Geology

Complete your collection of

Golden Guides® and Golden® Field Guides!

Golden Guides

Bats of the World • Bird Life • Birds • Butterflies and Moths • Casino Games •

Dinosaurs • Endangered Animals • Exploring Space • Fishes • Fishing • Flowers • Fossils • Geology • Insects • Mammals • North American Indian Arts • Planets • Pond Life • Reptiles and Amphibians • Rocks and Minerals •

Seashells of the World • Seashores • The Sky Observer's Guide • Spiders and T heir Kin • Stars • Trees •

Tropical Fish • Venomous Animals • Weather • Weeds • Whales and Other Marine Mammals

Golden Field Guides

Birds of North America

Eastern Birds Reptiles of North America

Rocks and Minerals Seashells of North America

Skyguide Trees of North America Wildflowers of North America

ISBN 0-307-24349-4

GEOLOGY

by

FRANK H. T. RHODES

Illustrated by RAYMOND PERLMAN

FOREWORD

"THE EARTH

- l ove it or leave it, " is a pop u l a r s l og a n evol v i ng from our recog n ition 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 undered p l a net . E ffec­ tive solutions to our societa l problems demand an effec ­ tive knowledg e of the e a r t h on which w e l ive .

The ob ject of this book i s to i ntroduce the earth: its re lation to the rest of the u n iverse , the rocks a n d m i nera l s o f which i t i s made u p , t h e forces that shape i t , a n d t h e 5 bil l ion year s of h i story that have g iven it its present for m . Many topics that a re d i scussed i n the book have a very practi ca l and u rgent sig n ificance for our society, th i ng s such a s water s u p p l y, i ndustrial mi 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 ides, a r i d reg io n s , a nd so o n . The e a r t h provides the u l t i mate basis of our 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 our p l a net.

The study of the earth opens our eyes to a vast sca le of t i m e that provides us with new d i me n s i o n s , m ea n i ng s , and 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 scri pt of the revised editio 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 IN SPACE

.

. . . 1 0

THE EARTH'S CRUST: COMPOSITION

..

21

THE CRUST: EROSION AND

DEPOSITION

. . . . .

.

. . . . ... . . .

32

WEAT H E R ING . . . ... ... . . . ... . .

34

GLAC I E RS AND G LACIATION .

.

. . . .

56

T H E OC EANS . . . ... . .

63

WINDS . . . ... . . .. .. . .

.

. .

74

PRODUCTS OF D E POSITION . . . .. .

. 78

THE CRUST: SUBSURFACE CHANGES

. .

82

VOLCANOE S . .

.

. .. . ... . . .. .

83

C LASSIF ICATION OF IGNEOUS ROC KS ..

.

. . . . .... .. ... .

.

. ....

89

METAMO R P H ISM .. .. ... . . . .. . .

94

MINE RALS AND CIVILIZATION .

.

...

96

T H E CHANGING EARTH . . ... . ..

104

ROC K D E F O RMATION . . ... .

.

. . .

106

ROC K F RACTURE S .

. . . 110

MOUNTAIN BUILDING . . .

115

THE ARCHITECTURE OF THE EARTH.

.

122

EARTHQUA K E S . . . . 124

THE EARTH'S INT E RIOR .

. . . 126

THE OCEAN FLOOR .

. . . 136

P LATE T ECTONICS . . . 144

HOW THE EARTH WORKS .

. . . • .

. 146

THE EARTH'S HISTORY . . . 1 48

OTHER INFORMATION . . . • .

. . . 1 56

INDEX

.

. . . 157

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 is only about 200 yea rs old, astronomy was actively studied by the Egyptians as long as 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 familiar with such natural disasters as earthqua kes and volcanic eruptions.

Gradual ly, human society beca me more dependent upon the earth i n i ncrea singly complex ways . Today, behind the insulation of our modern l iving conditions, civi ­ l i zation rema ins basica lly dependent upon our knowledge of the earth . All our m i nera ls come from the earth's crust . Water supply, agricu lture, and land use a lso depend upon sound geo logic information.

Geology sti mulates the mind . It ma kes use of a l most a l l other sciences a n d g ives much to them i n return . I t is the basis of modern society.

T H E BRAN C H OF G E O LOGY emphasized here is physical geology. Other Golden Guides of this series, Rocks and Minerals and Fossils, dea l with the branches of m i nera logy, petro logy, and paleonto logy.

PH YSIC AL GEOLO G Y is t h e

o v e r a l l s t u d y o f t h e e a r t h , embracing most other branches o f g e o l o g y b u t s t r e s s i n g t h e dynamic a n d structura l aspects . It includes a study of landscape development, the earth's inte­ rior, the nature of mountains, and the composition of rocks and m i nera l s .

HISTORIC AL GEOLOGY is the

study of the history of earth and its inhabitants. It traces ancient geographies 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, fuels, and water.

STRUCTUR AL GEOLOGY

(tec­ tonics) is the study of earth struc­ tures and their relationship to the f o r c e s t h a t p r o d u c e t h e structures .

GEOPH YSICS

is the study of the earth's phys i c a l propert i e s . It includes the study of earthquakes (seismology) and methods of min­ eral and o i l exploration.

PH YSIC AL OCE ANOGRAPH Y

is closely related to geology and i s concerned w i t h t h e seas, major ocean basins, seafloors, and the crust beneath them .

T H E S I ZE A N D SHAPE 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 is a sphere . For instance, as a ship a pproaches from over the horizon, masts or funnels are visi ble . As the ship comes closer, more of its lower pa rts come i nto view. F i nal proof, of course, was provided by circumnavigating 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 as 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 .

RECENT D ATA

f r o m o r b i t i n g earth satellites have confirmed that the earth i s actually slightly flattened at the poles. It i s an oblate spheroid, the polar c i r­ cumference being 27 mi les less

than at the equator. The fol low­ i ng measurements are currently accepted: Avg . diameter Avg . radius Avg . circumference 7 , 9 1 8 m i . 3 , 959 m i . 24, 900 m i .

LAR G E AS T H E EARTH I S , it is m i nute i n comparison with the un iverse, where distances a re measured i n light yea rs­ the distance 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 million, m i llion kilometers. Using these units of measurements, the moon is 1 . 25 light seconds from the earth , the 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 diam­ eter. The most d i stant galaxies a re 8 billion light yea rs from earth . It is esti mated that there a re at least 400 m illion galaxies "visible" from earth using radio 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 as 5 , 000 stadia (about 575 miles). Although the observer at Point A saw the sun d irectly over­ head at noon, the observer at B found the sun was incl ined at a n angle of 7° 1 2' (Y) to the vertical . Since a reading o f 7 ° 1 2' corre­ sponds to one-fiftieth of a full cir­ c l e ( 3 6 0 ° ) , Er a t o s t h e n e s r e a s o n e d t h a t t h e m e a s u r e d ground d istance of 5 , 000 stadia must represent one-fiftieth of the earth's circumference . He calcu­ lated the entire circumference to be about 28, 750 m i les.

T H E EARTH'S S U R FAC E, for the purpose of measure­ ments, is commonly assumed to be un iform , for mountains, 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 play a major role i n contro l l i ng the cli mate of the continents; they have profoundly i nfl uenced the patterns of human migration 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 great thicknesses of sedi­ mentary a nd volcanic rocks, many of them of marine ori­ gin. Thei r i ntense folding and faulting are evidence of enormous compressive forces.

Mounta ins a re not l i mited to the land . The ocean floor has even more relief than the continents . Most of the continenta l margins extend as conti nenta l shelves to a depth of a bout 600 feet below the level of the sea , beyond 8

7 6 5 4 3 2 I ocean

which the seafloor (conti nenta l slope) plunges 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 Phili ppines, 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 long these ridges new seafloor is constantly being created .

T H E EARTH'S C R U ST, derived from the denser, underlying mantle (pp. 1 28- 1 29), consists of two kinds of roc k . The continenta l crust differs from the ocea nic crust i n bei ng lighter (2 . 7 g m . /cc. compared with 3 . 0) , thicker (35 km-70 km versus 6 km), older (up to 3.5 billion years versus a maximum of 200 m i l l ion years), chemica l ly d ifferent, and much more complex i n structure . These differences reflect the different modes of formation of the two kinds of crust (pp. 1 40- 1 45 ) .

e Pluto

THE EARTH

RELATIVE

DISTANCES EARTH is one of n i ne planets revolving i n OF PLANETS

1 · 1

₍

11· · I) b" d FROM THE near y c1rcu ar e 1pt1ca or tts aroun our SUN star, the sun . Earth is the third pla net out from

Neptune

Uranus

the sun and the fifth largest planet 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.

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.

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 , Saturn and Pluto 1 .

Jupiter

COMETS

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

Cl e Venus Mercury e

IN SPACE

system, having a diameter of about 7, 9 1 8 miles. 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/a1 the weight, and 3/s the density of our planet. The moon completes one orbit around the earth every 27113 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 FALL "SLLMMER _.. .. - .... ,, ----�, .. .. _.. I I

\

s ' w I _{w w} I I I ,. \ N s' N s \ _..

Ti lted axis determ i nes d ifferent positions of sun at sunri se, noon, and sunset at d ifferent seasons i n middle north latitude.

THE EARTH'S MOT I O N S determ i ne the daily phenomenon of day and night and the yearly phenomenon of seasona l changes. The earth revolves around the sun i n a slig htly e l l i ptica l orbit and a lso rotates on its own axis. Si nce the earth's axis is tilted about 231/2° with respect to the plane of the orbit, each hemisphere receives more light a nd heat from the sun during one half of the year than during the other half. The season in which a hemisphere is most di 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

about the sun in a 600- m i l l ion­ mile orbit, as it completes one orbit about every 3651/• days traveling at 66, 000 mph.

ROTATION is a whirling motion

of the earth on its own axis once in about every 24 hours at a speed of about 1 , 000 m ph at the equator.

NUTATION

is a daily circular motion at each of the earth's poles a bout 40 ft. in diameter. 1 2

PRECESSION is a motion at the

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 Milky Way Galaxy. Our portion of the Milky Way makes one revolution each 200 m i l lion years.

GALAXIES seem to be receding

from the earth at speeds propor­ tional to their distances.

THE

S U N is the source of a l most a l l energy on earth . Solar heat creates most wind and also causes eva poration from the ocea ns and other bod ies of water, resu lting i n precipi­ tation. Ra i n fi l l s rivers and reservoirs, and ma kes hydro­ electric power possible. Coa l and petroleum are fossi l remains 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 is yet s o vast that it could conta i n over a m i l l i o n earths. Its diameter, 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 325 , 000, 000°F at the center) that the gases are inca ndescent. As a huge nuclear furnace, the sun con­ verts hyd rogen to heli u m , simulta neously chang i ng four m i l lion tons of matter i nto energy each second .

THE MILKY WAY,

l ike many other galaxies, is a whirling spiral with a centra l lens-shaped disc that stretches into spiral arms. Most of its 1 00 billion stars are located i n the disc. The Milky Way's diameter is

a b o u t 8 0 , 000 l i g h t yea r s ; i t s t h i c k n e s s , a b o u t 6 , 5 0 0 l i g h t years. (A light year is the distance light travels i n one year at a velocity of 1 86, 000 m i . per sec . , o r a total o f about 6 trillion miles . )

GALAX I E S are huge concentrations o f stars. With i n the universe, there are innumerable galaxies, many resembling our own Milky Way. Sometimes ca l l ed extraga lactic nebu­ lae or island 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 rregular nebulae known as 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. All 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 galaxies belong to multiple systems conta in i ng two 14

or more extraga lactic nebulae. Our galaxy is a member of the loca l G roup, which contains about a dozen o ther galaxies. Some a re e l l i ptica l i n shape, others irregular. G a laxies may conta i n up to hundreds of b i l l ions of stars and have d i ameters 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 years.

Many g a la xies rotate on their own axes, but all galaxies move bod ily 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 expanding, movi ng away from us at g reat speed s . Our nearest galaxy, i n Andromeda, is 2 . 2 m i l l ion light years away.

About 1 00 m i l lion ga laxies are known, each conta i ning many billions of stars. 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 systems similar to our own . It has been estimated that there may be as many as 1 019 of these . Chances of l ife occuring on other planets 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 tral part

GREAT SPIRAL NEBULA M31

in Andromeda i s s i m i l a r i n form but twice the size of our awn galaxy, the Mi lky Way.

T H E C H EMI CAL ELEME NTS a re the s i m plest components of the universe and cannot be broken down by chem ica l means. N i nety-two occur natura l ly on earth , 70 i n the sun . They develop from thermonuclear fusion within the sta rs, i n which the elementary partic les of the lightest elements (hydrogen a nd helium) are transformed i nto heavier

elements . /

T H E O R I G I N OF T H E U N IV E R S E is unknown , but a l l the bodies in the universe seem to be retreating from a common point, thei r speeds becoming g reater as 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 . Only 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 - expansion beg a n , the chem i­ ca l elements were formed , and turbu lent cel l s of hot 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 "Big Bang" theory, a term descri ptive of the initia l event, perhaps as long as 1 0- 15 bi l lion yea rs ago. T H E O R I G I N O F TH E SOLAR SYSTEM is not fu l ly under­ stood , but the similar ages of it s components (Moon , meteorites, Earth at about 4 .5 -4 . 6 b i l l ion years) a n d the similar orbits, rotation, a nd d i rection of movement a round the sun, all suggest a single orig i n . The theory currently most popu lar suggests that it formed from a c loud of cold gas, ice, and a little dust, which began slowly to rotate and contract. Conti nuing rotation and contraction of this disc-shaped cloud led to condensation and thermonuclear fusion - perhaps triggered by a nearby supernova , from 1 6

which stars such as the sun were formed . Collision of scattered materia l s in the disc gradua lly led to the forma­ tion of bodies - planetisma l s - which beca me protopla­ nets . The g rowi ng heat of the sun probably eva porated off the light elements fr om the inner planets (now represented by the d e n s e , rocky " te r restr i a l" p l a nets - Mercu ry, Venus, Earth, Mars, a nd the Moon) . The outer planets, because of their g reater distance from the sun, were less affected and reta i ned their lighter hydrogen, heliu m , and water compositio n . Perhaps they formed from m i n i-solar­ pla net systems withi n the larger disc. This composition may wel l reflect that of the parent gas cloud .

Each pla net seems to have had a disti nct "geologic" history. Some, like 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 active history, but are now "dead . "

This theory, i n a n earlier version, has a long history, going back to Imma nuel K a nt, the philosopher, in 1 755, and the French mathematician P ierre-Simon de La place ( 1 796).

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 lie 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 as 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 gigantic insulator for the earth by fi ltering out most of the u l traviolet and cosmic radiation but a l so burn up m i l lions 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- distance rad io commu­ nications p ossible by reflecti ng radio waves from the earth . It a lso p robably reflects much interstellar "noise" i nto space, which would make radio and tel evision as we know them i mp ossible. Composition of air at altitudes up to about 45 m i les nitrogen 78% oxygen 2 1% Composition of air at a ltitudes a bove 500 m i les helium 50% hydrogen 50% argon0.93% carbon dioxide 0 . 03%

other gases o.o.! %. _____ __.

THE RATIO OF GASES

in the atmosphere is shown i n the chart at left. C louds form i n the tropo­ sphere; 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 iles) contains layers of charged particles (ions) that reflect radio waves, permit­ ting messages to be transmitted over long distances . Faint traces of atmosphere exist i n the exo­ sphere to about 500 m i les from the earth's surface .

aurora borealis --900 vJ/>.ll/l OZONE -67° ultraviolet rays fmmsun unmanned balloon 27 miles � TROPOSPHERE 300 250 200 150 100 90 80 70 60 50 40 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 ATMOSPH E R E and oceans were probably derived by degassing of the semi-molten earth a nd conti n­ uing later additions from volcanoes and hot spri ng s . These gases - such as hydrogen , nitrogen, hydrogen chlorides, 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 , probably escaped . The later development of living organisms capable of pho­ tosynthesis slowly added oxygen to the atmosphere, ulti­ mately a l lowing the colon ization of the land by providing 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 brian rocks and fossils, which suggests a transition from a non-oxygen to free-oxygen envi ronment.

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 about 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 disti nctly layered structure. Studies of rock density and composition, heat flow, and mag netic and gravitational 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 layers, and suggested physica l and chemical cha racteristics form an important part of modern theories of the earth {p. 1 26).

The crust of the earth is 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 C a n yon of Colorado R i ver, Arizona, i s 1 m i l e deep, but exposes o n l y a s ma l l port of u pper portion of earth's crust.

MI N E RALS a re natura l l y occurring substa nces with a char­ acteri stic atomic structure and characteristic chemica l and phy si ca l properties . Some mi nera l s have a fixed chemical composi ti on; others va ry wi thi n certai n li mits . It is their atomi c structure that distingui shes mi nera l s from one a nother.

Some m i nera ls consist of a single element, but most minera l s a re composed of two or more elements . A dia­ mond , for i nstance, consi sts only of carbon atoms, but quartz is a compound of sili ca and oxygen . Of the 1 05 elements presently known , nine make u p more than 99 percent of the mi nera ls and rocks.

"'

�

"'

�

₀ "' " 2.0 2.59 2.83 3.63 5.00 A l u m i n u m 8.1 3 -= _c: ""'''"''"''.,..··"-'"' "'

�

_E

liii�.ik��:r;::!;.', :..

" -.; c: D ..., _c: " ...a D "' 0 :::ii: 22 .00 Tota l 98.59

OXYGEN AND SILICON

are the two most abundant elements i n the earth's crust. Their presence, i n such enormous quantities, indi­ cates that most of the m i nera l s a r e silicates (compounds of met­ als with silicon and oxygen) or aluminosi l i cates . Their presence i n rocks is also an i ndication of the abundance of quartz (SiO, , silicon dioxide) i n sandstones and granites, as well as i n quartz veins and geodes .

The most strik i ng feature of m i nera l s is their crysta l form , and this is a reflection of their atomic structure. The simplest example of this is rock sa lt, or halite ( N a C I , sodium 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 negatively charged chlor­ ine ions by their u n l i ke electrica l charges. We can imagine these ions as spheres, with the spheres of sod i u m having a bout half the radius of the chlorine ions ( . 98 A as agai 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 cm). T h e u n i t is named for Anders A ngstrom , a Swedish physicist.

X-RAY STU DIES

show that the internal arrangement of halite is a definite cubic pattern, i n which i o n s o f s od i u m a l te r n a t e w i t h those o f chlorine. Each sodium ion i s thus held in the center of and at equal distance from six symmetrica l l y arranged chlorine ions, and vice versa . It i s this b a s i c a to m i c a r r a n g e m e n t or crystalline structure that g ives halite its d istinctive cubic crysta l form and its characteristic physi­ cal properties .

H a l ite crysta l

SODIUM ATOM jo i n s CHLORI N E ATOM

to form i o n i c crysta l sod i u m c h l oride

T H E ATOMIC STRUCTU RE of each minera l is disti nctive but most m i nera l s are more com plicated than hal ite, some beca use they comprise more elements, others because the ions a re l i n ked together in more complex ways . A good example 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 different physical properties. Diamond is the hardest m i neral known , and graphite 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, consists of pure carbon atoms. Each carbon atom is l inked with four others by elec­ 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 crystal form i s a reflection of its structure and of the conditions under which it was formed. Dia­ mond is usua lly pale yel low or colorless, but i s found also 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, is soft and g reasy, and widely used as an industrial lubri­ cant. In graphite, carbon atoms are arranged in layers, giving the m inera l its flaky form. The atoms w i t h i n e a c h l a y e r h a v e v e r y strong bonds, but those that hold successive layers together are very weak. Some atoms between l a y e r s a r e h e l d t o g e t h e r s o poorly that they move freely, giv­ ing the graphite its soft, s l ippery, lubricating properties. Because of its poor bonding , g raphite is a good conductor of electricity. Its b e s t - k n o w n u s e is in " l e a d " pencils.

CRYSTAL 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 are ra re . Most minerals 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 classified on the basis of the i ntersection of their axes . Axes are imaginary 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

axes of equa l length meeting at right angles, as i n galena, gar­ net, pyrite, and halite.

TETRAGONAL CRYSTALS have

three axes at right angles, two of equal length, as i n zircon, rutile, and scapolite.

-,

Quartz Staurol ite

MONOCLINIC CRYSTALS have

three unequa l axes, two forming an oblique angle and one perpen­ dicular, as i n augite, orthoclase, and epidote.

TRICLINIC CRYSTALS

have three axes of unequal lengths, none forming a right angle with others, as in plagioclase feldspars .

Galena Zircon

HEXAGONAL CRYSTALS

have

three equa l horizontal 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 line.

ORTHORHOMBIC CRYSTALS

have three axes at right angles, but each is of different length, as in barite and staurolite.

E p i dote

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 aboratory tests (such as X -ray d iffraction) a re requi red to identify some of them . But many of the common m i nera l s can be recognized after a few simple tests . Six important physical properties of m i nera ls (hardness, l us­ ter, color, specific g ravity, cleavage, and fracture) a re easily determi ned . A ba la nce is needed to fi nd specific gravity of crysta ls or m i nera l fragments . For the other tests, a hand lens, steel fi le, knife , and a few other common items a re helpfu l . Specimens can sometimes be recogn ized by taste, tenacity, tarnish, transparency, i ridescence, odor, or the color of their powder streak, especially when these observations a re combi ned with tests for the other physica l properties.

diamond

MOHS' SCALE OF HARDNESS

HARDNESS

is the resistance af a minera l surface Ia scratching . Ten wel l-known m i neral s have been arranged i n a scal e af i ncreasing hardness (Mohs' sca l e ) . Other m i neral s are assigned compara­ ble numbers from l to l 0 to rep­ r e s e n t r e l a t i v e h a r d n e s s . A mineral that scratches orthoclase (6) but is scratched by quartz (7) would be assigned a hardness value of 6.5.

LUSTER

is the appearance of a m i neral when light i s reflected from its surface . Quartz is usually g lassy; galena, meta l l i c .

SPECIFIC GRAVITY

is the rela­ tive weight of a mineral com­ pared with the weight of an equal volume of water. A balance is normally used to determine the two weights. Same minera ls are similar superficially but differ i n d e n s i t y . B a r i t e m a y r e s e m b l e quartz, but quartz has a specific gravity of 2 . 7; barite, 4 . 5 .

COLOR

varies i n some minera l s . Pigments or impurities m a y b e the cause. Quartz occurs i n many hues but i s sometimes colorless. Among minerals with a constant color are galena ( lead gray). sul­ fur (yellow), azurite (blue). and malachite (green) . A fresh sur­ face i s used for identification, as we a 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 parallel to their crystal faces . A hammer blow or pressure with a knife blade can cleave a m i nera l . Gal­ ena and hal ite have cubic cleav­ age. Mica can be separated so easily that it is said to have per­ fect b a s a l c l eava g e . M i n e ra l s w i t h o u t a n o r d e r l y i n t e r n a l arrangement of atoms have no cleavage.

FRACTURE

is the way a mineral breaks other than by cleavage . Minerals with little or no cleavage are apt to show good fracture surfaces when shattered by a hammer blow. Quartz has a she l l ­ like fracture surface . Copper h a s a rough, hackly surface; clay, a n earthy fracture . Rhombohedra l c l eavage: ca lcite Conch o i d a l fracture: o b s i d i a n U n e v e n fractu re: arsenopyrite 2 7

COMMON ROCK-FORM 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 line waters , and by the action of ground water.

MINERAL CARBONATES, SULFATES, AND OXIDES

LIMONITE

is o group nome for hydrated ferric oxide minerals, Fe,O,. H,O. It is on amorphous minera l that occurs in compact, smooth, rounded mosses or in soft, earthy mosses. No cleav­ age. Earthy fracture . Hardness (H) 5 to 5 . 5 ; Sp. Gr. 3 . 5 to 4 . 0 . Rusty or blackish color. Dull, 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 minerals.

CALCITE is o calcium carbonate,

CoCO,. It has dogtooth or flat hexagonal crysta l s with excel lent cleavage. H. 3; Sp. Gr. 2. 72. C o l o r l e s s o r white. I m purities show colors of yellow, orange,

GYPSUM is o hydrated calcium

sulphate, CoS0,. 2H,O. Tabular or fibrous monoclinic crysta ls, or massive. Good cleavage . H. 2 . S p . Gr. 2 . 3 . Colorless or white. Vitreous to pearly luster. Streaks ore white. F lexible but no elastic flakes. Sometimes fibrous. Found in sedimentary evaporites and as single crysta ls in block shales. The compact, massive form is known as alabaster.

brown , and gree n . Transparent to opaque. Vitreous or dull lus­ ter. Major constituent of lime­ stone. Common cave and vein deposit. Reacts strongly in di lute hydroch loric acid.

QUARTZ

is s i licon dioxide, SiO, . Massive or prismatic . No cleav­ age. Conchoidal fracture. H. 7; Sp. Gr. 2 . 6 5 . Commonly color­ less or white. Vitreous to greasy luster. Transparent to opaque . Common in acid igneous, meta­ morphic, and c lastic rocks, veins, and geodes. The most common of all minera l s .

Quartz crysta l

ROCK-FORMING SILICATE MINERALS

FELDSPARS

a re alum ina-silicates

af e i t h e r pota s s i u m (KA I Si308 orthoclase, m icrocline, etc . ) ar sodium and calcium (plagioclase fe l d s p a r s NaA I S i,08 , C a A I , ­ Si,08}. Wel l-farmed monoclinic ar triclinic crysta ls, with good cleavage. H. 6 to 6 . 5 ; Sp. Gr. 2 . 5 to 2. 7 Orthoclase feldspars are white, g ray, or pink, vitreous to pearly luster, and lack surface striations . Plag ioclase feldspars are white or gray, have two good cleavages, which produce fi ne parallel striations on cleavage surfaces . Common in igneous and metamorphic rocks, and arkosic sandstones .

MICAS

a r e s i l i c a te m i n e ra l s . W h i t e m i c a ( m u s c o v i te} i s a potassium alumino-silicate. Black mica (biotite} is a potassium, iron, m a g n e s i u m a l u m i no-si l i ­ cate . Both occur in thin, mono­ c l i n i c , p s e u d o - h e x a g o n a l , sca lelike crysta l s . Superb cleav­ age g ives thin, flexible flakes . Pearly to vitreous l uster. Micas are common i n igneous, meta­ morphic, and sedi mentary rocks.

B i otite ( b l a c k m i c a )

30

PY ROXENES

i n c l u d e a l a r g e g r o u p o f s i l i c a t e s o f c a l c i u m , magnesium, and iron . Augite, ( C a Mgfe A I ),•( AIS i ), 06, a n d hypersthene, ( FeMg ) S i03, are the most common. Stubby, eight­ sided prismatic, orthorhombic or monoclinic crystals, or massive. Two cleavages meet at 90° (com­ pare amphiboles), but these are not always developed . Gray or green, grading into black. Vitre­ ous to dull l uster. H. 5 to 6. Sp. Gr. 3 . 2 to 3 . 6 . Common i n nearly all basic igneous and metamor­ phic rocks . Sometimes found in meteorites.

AMP H I BO L E S

a r e c o m p l e x hydrated sil icates o f c a l c i u m , magnesium, iron, a n d aluminum. Hornblende, a common amphi­ bole, has long, slender, pris­ matic, six-sided orthorhombic or monoc l i n i c crysta l s ; sometimes fib r o u s . Two 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 igneous and metamorphic rocks. Asbestos is an amphibole.

OLIVINE

is a magnesium-iron s i l i c a t e , ( FeMg ), S i O,. S m a l l , g lassy grains. Often found i n large, granular masses. Crystals are relatively rare. Poor 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 yel ­ lowish. Transparent or translu­ cent . Vitreous luster. Common in basic igneous and metamorphic rocks. Olivine alters to a brown color.

C O M M O N OR E MIN E R A L S

GALENA

i s a lead sulphide, PbS. Heavy, brittle, granular masses of cubic crystal s . Perfect cubic cleavage, H. 2 . 5 ; Sp. Gr. 7 . 3 to 7 . 6 Silver-gray. Meta llic luster. Streaks ore 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 sulphide, ZnS. Cubic crystals or granular, compact . Six perfect cleavages at 60°. H. 3 . 5 to 4; Sp. Gr. 3 . 9 t o 4 . 2 . Usual l y brownish; some­ times yellow or black. Translucent to opaque. 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 . Important zinc ore. Common vein

mineral with galena.

PYRITE

i s a n iron sulphide, FeS, . C u b i c , b r a s s y c r y s t a l s w i t h striated faces. May b e granular. No cleavage. Uneven fracture. H . 6 to 6.5; S p . Gr. 4 . 9 to 5 . 2 . Brassy yellow color. Metallic lus­ ter. Opaque and brittle. Also c a l l e d f o o 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:

Gradation incl udes the various surface agencies ( i n contrast t o t h e two interna l p rocesses below), w h i c h break down the crust (degradation) or build it u p (aggradation). Gradation is brought a b9 ut by running water, winds, ice and the ocea n s . Most sed i ments a re finally deposited in the seas .

Diastro p h i s m i s the name given t o a l l movements o f the solid 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 as marine sed i ments gradually rose until they now stand thousands of feet above sea level . Other dias­ trophic movements may i nvolve intensive fol d i ng a nd frac­ ture of rocks.

COASTLIN ES

the w o r l d ove r provide evidence of changes i n the earth's unstable crust . The photograph shows c l i ffs made of rocks that were deposited under the seas that covered the area about 130 m i l l ion years ago . These were uplifted and folded , so their 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 c o a s t l i n e s a r e formed are themselves t h e result of earlier gradationa l events .

NATURAL ARCH

reflects process of erosion, while deposition has p r o d u c ed the bea c h . D o rs e t , England.

Erupt i o n of Kapoho, Hawa i i , show i n g paths of molten lava

VU LCAN ISM i ncludes a l l the processes associated with the movement of molten rock materia l . This incl udes not only volca nic eruptions but a lso the deep-seated intrusion of 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 dynamic equ i l i b­ rium between them , a lways reflecting the cli mate, season, a ltitude, and geologic environment of particular areas. As a n end product of degradation, the conti nents would be reduced to flat plains, but the ba lance is 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).

Yosemite va l ley i s the result of i nteraction of various types of erosional processes.

EROSION

i nvolves the breaking down and remova l of material 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 volleys (p.

58), so characteristic of glacial topography. The level of the main vol ley floor lies 3 , 000 feet below the upland surface of the Sierras . D ifferences in topography ore portly the result of differences in jointing and resistance of under­ lying granites. H a lf Dome and El C a p i t a n ore resistant 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 reg i o n thus shows t h e effect of many degrodotionol processes . But the 300-ft . -thick sediment on the vol ­ l e y floor reveal s t h e continuing aggradational effects that ore a lso at work.

WEATHERING

Weathering is the genera l name for a l l the ways i n which a rock may be broken down. It takes place because m i nera ls formed i n a particular way (say at a high temperature in the case of a n igneous rock) a re often unstable 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 kinds of weathering ulti mately produce broken m i neral a nd rock fragments and other products of decompositio n . Some of these rema i n in one place (clay or laterite, for exam ple) while others are dissolved and removed by running water.

The earth's surface, above the level of the water ta ble (p. 50), is everywhere subject to weathering . The 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 distribution of the regolith 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 strikingly 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 vegetation. Bryce Can· yon, Utah , shows the effects of the bedding and differing resis· lance of rocks i n producing dis· t i n ct i v e e r o s i o n a l l a n d fo r m s . Weathering is of great impor· lance to humankind . Soils are the result of weathering processes, and are enriched by the activities o f a n i m a l s a n d p l a n t s . S o m e i mportant economic resources, such as our ores of iron and alu· minum, are the result of residual weathering processes.

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

o c c u r s i n w e l l - j o i n t e d r o c k s , because weathering tokes place m o r e r a p i d l y at c o r ne r s and edges (3 and 2 sides) than on sin­ gle faces .

MECHANICAL WEATHERING

involves the disintegration of a rock by mechanical processes . These include freezing and thaw­ ing of water in rock crevices, dis­ r u p t i o n by p l a n t r o o t s o r b u r r o w i n g a n i m a l s , a n d t h e c h a n g e s i n v o l u m e t h a t r e s u l t from chemical weathering within the rock. This weathering i s espe­ cially common in high latitudes and altitudes, which hove doily free z i ng and t h a w i n g , and in d e s e r t s , w h e r e t h e r e is l i t t l e w a t e r o r v e g e t a t i o n . R o t h e r a ng 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 doily tem­ p e r a t u r e c h a n g e s c a u s e d mechanical weathering, but this now seems uncertain.

C H E MIC A L W E AT H E RING

invo lves the d e c o m p o s i t i on of rock by chemical changes or solu­ tion. The chief processes ore oxi­ d a t i o n , c a r b o n a t i o n a n d hydration, and solution i n water above and below the surface . Many iron minerals, for example, ore rapidly oxidized ("rusted") and l i m e s tone is d i s s o l ved by water containing 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 climates . Blankets of soil or other material ore produced which ore so thick and extensive that solid rock is rarely seen in the tropics. Chem­ ical weathering is more wide­ s p r e a d a n d c o m m o n t h a n mechanical weathering , a lthough 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 plant life . The thickness and character of soil depend upon rock type, rel ief, cli mate, and the "age" of a soi l , as wel l as the effect of living organisms.

I mmatu re soils are little more than broken rock frag­ ments, grading down into so lid rock . Mature soils include quantities of humus, formed from decayed pla nts, so that the upper surface (topsoi l ) becomes dark. Orga nic acids and carbon dioxide released during vegetative decay d i s­ solve lime, i ron, and other compounds and carry them down i nto the lighter 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 uminum oxides with a l most no humus. Tra nsported soi ls have been carried from the parent rocks from which they formed ond deposited elsewhere. Wind-blown l oess ( p . 77), a l l uvia l deposits (p. 44) , a n d glacial t i l l (p. 5 9 ) a re common exa mples of transported soi l s .

LAT E RITIC SOI L P R O F I L E

Fr iable c l a y Con creti o n s rich i n iron and manganese oxides Iron-rich clays zone serpe ntine MAT U R E SO I L P RO F I L E

}

H u m u s-ri c h c l a y

l

C l a y w i t h l i mestone fra g ments Fre s h l i mestone 3 7

' , �

\

\

rock f a l l I ' _I I

ROCK FALLS

forming talus s lopes ore on example of moss wasting. The finer material tends to be concentrated near the bose of the slope. Such fa l l s may be either small and irregular or massive and sudden, causing a rock ava lanche.

ion

MASS WAST I N G is the name given to all downslope move­ ments of regol ith under the predominant i nfl uence of g rav­ ity. Weathered material is transported from its p lace of origin by g ravity, streams, winds, glaciers, and ocean currents. Each of these agencies is a depositi ng , as wel l as a transporting 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 impor­ tance in a l l pa rts of the world . Engi neering and m i n i ng activities usua l ly require 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 reservoir slide i n Ita ly in 1 963 clai med 2 , 600 l ives . Ca refu l geologica l site surveys can prevent such disasters .

EARTH FLOWS

may be slow 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 m a n e n t l y frozen . These a r e a s cover about 20 percent of the earth . On slopes, the thawed upper Ioyer slides on the frozen ground 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 slides in soft, uncon­ s o l i d a ted sed i m e n t . Some a r e submarine i n orig i n . Fossil slumps are recognizable i n some strata . Usua l ly, slumps are on a rather small sca le, as when sod breaks off o stream bank.

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 (chemi­ 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 also cause subsidence .

LANDSLIDES

a r e m a s s move­ ments of earth or rock along a d e fi n i te p l a n e . T h e y o c c u r i n areas o f high relief where weak planes-bedding, joints, or faults ( p . 1 1 0 - 1 1 1 ) - a r e s t e e p l y i n c l i n e d ; w h e r e w e a k r o c k s 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 .

H i l l creep affects fences a n d trees, a s we l l a s bend i n g of vert i c a l strata .

Typ i c a l l a n d s l ide topogra phy i n Wyo m i ng shows debris o f h u g e bou l ders.

R o c k g l a c i e r s 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, glaciers, and ocean waves are a l l confined to rela­ tively l i m ited land a reas, 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 it. I n the United States, this erosion of the land surface takes p lace at a n average rate of about 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 poration, condensation, and precipitation. Of the water that fa lls on the land, up to 90 percent is evaporated . 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 ground . The relative amounts

moist air mass

_>

moves to continent

of water fol lowing these paths vary considerably 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 peratu r e . The h y d r o s p h e r e i s estimated t o include 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 is i n the oceans.

condensation pr c i p i tation

E<--..f

eva poration i n fa l l i ng

eva poration from

/. .,.

IJ.i,. JOO O.f: o,

�

-,

I I

runoff

.

./" <>,._. '>� _{<>_.}

�

/

_I I l o s s by stream ru noff t::' ... _ / _I ...._.

...

..-

�

11:..

... ... _ _ _ _ _ _ _ _ _ _

... .-' /II

OCEAN ground water moves to ...,. -"

...

...

rivers, l a kes, ocea n s _ ... ...

---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 important in erosion, especially in areas with sparse vegetation and unconsolidated sed i ments . Runoff water rarely travels far as a conti nuous sheet, for it is broken u p i nto rivu lets a nd strea ms by surface irregulari­ ties i n rock type and re lief.

Running water carries its load of rock debris partly in suspension, partly by rol l i ng and bouncing it a long 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 factors. Water runs down h i l l under the influence of gravity, the flow of the water being cha racteristica l l y turbu­ lent, with swi rls and eddies. The overal l long profile of a river val­ ley is concave upwards, however much its g radient (its slope or fall in a g iven distance) may vary. The velocity of the stream i ncreases w i t h t h e g r a d i e n t , b u t a l s o depends on other factors, includ­ ing the position within the river channel, the degree of turbu­ lence, the shape and course of the channel, and the stream load (transported materia ls).

DENDRITIC DRAINAGE PAT­

T E R N

o f D i a m a n t i n a R i v e r , Queensland, Australia, is typical of river development i n areas where the underlying rocks are relatively uniform i n their resis­ tance to erosion. This pattern may be modified by continued downcutting of the river.

Diagrammatic model of a river system, showing cross sections of channel at three points

N i a g ara F a l l s are formed by res i sta nt bed of dolom ite.

RIVER PROFILES reflect the varying development of drain­ age systems. During early development, changes a re reflected by di rection and shape of stream courses . U lti­ mately, erosion produces an equi l ibrium between the slope and volume of the river and its erosional and depositional power. This resu lts i n a g raded profi le, or profi le of equ i l i b­ rium. F i n a l equ i librium is never reached because of sea­ sonal and geologic changes . The downwa rd l i m it of erosion by a river is the base leve l , below which the river cannot downcut a ppreciably because 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 labeled as "youth, " "maturity, " a nd "old age . " Although these stages describe certai n characteristics, they i mply no par­ ticular 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 higher reaches . For that reason , the terms are rather misleading, and a re now rarely used .

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 profi l 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 n g itud i n a l profil e ....

REJUVENATION

or uplift may i nterrupt the cycle of erosion at any stage to provide new energy for downcutting . The character of the stream is then often a combi­ n a t i o n o f r e c e n t l y c u t , s t e e p g o r g e s i n a n o l d e r m e a n d e r course . Ancient flood-plains are often left "stranded" as terraces. Upl ift and warping may be rela­ tively sudden or slow, frequent or rare, local or regional i n extent. Terraces are cut as river swings from one side of va l ley to other.

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 Yellowstone

River (the lower of which is twice as high as N iagara Falls) result from resistant lava flows . N iag­ ara Falls is held u p by an SO-foot­ thick bed of dolomite, which is more resistant to erosion than the u n d e r l y i n g s h a l e s a n d t h i n l imestones .

Other fal l s result from over­ deepening of a main va lley, often by ice, as i n the Bridal Vei l Falls i n Yosemite, so that the tributar­ ies are left "hanging" (p. 34) . Continued erosion by a stream leads to upstream migration and u l t i m a t e s m o o t h i n g o u t o f waterfal l s .

U N E Q U A L E R O SIO N

h a s already cut N iagara Falls back about 7 m i les from the N iagara escarpment, 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 er m i n e d , and eventu a l l y c o l l a p s e s . T h e N i a g a ra 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 drained by upstream migration of the fa l l s .

STREAM PIRACY

t a k e s p l a c e w h e n t h e t r i b u t a r i e s o f o n e s t r e a m ( A ) e r o d e f a s t e r t h a n those o f another (B) i n the same area . The headwaters of the less a c t i v e s t r e a m a r e u l t i m a t e l y diverted or cut off. When such "beheaded" strea m s a b a n d o n their path through a ridge, a wind gap results.

DELTAS

are masses of sediment deposited where rivers lose their velocity as they enter lakes or seas. Thick, relatively uniform layers of sediment accumulate on the steep outward slope . Deltas of the Mississippi, 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 C l i nton Li mestone _{---...__} ..;. a n d Sha l e

�

Thoro l d

�

Sa ndstone

_/

. .. Whirl pool •. Sand stone

POTHOLES

are circular hol lows.

in a stream bed , dril led out by swi rling currents of water carry­ i n g g r a v e l a n d p e b b l e s . T h i s "hydraulic drilling" i s a n i m por­ tant method of d o w n - c u tt i n g , even i n hard rock .

DENDRITIC

drainage patterns are t h o s e that show t r e e l i k e branching because the bedrock has a uniform resistance to ero­ sion and does not i nfluence the direction of stream flow.

SUPERIMPOSED

dra i nage gen­ era l l y has stream courses that are i ndependent of rock structure . A stream's ero s i o n a l power m a y have been strong enough t o main­ tain its a ntecedent course during

DRAI NAGE

CONSEQUENT STREAMS

flow

in directions determined largely by the orig inal slope and shape o f t h e g r o u n d . W h e n t h e i r courses are modified by features of the geology, such as val ley cut­ ting i n soft strata , the adjusted tributaries are known as subse­ quent stream s .

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 folded rocks . I n this rec­ tangular pattern , the tributaries are nearly perpendicular to the main strea m .

uplift and development o f a new geological structure . Or it may keep its same course after it cuts through younger, overlying , flat, sedimentary rock to an older, irregular rock mass.

PATTERNS

RADIAL

p a t t e r n s deve l o p o n young mountains, such a s volcan­ oes where stream s radiate from the high central area .

Piracy m a y c a u 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 piracy (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 vol leys by ice,

b l o c k i n g o f d r a i n a g e b y i c e , morainic deposits, and meltwa­ ter, which may produce glacial lakes and new outlets . Changes i n sea level may also modify drainage .

Great lakes basins were carved by ice from soft strata . Original drainage was blocked by ice, producing local crustal depressions.