a n d )
DELTAS ( p .
45) are formed b y ra pid deposition of material car ried by a river when i t enters the deep water of a lake or the sea and loses its velocity. All deltas have a s i m i l a r pattern of deposi t i o n a n d sed iment d i stributio n . Shape a n d size vary, depending upon local cond i t i o n s .
STORM BEACHES
(or b e r m s ) a r e formed by sto r m s that throw g ravel and boulders u p a bove the normal high-tide leve l .
Wide, sandy beach with storm beach near c l iffs at Lavernock, Wa les
Globigerina Calca reou s - Rad i o l a r i a n S i l iceou ! Terr igenous } } ooze ooze - Red Clay - Pteropod d i atom
MAR I N E S E D I M E N T S
a re classified in three broad cate gories: l ittora l , neritic, a nd deep-sea sedi mentation . lit tora l sed i ments form between high- and l ow-tide leve l s ; neritic sed i ments accum u late on t h e conti nenta l s h e l f . These two g roups of sha l l ow-water sed iments cover only a bout 8 percent of the ocean floor. They are made up of a var iable mixture of terrigenous or land-derived debris, chemica l prec i p i tates, a n d org a n i c deposits . They d i ffer from p l a ce
to p lace due to va riations in coastli nes, rivers, and changes i'n sea level . I n genera l , in sha l l ow-water sed i ments , there is a d i rect re lationshi p between the size of a sed iment pa rticle and the d i stance to which a given current wi l l carry it, the l a rger particles bei ng deposited nea rer the source . This pattern is modified by the action of waves, currents , and turbidity currents . Deep-sea sed i ments deposited outside the conti nenta l shelf form a layer genera l l y less than 2 , 000 feet thick over the deeper parts of the ocean floor. They are much thinner and younger i n age than we shou ld pred ict from knowledge of present rates of sed imentatio n . The a byssa l parts of the 72
ocean show much more un iform sed i ments than the conti nenta l margins, where most terrestrial debris is deposited . Those of the bathya l zone, deposited on the conti nenta l sl opes to a depth of a bout 1 2 , 000 feet, i n c l ude muds of va rious k i n d s . The sed i ments of g reater a byssa l depths are red clays a nd va rious oozes which cover 30 percent and 47 percent of the ocea n floor respectively.
CALCAREOUS OOZES are fine
med i u m -grai ned sed i ments that c o v e r a l m o s t h a l f the o c e a n floors. They occur down t o a depth of a l most 1 5 , 000 feet . Below t h a t depth, t h e calcareous tests of the planktonic fora m i n i fer G / o b i g e r i n a a n d pteropod molluscs, which form most of the sediments, are disso lved .
SILICEOUS OOZES
are derived from the rem a i n s of surface-living o r g a n i s m s ( d i a t o m s and r a d i o/aria) for m i n g very slowly a t g reat depth i n t h e ocea n s . Dia tom oozes a re found chiefly in polar seas where predatory crea tures are less c o m m o n . Rad iolar ian ooze i s most commonly found i n the warm, tropica l waters.
DARK RED CLAY
i s formed from meteoric dust and from very fi ne terrigenous o r volcanic particles carried by the wind or i n suspen sion i n sea water. It may form as s l owly a s one inch every 250, 000 yea r s . It may i n c lude volcanic ash laye r s . Other sed i ments, such a s manganese nodules (shown here), are present i n some parts of the ocean floor.
73
WIND is most effective i n trans
port and deposition i n deserts, n e a r s h o re l i n e s , and in o t h e r places where there is a supply of dry, fine-grained, l oose sed i ment with little vegetation to hold it tog ether. Great S a n d D u n e s National Monument, shown here, is formed by deposition of wind borne sand a g a i nst mounta i n range. W I N DS
Winds a re movements of the atmosphere brought a bout not only by the rotation of the earth but by unequal tem peratures o n the earth . The heat of the sun, the c hief source of this c i rcu lation, i s more concentrated i n the tropics than i n high latitudes. This prod uces vast atmo spheric convection currents , having a broa d , consta nt overa l l d i stribution that reflects the earth's rotation . It a l so shows wide local va riations in speed and d i rectio n due to d i ff e r e n c e s i n t o p o g r a p h y a n d o t h e r a t m o s p h e r i c conditions. A major role is played by the wind in the d istribution of water from the oceans to the land . Water vapor, i n turn, has a b l a n keting effect that keeps the earth's surface tem perature higher than it would otherwise be . Wind is an agent of transport and, to a lesser extent, of erosion . In this, it resembles flowing water, but because of its much lower density (only a bout 1 /800 that of water), it i s fa r less effective a nd genera l l y tra nsports only the fi ner d ust par ticles. Dust from volcanic explosions wi l l often g ive bri l l iant sunsets i n d istant l a nds for many months after the explo sion . Winds transport through the atmosphere compara tive ly large q uantities of salt crysta ls gathered from the ocea n's surface . 74
WI N D E R O S I O N is very l i m i ted in extent a nd effect . It is largely confined to desert a reas, but even there it i s l i m i ted to a height of a bout 1 8 inches above g round leve l .
DESERT PLATFORMS
are clean, wi ndswept areas where pebbles may have been rol led and bounced a long by the force of strong winds . larger cobbles and boulders are left behind .
VENTIFACTS, found
in deserts, are pebbles or cobbles that have devel o ped polished surfaces and sharp edges under wind abrasion .
DESERT EROSION
tends to expose bare rock surfaces, which may stand up without a cover of vegetation, as i n Paki stan . Semiarid landscape has di sti nctive erosiona l character.
W I N D D E PO S I T S
consist of transported particles that a re effectively sorted accord i ng to size because of the l i m ited carrying capacity of the wind . Sand dunes, for exa m p l e , genera l ly consist o f s a n d g r a i n s o f more or l ess u niform size, which a re rounded and pitted or frosted by abrasi o n . Sand dunes a re found i n a reas where there is a large supply of d ry, l oose, fi ne-grai ned materia l . Like snow drifts, they form a round loca l obstructio n s . They a l so m i g rate downwi nd . Their particu l a r size and form depend upon the sand supply, the presence of vegetation, and the vel ocity and con stancy of d i rection of the preva i l i n g wi nd . Dunes may be tra nsverse or long itud i n a l to the wind di recti o n .
BARCHANS
a r e crescentic dunes that often build up to 400 yards long and 1 00 feet h i g h , and are formed mostly i n deserts with more or less constant wind di rec tions. They are not static and may mig rate up to 60 feet per yea r.
The crescent points show down wind di recti o n . Winds a l so pro duce giant ripple marks on sand surfaces. Barchons usua l l y ore found in g roups, or swarms, and moy form long l i nes, or chains, across a p l a i n .
SAND-DUNE SHAPE
typica l l y h a s gentle wi ndward slope and steep leeward s l ope, dawn which sand grains slide o r ro l l . Dotted line shows haw continuous move ment of sand g r a i n s produces migration of whole sand dunes . Dunes take many for m s . It wou l d n o t b e e a s y t o compile a complete list of all the varieties. Va riations in form include sca l l oped sides and i rreg u l a r i ties i n plan of the crest .
Cros s-bedd i n g i n stationary d u n e
Cross-bed d i n g i n m i g ratory d u n e
AN CI ENT D U N E DEPOSITS,
preserved i n such sed imentary rocks as those of the Nava j o Sandstone i n Zion Canyon National Park, display aeo l i a n bedd i n g , sorti n g , and s a n d grain round ing similar to those of pres ent-day dunes . Ca reful mapping of b e d d i n g d i r e c t i o n s revea l s ancient wind d i rections. I n this way, i t has been possi b l e to make a map of the Permian winds of southwestern U n i ted States 225 m i l l ion years a g o . ..
LOESS DEPOSITS,
formed of fi ne-grai ned s i l t , lack ony bed d i n g b u t o f t e n h a v e ve r t i c a l joints . Transpo rted b y wind from d e s e r t s , f r o m d r i e d - u p fl o o d plains, from river courses, o r from g l acial deposits, they are c o m m o n in m i d w e s t e r n U n i ted States, C h i n a , E u rope, and in many areas surrounding the wor ld's deserts a n d g l a c i a l out wash areas. Loess produces fer tile s o i l s , partly beause of its very high porosity. loess i s ye l l ow or buff i n color and often forms ver tical c l iffs . Artifi c i a l caves i n eas ily worked loess may provide homes.
.
�JJ:
Cross-beddi n g , in wind -depos i ted sandstone, reflects its for· motion i n ancient sand dunes, east of Echo C l iffs, Arizona .
77
PRODUCTS OF D E PO S I T I O N
Sed i mentary rocks a re genera lly formed from the brea k down of older rocks by weathering and the agents of erosion descri bed on p p . 34-76 . A few are chemical pre cipitates, or orga nic debri s . Sed i menta ry rocks cover about 75 percent of the earth's surface. C LAST I C O R D E T R I TAL S E D I M E N TARY R O C KS a re formed from the debris of preexisting rocks or org a n i s m s . T h e weathered r o c k material is genera l l y transported before it is deposited . This movement often g ives round grains. The debris i s eventually laid down in horizonta l layers, usua l l y a s marine deposits but sometimes as depos its from rivers, lakes, g laciers, or wind . C lastic rocks a re solidified sed i ments .
CONGLOMERATE c o n s i s t s o f SANDSTONE
rounded pebbles or boulders held tightly i n o finer-grai ned matrix . The pebbles ore usua l l y of quartz at l e a s t '/• i n c h or m o r e i n dia meter.
consists of sand size particles, usua l l y of quartz. It may show considerable varia tion i n cementing minerals, i n rounding and s o r t i n g of parti cles, and i n forms of bedding .
ARKOSE
CALCARENITE
is a q u a r t z - fe l d s p a r sandstone u s u a l l y formed in desert a reas by rapid erosion and d e p o s i t i o n of f e l d s p a r - r i c h igneous rocks .
consi sts of bro ken shel l s or other organ i c mate rial and fragments from older li mestones . It i s deposited as sed i mentary debri s .
GRAYWACKE i s a poorly sorted SHALE c o n s i s t s o f v e r y fi n e
m i x t u r e of r o c k f r a g m e n t s , quartz, and feldspar fragments i n a clay matrix. Often formed by turbidite flows {p. 67), g ray wacke a lways i n d i cates rapid e r o s i o n a n d d e po s i t i o n u n d e r unstable conditions.
grai ned particles o f quartz and clay m i nera l s . I t i s consol idated mud that has been depo s i ted in lakes, seas, and s i m i l a r environ ments. About 45 percent of a l l exposed sed i mentary rocks are sha les.
O R GA N I C S E D I M E N TARY R O C KS a re formed from orga nic debri s - the deposits or rema ins of once - l iving organisms (she l l s , cora l s , calcareous a lgae, wood , p l a nts, bones , etc . ) . Although they a re a form of c l a stic rock, orga nic rocks may conta i n more and better-preserved foss i l s , as they a re laid down near the place where the a n i m a l or plant once l ived . C H EM I CALLY F O R M E D S E D I M E N TARY R O C KS
consist of i nterlocking crysta l s precipitated from soluti o n . They, therefore, lack the debri s-cement compositio n of other sed i mentary rocks . A decrease i n pressure, a n i n crease in temperature, or contact with new mater i a l s may cause m i nera l s to precipitate from solution .
LIMESTONE
consists chiefly of calcite from concentrated shel l , cora l , algae, a n d other debr i s . I t may g rade i nto dolomite, char acteri zed by the presence of cal cium and magnesi u m carbonate (p. 28). Chalk i s a fi ne-grai ned l imestone of m i nute cocco l i t h s . Travertine i s l i mestone prec i p i tated by spring s .
r-
bitu m inou: oa l
EVAPORITES
are chem ical pre cipitates, farmed by evaporation i n s h a l l ow, land-locked basins of water. They vary g reatly i n tex ture and composition . Rock salt, gypsum, anhydrite, and potas sium salts are the most common. I m portant i ndustria l m i nera l s . S E D I M E N TA R Y ROCKS
COAL
i s c o n s o l i d a te d p e a t , formed b y the decomposition of woody plant debris (p. 9 8 ) . I t is an organ i c rock, and plant struc tures may sti l l be preserved i n i t . C o a l s g rade from l i g n ite to anthracite, which h a s a bout 95 percent carbon in i t .
a r e i m portant natural resources . Shales and l i mestones a re used for cement, clays for ceramics; other rocks are used for road meta l . Sed imen tary iron ores, bauxite, and coa l form the ba sis of much heavy industry; soil i s the ulti mate basis of most of our food supplies.
T H E S I Z E , S H A P E , A N D S O RT I N G of sed i mentary struc tures with i n rocks may provide c l ues to the depositional environment that exi sted during their formati o n . Rounded , frosted , we l l -sorted grains, for example, i nd icate wind deposited sand .
CROSS-BEDDING
is a term applied t a sweeping, a r c l i ke beds that lie at a n acute angle ta the genera l harizantal stratification . It is camman in stream and deltaic depo s i t s , i n deeper marine waters, a nd i n sand dunes . Crass bedd ing reflects the d i rection af current flaw.
scour-and•fi l l g raded bedd i n g
GRADED BEDDING
i s due ta dif f e r e n t i a l s e t t l i n g af m i n e r a l grains, a n d i s useful far determin i n g the c o r r e c t "way u p " i n folded strata . T h e layers af coarse rack have a sharp base and gradua l l y g rade upward into finer-g rai ned materia l s .
VARVED BEDDING i s a type af thin, g raded bedding that has a lternate l a m i nations af coarse and fi ne-grai ned materia l . E spe c i a l l y camman i n g l a c i a l lake deposits, i t i s characteristic of seasonal deposition, each pair af varves representing a yea r. (See page 60 . )
MUD CRACKS
farm where lake and mud deposits are dried by the sun and are preserved by burial under mud.
SCOUR-AND-FILL
structures are farmed by erasion and subse quent fi l l i n g af val leys and chan nels i n a river bed .
RIPPLE MARKS
are produced by wind in desert sand deposits and by waves or currents i n various aqueous envi ronments.
81
THE CRUST: SUBSURFACE CHANGES R u n n i ng water, ice, wind, and other agents of erosion slowly wea r down the surface of the continents. Over the earth as a who l e , some 8 b i l l ion tons of sed i ment is ca rried by rivers i nto the sea every yea r, equ iva lent to some 200 tons per sq uare mile of land surface . This represents an average lowering of the rivers' drai nage areas by approx i mately one foot every 9, 000 years. The u lti mate effect of this erosion wou ld be to reduce the conti nents to a flat surface, but there a re two earth pro cesses that tend to interrupt this conti n u i n g erosion and restore the balance: the tectonic upl ift of a reas of both the existing conti nenta l and offshore a reas of sed i mentation (diastrophism), and the broadly related process of igneous activity (p. 83). Most of these forces act slowly, over long periods of time, and the earth's crust is a lways in a state of dyna mic eq u i l i br i u m , though conti nuously chang i ng , as a result of thei r va rying i nteraction . T H E ROCK CYCLE
Volcanic cones stand out as h i l l s near Springv i l l e , Arizona .
Ig neous rocks form the foundations of the continents, but most of the surface of the continents is made u p of sed i mentary rocks of various ages . These layers have been deposited on and a round a ncient continenta l cores or shields, which a re made c hiefly of granitic igneous a n d meta morphic rocks. T h e cores o f mounta i n chains genera l l y revea l t h e s a m e rocks. These ig neous rocks were genera l l y formed at great depths a nd later upl ifted , eroded , and covered with a relatively thin veneer of sed i ments . VOLCANOES
Volca noes are mounta i ns or h i l l s , ra nging from sma l l coni ca l h i l l s t o peaks with 1 4 , 000-foot relief, formed b y lava and rock debris ejected from within the earth's crust. Of the more than 500 active volcanoes, some extrude m olten lava , others erupt ash and solid (pyroclastic) fragments. Sti l l others eject both , and all emit large quantities of stea m and va rious gases . Some eruptions a re relatively "quiet , " others expl osive .
83
VOLCA N I C ACT I O N results in the formation of five basic types of vol ca noes, but no two a re ever quite a l i k e . The kinds of materi a l s that erupt from a vo lcano l a rgely deter mine the shape of its cone . F l uid streams of lava travel fa r and usua l l y produce wide-based mounta i ns; ash, viscous lava s , a nd c i nders usua l l y build up steep cones .
CIN DER CONES
are steep sided, symmetrica l cones, such a s Vesuvius, formed b y the eruption of ci nders, ash, and ather pyro clastic products.
SHIELD VOLCANOES, l i ke those
i n H a wa i i , a r e broad d o m e s formed b y lava flows from a cen tra l vent o r from fissures a nd par asitic vents.
COMPOSITE CONES
are formed from interbedded lava flows and pyroclastic debr i s . They a re i ntermed iate i n form between cin der and shield vo lcanoes .
CALDERAS
are formed by the c o l lapse of the top of a volcano f o l l o w i n g an e x p l o s i o n . N ew cones may be born in the ca ldera . Crater Lake i n Oregon i s an exa m p l e o f this type o f volcano.
84
PLATEAU BASALTS cover g reat
areas i n the Columbia R iver Va l ley, Iceland, and I nd i a , and a re apparently extruded , to spread i n thin sheets, not from central vents but from cracks or fissures .
VOLCA N I C P R O D U CTS i n c l ude bombs, cinders, ash, and dust, as we l l a s lava and gases . Most l ava has a basa ltic composition (p. 93) consisting of plag ioclase feldspars, p y r o x e n e , a n d o l i v i n e . E r u p t e d a t t e m p e r a t u r e s of between 900° a n d 1 , 200°C , it can flow for g reat d i stances at considera b l e speed s .
THE TYPES OF LAVA
differ i n form a n d texture . P i l low l ava ( l eft) is formed by submarine vol canoes . B l ocky lava h a s a jogged
surface and i s produced by s u d d e n gas escape . R o p y l a v a (right) forms at a higher temperature than blocky lava .
VOLCANIC BOMB
VOLCANIC T U F F
d le-sha ped form .
shows spin
DISTRIBUTION OF VOLCANOES
in na r r o w b e l t s r e fl e c t s e a r t h ' s major plates ( p p . 1 44- 1 47) a n d is related to deep interior processes. Many lie around the Pacific Ocea n .
i s a fi n e grained, pyroc lastic roc k .
Others o r e formed i n such areas of r e c e n t t e c t o n i c a c t i v i t y as t h e Mediterranean o r the African Rift Vol leys . Most oceanic islands ore volcanic.
I NT R U S I V E I G N E O U S R O C K S a re formed from magma rising within the earth's crust. U n l i ke the extrusive vo lcanic rocks, i ntrusive rocks crysta l l i ze below the earth's surface, and the i r presence becomes obvious only after the country rock i nto which they were intruded has been removed by erosio n . I ntrusions show g reat va riation i n for m . Some c u t across bedding pla nes (discorda nt), while others run pa ra l l e l with them (concordant) . They range i n size from d i kes measur ing a few i nches wide to bathol iths hundreds of miles across . I ntrusive rocks a re often associated with i m portant meta l l i c m i nera l deposits, such as copper or nickel .
FORMS OF IGNEOUS INTRUSIONS PLUTONIC, INTRUSIVE ROCKS, DIKES are usua l l y vertical intru
similar to granite, are identified by having a coarser crystal tex ture, produced by s lower coo l i ng than that in extrusive rocks. In genera l , rocks formed ot sha llow depths i n the crust hove o n i nter mediate texture.
s i ve sheets , often d i sc o r d a n t . They may occur in vast d i k e swarms, asso_c i a ted with a centra l volcanic neck or intrusive centers. Dikes and sills frequently have fi n e - g r a i n e d , c h i l l e d c o n t a c t marg i n s .
Dikes, such as those on left, s h o w c h i l led marg i n s o f li ne-gra i ned texture, where i n contact with the country rock, as on r i g ht.
Old volca n i c neck at S h i prock, New Mexico, is c i r c u l a r in outl ine and fed a rad iating series of once molten d i kes.
VOLCANIC PLUGS,
or necks, ore more or less cylindrica l , ver tica l -wa l l ed intrusions, genera l l y of porphyritic roc k . Whether o r n o t they p a s s u pward i nto Iovas, or breccias, depends to o g reat extent upon the depth of local weathering .
SILLS
o r e h o r i z o n t a l i n t r u s i ve sheets, either concordant or dis cordant. Such s i l l s os the 900-
foot-thick P a l i sades on the Hud son River show vertical differen tiation because of g ravitational c r y s ta l l i za t i o n . Some s i l l s a n d dikes o r e o n l y o few feet i n thick n e s s ; some d e v e l o p c o l u m n a r jointing s i m i l a r t o that o f lovo flows . S i l l s (A) ore d i stingui shed from lovo flows (B) by the baking of ove r l y i n g a d j a c e n t c o u n t r y rock b y s i l l s and erosional con· tocts i n buried lovo flows .
fine-gra i ned c h i l led zone 1 % olivine PALI SADES SILL 25 % o l i v i n e fi ne-gra i ned c h i l led zone 1 % olivine
LACCOLITHS
ore dome-shaped intrusions having o flat bose but on a r c h e d c o n c o r d a n t r o o f ,
compact basalt
col u m na r
formed b y intrusive pressure, and o rather flat floor. They may show differentiatio n .
country rock sate l l ite stock '-.....,. roof
roof pendant
I
BATHOLITH
BATHOLITHS,
or pl utons, ore major, com plex, intrusive masses, genera l l y g ranitic and often several hundred m i les in exten t . They o r e t h e largest i ntrusions and a r e found in a reas of major tectonic deforma tion, as i n the Idaho Batho l i th and in the conti nenta l shields. Although many plu tonic granites show sharp contacts with older rocks, and ore, therefore, intrusive in a strict sense, others show complete transition with surrounding country rocks and seem to resu lt from the metamorphism or granitizotion of older sedi m e n t a r y and m e t a m o r p h i c Stock a n d a s soc iated laccol ith, structures. Henry Mou nta i n s , Utah
LOPOLITH S A N D LAY E R E D INTR USIONS o r e s a u c e r l i k e
STOCKS
ore discordant, intru sive masses, a few m i les in diam e t e r . S o m e ore p l u t o n i c , b u t others pass upward into ancient volcanic plugs. They ore smaller than batho liths.
ba s i c lopolith
i ntrusions, u p to 200 mi les across, with complex geologic histories. D i sti nct m i nera l og i c a l l a y e r i n g is present, the more basic minerals genera l l y being i n t h e l ower layers, as i n t h e Bush veld complex i n South Africa and the Sudbury, Onta r i o , lopolith . Gravity settling and convection currents seem responsible for this layering .
roof rocks
gran ite
M I N ERAL COMPOS I T I ON
K-feldspar
z
i5 iiii
0
... > ;;; j .. .... X ... ... > ;;; j .. ....
:!:
Texture
,,1o->o
B i otite
Plagioclase Olivine Pyroxene Am hibole
Pyroclastic
Tuff and breccia
Glassy
Obsidian (massive) Pumice (frothy)
Apha nitic (very fineg r a i n e d)
Rhyo l ite
Andes ite
Basalt
Phaneritic (coarsegrai ned
Gran ite
Diorite
Gabbro
LIGHTER
INTERMED IATE
DARKER
COLOR
C LASS I F I CATI O N O F I G N EO U S ROCKS
We have a l ready seen that ig neous rocks va ry i n their occurrence, which tends to produce differences in texture (crysta l size, shape, and a rrangement) . They a l so vary g reatly i n m i nera l content, a nd thus i n chemical composi tion. Acid rocks (those conta i n i ng qua rtz) m a ke u p the b u l k o f plutonic i ntrusions b u t seem t o b e confi ned to t h e conti nents , whereas basa ltic rocks account for most of the volcanic rock of both the conti nents a nd the ocea n s . Ig neous rocks a re commonly classified by t h e i r texture (the size, shape, and variation i n the i r crysta l l i n e form) and their chemica l composition (represented by thei r con stituent m i nera l s ) . These two factors reflect their rate of cooling and o r i g i n a l magma compositio n . 89
1 1 00°
I
TEMPERATU R E (Centigrade)
Mu scovite mica
zeol ite Qua rtz (Sti l bite)
Reaction series of common s i l icate B i otite m i n e r a l s f r o m i g n e o u s r o c k s . H i g h m i ca te mperature m i n e ra l s are shown on left of diagram. Ol ivine MAGMA
is the molten s i l icate source materi a l from which i g neous rocks are derived . Although lava provides a sur face sample of magma, the increased pressures and tem peratures of deeper magmas permit a hig her gas and water content than at the surface . Ig neous rocks show g reat variation i n chemical compo siti o n , but thi s does not mean that each of the many types has crysta l l i zed from a different kind of magma . It seems probable that a single kind of basa ltic magma i s the parent of a l l va rieties of igneous rocks, and that different chemical compositions resu lt from crysta l l i zation differentiation . Field and l a boratory studies o f igneous rocks show that igneous m i nera ls have a defi nite sequence of crysta l l i za tion : Iron, magnesi u m , and calc-sil icate m i nera l s (such as o l i v i n e , p y r ox e n e , and c a l c i u m - p l a g i o c l a se fe l d s p a r s ) form before sod i u m and potassium feldspars and quartz . This sequence is seen in differentiated i ntrusions. Although crysta l l i zation of a basa ltic magma wou ld norma l l y g ive a basa lt (if the early-formed m i nera l s a re sepa rated from the bulk of the magma by g ravity settling or tectonic pressure) , the rema ining magma wou ld be 90
C R YSTAL S ETTLI N G
acid and relatively rich in si lica and potassium and i n sod i u m a l u m i nosi l i cates. Conti n ued crysta l l i zation a n d sepa ration wou ld then produce a rhyo l itic magma . About 90 percent of the orig i n a l magma wou l d rem a i n as c r y s t a l l i n e r o c k s of b a s i c composition . The hypothesis of a single parent basa ltic magma exp l a i ns many oth erwise puzz l i ng features of igneous rocks , i n c l u d i n g the preponderance of basa lt lavas and the freq uent sma l l a n d late rhyo l itic flows i n basa ltic vo l c a n i c e r u p t i o n s . U n d i s t u r b e d c o o l i n g w o u l d p r o d u ce g r a n i t e s , which wou ld overlie basic rocks . Other facts, however, suggest that such a n expla nation cannot account for a l l granitic rock s . The abundance of granites i n mounta i n ranges, and their apparent conti nuity with meta morphic rocks, i m p l ies that many gran ites are formed by the "graniti zation" of deeply buried sed i men tary rocks i n the roots of mounta i n cha i n s . T h i s metamorphic o r i g i n of granitic, p l utonic rocks m i g ht then prov i d e " i n t r u s ive m a g m a s " a t higher leve l s i n the crust. I t seems u n l i kely that such huge masses of granite could have formed by differ entiation of basic lava s .
Evolution o f granitic magma from basaltic magma A . BASALTIC MAGMA 50% S i 02 1 0% Fe0 + Mg 0 40% ather
B. O l i v i ne, p l a g i o c l a s e fe l d s pa r, a n d pyroxene qysttl l s form at�d settre
magnesium s u btracted ••
D. G RANITIC MAGMA 70% S i 02 2% FeO + MgO melt 28% other
91
Gran ite la ndscape in S i erra Nevada shows sheet l i ke weathe r i n g .
TH E F O R M A N D TEXTU R E
of ig neous rocks differ g reatly in deta i l s of a p pearance and in the g ross forms of the entire rock bod ies. In contrast to vo lcanic lava s , which a re extru sive ig neous rocks, those formed at great depth a re known as i ntrusive or pl utonic (p. 89). These tend to crysta l l ize more s l owly and, therefore, have la rger crysta l s than extrusive rocks . The texture of a n igneous rock depends upon its rate of coo l i n g , and thus on its geo logic mode of formation ( p . 89) . The chemica l and m i nera l content of igneous rocks depends upon the composition of the mag mas from which they were formed . Mag m a s (or melts) rich in s i l i ca produce granitic (acid) type rocks . Those rich in i ro n and magnesium tend to be more mobile, and these produce basa ltic (basic) type rocks. Because of their dura bil ity, many igneous rocks a re used as road materia l . La rge, speci a l l y cut b locks are used a s orna menta l building stones. The color of a n ig neous rock is related to its composi tion . Acid rocks genera l ly tend to be l i g hter i n color than basic ones. 92
GRANITE,
COMMON INTRUSIVE IGNEOUS ROCKS G R A N IT E P O R P H Y R Y
usua l l y l i g ht-colored and c o a r s e - g r a i n e d , c o n t a i n s about 30 percent quartz a n d 60 percent potash feldspar. It may be pinkish red o r black-spotted . Granite is common in many large intrusions and often associated with m i neral deposits.
has ground mass with longer crysta l s (phenocrysts) of feldspar, quartz, o r mica . Very coarse grained acidic rocks (pegmatites) have crysta l s over 40 feet long . S m a l l p o r p h y r i t i c c r y s t a l s a re also found in lava s .
GABBRO
i s dark-colored and has a coarse, granitic-type tex t u r e c o n s i s t i n g of p l a g i o c l a s e feldspar and pyroxene with traces o f other m i nera l s , but i t conta i n s n o quartz .
RHYOLITE
COMMON EXTRUSIVE IGNEOUS ROCKS
is a light, fine grai ned volcanic rock of gra nitic compositio n , often porphy ritic, with phenocrysts of quartz and orthoc lase .
OBSIDIAN AND PUMICE have BASALT, t h e m o s t common lava ,
s i m i l a r composition to rhyo l i te . Both ore rapi d l y cooled . Obsid ian i s a translucent g lass. Pumice is a r h yo l i t i c f r o t h c h a r a c t e r i zed b y cavities left by t h e release of g a s .
is dark, fi ne-grai ned , and rather heavy. It consi sts of pyroxene and a plagioclase feldspar. C i ndery basa lt (scoria) may have second ary m i nera ls within the cavities formed by gas .
93
METAMO RPH I SM
Metamorphism i s the process of change that rocks with i n t h e earth undergo when exposed t o i ncreasing tem pera tures and pressures at which their m i nera l components are no longer sta b l e . Metamorphism may be local - contact meta morphism is d ue to ig neous intrusion ( p p . 86-99) - or regional - a s takes place i n mounta i n b u i l d i n g , when slate, schist, and gneiss a re formed . Metamorphism may take place i n a sol id state, without melting . Effects of meta morphism depend upon the composition, texture, and strength of the orig inal rock, and on the temperature, pressure, and amount of water under which meta morphism ta kes place. Meta morphosed rocks may differ i n texture, m i neral content, and tota l chem ica l com position from the parent roc k .
TEXT U R A L C H A N G E S
are shown by m a s t m e ta m o r p h i c racks. C leavage i n slates (their fissi l ity a l ong defi n i te pla nes) i s produced b y para l lel rea l i g nment of such flaky m i nera ls a s mica, and is often i n c l i ned sharply Ia original beddi ng i n the rack s . Cleavage is m a i n l y t h e resu lt o f the increased pressure of dyna mic metamorph i s m .
FOLIATION
i s t h e development of wavy or contorted layers under more intense metamorphism . It invo lves structura l and m i nera l o g i c a l c h a n g e s . S c h i s t s h a ve closely spaced foliation .
PHYLLITES
have a rather g lossy, s l a t y a p p e a r a n c e c a u s e d by r e c r y s ta l l i za t i o n o f fl a k y m i n era l s a l o ng c l eavage planes. Characteristics a r e i n between schi sts and slates .
GROWTH OF NEW MINERALS
resu lts from i ntens ive meta morphi s m , such a s reg ional meta morphism i nvo lved in mounta i n bui lding . l o s s of s o m e chemical components and add ition of oth ers may produce changes i n tota l chemical composition of the orig inal roc k . Such flaky m i nera l s as mica are unsta ble under these conditions and high-grade meta morphic rocks often have a gran u l a r a p p e a r a n c e , d e ve l o p i n g whole new suites o f meta morphic m i nera l s . The particular assem blage depends upon the compo sition of the parent rock and the metamorphic environment. This has led to a concept of metamor phic facies.
MARBLE, fine to coarsely granu
lar, i s composed chiefly of calcite or dolomite . It derives from meta morphosed l i mestone.
meta morphosed sed imentary rock a u reole, zone of contact
(granitic i nt rusion )
RECRYSTALLIZATION
of some m i nerals and the conversion of others i s a feature common to many metamorphic rocks, espe c i a l l y those formed at high tem peratures, such a s those around intrusions (thermal metamor phism). Rocks around intrusions often display this as a resu lt of contact metamorph i s m .
Black Marble
QUARTZITE
i s a tough rock of metamorphosed quartz sand stone with a sugary texture . Its color i s white to pink-brown .
GA R N ET I F E R O U S S C H I ST
shows fo liation i n para l l e l arrangement of platy m i cas, and g r o w t h of g a r n e t as a n e w m i nera l .
ECLOGITES
conta i n garnet and pyroxene, formed from basic rocks at h i g h pressure .
Eclog ite
Garnetiferous s c h i st
Offshore d ri l l i ng rig symbol i zes man's search for petro l e u m . M I N E RALS AN D CIVILIZATI O N
The foundation of 20th-century civi l i zation is i ndustri a l , a n d the basis for i ndustry i s fuel s a n d meta l ores . T h e thi ngs that form the basis of l ife i n the developed wor l d - clean water supply, bu i l d i ngs, hig hways, automobi les, hard wa re, tool s , ferti l i zers, plastics, fuel s , chemica l s , and more - u ltimately come from the crust of the tiny p l a net on which we l ive . E c o n o m i c m i n e r a l s a re v e r y u n eve n l y d i s t r i b u ted . Although most m i nera ls themselves are widely scattered , deposits sufficiently rich to mine de'p end upon ra re combi nations of geologic processes. They a re often found in isolated a reas, and long geo l ogic study may be needed to l ocate a nd exploit them . Al most 90 percent of the world's nickel supply, for exa mple, comes from a s i n g l e i ntrusion nea r Sudbury, Ontario . N ew i ndustrial processes and i nventions bring demands for new m i nera l s and fue l s . The need for radioactive m i n era l s spurred development o f new techniques and d i men96
Iran
58.0 (9. 0 % )
Mexico
Kuwait
68.5 ( 1 0. 7 % )
I raq
3 1 .0 (4 . 8 % )
U .A . E m i rates
29.4 ( 4 . 6 % )
U n ited States 26.5 (4. 1 % )
Li bya
23.5 (3.7% )
People's Repu b l i c of China 20.0 (3. 1 % )
WORLD C R U D E O I L proved reserves tota l a bout 640 b i l l io n barre l s ;
d i stribution i s concentrated i n M i d d l e East. (After U . S . Dept. o f Energy)
sions i n geo logic surveys after World Wa r I I . The d iscovery of new m i neral deposits ca n ra pidly revo l utionize the civi l i zation of whole nations, as it has i n the Middle East, where some of the world's l a rgest petroleum resources have brought g reat wea lth to Arab countries . A l l m i neral deposits are exhausti b l e . Once we have mi ned a vei n of si lver or a bed of coa l , there is no way of replenishing it. This demands carefu l conservation of m i n era l deposits as wel l as long -term exploration and p l a n n i n g for new supplies. Depletion of some essential m i nera l s now poses seri ous long-term problems. It i s estimated by some that 80 per cent of the world's economica l l y recovera b l e supplies of petroleum wi l l be exhausted withi n a century. Some meta l s , i n c l u d i n g l e a d and copper, have compara b l y l i m i ted reserves. These estimates are based upon present rates of consumpti o n , but a n explod i ng world population could quadruple our m i nera l needs . 97
COAL F I E L D S O F
T H E U N I T E D STAT E S
Anthracite Bitu m i n o u s Sub-bitu m i n o u s a n d l i g n ite M I N E RA L F U E LS
are basic to an i ndustri a l economy not only for heating, lighting, a nd transport but a l so for the i ndustrial power needed i n m i neral processi n g , m i n i n g , and in manufacturing . Hyd roelectricity, though of g reat importance in some a reas, provides only a sma l l fraction (less tha n 2 percent) of the world's power. Mineral fuel s (oi l , coa l , and g a s ) provide a bout 9 8 percent. Coa l a n d petroleum a re fossil fue l s .
COAL
B i t u m i n o u s coal
is a s e d i m e n t a r y r o c k formed from the rema i n s of fossi l plants. Buried peat, under pres sure, loses water and volatiles and achieves a relative l y high carbon content . Peat has about 80 percent moisture; i i g n i te (on i ntermed iate step between peat a n d c oo l ) , a b o u t 40 p e r c e n t ; bitumi nous coo l , o n l y a bout 5 percent. Anthracite, formed under cond itions o f extreme pres sure, conta i n s 95 percent carbon compared with bitu m i nous coo l , which has o n l y a bout 8 0 percent. Most of the world's g reat coo l deposits ore in rocks of Pennsyl vanian or Perm ian age.
ATOMIC FUELS
Carnotite, a u ra n i u m mi neral
98
at present supply only a fraction of the world's resources, but they wi l l become more importa n t . Mineral fuel s used a re o r e s of urani u m .
Cross section of o i l fie l d shows s u b s u rface structure; petroleum i s trapped i n crest o f a n a ntic l i ne sealed b y i m pervious c a p rock.
PETROLEUM
i s a general term far a m i xture of gaseous, liquid, and solid hydrocarbons. When burned , this foss i l fuel releases solar energy stared m i l l i ons of years a g o . Petroleum m i g rates from the source rock, where it f o r m s , to o t h e r r o c k s . M o s t petroleum remains d i spersed i n r o c k pores and much escapes at the surface of the earth . But in commercia l fields, i t is trapped b e t w e e n an i m pe r m e a b l e c a p rock and a permeable reservoi r rock, often floating on water, as i l l u strated .
OIL SHALE
is carbonaceous shale that yields hydrocarbons when distilled . At present, the cost of oil and gas production from i t is too high to make oil sha le eco nomically i m portant, but it may be used i n the future.
FAU LT O I L TRAP
STRATIGRAPHIC OIL TRAP
PETROLEUM EXPLORATION
i nv o l v e s b o t h g e o l o g i c a l a n d geophysical studies. S i nce most of the more obvious surface traps have now been d i scovered , seismic and other surveys are i ncreasingly used to d iscover sub surface and s u b m a r i ne struc tures, such a s those beneath the North Sea .
SALT DOME O I L TRAP
99
O R E D E PO S ITS a re natura l concentrations of meta l l i c m i n era l s i n sufficient quantity t o make their exploitation com mercially worthwh i l e . Most i ndustrial meta ls, other than iron, a l u m i n u m , and magnes i u m , a re present a s o n l y a fraction of 1 percent i n the average igneous rocks of the earth's crust . P lati n u m , for exa m p l e , has a n a bunda nce of . 0 0 0 , 000 , 5 p e r c e n t by we i g h t ; s i l v e r a n d m e r c u r y , . 000 , 0 1 percent. An o r e with 1 percent a nti mony conta i n s 1 0 , 000 ti mes a s much as t h e average ig neous roc k . There a re several geolog ica l processes by which ore-bea ring m i nera l s a re concentrated i n the crust. MAGMAT I C O R E S are concentrated from molten rock where crysta l s settle during coo l i ng . Some of the world's g reatest ore bod ies were formed in this way.
Pretoria Series
gran ite
c h rom ite layers
THE BUSHVELD
_6. mag netite and chram ite deposits of South Africa occur i n layered lopoliths of norite (gabbro with hyper sthene pyroxene). See p . 8 8 .
..... THE SUDBURY nickel depos
its of Ontario occur as stringers in a norite intrusive complex.
DIAMONDS
are formed deep i n volcanic p i pes, crysta l l i z i ng from a disti nctive u ltrabasic rock, k i m berlite. Exposure by weathering may subsequently concentrate diamonds in river placer deposits .
M E TAMO R P H I C O R E S
are formed a round some i ntrusions by contact metamorphism of the country rocks . Asbestos , once used f o r insulation a n d fireproofi n g , is a common nonmeta l l i c metamorphic m i nera l .
H Y D R OT H E RMAL O R E S
include many o f the largest deposits of lead, zinc, cap p e r , and s i l v e r . D e p o s i t ed from hot, aqueous solutions, their distribution is usua l l y contro l led by joints, faults, bedd i n g , and l i thology of the country roc k s . The m i nera l i z ing solutions a r i s e f r o m mag matic sources, a l though the parent ig neous rock may not a lways be ex posed . The cop per deposits of Butte, Mon tana , and Utah, si lver of C o m st o c k Lode, N evad a , and gold o f Cripple C reek, Colorado, are all hydrother mal deposits. A large portion of the wor l d 's gold is m i ned from deposits that origi nated from hydrotherma l solutions.
Gray porphyry
Tucson fa u l t Parting/ quartzite White Cambri a n q ua rtzite Diagrammatic cross section of structu ral ore control i n l e a d a n d z i n c m i n e , L e a d v i l l e , C o l o ra d o (After Arga l l )
1 01
Dri l l i ng i n tacon ite i ro n ore rocks req u i res s pec i a l eq u i pment.
METALLIC ORES,
such a s the i ran around B i r m i ng h a m , Alabama, and Northamptonshire, England, are sedi mentary rocks rich i n original hematite. Carnotite, a sed i mentary u r a n i u m o r e , i s found i n sandstones of the Colo rado Plateau . S E D I M E N TARY O R E S form in a variety of different ways . Some a re formed as d i rect sed i mentary deposits or by eva poration ; others have an ig neous orig i n a nd a re con centrated by sed imentary processes .
EVAPORITES
are m i neral salts ( h a l i te , g y p s u m , pota s h , etc . ) formed by evaporation of restricted bodies o f water. The great salt deposits of Germany, Utah, and N ew Mexico are exa m ples . Evaporites may be i m pure, conta ining clay, sand, or carbonate s .
RESIDUAL MINERAL DEPOSITS
are formed by leaching, with e n r i c h ment of deposits, many
weathering and c o r re s p o n d i n g l ow-g rade ore o f sed i mentary
PLACER DEPOSITS
are formed by concentration of fragments in stream deposits. Such heavy m i n era l s as gold, diamonds, magne t i t e , a n d t i n o x i d e a r e o f fe n concentrated i n this way. Exam p l e s a r e t h e t i n d e p o s i t s of Ma laysia and the gol dfields of Cal iforn i a and Austra l i a . origi n . T h e g reat i ron deposits of the Great Lakes and the a l u m i num (bauxite) deposits of Arkan sas have been concentrated i n s i t u b y this process.
Iron ore pit, V i rg i n ia, Minn.; rocks a re Preca m bria n .
C O N S TR U CT I O NAL A N D I N D U ST R I A L M I N E RALS,
a lthough more a bundant than o r e m i nera l s , a re used i n much g reater quantities . They a re genera l l y quarried rather than m i ned .
BUILDIN G MATE RIALS
are extracted from the earth, whether brick, stone, steel g i rd e r s , or g l a s s . B u i l d i n g stones m u s t be d u r a b l e , easily quarried, and eas i l y w o r k e d . T h e p a r t i c u l a r stones selected often depend upon t h e local weathering condi tions (determ i ned by amounts o f industria l gases) a n d local ava i l a b i l i ty of stone s . Other industri a l m i nera l s , such as sul phur, salt, potash, gypsum, and asbestos, are not so widely d i stri buted , but are used i n many industrial processes . They occur i n very d i fferent geo l o g i c setti n g s .
CLAYS are u s e d f o r b r i c k m a k
ing, i n chem i c a l industries, in cera m i c s , and i n the manufacture of many other products . Each use demands slightly d i fferent q u a l i ties. M a n y varieties of clay a r e known and mined.
SAND AND GRAVEL
are widely used in c o n c r e t e c o n s t r u c t i o n . Most supplies come from g l a c i a l and river deposits. Reserves are plentifu l , but d i stribution is patchy.
STONE AGGREGATE
for high way, a i rfield, and dam construc tion is a lso used in great quantities. I t is resistant and cheaply quarried . I n some tropi cal areas, it has to be i m ported .
LIMESTONE
is used in the manu facture of cement, as a meta l lurgical flux , a s a n agg regate, and i n agriculture. Limestones are widely di stri buted, and over 500 m i l l ion tons are quarried annua l l y in North America a l one.
T H E C H ANGING E A R T H If the processes o f erosion a n d deposition were counter acted only by igneous activi ty, we should expect the conti nents to be featureless plains , broken only by active volca noes or p lateaus of lava . But there are considera b l e move ments o f t h e earth's crust.
EARTHQUAKES
provide dra matic evidence of crustal move ments . Sma l l relative movements of the crust, both vertica l and horizonta l , can very often be measu red after earthqua kes have taken place (p. 1 26 ) .
RAISED BEACHES
and buried forests often i nv o l v e reg i o n a l warpi ng rather than simple uplift. Raised beaches show a rise i n the C a l ifornia coast line over the past years, but on the other hand , ports af Denmark are sinking (see p. 62).
OTHER COASTAL FEATURES
Ancient caves, high a bove present beach, i n d icate former position of sea level, N. Ireland.
such a s wavecut platforms indi cate relative upl ift of the land; drowned va l l eys indicate relative sinking (p. 69) . Raised coral reefs often show upl ift, t i l t i n g , and even reversal of movement .
FOSSILS
af marine a n i m a l s i n land areas indicate relative upl ift of the land. The presence of coal seams with land plants thousands of feet b e l ow p r e s e n t g r o u n d level shows that s i n k i ng h a s a l so taken place. lake sediments are often interbedded with sediments that were deposited i n the sea .
Crusta l movement is a common feature of the earth . It i s found throughout t h e wor l d , i n rocks ra n g i n g i n age from the o ldest to the youngest. It i nvolves va rious kinds of movement: gentle, slow u p l ift or s i n k i n g , reg ional wa rp ing, rapid earthquake movements, and reg ional stresses that are strong enough to buckle and break g reat masses of rock { p p . 1 07- 1 25).
INCISED MEANDERS
of rivers ore thought to result from relative uplift of the land surface ( p . 44). The Grand Canyon, which mea sures more than a m i l e deep, is a n exa m p l e .
RECENT MOVEMENT
of the crust i s indi cated b y famous tem ple ruins near Naples. The p i l l a r s , erected o n land, were bored by marine m o l l uscs, indicating submergence by the seo and the subsequent upl ift of the land.
UNCONFO RMITIES
(p. 1 1 4) represent breaks i n the deposi tion of sed i ments, sometimes indicating periods of sign ifica nt crustal disturbance.
FOLDS
i n a ncient strata range from sma l l warps, o n l y one or two feet in height, to g reat domes, m i les across. I n areas where rocks have responded i n a brittle man ner, brea ks (faults) have deve loped .
FAULTS
i nvolve fracture and rel ative movert]ent of rock units (p. 1 1 1 ). I n near l y all cases, the rocks i nvolved were origina l l y in a horizontal position .
Roman tem p l e , b u i l t on dry land, i s now floode d . Devon i a n sandstone U N C O N FO RMITY
Typical unconformity showing effects of u p l ift i n pre-Devo n i a n Fol d i n g i n l i mestone, Victoria1 Austra l ia T
ROC K DEFORMAT ION Although sed i menta ry rocks a re genera l l y deposited in a l most horizonta l beds, we genera lly find them d i storted and tilted if we fol l ow them over a ny considera b l e d i stance. Such structures are the resu lt of l a rge-sca le crusta l defor mation, which produces corresponding changes i n volume, shape, and sometimes chemica l composition of the rocks themselves . The i ntensity of the changes is proportiona l to the i ntensity of deformation and the depth of buri a l (see Metamorphism, p. 94) . U nder some stress cond itions, rocks behave a s though they were elastic, but a s stress increases, they undergo permanent (plastic) deformation and may u lti mately fracture . The most i ntense zones of deformation are associated with mounta i n cha i n s .
DIP
o f o bed i s a measure of its slope or tilt i n relation to the hor izonta l . The direction of dip i s the di rection of maximum slope, or the d i rection a ball would run over the bed i f its surface were perfectly fla t . The angle of dip is the acute angle this di rection makes with a horizonta l plane. The strike of a rock bed i s the d i rection of the i n tersection of its d i p d i rection with o horizontal plane. It is expressed as a com-
D i p·slrike map symbo l :
.,_ 35
bedd i n g plane l i ne of stri ke
a n g l e of d i p
pass bearing a n d lies at right ang l es to the d i rection of d i p . Di p-strike symbo l s a r e used o n most geologic maps. C l i n o m e t e r s are e l a b o r a te i nstruments used by geologists to measure d i p . A s i m p l e instru ment, however, may be made from a plastic protractor fixed to a flat base with a weig hted thread to measure the maximum d i p . The strike of the d i p is then measured with a compass .
Sheep Mou nta i n , Wyo m i n g , is a fine exa m p l e of pitc h i n g anti c l i n e . FOLDS
are wrin k l es or flexu res i n stratified rock s . They range from m icroscopic s i zes in meta morphic rocks to great structures hundreds of m i les across. They sometimes occur i n isolation, but more often they a re packed together, espec i a l l y i n mounta i n ranges . U pfolds a re cal led anticlines, a nd downfolds a re ca l l ed s ynclines . Fol d s with one limb more or less horizonta l a re ca l l ed monoclines . A l l folds tend t o die o u t as they a r e traced a long t h e i r lengths.
FORMATION OF FOLDS shown
i n stage s . I n Stage 1 , the rocks are deposited . In Stage 2 , they are folded . I n Stage 3, they are uplifted and their tops eroded , with only their dipping limbs rema i n i n g . The o ldest beds ( 1 ) are a lways i n the core of a n anti cli ne, but on the flanks of a sync l i n e .
STAGE 2
a n g l e af p l unge
�
N
t
Bed 7 "'
... ...
7
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... ..
-a -x i-s - -!"� 5 6
9
1
10
10 8
9 _,_
J.-
Bed 6
N
t 14 -.J....
-1
Re l i ef d iagram and geolog ic map of a p l u n g i ng anti c l i ne
THE STRUCTURE O F FOLDS SYNC LINES o c c u r w h e r e t h e (above). The axial plane i s drawn sa that it bisects the angle of the fold . The axis i s its trace on a beddi ng plane. If the axis is not horizonta l , the fo ld i s said to pitch or plunge.
beds d i p toward the axi s . They can pitch a nd be either symme trical or asymmetrica l . A struc t u r a l b a s i n is a b a s i n - s h a pe d sync l i ne where dips converge on a central point or area .
geologic maps by the d i p ar rows p o i n t i n g away f r o m t h e a x i s . A p i t c h i n g f o l d g i ves a
"closing" outcrop patter n . A dome is an anti c l i ne that has dips poi nting i n a l l d i rections from a central point or a rea .
ANTICLINES can be spotted on
K I N DS OF F O L D S
SYMMETRICAL FOLDS (a) have OVERTURNED FOLDS
limbs dipping in opposite direc tions at the same incli nations. The axial plane (ap) is vertica l .
(c) have an i n c l i ned axial plane and l i mbs dipping i n the same di recti o n . O n e l i m b i s inverted .
ASYMMETRICAL FOLDS (b) are ISOCLINAL FOLDS those having an i n c l i ned axial plane and, l i ke symmetrica l fo l d s , have l i m b s d i p p i n g i n opposite d i rections, but a t differ ent i n c l inations.
(d) have equa l d i p s o f two limbs; axial plane dips i n same d i rect i o n .
RECUMBENT FOLDS
horizontal axial planes .
(e) have
'
Folded roc k s form cl iffs 1 , 500 feet a bove San J u a n River, Uta h . F O L D PAT T E R N S a re rarely as simple as idea l i zed ones shown on these pages . They often pass i nto fa ults. Some beds yield to stra in more readily than others . Such i ncom petent rocks as sha l e and rock sa lt, for exa m p l e , often yield by flowing a nd s l i pping i n fo l d s . F l owage of salt may prod uce structura l domes, leading to petro leum reservo i r s . S u c h incompetent r o c k movements a re sometimes so strong that, as in some Midd le East oilfields, major fo lds at depth a re not reflected at the surface . Folds that do reach the surface a re best exposed i n arid and sem iarid a reas and in rock faces and cl iffs, but reg ional mapping of other vege tation-covered a reas, such as the Appa lachians, often shows fol d i n g over g reat a reas . Anticl i n a l fold structures sometimes provide petroleum reservoirs.
Section across southern Appa lachian Mounta i n s Cumberland Pl ateau NW
'
. P1edmont Plateau R .1 d ge · h,,
��
Atlantic coa sta l p 1 01 n
Carol i n a s 1 a 1 e b e It
.l9).,'V ':;r··,...
•
SE ,,
1 09
ROC K FRACTUR E S J O I NTS
a n d fractures a re a nother way that .rocks yield to stres s . J o i nts a re fractures or cracks in which the rocks on either side of the fracture have not undergone relative movement. Common in sed i mentary rocks, they are usua lly caused by release of buria l pressure or by d i a strophism . They play a n i m portant part in rock weathering as zones of weak ness a nd water movement.
�§;:�\:::.c;;:����� � JOI NTS IN S E D I M E N TARY ROCKS occur i n para l lel sets at right ang les to the bedd i n g . Ten siona l , com pressiona l , and tor sional stresses a l l produce d i stinctive j o i n t s .
JOINTS IN IGNEOUS ROCKS
may result from shrinkage during coo l i ng . I n fi ne-grai ned rocks, there i s a characteristic polygo nal arrangemen t . Granite masses may show sheet jointi n g . Giant's Causeway, Northern Ireland, shows hexagonal col u m n s formed b y coo l i n g of basalt lavas.
FA U LTS are fractures where once-continuous rocks have suffered relative d i splacement. The amount of movement may va ry from less than a n inch to many thousands of feet vertica l l y and to more than 1 00 mi les horizonta l ly. Some , such as the San Andreas Fault, are major earth features . Different types of faults are produced by d i fferent com pressional a nd tensiona l stresses, a nd they a l so depend upon the rock type and geo l ogical setti ng . Fau l ts are the cause of ea rthquakes, which suggests that repeated sma l l movements rather than one "catastrophic" brea k cha racterize many fa ults. Distinctive, l a rge-sca l e fracture zones (transform fau l ts) d i sp lace t h e m i d -ocean i c ridges i n severa l a reas ( p p . 1 36 and 1 40) .
KINDS OF FAULTS NORMAL, GRAVITY, OR TEN throw of t h e fault is the vertica l SIONAL FAULTS are n a t neces displacement of the bed (ac); the
sarily the mast cam man fau l t type i n a g iven area . They are faults in which relative downward move ment ·has taken place down the upper face or hanging wa l l of the fault plane. (We cannot gener ally prove whether both beds have moved, or o n l y one . ) The
heave is the horizontal displace ment (be). The angle abc i s the dip of the fau l t plane, and the complement of this i s the hade. The dip is usua l l y stee p . Some times there may be more than ane episode of movement a long the same fault plane or zone.
B l o c k d i a g r a m of n o r m a l fa u l t before weathe r i n g
B l ock d ia g r a m of n o r m a l fa u l t after weath e r i n g
thrust plane
B l ock d i a g ra m of h i g h - a n g l e reverse fa u lt
REVERSE OR THRUST FAULTS
have relative upward movement of the hanging wa l l of the fa ult plane . They occur in a reas of compression and fo lding such as mounta i n belts. lateral di splace ment may be many miles. They
kl i ppe of overt h r u st
w i ndow o f
D i a g r a m m a t i c c r o s s s e ct i o n o f eroded low-a n g l e t h r u s t fa u lt
often have a low d i p and resu lt i n repetition and apparent reversal of stratigraphic order i n a verti cal sequence. C h i ef Mounta i n in Montana i s a n eroded remnant of a large thrust fa u l t .
A GRABEN is a block t h a t h a s
Topogra p h i c depression marking San A n d reas Fa u l t i s occupied by lagoon, Bolinas Bay.
a
been dropped d o w n between two normal faults. An u p l ifted fau l t block is a hors t . R ift valleys are g ra ben s t r u c t u r e s h u n d reds of miles in length . The most spectac ular is that a long the Red Sea, but they a re a l so found in East Africa, the Rhine, and Califor nia, and they a l so occur beneath the ocean s along the crests of the mid-ocea nic ridges.
TEAR FAULTS, OR STRIKE-SLIP FAULTS, AND RELATED TRANS FORM FAULTS are those where
shearing stress has produced hor i z o n t a l move m e n t . The S a n Andreas Fault i n C a l ifornia i s 600 m i l es long and has a d i splace ment of over 350 m i les. The 1 906 and 1 989 San Francisc o earth quakes were caused by the move ment of the San Andreas Fault, which is sti l l active .
FA U LTS I N T H E F I E L D are genera l ly more complex than those shown in these diagra m s . Rotational movements often com p l i cate the simple vertica l and hori zonta l move ments , a nd the throw and hade of a fau l t may change a long its length . The fa u l t plane is often poorly defi ned , and is represented by a fa u l t zone made u p of broken and distorted rocks . Fau lts often occur in groups (fa u l t zones) made up of many indivi dua l fau lts . Except i n desert a reas, cl iff faces , a nd quarries, fa ults a re rarely seen at the surface, but their presence is i ndicated by one or more of the fol l owing features:
FAULT BRECCIA
occurs where the rocks of the foult zone are shattered i n to angular, i rregu larly si zed frag ments. Some may be reduced to a g ritty clay.
SLICKENSIDES,
po l ished stria tions or fl utings, are often found i n the fau l t p l a ne or zone. They may indicate the d i rection of re l ative movemen t .
TOPOGRAPHIC EFFECTS
occur when faults bring together rocks of d iffering hardness. Denuda tion may indicate the faulting by showi ng sharp, "unnatura l " top o g r a p h i c c o n t a c t . T h e Te t o n range i n Wyo m i n g is a n example, where resistant Precambrian igneous rocks are faul ted agai nst s o f t e r Te r t i a r y sed i m e n t s ( p . 1 1 8) . Other topographic effec t s - b a y s o r v a l l e y s , f o r example - may resu lt from the weakness of a fault zone,which itself may undergo strong differ ential weat hering .
SPRINGS
may be produced by faults when pervious and i m per vious strata are brought into con tact with one a n other (p. 5 1 ) . Li nes of springs often indicate the existence of a fau l t .
DISPLACEMENT OF OUTCROP
i s another i n d i cation of a fau l t . A s we l l as displacing rocks verti cal ly, f a u l t s i n dipping b e d s w i l l displace the i r outcrop patterns . In geologic mappi ng, faults are often i nferred from outcrops that "won't match . 11
Smal l-scale fa u l t i n g i n l i mestone
U N C O N FO R M I T I E S a re ancient erosion surfaces in which a n older group of rocks has been upl ifted , eroded , and subsequentl y buried by a group of younger rocks. U ncon form ities can be used to date periods of crustal movement i n the reconstruction of earth h i story. The time spa n repre sented by the erosion surface (a bsence of deposits) may va ry from very long to very short period s . A "break" in foss i l sequence, often a characteristic feature of uncon formities, can be measured by compa ring the fossi l s with those of uni nterrupted rock sequences . U nconformities va ry in form , and in the erosiona l interva ls they represent.
DISCONFORMITIES
occur where the beds obove ond below the erosion surfoce ore para l le l . If the o lder g roup of rocks has been folded and eroded before the deposition of the younger group, the i r beds will not be para l l e l , a n d they represent an angular unconformity.
T h e eroded upper surface o f t h e underlying g r o u p of rocks may be flat or it may have consid erable relief. O l d soils are some t i m e s p r e s e r v e d . T h e l ow e s t overlying beds a r e often con g lom erates made u p of eroded f r a g m e n t s of t h e u n d e r l y i n g group o f rock s .
STAG E 2 U p l ift a n d fold i n g
STAGE 3 S u rface eros ion
S u bmergence and renewed deposition Plane of u n conformity STAGE 4
1 14
The Matterhorn, Switzerland, s h ows s harp outl i n e formed by g l a c i a l ero s i o n t y p i c a l of many mounta i n s .
MOU NTA I N B U I L D I N G
Mounta i n s a re among the most conspicuous features of the earth's surface. Although they a re formed i n va rious ways and a re of various ages, most a re concentrated in g reat folded belts that run a long the continenta l marg i n s . Not all are confined to the l a nd . The Mid-Atlantic Ridge stretches thousands of m i les, rising a l most 1 0, 000 feet a bove the ocean floor. S i m i l a r ridges a re found in other ocean s (p. 1 36 ) . A n y isolated , upstanding m a s s m a y be cal l ed a m o u n ta i n . There is no m i n i m u m h e i g h t or particu l a r shape i nvolved . Some of the ways i n which mounta ins may be produced are shown on the fol l owing pages . 1 15
VOLCANIC MOUNTAINS i n c l ude some of the world's most beautiful and fa mous mounta i n s . Mt. Fuj iyomo i n J a pa n , Mt. Vesuvi us i n Ita l y, Mt . Hood i n Oreg o n , and Mt. St. Helens and Mt . Rainier i n Washington o re a l l examples of characteristica l ly steep and symmetrical volca n i c moun ta i n s . Other volcanic mounta i n s , such as Mauna loa in Hawa i i , ore formed by shield volcanoes ( p . 84) . They tend to be rounded and flattened , but may sti l l be high and Iorge. Mauna loa rises a l most 1 4 , 000 feet a bove sea leve l and a bout 3 2 , 000 feet above the seafloor. With its d i a m eter o f 60 m i l es , it is t h e world's la rgest mounta i n i n terms of volume. Volcanic mounta i n s may form very rapid ly. Mt . Poracu tin i n Mexico beg a n to g row in Februa ry, 1 943 . Within a wee k , its cone was 500 feet h i g h , and with i n two years i t h o d reached 1 , 500 feet. Some volcanic mounta i n s occur as isolated structures , but others form ports of extensive volca n i c cha i n s . Volcanic islands may form g reat island orcs, of which the 1 , 000m i le Aleutian cha i n is on exa m p l e . M t . F u j iyama, Japan's sacred mounta i n , i s a volca n o .
B l u e Mounta i n s , New South Wa les, A ustra l i a , ore eros i o n a l i n orig i n , con s isting c h i efly o f horizonta l Perm ian and Tri a s s i c strata .
E R O S I O N A L M O U N TA I N S are found in reg ions of crusta l upl ift. These a re wide a reas where crusta l upl ift (epeir ogeny) has prod uced el evated plateaus and thereby pro vided new erosiona l energy to rivers . The steep gorges with preci pitous edges of Grand Canyon-type topography, for exa m p l e , are genera lly thought of as a dissected pla tea u rather than a mounta i n . Some we l l - known mounta i ns, such a s the B l ue Mou nta ins of New South Wa les, Austra l i a , a re formed i n this way. If erosion continues long enough , hug h i so l ated , resi sta nt remna nts a r e left. A n exa m p l e i s Mt . Monadnock i n N e w H a m pshire, which ri ses 1 , 800 feet above a "penepla ne" and g ives its name to such isolated structures (monadnocks) . These structures a re thus outl iers of younger rock , resti ng on an exposed basement of older rock s .
1 17
DOME MOU NTA I N S
are the sim plest kind o f structural moun ta i n s . They tend ta be relatively sma l l , isolated, structural domes, uplifted without intense faulting . They may be associated with such igneous intrusions as lacco l i t h s . The Black H i l l s of South Dakota a r e a n exa m p l e . S T R U CT U RAL M O U N TA I N R A N G E S
are by fa r the m ost n umerous a n d extensive mounta i n ranges and differ from volcanic and erosional mounta i n s in the structura l defor mation and upl ift they have undergone. Some of the o l der mounta i n ranges, such as the Appa lachians, a re less i m pressive than the younger, such a s the A l ps, but they share certa i n common characteristics, six of which a re d i scussed below. It is these, more than just height, that are a clue to a n understa nding of the nature of structura l mounta i n s .
FAU LT BLOCK MOU NTAINS
may be formed i n any type of rock . They are often, though not a lways, structura l l y undeformed where differentia l vertical dis placement or tilting by steep nor mal faults g ives relatively up l i fted and downsunk blocks. Rift val leys are often associated with the mounta i n scarps, which tend
to run para l lel with the faulting . S u c h m o u n t a i n s m a y be v e r y large. T h e S ierra N evada repre sents the uptilted edge of a b l ock of granite 400 m i l es long and 1 00 m i les wide, with an eastern scarp 1 3 , 00 0 feet a b ove sea l e ve l . There are several such ranges from Oregon to Arizona, which produces spectacular scenery.
, , S m l �s ,
1 18
'
MOUNTA I N C H A I N S
)C O N T I N E NTAL S H I E LDS
Cenozoic Mesozoic P a l eozo ic
F O LD E D MO U N TA I N RANGES all have s i m i l a r character istics . Most of the world's g reat mounta i n cha i n s , such as the "young" H i ma layas, Al ps, Rockies, and Andes, a nd the older U ra l s and Appa l achians, belong to none of the g roups d iscussed on pp. 1 1 6- 1 1 7 . I n spite of many i ndivid ual d ifferences, all a re funda menta l l y folded ranges and share these six i m portant characteristics:
1 . Linear d i st r i b u t i o n of mounta i n ranges ind icates they a re not haphazardly scattered across the earth's surface; they are found i n long, na rrow belts . The Appa l achians, for exa m p l e , stretch 1 , 500 m i l es from N ewfoun d l a nd to Alabama and a re up to 350 m i les wide . The Rockies are 3 , 000 m i les long and continue southwa rd for another 5 , 000 m i les i n the Andes of South America a nd then i nto Antarctica . Their location near the marg i n s , or former margins, of continents is a major c l ue to thei r orig i n .
1 19
Precambrian
Cambrian
Ordovician Devon i a n
Carboniferous
Perm ian Triassic
Cretaceous
Tertiary
Genera l ized section across the Rockies from eastern Idaho to western Wyom i n g (see p. 1 52 for mea n i ng of geo l og i c ages) 2 . T h i c k, sed i m enta ry rocks that are found i n a l l the world's major mounta i n chains are genera l l y 30, 000 to 40, 000 feet (6-8 mi les) i n thickness . Fossi l s and structures show most are marine rocks, formed in elongated crusta l downwa rps. They incl ude ch iefly clastic sed i ments and vo l canic rocks. Strata o f eq uiva l ent a g e on t h e bordering continents are often only a tenth of this thickness and genera l l y have a higher proportion of carbonates and coarse clastic rocks. 3 . D i st i n ct i v e i g n e o u s a n d m e ta m o r p h i c r o c k s are found i n mounta i n cha i n s . Clastic sed i ments a re often i nterbedded with g reat thicknesses of volcanic lavas, and eroded cores of mounta in chains include vast g r a n i tic bath o l iths, as wel l as m igmatites . The bathol iths a nd migmatites were proba bly formed from the effect of depth and heat on deeply bu ried sedi ments . In you nger mounta i n ranges - those formed during the l a st 1 20 m i l l ion years (see p p . 1 1 9 a nd 1 52 ) - active vulcanism continues . The vo lcanoes of the Cascades, Andes, a nd Antarctica are exa mples. I n o l der mounta i n ranges, a l though the elevation seen in more recent moun ta ins is reduced by erosion, the disti nctive rock types and deformation patterns can sti l l be recog n i zed .
1 20
4. I n te n se fo l d i n g a n d fa u l t i n g i n major mounta i n c h a i n s often involve thrusting on t h e neighboring continen ta l marg i n , with displacements of many mi les a nd conse quent shortening of the crust. Active earthqua kes and u p lift mark "younger" mounta i n ra nges . 5 . R e p e a ted u p l i ft a n d e ros i o n have ta ken place i n folded mounta i n s . O l d e r ranges m a y b e eroded down a l most to a flat surface (a penep l a i n ) and then be u p l ifted aga i n . The Appa lachians, formed in the Late Pa leozoic, were eroded to a penepl a i n i n the Triassic and a g a i n i n the Cretaceou s . Their present 6, 000-feet-high rel i ef res u l ts from Tertiary upl ift and recent erosion . Conti nenta l shield a reas, now rather flat, represent a ncient eroded mounta i n cha i n s . 6 . Mou nta i n s h a v e roots .
They a re n o t s i m p l y l u m ps of rock resting on a uniform surface. They have roots of light material extend ing far down below their norma l depth s . This is suggested b y t h e gravity a nomaly o f mounta i n s , by the paths of earthquake waves through them , and by high heat fl ow associated with mounta i n s . Si nce mounta ins deflect a p l u m b l i ne less than the mere attraction of the i r su rface m a s s resting on "norma l " basement, they m ust be under l a i n by a l a rger amount of light materia l .
actua l mea s u red --- gravity
- 50
fo lded sed i mentary rock
I
C R U STAL ROCK MANTLE RQ4if
, ..
continent g uyot
I abyssal p l a i n
Ma i n features of t h e ocea n floor
THE A R CHIT E CTUR E OF THE EA RTH S E E N FROM S PAC E , earth i s a b l ue pla net, vei l ed i n a swi r l i n g , changing tracery of clouds. It is blue beca use it i s a watery p l a net, two-thi rds o f i t s surface covered by ocea ns; it is vei l ed because its enveloping atmosphere is i n constant motion, i nteracting everywhere w i t h land and water. The fi rst part of this book has described this i nter actio n , showing the changing pattern of rock formati o n , eros i o n , and depos i t i o n . But beneath these su rface changes, there a re a l so changes within t h e earth, which have created its major a rchitectural features . The second part of this book describes these major earth features a nd the processes that produced them . The continents consist of l a rge shields of ancient Preca m brian rocks, a round which younger rocks have been formed . Mountain ranges of severa l different ages are found i n l i near belts around the marg i ns o f the conti nents . They a re
1 22
( '
abyssal p l a i n
"'
(After
The S tory of the Earth,
H.M.S.O.)
i ntensely folded a nd fau l ted and conta i n a variety o f sed i menta ry, igneous, and meta morphic rocks ( p . 1 1 9) . Volcanoes a nd earthquakes a r e not randomly d i strib uted , but are concentrated i n narrow belts, especia l ly i n a "ring of fire" a round the Pacific Ocean , in a reas of recent mounta i n b u i l d i n g , and a long the mid-ocean ridges and rift va l l eys ( p . 85) . M id-ocean ridges form a g lobal network of submarine mounta i n chains, 25, 000 m i les long and reaching 1 8 , 000 feet in height a bove the seafl oor. Rift va l l eys, tra nsform fa ults, sha l l ow-focus earthq uakes, and volcanoes mark the crest of the m id-ocean ridge (p. 1 36). Island arcs and submarine trenches, some over 6 m i les deep, found in the Pacific and el sewhere, are marked by vulcanism a n d i ntense earthquake activity (pp. 1 3 8- 1 39) . The ocean floor i s made up of volca n i c a n d sed i mentary rocks, all of relatively young age. I n contrast to the a ncient rocks of the conti nents (some 3 . 6 billion years old), the o ldest ocea n i c rocks are "only" 1 75 m i l l ion years o l d . 1 23
T H E M E C H A N I SM OF M O U NTA I N B U I L D I N G ( O R O G E N Y)
is o n e of t h e great d i scoveries o f recent geological science s . Although surface processes play a sign ificant role in supplying the sed iments that make up mounta i n ranges and i n erod ing and accentuating them after they a re u p l ifted , the chief processes operate with i n the earth's interior. The fo l lowi ng pages trace the way in which recent d i scoveries have led, step-by-step, to a new theory of mounta i n b u i l d i ng - plate tectonics - that accounts for the features just descri bed . To understand this theory, we need information on the structure of the earth and its physical properties . The fo l l owing pages describe these features.
EART H Q U A KES Ea rthquakes a re rapid movements of the earth's crust ca used by fa u l t movements . Al most a m i l l i o n occur each year, but most a re so weak they may be detected only by very sensitive recording i nstruments (seismographs). About 90 percent of a l l earthqua kes seem to origi nate at a focus, the point of maximum intensity of the earthquake, in the outer 40 m i les of the crust. A few deep-focus earthquakes origi nate at depths as g reat as 400 m i les. The location and pattern of earthqua kes a re major cl ues to earth's structure .
TYPICAL EARTHQUAKE EFFECTS are shown an black dia
1 24
gra m . Earthquake waves radiate o u twa r d s . T h o s e t h a t t r a v el d i rectly t o the surface a r e shown . lsoseismal l i nes j o i n a l l places where the earthquake i s recorded with the same intensity. These genera l l y form approxi mate cir cles or ova l s of decreasing i nten sity around the epicenter, which is di rectly above the focus.
Destruction cau sed by the C a l ifornia earthq uake of October, 1 989 E ARTH Q U A K E D E S T R U C T I O N occurs when shock waves a re absorbed by b u i l d i n g s . Those on thick soil or rock debris a re most affected . Steel -fra me b u i l d i ng s on solid rock are more l i kely to su rvive . Violent earth movements may be accompanied by earth fl ows and s l u m p s , cracks, sma l l fa ults (with hori zonta l or vertica l movement u p to a bout 30 feet) , and temporary founta i n s . F i res often resu lt from broken gas l i nes . Tsuna m i s are g i a nt waves in the ocea ns that result from earthq uakes . Movement of a part of the ocean floor d i s turbs t h e water and l e a d s t o h u g e osc i l l atory waves, often over 1 00 m i les long . Moving at speeds of up to 500 m p h , they p i l e u p a l ong shores, often causing g reat destructio n .
EARTHQUAKE BELTS
encircle the ea rth . About 415 o f o i l earthquakes and a l most all deep-focus ones occur i n a belt around the Pacifi c . A less active belt runs through the H ima layas and A l p s . (Compare with p . 1 45)
EART H Q U AKE R E C O R D S , or seismogram s , show that va rious types of earthquake waves travel through the earth by d i fferent paths . There a re two types of waves that trave l through the ea rth: P {push-pu l l ) waves that are compres siona l and travel by moving the particles backward and forward through a ny materi a l , and S (shake) waves that pass only through sol i d s . S-waves shake the particles per pendicularly to the di rection of their movement, l i ke waves made in a rope by holding one end sti l l a n d shaking the other u p and down . l (surface) waves travel a round the
A S E I SMOGRAPH
is u s e d to record earthquake waves . The suspended weight, because of its inertia, i s unaffected by earth quake movements, which a re re corded o n the rotating dru m . Seismographs d iffer i n size and sensitivity.
surface of the earth and can pass through a ny materia l . From many observations, i t has been s hown that when record ing stations a re more than 7, 000 m i les away from an earthquake epicenter, the otherwise very reg u l a r travel ti mes of P-waves a re reduced . They arrive later than ca l culated , a nd S-waves a re e l i m i nated completely. T h i s occurs because changes of composition with i n t h e earth's i nterior infl uence the paths and speeds of the various waves . By using a worldwide seismograph network and studying the paths and travel times of these waves, it is poss i b l e not only to l ocate a n earthquake at its source but a l so to build up a model or picture of the nature of the ea rth's i nterior.
EARTHQUAKE WAVES
passing through the earth are i l lustrated b e l ow, s h o w i n g t h e effect af refraction o n P -woves. S-woves
ore complete l y e l i m i nated by the core . P -woves ore indicated by red l i nes, S-woves by the blue lines.
1 27
THE EARTH'S INTER I OR The behavior of earthquake waves indicates that the earth's i nterior is not homogeneous but is made up of a s e r i e s o f c o n c e n t r i c , l a ye r e d s h e l l s o f d i f f e r e n t compositio n . U s i n g data from seismographs, a hypothetical model of the i nterior she l l s can be constructed . We have no d i rect method of testing its accuracy, but there a re severa l l i nes of i n d i rect evidence that seem to support it (p. 1 29) . The three m a i n layers a re cal led crust, mantle, a nd core. The crust is sepa rated from the mantle by the Mohorovicic, or M-d isconti nuity, where earthquake waves speed u p by 1 5 percent. CONTI N E N T
ALTERNATIVE CLASSI FICA THE CONTINENTAL CRUST i s TION recognizes the rigid litho thicker t h a n t h e oceanic (20 3 8 -
s p h e re ( t h e o u t e r 62 m i l e s ) .
consisting o f the crust ond the upper port of the mont l e . This overlies the hot, soft astheno s p h e re ( 6 2 - 1 5 5 m i l e s d o w n ) , c h o r o c t e r i z e d b y l ow s e i s m i c wave velocities and high seismic attenuation, which i s thought to be capable of flowi ng . This over lies the stronger mesosphere, the lower mantle. It appears that the major plates of the earth rest on and move across the astheno sphere (see p . 1 44 ) .
1 28
m i les, as opposed to 5 m i les), older (up to 4 b i l l ion years, as opposed to a maximum of 1 75200 m i l l i o n yea rs), l ighter (2. 7 g m s per cc, as opposed to 3 . 0 gm/cc), and more structura l l y complex. The continental crust has a granitic composition, with thick sed imentary and metamor p h i c r o c k s , w h i l e the o c e a n i c crust i s basa ltic, with only a t h i n veneer of sed i ments. These d i ffer ences are explai ned by the theory of plate tecto n i c s .
THE MANTLE,
bounded by the Mohorovicic discontinuity, is marked b y 1 5-percent increases i n P- and S-wave velocities, sug gesting a change i n compositio n . It seems to consist of d a r k , heavy rocks, rich i n iron-magnesium s i l icates (ol ivine and pyroxene, p p .
THE CORE
i s recog n i zed as a m a j o r d i s c o n t i n u i ty w i t h i n t h e e a r t h where t h e S - waves a r e e l i m i nated and P-wave velocity i s sl owed down . T h i s indicates that the outer core i s probably a " l i q uid , " b u t t h e fact t h a t P-waves speed up again at a depth of 3, 1 60 m i les suggests that the inner core i s s o l i d . C i rculation movements i n the liquid outer core proba b l y generate the
30 and 89). A few surface out crops of supposed mantle rocks fit this general compositi o n . S l ow conve c t i o n m o v e m e n t s i n t h e upper mantle i nfluence the struc ture of the crust, a l lowing the m o v e m e n t s i n v o l v e d in p l a t e tectonics.
earth's magnetic field ( p . 1 35 ) . I t probably has a n i r o n - i r o n s u l p h i d e composition because of i t s h i g h density ( 1 5 f o r t h e i nner core), and by analogy with meteorites, w h i c h proba b l y formed a t the same t i m e . I t may, however, be a s i m i l a r composi tion to the mantle, its differences arising from variations created under very high pressure .
1 29
provide additional i nformation a bout the earth's interior a n d mounta in-build ing processes . Artificial shock waves, used i n seismic stud i e s of s u b s u rface structure, revea l t h e n a t u re a n d boundaries of sha l l ow layering with i n the earth's i nterior. Measurements of g ravity and mag netic va riations are use ful in subsurface studies, both "deep , " a s i n those beneath mounta i n roots, a nd "sha l l ow, " as in the location of petro leum tra ps or m i nera l deposits . Heat-flow studies a re used to ana lyze deep structures. Radar mapping is also of i m porta nce. Continuous, rapid geophysical survey mea surements are now being made from a i rcraft and ships .
G E O P H Y S I CA L MEAS U R E M E NTS
Heat flow rel ated 0 5 to d i stance from 3: )( 0 Mid·Atlantic Ridge. u 4 ;:;:: " Note h i g h heat 0 NVI 3 flow at a x i s . " E :I: � 2 a u
0 1 00 200 300 400 500 600 Km
HEAT-FLOW STUDIES
BAS INS
i n mines and boreholes show that the tem perature of t h e e a r t h 's c r u s t increases b y an average of 30°C per k i l ometer of depth, a lthough there are wide loca l variations caused by the geological setting and local conductivi ty. Over the conti nents , most of the back ground heat seems to originate from radioactivity in the crust. Where the crust is thick, as in mounta i n ranges, values tend to be high. Seismic s hot explosion produces a p l u m e of soi l . S e i s m i c stud ies a l so u s e "t h u m p e r " t e c h n i q u e s , without explosives.
s hot
Record i n g truck
geophones
Seismic record
V� -JJjj jjj
GEOPHYSICAL EXPLORATION
studies provide a structural pro fi le of layered rocks. Geophones p l a c e d a t m e a s u r e d i n t e r va l s record the trace and time of P-waves.
is w i d e l y used i n t h e location o f m i neral deposits and petro leum traps, and i n geologic mappi ng . The diagram shows how seism i c
ELECTRIC LOGG I N G
measured and p lotted . E lectric logging i s a n a i d i n subsurface correlation of ail wel l s . ( Landes)
o f the resistivity and self-potential o f r o c k penetrated by boreholes is RADIOACTIVITY LOG
Garno ray cu rve
Neutron c u rve
rad ioactivity
rad ioactivity
GEOLOGICAL LOG
ELECTRICAL LOG
Natural potenti a l shale l i me stone po s s i b l e poro u s zone shale sand stone
shale n hydrite shale sand stone
Res i st i vity
-
THE FORCE OF GRAVITY is present in every part of the u niverse, from the sma l l est particle to the la rgest star. The earth attracts everything a round it with a pu l l towards its center. The sun holds its pla nets in their orbits by g ravita tional attraction. The force (F) i nvolved i s not constant but va ries i nversely with the square of the distance between the two bod ies i nvolved ( D ) , and di rectly with their masses (M 1 and M2 ) . We can ca lculate it by using the equation devel oped by Newton , F GM 1 M 2 , where G is the earth's 02 g ravity constant (6 . 67 X 1 0· 1 1 cubic meters per k i l ogram second 2 ) . The force o f g ravity is not the same at every place o n the earth . It i s lower on high mounta i n tops, a n d shows a genera l decrease of about one-ha l f of one percent from the poles towa rd the equator. G ravity i nfluences a l most a l l geological p rocesses. Weathering a nd erosion, patterns of sed i ment d i stribution, the form of mountains, and even the coo l i ng of igneous rock, a l l reflect the influence of g ravity. =
P r i n c i p l e of g ravity is s hown by s p r i n g bala nce; relative extens i o n of s p r i n g i s proportional t o dens ity of u n d e r l y i n g rock.
A GRAVIMETER is used to make
rapid and accurate determina tions of relative differences i n the earth's gravity fie l d . It i s a very sensitive spring balance, an increase in gravity being recorded by the stretching of the spring . Gravimeters employ opti c a l a n d e l e c t r i c a l m e t h o d s to "magnify" the m i nute increase in s p r i n g l e n g t h t h a t h a s t o be measured .
GRAVITY PROFILE m a y a l s o
s h ow l o w g r a v i t y ( n e g a t i ve a n o m a l y ) d u e to p r e s e n c e o f lig htweight s a l t in a sa l t plug, invisible from the su rface (see p. 99).
Bouger Anomaly Free-a i r Anomaly meters
m. gals.
+ 50 0
2000 1 00 0 0 1 000 200 0 ���������������������
THE RED SEA AND GULF OF MOUNTAIN CHAINS often hove ADEN, u n l i k e other r i f t volleys, a major negative g ravity anom
hove positive g ravity anoma l i e s . T h e y ore underla i n by b a s i c rocks a n d o r e b o u n d e d by p a r a l l e l fau lts. This suggests they were formed by the crustal separation of Arabia and Africa, the "gop" between them being fi l l ed by ris ing basic materia l from the mantle.
aly caused by a root of l i g h t weight granitic material . Negative anomalies across rift vol leys ore portly explicable by thick sed iments within the vol leys themselves, and may result from the concentration of l i g htweight a l k a l i ne magmas and volcanoes in rift area s .
is the state of equ i l i b rium t h a t exi sts in t h e earth's crust. Because mountains hove roots (p. 1 2 1 ) and a l so stand above the overage level of the genera l l y s i m i l a r rocks of sur rounding areas, a balance must exist between them and the den s e r m a t e r i a l on w h i c h t h e y
'"float . " As erosion reduces the moss of the mounta i n , uplift tokes place below i t as plastic material flows under i t - just as unloading cargo from a s h i p causes i t to r i se i n the water. The postg lacial uplift o f Scandinavia i s o n exam ple of i sostatic adjustment ( p . 62).
ISOSTASY
Ai ry's i sostatic hypothe s i s { 1) suggested eq u i l i b r i u m results i f cru sta l blocks of s i m i l a r dens ity have d i fferent h e i g hts; P ratt's hypothes i s {2): b locks of d i fferent d e n s ity have o of un iform level c o m pe n sa t i on . Modern hypotheses suggest variation i n d e n s ity i n a n d betwee n col u m n s .
G E OMAG N ET I SM
reflects the earth's behavior as though it were a giant bar magnet surrounded by a magnetic field . The force causes a compass to rotate so that it p o ints towa rds the mag netic north po le. The earth's magnetic field i s probably caused by convection currents i n the outer core . Geog ra p h i c North
D i rection of the tota l field
MAGNETIC STORMS
are sudden fluctuations in the earth's mag netic field caused by charged particles from the sun (the solar w i n d ) . Magnetic storms often precede aurora d isplays. Airborne magnetometer
MAGNETIC DECLINATION
is t h e angle between geographic ("true") north and magnetic north . The vertical angle between the horizontal and a freely d i p ping magnetic need le is the incli n a t i o n . The dec l i n a t i o n s h ows d a i l y c h a n g e s a n d a l s o s l ow, measurable changes over long periods of time. The magnetic poles change position relative to the geographic poles at a present rate of about four m i les every year, a lthough the devi ation is never large.
MAGNETOMETERS measure the
local intensity of the earth's mag netic fie l d . Variations ( anoma l i es) are caused by rocks of d ifferi n g magnetic properties. Magnetic traverses can be made on the ground or by a i rborne or seaborne magnetometers . Reg ional surveys are used i n m i n eral exploration and i n study o f t h e ocean floor.
PALEOMAGNETISM
i s the rem nant magnetism found i n rocks espec i a l l y l avas and some sedi m e n t a r y r o c k s - refl e c t i n g the ancient magnetic fields at the time of their formation . Magnetic particles i n the rocks orient them s e l ve s l i k e c o m p a s s n e e d l e s , reflecting the field i n which they formed .
North
THE MAGNETIC FIELD
of the earth has been measured and m a pped . The l i nes of force, m a rked by a rrows, show its d i rection a t various places. It a lso varies i n intensity (being twice a s great at the poles as a t t h e equator), and t h e inclination ranges from 0° at the magnetic equator to 90° at the magnetic poles. local anomal ies are often produced by d i stinctive magnetic properties of some rock bod ies.
North Geog ra p h i c ! Pole I
REVERSALS
i n earth's magnetic field have occurred every few hundred thousand years during the last 70 m i l l ion years . A time sca le of reversals has been recon structed from sequences of lavas and deep-sea sed iments.
show a d i stinc tive para l l e l pattern of mag netic reversa l "anom a l ies" across the m i d-ocean ridges, and provide a c l ue to the h i story of the ocea n (p. 1 40) . MAG N ET I C S U RVEYS OF OCEAN FLOOR
POSITIONS OF ANCIENT CON TINENTS are reconstructed b y measurements of remnant m a g n e t i s m in r o c k s o f successive ages . For E urope and North America, the traces o f ancient polar positions are simi lar, but
are di splaced para l lel to one another, suggesting either that the poles have wandered or that the conti nents have m oved apart. Other i ndependent evidence sup ports the i nterpretation of major continental movements.
TRIASSIC EQUATOR
i n North America based on paleomagnetic data (after Irving ) . Zero paleo m e r i d i a n is a r b i t r a r i l y t a k e n through N e w Yor k . P a l eomag netic data show North America was o n c e j o i n e d t o E u r o p e , Africa, a n d South America within a single continent, Pangea, which began to s p l i t apart about 200 m i l l ion years ago. Pa l eoeq uator
Ty pica l profi l e (3,000 m i les lo n g ) across Atl a ntic floor Crest of M i d Atl a ntic R i d g e world avera !!!..---"'""---North American Heat flow profo. le across Atlantoc plate N. America sed i ments Mid-Atla ntic Ridge .
�
1-
0 10
� 20 g 30
Africa
Basaltic Continental crust vol ca n i cs General ized cru sta l structure across North Atlantic
;2
STRUCTURE OF THE OCEAN F LOOR M I D - O C E A N R I DG E S
form a g loba l network of mounta in cha ins, 25, 000 mi les long, up to 1 , 500 mi les wide, and up to 1 8 , 000 feet high (p. 66) , formed of young vo lcanic rocks. They a re repeated ly offset by tra nsform fa u l ts and have crests marked by rift va l leys , up to 1 2 , 000 feet deep and 30 m i les wide . In some places (e . g . , Iceland and East Africa) ridges emerge above sea level .
MI D-OCEAN RIDGES
are marked by abnormally high heat flow, shal low earthquakes, vol canic activity, rift va l l eys, and transform fau lts. All these imply that the ridges are places of strong crusta l tension. Geophys ica l studies, d redg ing, and d r i l l ing s h o w t h e ridges have a t h i n Mid-Ocea n Ridges Tra n s form Fau lts
1 36
veneer of deep-sea sed iments, but consist chiefly of basa ltic p i l low lava s , ove r l y i n g v e r t i c a l feeders, and deep gabbroic crys ta l l ine racks . All these features imply that the ridges are sites for the submarine extrusion of new crusta l mater i a l (see pp. 1 401 4 1 ).
ABYSSAL PLA I N S
cover over 60 percent of the ocean floor. They a re flat basins, hundreds of m i les across, gen era l ly broken only by ocea n ridges, canyon system s , trenches, vo lca n i c i s l a n d s , or sea mounts . They lie at depths below 1 5 , 000 feet . Geophysical stud ies show that they have a thin cover of young sed i ments .
ROCKS OF THE OCEAN FLOOR
are younger than the continents, none being o lder than 1 75 m i l lion years; some continental racks are over 20 times that age . Ocean sed i m e n t s t h i c k e n and i n c l u d e successively older layers i n par-
a l l e l bands away from t h e m i d ocean ridge. U nderlying subma rine lavas show the same feature ( p . 1 40 ) . These observations are i m portant i n the development of a model to expla i n the h i story of the earth .
VOLCANIC SEAMOUNTS
r i se up to 1 2 , 000 feet above the abyssa l p l a i n s . Guyots are s i m i l a r , b u t have f l a t tops, presum a b l y having been eroded b y wave action and later submerged . M o s t l i e a t 3 , 0 0 0 - 5 , 00 0 f e e t below present s e a leve l . N Ma i n e
Bermuda I s .
Puerto Trench
VO LCA N I C I S LA N D A R C S ,
common a round the Pacific Basi n , a re major earth features, genera lly bordered on the i r convex ocea nic side by na rrow trenches, up to 36, 000 feet deep . They a re sites of vulca n i s m , earthq uakes, neg ative g ravity anomal ies, a nd crusta l i nsta b i l ity. Their andesitic lavas, i ntermed iate in composition between those of the conti nents and ocea ns, suggest a m i xture of the two . Nearly a l l deep-focus earthquakes occur below these a rcs . Geophysica l studies ( p . 1 39) ind icate that i s l a nd a rcs reflect the col l i sion of two p lates of the earth's crust. The trace of earthquake foci marks the buckling under (subd uc tion) of one plate . The islands seem to be formed from melted material from the subducted plate r i s i ng through the overrid i ng plate, for m i ng volcanoes and i ntrusions. J a pa n and the Aleutian and Ma rianas islands are exa m ples. Destruction of "old" ocean crust by subduction (p. 1 39) ba la nces new crust formed at the ridges . 1. 2. 3. 4.
Aleutian Kurile Japan N a n s e i S hoto S. Mariana 6 . Pa l a u 7 . P h i l i pp i n e 8 . Weber 9 . Java 1 0. New Brita i n 1 1 . N e w Hebrides 1 2 . Tonga -Kermadec 1 3 . Peru·Ch i l e 1 4. Aca p u lco Guatemala 1 5 . Cedros
land
.
volcanoes
£.A,
EARTHQUAKE EPICENTE R S :
\•
s h a l l ow focus
0 0 0
i ntermed i ate focu s
•. •
deep foc u s
Diagram of a typ ical i s l a n d a rc a n d trench structure, showing relative p o s i t i o n o f v o l c a n o e s ; s h a l l o w - , i n t e r m e d i a t e - , a n d d e e p -f o c u s earthquakes; a n d ocea n i c trench. A B represents l i n e o f sections below; length a bout 1 ,200 ki lometers. A
B sea levei O crust
TOPOGRAPHIC PROFILE
shows very steep sides of typical i sland arc trench . B
B
A
0
GRAVITY PROFILE
shows i sland arc trench areas a s belts of strong negative anoma l i e s . B A 34 ' V •.r okm •
-2 A)O� / 58 ' ��� ·. r9' 1 • 0 700km Al • :l in volcanic d i pping toward the conti areas but low across trenches r e fl e c t s " c o a l " o c e a n i c p l a te nents at 30°-60°, reflects subduc tion af ocean i c plate. being melted and uprising .
�
HIGH HEAT FLOW
EART H Q U A K E E P I C E N T E R PLOT,
MODEL OF ISLAND ARC shows
trench, earthquakes, vo lcanoes, and heat and gravity features. how subduction of plate creates i s l a n d a rc ocea n i c crust (After The Story layer 1 ����----���� --�-- of t h e Eor t h H . M . S . O . ) layer 2 layer 3 active volcano l ithosphere ocea n i c tre n c h mantle s u b d u ction zone Benioff Zone ea rth q u a ke foci to depths of 600-700 km 1 39 ,
-lt----��i;;;;�!'J���i!;;;iif
Tra n sform fau lts
.. .
Plei stocen e 2 P l iocene 7 Miocene 26 01 igocene 38 Eocen e 54 Pa l eocen e 65 C retaceou s 1 36 gest seafl oor spread i ng . N ote concentra t i o n of e a r t h q u a k e s ; deeper f o c u s ones ore confi ned t o t r e n c h o r e o s show n .
Mid-ocean ridges Tra n sform fault zones Deep ocea n trenches - Earthquake epicenters �;· _ _ _ _
AGES OF ROCKS,
shown by magnetic anomalies, existing as m i rror images a cross the Pacific mid-ocea n i c ridge, strong l y sug-
S EA F L O O R S P R EA D I N G i s a mechanism that accounts for the major features of the continents and ocea n s . New ocea n floor i s created a long the length of the mid-ocea n ridges, from which it then moves away i n both d i rections at rates of between 1 12 i nch and 3 i nches a yea r. This seems to be part of a l a rger movement i n which the outer shell of the earth (the l ithosphere) g l ides s l owly across the molten upper layers of the mantle (the asthenosphere, p. 1 28 ) , w h i c h a c t s as a conveyor b e l t f o r t h e rigid plates that i nc l ude both ocea ns and conti nents . Both the basa ltic rocks of the ocea n crust and the overlying sed i ments become successive ly older away from the ridges, but a l l are less than 1 75 m i l l ion yea rs o l d .
1 40
•
0 "' a � a " >0 � c .!!
P R E S E N T O C E A N BAS I N S are young features of the earth, ea rlier ocea n basins having been destroyed by plate tectonic movements in subduction and col l i sion . TRAN S F O R M FA U LTS
( p . 1 40) offset the m i d-ocea n ridges. They are probably caused by d i fferent sprea d i n g rates a l ong t h e ridge . Ea rthquakes occur only a long parts of fa u l ts between the two ridge seg ments , because spread ing movement across other parts of fau l t i s i n same d i rec tion . The San Andreas Fault is a tra nsform fault that outcrops on l a n d , with the Pacific P late s l i d i ng at a rate of 4-6 em . a yea r past the American Plate. Sudden move ments a long this fa u l t produce destructive earthqua kes .
RAT E OF S P R EA D I N G from mid -ocean ridge is measured from sym metrical pattern of magnetic reve rsa l s observed in rocks of equal age. These reversa l s a re dated by studies of s i m i l a r reversa l s i n lava seq uences . The d i stance of bands of equ a l age from the ridges shows d i fferences in rate of sprea d i ng . The East Pacific i s opening u p at a bout 1 2 em. (% i n . ) a yea r, the N orth Atlantic by only a bout 2 e m . a year. Age ( m i l l ions of yea r s ) 4 3 2 1 0 1 2
ANOMALIES
i n the remnant magnetism o f the oceanic crust s h o w r e v e r s a l s , r e fl e c t i n g changes i n the magnetic field at the various times of crust forma tion. Studies of lava flows on land g ive a t i m e scale of magnetic reversa l s for the past 4 m i l l ion years, and this i s confirmed in deep-sea cores . S i m i l a r anoma lies exist i n symmetrical bands, para l l e l to the m i d -ocea n i c ridges. This suggests that new crust is being formed and spread ing at rates up to 1 2 centi meters per year.
MID-
Model shows rise of new seafloor, " i m pri nted" with reversa l s of suc cessive magnetic periods and reversa l s .
141
3 4
S U BM E R G E D VO LCA N O E S a re scattered a c ross the ocea n floor, either singly or in c l u sters, with more than 1 0 , 000 in the Pacifi c . Ma ny formed a bove sea leve l , but have since been submerged by plate movements (p. 1 44 ) .
THE PRATT-WELKER
c h a i n of guyots runs from the Gulf of A l a s k a t o w a r d the A l e u t i a n Trenc h ( p . 1 37 ) . Guyot GA- l has a surface lying a lmost 6, 000 feet
below the genera l level of all the other guyots . It has been carried downwards by the subsidence that produced the Aleutian Trench .
0 1 000 2000 3000
SOUTH EASTWARD MIGRA TION o f volcanic activity in sev
eral i s land chains is explai ned by
a stationary plume or hot spot, rising from the mantle through a northwest movi ng plate.
SEAMOUNT
<1] 5 0 Hawa i i a n Em peror Bend .., � 0 e0 "'\, 4 "' G; 30 �0 ..... 20 � '� (. \l 1 0 ")i Hawa i ia n Ridge Hawa i i <"!
.SO �
Pacific Ocean
chain has active vul canism l i m i ted to the i s land of Hawa i i at the extreme southeast tip. Extinct vo lcanic i s lands a re progressively alder towards the n o r t hwest . H a wa i i a n E m pe r o r B e n d shows c h a n g e i n d i rection of plate movement 40 m i l l i o n years ago . Neighboring i s land chains (Tuamotu line and G i l bert Aus tra l ) show similar trend s . (After various authors)
C O R A L R E E F S develop a round islands, espec i a l l y the vol ca nic islands of t h e Pacific, where they m a y form fringing reefs (g rowing on the fringe of the island), barrier reefs (sepa rated from the i s l a nd by a lagoon), or ato l l s (ring l i ke ram pa rts of lagoons, with no main island core) . One hundred and fifty yea rs ago, Charles Darwi n suggested these three types of reefs represented th ree stages in the sinking of the ocea n floor a round a volca n i c i s l a n d , the cora l g rowth at sea l eve l keeping pace with the sinking of the floor. Geophysical surveys and borings on Pacific ato l l s con firm this theory. O n E n iwetok , 4, 000 feet of cora l sed i ments overlie o l ivine basa lt. It m a y n o t b e a complete explanation, however, because of the genera l l y u niform depth of lagoons (from 1 50 feet for the sma l l e r ones to 270 feet for the l a rger ones), which may be the result of Pleistocene erosion d u r i ng periods of lower sea l evel . Sea floor sprea d i n g (p. 1 40) accounts for the subsidence and latera l movement of the ocean floor i nvolved i n the forma tion of reefs a nd volcanoes .
PLAT E T E C TO N I C S theory offers a unified expla nation for most features of the earth . The earth's surface consists of seven r i g i d , movi n g , i nteracti ng plates and several m i nor ones, each a bout 1 00 kilometers (60 m i l es) thick, carrying both conti nents and ocea ns. New crust is created at spreading ridges (constructive or divergent margins, p. 1 45); it moves away from these at s peeds of up to 1 8 centi meters a yea r, and is destroyed by subduction into the mantle at destructive or convergent margins, such as trenches, where one plate typica l l y overrides a nother ( p . 1 45 ) . P l ates m a y a lso s l i d e latera l ly past one a nother at transform faults (p. 1 4 1 ) . The process seems to be cont i n u o u s and broadly ba l a nced , so t h e earth does n o t change i n size. - S h a l low,
e =
intermediate,
a n d deep e a r t h q u a k e s M i d -ocean r i d g e s
... R. e l o t i v o
v
0 0 oO
lote movement
ocea n trench (convergence)
Association of Earthquakes
r i s i n g magma
OCEAN RIDGE topography, vu l ISLAND ARC TRENCHES reflect
c a n i s m , s h a l l ow e a r t h q u a k e s , a n d symmetrica l magnetic anom alies reflect lateral spreading of new crustal material from ridge a rea s . (After Press and Siever)
the subduction of ocea nic plate; melting and rising material con t r i butes to v o l c a n o es; f r i c t i o n from downward plate movement produces earthquakes .
MAJ O R E A R T H FEAT U R E S
a re exp lai ned by plate tecton ics . For exa m p l e , young ocea nic rocks (less than 1 75 m i l l i o n years o l d ) reflect subduction o f older ones . Older continents ( u p to 3. 8 b i l lion yea rs) a re genera l l y too " l i g ht" to be subducted . Other exa m ples are i l l ustrated here . MOU N TA I N B U I L D I N G
takes pl ace at convergent bound of p l ates, where collision produces intense com pres sion . There a re severa l va rieties of co l l ision .
aries
ANDES a r i se from subduction of HIMALAYAS formed from colli
the oceanic N a zca plate below the continental South American plate . Mounta i n b u i l d i n g , vulcan i s m , earthquakes, u p l ift, and the deep C h i l e trench result, as i n fig ure above .
PERMIAN GLACIAL DEPOSITS,
n o w scattered across southern continents, were formed within a supercontinent, Gondwana land . This drifted apart, and the indi vidual continents are now iso lated by plate movement.
sion 40-60 m i l l i o n yea rs ago of Indian and Eurasian plates; over t h r u s t i n g p r o d u c e s m o u n ta i n s . The Alps reflect c o l l i sion o f Afr i can a n d Eurasian p l a t e s , 60-80 m i l lion years a g o .
H OW T H E EARTH WO R KS I F S E AFLO O R S P R E A D I N G
and the interaction of the rigid p lates of the earth's crust (plate tectonics) can account for a l l the major features of the earth , we have to ask what can account for the va rious phenomena of plate tectonics . What is the d riving force that moves the huge, rigid slabs that make up the surface of the earth? How can any such force be powerfu l enough to cause ea rthqua kes and ra i se up mounta i ns? What is the earth engine that dr ives pl ate tecton ics? Most earth scientists now concl ude that the mantle ( p . 1 29) - the dense layer underlying t h e crust - is a sol i d , s o h o t that it can flow very slowly. " F loating" on t h a t foun datio n , the l i g hter crusta l plates a re d ragged a long by these slow movements in the mantle. How cou ld that work? There i s no agreement on this, but three possible ways a re descri bed on p. 1 47 . Whatever movement is proposed must be capa ble of exp l a i n i ng lat eral movement of plates away from spreading ridges and downward movement of plates at trenches .
SIMPLIFIED MODEL
of how plates of crust may be driven by convection currents in the mantle. (After several authors) m i d ·ocea n ridge spread i n g
FO R C E S T H AT MOVE PLAT E S are sti l l not clear. I l l ustrated below a re three possible mode l s . p u shed
pl ate
PUSH-PULL FORCES
m a y pro duce plate movement. The height and weight of spreading ocean ridges moy push the plate lat erally, while the coo l , heavier subducted plate may pull i n the same d i rect i o n .
CONVECTION CURRENTS
in t h e m a n t l e m a y f a r m s e ve r a l s l o w l y c o n ve c t i n g c e l l s , r i s i n g below t h e ridges, sinking below the trenches, dragging the crust with them .
PLATES
may be cooled portion of upper mantle formed by convec tion currents rising at ridges and coo l i ng as they spread . ( I l l ustra tions after Press and Siever)
E AR L I E S T H I STO RY OF T H E EARTH is sti l l obscure, but it proba bly i nvolved the accretion about 4 . 7 b i l l i o n yea rs ago of cold m ateri a l s . The i m pact of this material gradu a l l y heated u p the growing earth, as d i d compression a n d radi oactivity. T h e temperature slowly increased unti l , per haps 1 b i l l i o n or so yea rs after its formatio n , the earth was hot enough to melt the iron present, which sa nk towards the center, o r core, creating more heat and causing d i ffer entiation of l i g hter materia l i nto crust a nd i ntermed iate mantl e . As the temperature rose , slow convection m ove ment bega n to take place, and the differentiation i nto a layered earth conti nued , with lig hter material form i n g the conti nents . The atmosphere a nd oceans proba bly accum u lated b y outgassing from withi n t h e earth . P late tectonics is part of this later history of the earth . 1 47
THE EARTH'S HISTORY The age of the earth has been a subject of speculation si nce early days of humankind, but only in the last century have attempts been made to measure it. Geologists studying ea rth processes a re concerned with the seq uence of rocks and structures i n time, and thus with the h i story of the earth itself.
RATE OF COOLING w a s once SALT C O N T E N T O F T H E thought to show the earth was OCEANS i s presumed to have
only 20-40 m i l l ion yea rs o l d . D i s covery o f h e a t produced by radioactivity provides n e w data that inva l idate this conc l u s i o n .
TOTAL AVERAGE THICKNESS
of sed i ment over the earth , if reg u l a r l y deposited , was thought to provide the earth's age, if divided b y the average annual addition to new sed iments. While this method may be acceptable for a few local deposits, there are f a r too m a n y v a r i a b l e s a n d unknowns (such as redeposition of sedi ment) to a l low its use for determining the age of the earth.
come from weathering of rocks and was d ivided by the annual i ncrement to give on age for the oceans of a bout 90 m i l l ion yea r s . T h e s a m e l i m itations apply to t h i s factor as to sedi ment-th ickness calculations. Both fig u res i nvolve corrections that would g reatly increase their value.
RADIOACTIVE DECAY
provides the best present method of mea suring the age of rocks. Radio active elements undergo sponta neous breakdown by loss of a l pha and beta particles into stable ele ment s . The rate of breakdown, which con be accurately mea sured, is i ndependent of any envi r o n m e nta I c o n d i t i o n s , such as t e m p e r a t u r e or p r e s s u r e . T h e ratio o f decayed t o parent ele ments thus provides o n indication of the age of the m i neral i n which it i s fou n d . D i fferent elements have very different decay rates. <111111 Succes sion of sedimentary rocks
in Grand Canyon, Arizona, s hows thei r relative geolog ic ages, but r e p r e s e n t s o n l y p a r t of t o t a l geologic time ( p . 1 5 1 ) .
newly-formed m i neral
URANIUM {U238)
is a commanly used element, going through 5 d i s i nteg rations before it becomes the stable e l ement lead { Pb206 ) . D i fferent elements have different rates of disintegration . U 2 3 8 has a half-l ife (the time taken for h a l f its atoms to d i s i n t e g r a t e ) of 4 , 500 m i l l i o n (4 . 5 b i l l ion) years. Some other radioactive e l ements commonly used in age studies ore thorium, potassium, a n d rubid i u m .
THE OLDEST ROCKS measured
by radi oactive methods have a n a g e o f about 3 , 800 m i l l ion ( 3 . 8 b i l l ion) yea r s . This i s younger than the earth itself, which is probably about 4 . 5-5 . 0 b i l l i o n years o l d . Stud ies of meteorites, which are probably samples of the planetary mater i a l s from which the earth origi nated, a l l indicate a n a g e o f about 4 . 5 b i l lion yea rs. The o ldest known los-
1 00
�
�
m
u ra n i u m
• 23 8
lead 206
75
"
� E .!:
50
7/o gone
o
� 25
� � �
1 2 112 0 Increasing t i m e ( i n billions of years)
s i l s ore about 3 . 4 b i l l i o n year s old, but common foss i l s are found only i n rocks younger than 6,00 m i l lion years o l d . Radiocarbon dating i s usefu l for rocks that conta i n wood frag ments a n d a re younger t h a n a bout 70, 000 year s . C 1 4 from the atmosphere i s i ncorporated in plant tissues a n d disi ntegrates to N 1 4 with a h a l f - l ife of 5 , 570 Rate at which C" decays and becomes N1" is known
Neutro n s bombard
N1" + neutron C 1 .. (rad iocarbon + proto n ) =
Tree absorbs C "O,
( 1"02 a n d ( 1 2 Q2 rema i n con sta nt i n l i v i n g tree Section of l i v i n g tree conta i n s x amount of ( 1"
U N D E RSTA N D I N G EARTH H I STORY a l so i nvolves the study of the deve lopment of a n i m a l s and p l ants and of the conti nuously chang i ng ancient geog raphies. Particu l a r sedi mentary rocks, dated b y va rious methods, c a n then be related to a general geologic time sca l e . Most radi oactive m i nera l s used for age determinations occur i n i gneous rocks, a lthough g lauconite is a m i neral used for age studies of sed i mentary rocks.
SUPERPOSITION
o f s ed i m e n tary rocks i nd icates their relative a g e s . In u n d i s t u r bed s e ct i o n s , younger rocks overlie older.
STRATIGRAPHIC CORRELA TION of strata i n one p l a c e with
those of the same age, deposited at the same period of time i n another p l a c e , i s fundamenta l i n the i n t e r preta t i o n o f g e o l o g i c history.
FOSSILS a re i m portant i n corre
lation of sed i mentary roc k s . Fos sils are the remains of, or direct indication of, prehi storic a n i m a l s and plants. A l t h o u g h i n fluenced by envi ronment, s i m i l a r assem blages of foss i l s genera l l y indi cate s i m i larity of age i n the rocks that conta i n them .
ROCK FACIES, the sum tota l of
the characteristics of a rock's d e p os i t i o n a l e n v i r o n m e n t , a r e independent of geological t i m e . A n awareness of t h e m , however, i s i m portant in correlati o n . F i g u r e shows h o w a sha l l ow sea transgressed over a deltaic and near-shore environment in west ern U . S . in Cambrian times, about 500 m i l l i o n years ago.
LITHOLOGICAL CORRELATION GEOPHYSICAL CORRELATION
u s e s the s i m i larity of m i neralogy, sort i n g , structure, bedd i n g , sequence, and other features a s i n d i ca t i o n s o f s i m i l a r ages of rocks. I t i s of l i m i ted value, since rocks of d i fferent l ithology often ore deposited a t the some time in adjacent area s .
makes u s e of s i m i larity of physi· col r o c k p r o p e r t i e s ( e l e c t r i c a l resistivity a n d self-potenti a l , for example) as o n i ndication of s i m · i l o r age . W i d e l y u s e d i n bore· holes, this method i s l i mited by the some factors a s lithological method s .
STANDARD GEOLOGIC COL· ROCK SYSTEMS i n t h e geologic UMN, which h a s been bui lt up by column o re m a j o r d ivisions of
rocks deposited during a partic ular period of geologic t i m e . Names of systems ore t a k e n from a r e a s w h e r e r o c k s w e r e fi r s t descri bed , such as Devonian from Devonshire, o r from the i r char acteristics, as Cretaceous from the chalk, which i s found i n many strata of this age. Egypt i a n Roman Assyrian Middle E m p i re E m p i re Kingdom
combining rock sequences from different areas , con be matched with a time scale based on mea· sured absolute ages of roc k s . This i s like a master motion picture fi l m i n which local rock sequences e a c h r e p r e s e n t a few s i n g l e frames. T h i s geologic time sca l e is shown on p . 1 52 . Wor l d Rena isWa rs sa nee
1 000
Li mestone Shale Li meston e a n d s a n d stone Sandstone a n d s h o l e T h e Earth's tota l h i story i s pieced together by comparison of rocks from m a n y area s .
ZION CANYON
1 35 1 66 181
230 280
300
345
600
PERIODS
u
0 N
0 z
t:l
QUATERNARY Recent Plei stocene TERTIARY P l i ocene Miocene Oligocene Eocene Pa leocene
dura- yea rs tion ago m i l l ions
CRETAC EOUS
2 5
9
1 1
16 2
65
70
u
1 35
0
0 i
J U RASSIC
"'
55 1 90
TRIASSIC
35
PERMIAN
55
P E N NSYLVANIAN
40
MISSISSIPPIAN
25
DEVON IAN S I LURIAN ORDOVICIAN
34 5
50 35
395 430
70 500
CAMBRIAN
70 570
1 52
E = Evaporite Deposits Equat o r
0
I
0
1 000 K i l ometers
PALEOGEOGRAPHY OF U.S.,
i n Middle Permian times, a bout 250 m i l l i o n yea rs ago. (After Dott and Batten) PA LEOG E O G RAPH I C MAPS a re reconstructions of the geography of past geologic periods. Past continenta l geog raphies can be pieced together by using paleomag netic data and by plotting the d i stribution of different rock types, foss i l s , and geologic structures, using the methods i l l u strated on pp. 1 50- 1 5 1 .
WORLD PALEOGEOGRAPHY about 200 m i l l i o n yea rs a g o . Shading
represents deposits of former ice cap (see p . 1 45 ) . (After Press and S iever) 80°
40°
LAU RASIA
PANTHALASSA
fl 0o
1 20°
80°
L � 400 0"?' ,.. oo .t,_. �""- t 20° \
( (
60• -aoo
•
so·
1 20•
1 60•
TETHYS S E A GONDWANALAND '
l-
1 53
GEOLOG IC MAP OF
Rocks and unconsolidated deposits of P l e i stocene a n d Recen t age
TERTIARY Rocks of Paleocene, Eocene, Oligocene, Miocene, a n d P l i ocene age
MESOZOIC
Rocks o f C a m b r i a n ,
J u r a s s i c , and
Ordov i c i a n , a n d
C retaceous age
Si l u r i a n age
LATE PALEOZOIC Rocks of Devo n i a n ,
1 54
EARLY PALEOZO I C
Rocks of Tr i a s s i c ,
PRECAMBRIAN
A variety o f i g n e o u s , meta m o r p h i c ,
M i s s i s s i p p i a n , Pennsylva n i a n ,
and s e d i m e n t a r y rocks ( i n c l udes
a n d Per m i a n a g e
some metamorphosed Paleozoic loco II)
THE U N ITED STATES 0
1 00
200
300 Statute m i les
---
0
1 00 200 300 K i l ometers
EXTRU S IVE IGNEOUS ROCKS C h i efly lava flows o f Tertiary a n d Quaternary age
I NTRUS IVE IGNEOUS ROCKS ( i n c l udes some meta m o r p h i c r o c k s ) G r a n i t o i d r o c k s of various ages
1 55
FOR MORE INFORMATION M U S E U MS AND EXH I B ITS provide exce l l e n t d i splays of general geologic topics and of reg ional geology. Many universities and most l a rge c i ties have museums with geologic exhibits.
STATE GEOLOGICAL S U RVEYS
publish maps, guide books, and elem entary introductions to geology. The U . S . Geological Su rvey, Wa shi ngton, D . C . 20242, a n d the Geo l o g i c S u rvey o f Canada a l so publish many u seful reports and mops.
NATIONAL PARKS AND MONUMENTS are genera l l y areas of great geologic interest. They provide gu ided tours, talks, museums, and pamphlets.
F U RT H E R READING BOOKS w i l l h e l p y o u develop your i n terest i n earth science.
Here are a f e w useful titles: Cattermole, Peter, and Patrick Moore. T h e S t o ry o f t h e Earth . New Yor k : C a m bridge U n i versity P r e s s , 1 98 5 . A s i m p l y written , u s e f u l overview. larso n , Edwin E . , and Peter W. Birkeland . Putnam's Geology, 4th e d . New Yor k , Oxford U n iversity P r e s s , 1 98 2 . We l l - i l l u strated, readable, college-level text. Press, Fra n k , and Raymond S iever. Earth, 4th e d . New York, W. H . Freeman and C o m pany, 1 98 6 . Com prehensive, college-level text. Rhodes, Frank H . T. , Herbert S . Z i m , and P a u l R . Shaffer. Fossils . New Yor k , G o l d e n P r e s s , 1 96 2 . S m i t h , D a v i d A . , ed . Cambridge Encyclopedia of E a rth S c iences . New Yor k , C a m bridge U n iversity P r e s s , 1 98 1 . Sta n l ey, Steven M. Earth and life Through Time . New Yor k , W. H . Freeman and Com pany, 1 98 5 . C o l l ege-level review of h i storical geology. Zim, Herbert S., and P a u l R . Shaffer. R o cks a nd M inerals. New Yor k , Golden Press, 1 95 7 .
PHOTO CRE DITS : U n less otherwise credited, photographs a r e by the author. p . 4 R. Wen k a m . p . 15 A m . Museum of Natural H i story. p . 31 E . Moench/ Photo Researchers . p . 3 3 R . Wenka m . p . 34 top, Kai Curry-lindahl, bot . , Douglass Hubbard. p . 52 bot . , New Zea land Geological S u rvey. p . 5 3 luray Cave s . p . 5 8 M . Rosal sky. p . 60 t o p , J . Wyckoff. p . 63 C . Tra utwe i n . p . 68 t o p , M . Rosalsky. p . 69 top, E. Druce. p . 75 J. Wyckoff. p . 76 J. Wyckoff. p . 80 H. Sch lenker/ Photo- Resea rchers. p . 8 3 M . Rosalsky. p . 85 J. Muen c h . p . 96 E. Druce. p . 1 02 l. Carlson. p. 1 03 top, J . J . Witkind. p. 1 05 M . Rosal sky. p. 1 1 2 C a l ifornia Geology. p . 1 1 5 M . Rosalsky. p . 1 1 6 H . Bristo l . p . 1 25 Matthew J. lee/Oakland Tr ibune. p . 1 34 Airborne Geophys ics Division, Lockwood, Kessler & Barlett, Inc.
1 56
I N D EX
Abyssal plains, 1 37 Ai ry's hypothesis, 1 33 Amphiboles, 30 Anticlines, 1 07, 1 08 Arcs, i s l a n d , 1 23 , 1 381 39 Aretes, 58 Arkose, 78 Artesian wel l s , 50 Asteroids, 1 1 Atmosphere, composition of, 1 8- 1 9 definition of, 1 8 At
����7u��·. ��
Barchans, 76 Bars, off- shore, 71 sand, 71 Boso l t , 93 Batholiths, 88 Bovs, and erosion, 68 Be0ches, 70 rai sed , 1 04 sto r m , 7 1 Beddi n g , cross, 81 graded, 81 varved , 8 1 Big-bang theory, 1 6 Blocky lava, B5 Brecc i a , and fau l t s , 1 1 3 Bridges, natura l , 4 8 B u i l d i n g mate r i a l s , 1 03 Bushveld deposits, 1 00 Calcarenite, 78 C a l careous ooze, 73 Calcite, 28 Calderas, 84 Carbonates, 28 Caves, and erosion, 68 and groundwater, 53 Chemical elements, 1 6 Chemical sed iments, 7273 Chemical weather i n g , 36 Cinder cones, 84 Cirques, 58 C l a y, in industry, 1 03 sediments, 73 varve, 60 C l eavage, i n m i nerals, 27 C l iffs, and erosion, 68
C l i nometers, 1 06 Coal, 80, 98 Coastal features, and crustal movement, 1 04 Coastl i n e s , emergent, 69 and erosion, 68 and marine deposition, 70 and sea l eve l s , 69 submerged, 69 Co lor, of minerals, 27 Comets, 1 0 Composite cones, 84 Composi t i o n , of atmosphere, 1 8- 1 9 of crust, 1 28 of sun, 1 3 Cones, ci nder, 84 composite, 84 Cong lomerates, 78, 1 1 4 Consequent streams, 46 Continental shelf and slope, 66, 67 Continents, 66, 1 22 Convection currents, i n mantle, 1 46 , 1 47 Coral reefs, 1 43 Core, of earth, 1 29 C revasse fi l l i n g s , 60 Cross-bed d i n g , 8 1 Crust, build-up of, 82- 1 03 composition of, 1 28 erosion of, 34-45 form of, 3 2-8 1 materia l s of, 2 1 -3 1 Crustal movements, 1 04- 1 47 and coastal features, 1 04 and foss i l s , 1 04 and incised meanders, 1 05 C r ystal form, of minera l s , 25 C u b i c cryst a l s , 25 Cuestos, 48 Currents, i n ocean s , 64 turbidi ty, 67 Dating, methods used for, 1 48- 1 5 1 Dec l ination, magnetic, 1 34
Deep-sea sed i ments, 72-73 Deformation, rock, 1 06- 1 1 4 Deltas, 4 5 , 60, 7 1 Dendritic drainage patterns, 46 Deposition, product of, 78-8 1 Deposits, Bushve l d , 1 00 d u n e , 77 g l a c i a l , 59, 60 loess, 77 marine, 70 ore, 1 00 outwas h , 60 placer, 1 02 residual minera l , 1 02 Sudbury, 1 00 w i n d , 76-77 Desert platforms, 75 Diamonds, 24, 1 00 Diastrophism, 3 2 Dikes, 86 Dip, of rock bed , 1 06 Disconformities, 1 1 4 Dome mounta i n s , 1 1 8 Drainage patterns, 46-55 dendritic, 4 1 , 47 modification of, 47 superimposed, 46 rad i a l , 47 trellis, 46 D r u m l i n s , 59 Dunes, 76-77 ancient deposits i n , 77 shape o f , 7 7 Dust-cloud theory, 1 6 , 1 7 Earth, age af, 1 48 crust of, 9, 2 1 -3 1 h i story of, 1 47 , 1 481 55 insta b i l i t y of, 1 04- 1 47 i nterior of, 1 2 8- 1 35 materi a l s in crust of, 2 1 -3 1 motions of, 1 2 a s planet, 1 0-20 size and shape of, 6-7 surface of, 8-9 , 1 221 23 Earth flows, 38 E orthquake(s), 1 04 , 1 24- 1 2 7
1 57
Earthquake(s) (coni . ) , destruction b y , 1 25 distribution of, 1 23 , 1 25 , 1 45 epicenter plot, 1 39 records, 1 26- 1 27 waves , 1 26- 1 27 Eclogites, 95 Economic geology, 5 E lectric logging, 1 3 1 Emergent coastl ines, 69 Eratosthene s , 6 , 7 Erosion, bays a n d , 68 caves and, 68 c l iffs and, 68 coasta l , 68 of earth's crust, 34-35 g l a c i a l , 57-58 hea d l a n d s and, 68 a n d landforms, 48 and mountains, 1 1 7 a n d rapids, 44 a n d rejuvenati o n , 44 river, 4 1 -4 3 by water, 40-4 1 a n d waterfa l l s , 44 wave-cut platforms a n d , 68 by w i n d , 75 Erratics, 59 Eskers, 60 Evaporites, 80, 1 02 Expa n d i n g - u n iverse theory, 1 6 Extru sive igneous rocks, 93 Fault block mounta i n s , 1 18 Faults, 1 05 , 1 1 1 - 1 1 3 i n mounta i n chains, 121 reverse, 1 1 2 strike-sl i p , 1 1 2 tear, 1 1 2 thrust, 1 1 2 transfo r m , 1 4 1 , 1 44 , 1 45 Feldspars, 2 9 F i l l structures, 8 1 F o l d s , 1 05 , 1 07- 1 09 in mounta i n chains, 121 Foliation, 94 Forests, buried, 1 04 Foss i l s , 1 04 a n d dating, 1 50 Fractures, of minerals, 2 7 rock, 1 1 0 Frost shatte r i n g , 36
1 58
Fuels, atom i c , 98 minera l , 98 Fumaroles, 5 1 Gabbro, 93 Galaxy, 1 2 , 1 4- 1 5 Galena, 3 1 Geologic c o l u m n , 1 5 1 Geologic map, 1 54- 1 55 Geologic time scale, 1 52 Geology, branches of, 5 history of, 4-9 Geomagnetism studies, 1 34 Geophysical correlation, 151 Geophysical exploration, 131 Geophysical measurements, 1 30 Geophysics, 5 Geysers, 52 Glaciation, histor of, 6 1 -6 2 p r e - P eistocene, 62 and sea leve l , 62 Glaciers, deposits of, 59-60 and erosio n , 57-58 and glaciation, 56-62 Graben, 1 1 2 Gradation, 3 2 G r a d e d bed d i n g , 8 1 Granite, 93 Granite porphry, 93 Graphite, 24 Grave l , i n i ndustry, 1 03 Gravimeter, 1 3 2 Gravity, anomalies, 1 3 2 , 1 33 faults, 1 1 1 force of, 1 32 profi l e , 1 3 2 Graywacke, 79 Great Spiral Nebula M3 1 , 1 5 Groundwater, 49-53 Gulf of E d e n , a n d rift vol leys, 1 33 Guyots, 1 37 , 1 42 Pratt-Welker chain of, 1 42 Gyps u m , 28
r.
Hardness, of m i nera l s , 26 Head lands, and erosi o n , 68
Heat flow, 1 30 , 1 39 Hexago n a l crysta l s , 2 5 H i storical geo l ogy, 5 H i story, of atmosphere, 20 of earth , 1 47 , 1 48- 1 55 of geology, 4 of glaciation, 6 1 -6 2 H o g b a c k s , 48 Horns, and g l a c i a l e r o s i o n , 58 Horst, 1 1 2 Hot springs, 5 1 Hydrologic cycle, 40 Hydrosphere , 40 Hydrothermal ores, 1 0 1 Ice ages, 62 Identification , of m i n e r a l s , 26-27 Igneous rocks, classification of, 89-93 common extrusive, 93 common intrusive, 93 as continental foundation, 83 form of, 92 intrusive, 8 6 joints i n , 1 1 0 and magmas, 92 in mounta i n ranges, 1 20 texture of, 92 I n c i sed meanders, 1 05 I ndustry, minerals used in, 96- 1 03 I nstruments, gravimeter, 1 32 seismograph, 1 26- 1 27 Intrusions, layered, 88 sha l l ow, 86 Intrusive igneous rocks, 86, 93 lslond arcs, 1 23 , 1 381 39 , 1 45 Island trenches, 1 38- 1 39, 1 45 Isostasy, 1 33 Joints, and rocks, 1 1 0 J u p i ter, 1 0 Kame terraces, 60 Kames, 60 Kettle holes, 60 lacco l i t h s , 87 landforms, erosi o n a l , 48 landslides, 39
lava, 85 li mestone, 79, 1 03 and porosity, 4 9 limonite, 28 lithological corre l a t i o n , 151 loess deposits, 77 loggin g, electri c , 1 3 1 lopoliths, 88 luster, of m i nera l s , 26 Mag m a , 90-9 1 , 9 2 Magmatic o r e s , 1 00 Magnetic lields, 1 34 , 1 35 Magnetic surveys, 1 35 Magnetometers, 1 34 Montie, 1 29 convection currents, 1 46 , 1 47 Mop(s), geologic U . S . , 1 54- 1 55 paleographic, 1 53 Marble, 95 Marine deposi t i o n , 70 Marine sed i ments, 72-73 Mass wastin g , 38 Measurement(s) , of age in rocks , 1 48- 1 49 of earth, 7 geophysica l , 1 30 of s u n , 1 3 Mechanical weatheri n g , 36 Mercury, 1 0 Metamorphic ores, 1 0 1 Metamorphic rock s , foliation in, 94 i n mounta i n chains, 1 20 recrysta l l i zation i n , 95 textural changes i n , 94 Metamorph i s m , 94-95 and new m i n e r a l s , 95 Meteors, 1 1 Meteorites, 1 1 Micas, 29 Mid-ocean ridges, 1 23 , 1 36 Milky Way, 1 4 Mineral(s), atomic structure of, 24 carbonates, 2 8 characteristics o f , 22-24 color of, 27 constructiona l , 1 03 common ore, 3 1 crystal form of, 25 fuels, 98 hardness of, 2 6 identification of, 26-27
Minerol(s) (con t . ) : and i ndustry, 96- 1 03 luster of, 26 oxides, 28-29 rock-fo r m i n g , 28-30 sulfates, 28 Monoc l i n e s , 1 07 Monoc l i n i c crysta l s , 25 Moon, 1 0 Mora ines, 59 Mounta i n - b u i l d i n g , 1 1 5- 1 24, 1 45 Mountain ranges, folded , 1 1 9- 1 2 1 and gravity anomalies, 1 33 structu r a l , 1 1 8 Mounta i n s , 1 1 5- 1 24, 1 33 and crustal movement, 1 1 5- 1 24 dome, 1 1 8 erosional, 1 1 7 erosion in folded, 1 2 1 fa u l t blocks, 1 1 8 and orogeny, 1 24 roots of, 1 2 1 uplift i n folded, 1 2 1 volca n i c , 1 1 6 Mud cracks, 8 1 Natural bridges, 4 8 N iagara Falls, 4 4 , 4 5 N utation , o f earth, 1 2 Ocean(s), 63-73 currents in, 64 floor, 1 23 floor spreading, 1 40- 1 4 1 salt content i n , 64, 1 48 structure of, 1 36- 1 43 tides i n , 65 water movement i n , 64-7 1 waves i n , 65 Obsid i a n , 93 Off-shore bars, 7 1 O i l shale, 99 Olivine, 30 Oozes, 73 Ore(s), deposits, 1 00 hydrotherma l , 1 0 1 magmatic, 1 00 meta l l i c , 1 02 metamorphic, 1 0 1 minerals, 3 1 sedim entary, 1 02 Orthorhombic crysta l s , 25 Outcrop and faults, 1 1 3 Outwash deposits, 60
Oxides, 28 Oxyg e n , i n e a r t h ' s crust, 22 Paleographic mops, 1 53 Paleomagneti s m , 1 34 Petro leum, 98, 99 P h y l l ites, 94 Physical geology, 5 Ph y sical oceanography, 5 P i l l o w l a v a , 85 P i racy, river, 47 strea m , 45 Placer deposits, 1 02 Planetesimal theory, 1 7 Planets, 1 0, 1 1 , 1 7 Plateau basa lts, 84 Plate tecto n i c s , 1 44- 1 47 Pluto, 1 0 Plutonic intru sive rocks, 86 Porosity, of l i mestone, 49 of rock and sand, 4 9 Pratt-Welker c h a i n o f guyots, 1 42 Pratt's hypothes i s , 1 33 Precession, of earth, 1 2 P u m i c e , 93 Pyrite, 3 1 Pyroxenes, 30 Quartz, 29 Quartzite, 95 Radial drai nage patte r n , 47 Radi oactive decay, 1 48 Radio carbon dating, 1 49 Raised beaches, 1 04 Rapids, and erosion, 44 Red Sea, and rift va l leys, 1 1 2 , 1 33 Rego l i t h , 35 Rejuvenation, and erosion, 44 Residual mineral deposits, 1 02 Reverse fault, 1 1 2 Revolution, of earth, 1 2 Ridges, m i d -ocea n , 1 2 3 , 1 36 Rift va l ley, 1 1 2 , 1 33 Ripple marks, 8 1 River(s), cycles, 4 2 f o r m of, 4 1 pi racy, 47 profi les, 42-43 Rock(s), deformation , 1 06- 1 1 4
1 59
Rock(s) (cont . ) , facies, 1 50 fractures, 1 1 0 igneous, 89-93 measuring age of, 1 48-1 49 metamorphic, 94, 95, 1 20 sedimentary, 78-8 1 systems, 1 5 1 Rock lo l l s , 3 8 Rock-forming m i n e r a l s , 2 8-30 Ropy lava, 85 Rotat i o n , of e a r t h , 1 2 Sand , in i n d ustry, 1 03 Sand bars, 7 1 Sand dunes, 76-77 Sandstone, 78 Sate l l ites, 1 0 Scour structures, 8 1 Seafloor, spreading of, 1 40- 1 4 1 Sea level s , and coastal features, 69 a n d glaciation, 62 Sea mounts, 1 37 Sea water, 64 Sedimentary ores, 1 02 Sedimentary rocks 78-8 1 c a l formed, 79 s i , dating of, 1 50 detrita l , 78 joints i n , 1 1 0 in mounta i n ranges, 1 20 organic, 79 Sed i m entary structures, 8 1 Sed i m ents, marine, 72-73 red-clay, 73 Seismograms, 1 26- 1 27 Seismograph, 1 26, 1 27 Septarian nodule, 49 Shale, 79 o i l , 99 Shell, conti nenta l , 66, 67 Shield volcanoes, 84 S i l iceous ooze, 73 Silicon, in earth's crust, 2 2 S i l l s , 86, 87 Slickensides, and faults, 1 1 3
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Slope, continenta l , 66, 67 Slumps, 39 Soi l , mass wasting of, 38 and weather ing, 37 Soil creep, 39 Solar syste m , movement of, 1 2 origin of, 1 6- 1 7 Space, planets in, 1 0- 1 1 Specific gravity, of minera l s , 27 Sphalerite, 3 1 Sp i � l s, 1 1 3 Stocks, 88 Stone aggregate, 1 03 Storm beaches, 7 1 Stratigraphic correlation, 1 50 Stream(s), consequent, 46 piracy, 45 Strike-sl i p f a u l t s , 1 1 2 Structural geology, 5 Studies and surveys , geomagneti s m , 1 34
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30 paleomagnetic, 1 35 Submarine canyons, 67 Submerged coastlines, 69 Subsidence, 39 Sudbury deposits, 1 00 Su lfates, 28 S u n , 1 0, 1 3 Superimposed drai nage pattern, 46 Synclines, 1 07 , 1 08 Tear faults, 1 1 2 Tensional faults, 1 1 1 Tetragonal crysta l s , 25 Thrust lo u l t , 1 1 2 Tides, 65 h s �u t , 1 4 1 , 1 44, 1 45 Tre l l i s drainage patterns, 46 Trenches, island, 1 38-1 39 submarine, 1 23 Triassic equator, 1 35 Tric l i n i c crysta l s , 25
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Tsuna m i s , 1 25 Tu rbidity currents, 67 U n conform ities , 1 05, J 1 4 Underground water, 49-53 U n iverse, o r i g i n of, 1 6 U p l ift, and erosi o n , 44 fau l t s , 1 1 2 Uranium, and age s t u d i e s , 1 49 Varve c l a y s , 60 Varved bed d i n g , 8 1 Ventifacts, 75 Volca n i c , activity, 8 3 , 8 4 bomb, 85 island arcs, 1 38-1 39 plugs, 87 prod ucts, 85 tuff, 85 Vo lcanoes, 83-85, 1 42 distribution, 85, 1 23 , 1 42 form of, 84 prod ucts of, 85 s h i e l d , 84 Vulcan i s m , 33 Water, and d rainage patterns, 46-55 and erosi o n , 40-4 1 as natural resource, 54-55 and hydro logic cyc l e , 40 origin o f , 2 0 supply of, 50 table , 50 undergro u n d , 49-53 Waterfa l l s , a n d erosion, 4 4 Waves, i n ocean s , 65 Weath e r i n g , 35 chem i c a l , 36 d i fferentia l , 48 effects of, 35 mecha nica l , 36 spheroida l , 36 and s o i l , 37 � ula, 1 5
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deposits, 76-77 Wor l d , size and s h a p e o f , 6-7 Yosemite Vo l ley, 34
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GEOLOGY
a, � CktcU,® FRANK RHODES, President of Cornell University, was educated at Solihull School and the University of Birming ham. He has held teaching positions at the University of illinois and the University of Wales, Swansea, where he was Professor and Head of the Geology Department for 12 years . He served successively as Professor of Geology, Dean of the College of Literature, Science, and the Arts, and Vice President for Academic Affairs at the University of Michigan. He is past Chairman of the Boards of the Carnegie Foundation, the American Council on Educa tion, and the Association of American Universities. He is a member of the National Science Board and President Bush's Education Policy Advisory Committee and holds honorary degrees from 20 colleges and universities . RAYMOND PERLMAN, teacher, designer, illustrator, and formerly Professor of Art (in charge of graphic design) at the University of illinois, in Champaign, holds degrees in fine arts from that university, and has a Master of Professional Arts degree from the Art Center School in Los Angeles . In the Golden Guide Series he has illus trated Fossils, Geology, and Rocks and Minerals. HOWARD FRIEDMAN prepared the illustrations on pages 97, 1 22123, 139 bottom, 142 bottom, 145 top, 146, 147, and 153.
