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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

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w ,.

s

N

I I I I \

- .... ,,

s'

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..

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"SLLM MER

..

w

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w N

..

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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

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0 "' " -= c:

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c: D ..., c: " ...a D "' 0

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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 1 0 ")i Hawa i ia n Ridge Hawa i i