extract from press and siever including some from index

8/7/2019 extract from press and siever including some from index

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Box 19-2 PLATE MOT IONS ON A SPHERICAL EARTH

rI

face. Box 19-2 explains how plate movements ona sphere can be descr ibed. With the application ofthese geometric principles to find spreading direc-t ions and magnetic anomalies to deduce spread-ing rates , the relat ive mot ions of the l ithosphericpla tes are being worked out wor ldwide. Some re-sul ts have already been pictured in Figures 18-21and 19-4. However, geophysicists are searchingfor ways to measure the absolute mot ions of indi -vidual pla tes rather than their mot ions relat ive toeach other. If the hot spots discussed in Chap-ter 15 turn out to be fixed in the mantle belowplates, then the string of extinct volcanoes trailingfr om the hot spo t would r ecord t he movement o findiv idua l p la tes a s they g li de over t he mantl e(Figure 15-38). This is currently a subject of activeresearch.

SEA-FLOOR SPREADINGAND CONTINENTAL DRIFT:RETHINKING EARTH HISTORY

GLOBALPLATETECTONICS: THE UNIFYING MODEL 451ology is now 'receiving much attention. New de-velopments repor ted in nearly every issue of thegeological journals show that the subject has def i-nitely been revitalized. Rock associations, volcan-i sm, metamorphism, the evolution of mountainchains-all are being rexamined in the frame-work of plate tectonics. Some ofthe new interpre-ta ti ons that we descr ibe in th is chapte r may notst and t he t es t o f time . I n t hi s connec ti on , f utu reedi tions of this book may show some changes , notso much i n the b ig p ic tu re o f p lat e t ec toni cs as inthe det ail s o f fi tt ing regiona l geo logy i nto t heove ra ll f ramework . The st udent ( as wel l as t heauthors of this book) should be cautioned againstcal ling on plate tectonics for easy explanations ofeverything geological. It is not clear, for example,how or whether the o rig in -o f such st ruct ur es a sthe Ozarks, the Black Hills, the Colorado Plateau,or such intracontinental, sediment-filled depres-s ions a s the Michi gan Basi n a re re la ted to p la temovement.

One ofus (F.P. )once helped wri te a paper dealingwith the permanence of ocean basins. Ifhe wereallowed to expunge from the scienti fic record theone contr ibut ion he regrets the most, thi s wouldbe it. The notion of the stability of global geo-graphic features was not only q_ main tenet o f t heo ld geo logy bu t s eems to be f irmly root ed in thehuman psyche. We now know that on the geologi -cal t ime scale the sea f loor i s far f rom permanentThe present ocean basins are being created byspread ing and r ecyc led by subduc tion on a t imescale of about 200 mil lion years , which is about 4percen t of the age o f t he Eart h. The l ikeli hood o ff indi ng extensi ve o lder remnant s o f seaf loor iss light. Continents, on the other hand, are mobilebut permanen t f eat ure s. They ar e t oo buoyan t tobe subducted. They may be fragmented, moved,reassembled, deformed, and eroded at their sur -faces, but their bulk does not seem to be muchdiminished. Old ter ra ins with ages of around 3.5to 3.7 bil lion years can sti ll be found. Continentsg row wit h time by the g radua l a ccumula ti on o fmater ia ls a long their margins . New continentalstrips can therefore be added on in differentplaces at different time's, dependingon the historyof fragmentation, movement, and reassembly.

Wi th the emergence o f thes e revo lu tiona ryi dea s, geo logi sts a re r et hinking Ear th h is to ry .Most o f t he ev idence fo r p la te te ctonic s comesfrom the sea f loor , a relat ively s imple place com-pared to the enormously complicated continents.Jus t how plate tectonics explains continental ge-

Rock Assemblages and Plate TectonicsThe only record we have ofpas t geologic events i sthe incomplet e one f ound i n the r ocks that havesurv ived e ros ion o r subduc tion. S ince only s eaf loor younger than 200 mil li on yea rs ( the la st 4percent ofEarth history) has survived subduction,we must focus on the continents to find the evi-dence for most of Earth history. Some of themethods of reading the rock record have beendescr ibed in ear lier chapters. Here we explore thenature of the rock assemblages that character izedif ferent pla te-tectonic regimes as a f ir st s tep inunravel ing the his tory of pas t pla te mot ions . Ouraim isto reconst ruct the process ofcont inent f rag-mentation and ocean development , to locate thes ites ofvanished oceans, and to recognize the su-t ure s that mark anc ient p la te coll is ions. .

Of the three kinds of plate boundaries, wemight expec t d is tinct su ite s ( ass emb lage s) o frocks to be associa ted with plate divergence andconve rgence . At tr ans fo rm fau lts no d ist inct o rcharacteristic rock assemblages are to be ex-pected. Discontinuities across the fault are found,however , s ince r ock forma tions fo rmed and a l-te red e lsewher e have sl ipped pas t one anot he r,and once-continuous formations or structural fea-tures are displaced.Think of all that happens at a zone of diver-gence, where plate accretion and spreading occur,and you can p red ict t he k inds o f r ock t ha t wouldcharacter ize the place and the process . Because

- Spreading zone- T ransform f ault~ Subduction

Geometry allows us to describe the separation oftwoplates on a sphere-for example, plate A and plate Binthe f igure-as a rotat ion ofB with respect to A aboutsome pole of rotation, called a pole of spreading. Noteon the diagram ofplates (inside back cover)that alongmid-ocean ridges where plates separate, the axis ofspreading is not continuous but is offsetby transformfaults, approximately at right angles to the axis. Whythis occurs isnot fully understood, but itappears tobeeasierfor plates to break apart this waywith the platestypically sliding by each other at the transform fault,rather than pulling apart or overlapping there. Becauseof this geometry, if one imagines latitudes and longi-tudes drawn with respect to the pole ofspreading, thetransform faults lie on lines oflatitude, and lines per-pendicular to them arelongitudes that converge at thepole ofspreading. Tounderstand why this must be so,consider the following analogy: If a tennis ball weres liced in two par ts and put back together, one couldrotate the two par ts a long the cut (as on a t ransfqrmfault). The cut would also describe a latitude centeredona pole ofrotation, which can belocated by drawinglongitudes perpendicular tothe cut. The intersection oftwo ormore suchlongitudes isthepole ofrotation. Ona model ofthe Earth, ifgreat circles are drawn perpen-dicular to transform faults between a pair of plates,their intersection locates the pole of spreading, whichtogether with the spreading rate completely describesthe relative motion of the two plates. The spreadingrate iszero atthe pole ofspreading and increases to amaximum 90away at the equator ofspre~ding, as the

West longitudeAfter W. J. Morgan, "Rises, Trenches, Great Faults,and Crustal Blocks," J. Geophysical Research, v. 73,pp. 1959-1982,1968.

figureindicates. This maximum equatorial value isfre-quently cited asthe spreading rate between plates.Toseehow a pole ofspreading islocated inpractice,refer againto theinside ofthe back cover,which showsthezone ofspreading andthe transform faultsthat sep-arate the African and American plates. Great circlesperpendicular to the transform faults are drawn in thefigurebelow.They intersect near the point 58N,36W,offthe southeast coast ofGreenland. This isthe pole ofspreading ofthesetwo great plates. Don't bother goingther e, fo r ther e is noth ing to be s een . The pole o fspreading has no physical significance. Itservesonly asa construction point, a convenience for describing therelative motion ofplates merely by givingthe latitudeand longitude of this point.


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452 THE BODY OF THE EARTH:INTERNAL PROCESSES

Deep-sea sediments:shales, limestones,cherts, turbid ites,fossils of pelagicmarine organisms

Basaltic pillow lavacut byd ikes

Gabbro, evidence ofhydrous metamorphism

Peridotites andother ultramaficrocks, often showinghydrous metamorphism

Figure 19-10Idealized section of an ophiolite suite. The combin-ation of deep-sea sediments, submarine lavas, andmafic igneous intrusions indicates a deep-sea origin.Many geologists now believe ophiolites to be frag-ments of oceanic lithosphere emplaced on a conti-nent as a result of plate collisions.

there.is extensive undersea volcanism, one wouldexpect to find submarine basaltic lava, perhapspillow lavas, the volcanic rock formed when hotlava is quenched by cold sea water (Chapter 15).Suboceanic crust and mantle are created here;dredge hauls and geophysical data show theselayers to consist of mafic rocks, such as gabbroand peridotite, often showing evidence of altera-tion in a water environment (hydrous metamor-phism). A carpet of deep-sea sediments would

" cover all ofthis. From Chapters 10 and 11we re-member that these deposits are recognized bythin layers of shale, limestone, and the siliceousrock chert, often with thin, discontinuous turbi-dites between them. Some or all of these layersmay contain fossil remains of open-ocean marineorganisms. A combination of deep-sea sediments,submarine basaltic lavas, and. mafic igneousinstrusions like that shown in idealized section inFigure 19-10, is called an ophioli te suite. Thepresence of narrow ophioli te zones in conver-gence features like the Alpine-Himalayan belt

(d)

Figure 19-11The development of a geosyncline on a rifted conti-nental margin off the Atlantic coast of the UnitedStates. A rift develops in Pangaea as the ancient con-tinent splits. Volcanics and Triassic nonmarine sedi-ments are deposited in the faulted valleys (a). Sea-floor spreading begins, the lithosphere cools and con-tracts, and the receding continental margins subsidebelow sea level. Evaporites, deltaic deposits, andcarbonates (b) are deposited and then covered byJurassic and Cretaceous sediments derived fromcontinental erosion (c and d).

and the Ural and Appalachian belts may indicatethat slices of oceanic crust and mantle originallyproduced at accreting plate margins were thrustonto land when an ancient ocean finally disap-peared as two continents converged. It is gener-ally believed that the Appalachians, for example,mark the si te at which the ancestral Atl ant icOcean (called Iapetus for one of the Greek gods)closed when Nor th America and Africa con-verged about 375 million years ago. The Atlanticreopened a few hundred kilometers east of thisold suture, about 200 million years ago, in aspreading episode that is still underway.

Arc: magmatic belt:volcanoes, intrusions,and high-temperature,low-pressuremetamorphism

t

453

Subduction melanges:low-temperature,high-pressuremetamorphism

Figure 19-12 .Geologic features and activities associated with plate collisions and subduction:ocean trenches, melange deposits, magmatic belts, metamorphism, volcanism, earth-quakes (dots). The drawing is not to scale; the thickness of lithosphere is about'70km, depth of the ocean trench 10km, and the dis tance from trench to arc is300-400 km.

Much was dropped onthe continental slope, onlyto be moved later to the continental r ise by tur-bidity currents. In deep water, very thick depositscan be built up in this way. As the shelf miogeo-syncline builds up, deposition may become domi-nated by shales and carbonate platform depos-its-indicator s of a decrease in the supply ofdetritus from the continent.

Think what might happen to these geosynclinesif the orderly, sequentially layered, gently dippingsediments were to become the leading edge of aplate in collision. In the following sections wedescribe some of the many possibilities.

Just as the events that take place in a conver-gence zone are different from divergence-zonephenomena, so do the rock assemblages have dif-ferent characteristics. The main features ofocean-ocean or ocean-continent collision areshown in t ransverse sect ion in Figure 19-12.Thick marine sediments, mostly turbidites,eroded from the continent or the island arc, rap-idly fil l the long marginal depressions. In de-scending, the cold oceanic slab stuffs the regionbelow the inner wall ofthe trench with these sedi-ments and with deep-sea materials brought withthe incoming plate. Regions of this sort are enor-mously complex and highly variable, as they in-clude turbidities and ophiolitic shreds scraped offthe downgoing slab by the edge ofthe overridingplate-all highly folded, intricately sl iced andmetamorphosed: They are difficult to map in de-tail but recognizable by their distinctive mix ofmaterials and structural features. Such a chaoticmess has been called a melange. The metamor-

Continent orocean basinbehind arc~

Continental-shelf deposits are sedimentaryrock assemblages that are laid down in an orderlysequence under tectonically quiet conditions in ageosyncline at a receding continental margin. Fig-ures 19-11 and 10-29 show the orderly sequenceof deposits in the geosyncline that is still formingoff the Atlantic coast of the United States. Thecontinental margin there was formed when theAmerican plate separated from the Europeanplate about 200 million years ago. Resting on theoffshore shelf is a wedge-shaped deposit of sedi-ments eroded from the continent and carried intoshallow water. Because the trailing edge of thecontinent slowly subsides as the spreading litho-sphere cools and contracts, the geosyncline contin-ues toreceive sediments for a longtime. The loadofthe growing mass ofsediment further depressesthe crust isostatically, so that the geosyncline canreceive still more material from land. For everythree meters ofsediments received, the crust sinkstwo meters. The result ofthese two effects isthatthe geosynclinal deposits can accumulate in anorderly fashion to thicknesses of 10kilometers ormore. At the same time, the supply ofsediments issufficient to maintain the shallow-water environ-ment ofthe geosyncline, or miogeosyncline, aswecalled it in Chapter 11.

The deposits show all of the characteristics ofshallow-water conditions (Chapter 11).Atthe bot-tom of the entire sequence are rif t val leys con-taining basaltic lavas and nonmarine depositsformed during the early stages of continental fis-suring. In the early stages of shelf deposition,sandy materials s tarted to fil l the depression.


3/7

454 THE BODY OF THE EARTH: INTERNAL PROCESSESphism is the kind characteristic of high pressureand low temperature because the material may becarried relatively rapidly to depths as great as 30kilometers, where recrystallization occurs in theenvironment of the cold slab. Somehow, perhapsby buoyancy and mountain-building, the materialrises back to the surface much later. Find amelange and you can't betoofar from the place ofdownturn of an anc ient p la te, l ong sinc e con-sumed, but leaving this relic of its existence.

Ref er aga in to Figure 19~12. Pa ra llel to themelange is a magmatic bel t tha t makes up the ar-cuate system of volcanoes, intrusions, and meta-morphic rocks formed onthe edge o f the overrid-ing pla te . Here the condi tions are dominated bythe r ise of magma from the descending pla te . Atthe interface, where the descending plate slidespast the overriding one, perhaps friction is greatenough to melt the upper part ofthe downturnedslab, including the subducted wet sediments andocean crust. The liqu ids rise buoyantly fromdepths of100to 200kilometers to erupt and buildthe volcanic chains onthe leading edges ofplates.The character istic igneous rocks produced areandesit ic lavas and grani tic int rusives. Island

~-a rc s, bu ilt up from the se a fl oor , may conta inlarger amounts ofbasalt; continental margins typ-ically erupt rhyolitic ignimbrite arid are intrudedby granitic batholiths below (see Chapter 15).Incontrast to that in a melange, the metamorphisminthe magmatic belts istypically the result ofre-crystallization under conditions of high tempera-tures and low pressures. This is because the hotfluids rise close to the surface, delivering muchheat to a low-pressure environment.

Paired belts of melange and magmatism(Fig. 19-12) are the signature of subduction. Theessential elements of these features of collisionhave been found in many places in the geologicrecord. One can see melange in the FranciscanFormation of the Cal ifornia Coast Ranges andmagmat ism in the paral le l bel t of the ~jerra Ne-vada to the east (Fig. 19-13). This paired beltmarks the Mesozoic boundary between the collid-ing Pacific andAmerican plates. It even shows thepolar ity of the conve rgence by the loca tion ofmelange onthe west and magmatism onthe east;the Pacif ic pla te was the subducted one. Otherpaired belts-for example, in Japan-can befound along the.continental margins framing thePacific basin. The central Alps, a European exam-ple,were produced bythe convergence ofa Medi-terranean plate with the European continent.

Seismic reflection profiles (see Box 17-1) arebeginning to provide "x-ray" views oflayers deep

- - - - - - - - - - - - - - - - , -. !\i

\Idaho __--~batholith_-.----- iiiiiI,..>:/ !-~ L--, ---I \i ii ii i. i

Franciscan melangeMagmatic belt: metamorphicand granitic rocks

o 500 km\--L---,'---'-o-----''--------,J300 mi

Figure 19-13 ,This paleogeologic map of the western United Statesshows the geology ofthe reg ion as i t was a t the be-ginning of Tertiary time. The paired melange andmagmatic belts indicate a collision of the Pacific andAmerican plates in Mesozoic time, the Pacific platebeing the subducted one. [After W.Hamilton andW. B.Myers, "Cenozoic Tectonics," Reviews of Geo-physics, v. 4, p. 541, 1966.]

within the crust. Figure 19-14, a remarkable ex-ample of this new technique , shows the Austra-l ian pla te being subducted under the Eurasianplate at the Java trench.

Orogeny and Plate TectonicsOrogeny means mountain-making, particularly by

-folding and thrusting ofrock layers. Inthe frame-work of plate tectonics, orogeny occurs primarilyat the boundaries of colliding plates, where mar-gina l sedimentar y depos its a re c rumpled andmagmatism and volcanism are initiated.

Consider first some scenarios of plate conver-gence. In Figure 19-15a, a plate with a continent at

TtSouth North

Seasurface

Subductedoceanfloor INDIAN OCEANSeafloor

km8 km

16 kmo 10 20 30 40 50Figure 19-14

Seismic reflection profile across the Java Trench subduction zone south of Bali, along longitude 112E.Subducted ocean floor (between large arrows) dips about 6 under overthrust wedge of highly deformedsediments . The ocean floor can be followed from the beginning ofsubduc tion at the north wall o f thetrench to a depth of12km below sea level. [Courtesy of R. H. Beck and P. Lehner, Shell InternationalePetroleum.]

Subduction zone

(a)

New subduction zone

Figure 19-15Possible stages in plate collisions. (a) Convergence between plates with continental andoceanic lithosphere at leading edges. Magmatic belt, folded mountains, and melange depositsare features of the overriding continental boundary. (b) Collision of continents, producinga mountain range, magmatic belt, and thickened continental crust. Since the continent istoo buoyant to be carried down into the mantle , pla te motions may be brought to a halt.(c)Alternatively, the plate may break off and a new subduction zone be started elsewhere.An extinct subduction zone may show as a scar in the form of a mountain belt within acontinent. Examples are the Ural Mountains and the Himalayas. [After "Plate Tectonics"by J. F.Dewey. Copyright 1972by Scientific American, Inc. All rights reserved.]


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456 THE BODY OF THE EARTH: INTERNAL PROCESSES

the leading edge collides with another plate carry-ing a continent. In the early stage, during whichthe convergence is between continent and sub-ducted oceanic lithosphe re , a magma tic belt,folded mountains, and melange deposits may befeatures of the overriding continental boundary.An example exists today along the Pacific coast ofSouth America , where the American and Nazcaplates are colliding. Look at the illustration insidethe back cover to see the setting ofthe plates. TheAndes, from which the name ofthe volcanic rockandesite is derived, lie in the magmatic belt; sub-duction is taking plac e under the Pe ru-Chiletrench.

In a later stage, continent may meet continent,as shown in Figure 19-15b. Since cont inentalcrust istoolight for much ofitto be carried down,

.the plate motions could be slowed or halted. An-other possibility, the one depicted in the figure, isthat the plate motions continue, with subductionceasing at the continent-continent suture butstarting up anew elsewhere. Cold and dense asthedescending slab is, chunks ofit may break off,fallfreely into the mantle, and be resorbed. As Figure19-15c shows, the suture ismarked by a mountainrange made up ofe ither folded or thrust rocks, orboth, coincident with or adjacent to the magmaticbelt, and by a much-thickened continental crust.A prime example of continent-continent collisionis the Himalayas, which began forming some 25million years ago when a plate carrying India raninto the Asiatic plate (the collision and uplift arestill going on).This may be how the root underly-ing the Himalayas origina ted (see Chapter 18) .The plate-tectonic cycle ofthe closing ofan oceanbasin, a continent-continent collision, and the for-mation of an intracontinental mountain belt hasbeen called the Wilson cycle, after the Canadiangeologist J.Tuzo Wilson, who first suggested theidea that an ancient ocean closed to form theAppalachian mountain belt and then reopened toform the present day Atlantic Ocean. =

Displaced TerrainsGeologists have come across blocks within conti-nents whose rock sequences, fossils, and paleo-magnetism are a lien to the ir sur roundings. Therock assemblages andthe fossils indicate differentenvironments and age s than the surroundingterrain, and the paleomagnetic poles imply thatthe block originated in a different latitude. Theseare now bel ieved to be fragments ofother conti -nents or of ocean c rust that we re swept up andplastered onto a continent in the process of plate

collisions and separations. Coastal New Englandand Newfoundland may be slices ofEurope; partsof Alaska, Bri ti sh Columbia, and Nevada mayhave been scraped off Asia ; and central Floridamay be a fragment of Africa. Displaced terrainshave also been found in Japan, Southeast Asia,China, and Siberia, but their original locationshave ye t to be worked out.

The Grand ReconstructionAt the c lose of the Paleozoic, some 250 mil lionyears ago, ther e was a single supe rcontinentPangaea, stretching from pole to pole (Fig. 1-14).The fragmentation ofPangaea as a result ofplatetectonics and continental drift over Mesozoic andCenozoic time to form the modern continents andoceans is documented in the well-preserved rec-ord of magne tic reversal s tripes on the oceanfloor. But what of the pre-Pangaean distributionofcontinents? What were their shapes and wherewere they located? There is growing evidencethat Pangaea was formed by the collision of con-tinental blocks-not the same continents weknow today but continents tha t existed ear lierin the Paleozoic. The ocean-floor record for thisperiod has been de str oyed by subducti on, sowe mus t re ly on the older evidence pre servedon .continents to ident ify and chart the move-ments of these paleo continents. Old mountainbel ts l ike the Appalachians and the Urals markthe collision boundaries of the paleocontinents.Rock assemblages there reveal ancient episodesof rifting and subduction. Rock types and fossilsa lso indicate the distr ibut ion of shallow seas,glac ie rs, lowlands, mount ains , and c limati ccondi tions. Paleomagnet ic data can be used tofind the latitude and the north-south orientationof the paleocontinents. Latit ude s can al so bechecked by paleoclima tic da ta. Al though it isnot possible to assign longi tudina l posit ion tothe paleocontinents, the relative sequence of con-t inents around the globe can be pieced togetherf rom the fossi l record. One of the f irst e ffor ts todepic t the pre-Pangaean configura tion of con-t inents using these methods is shown in Figure19-16. The ability of modern science to recover

_the geography of this strange world of hun-dreds ofmillions ofyears ago istruly impressive.Geologists may be able to continue to sort outmore details of this complex jigsaw puzzle,whose individual pieces change shape over geo-logic time.

Figur e 19-17 reconstr ucts the mos t re centbreakup ofPangaea aswe now understand it. Fig-

Mountains_ Evaporite minerals ~ Coals

(indicating hot, d ry (indicating warm,conditions) humid conditions)

LowlandsShallow sea c=J Glacial deposits

Figure 19-16(a)Paleocontinents in the MiddleOrdovician, about475-490 million years ago.At that timethe continents consisted of Gondwana (made up of South America, Southern Europe, Af-rica, the NearEast, India, Australia, NewZealand and Antarctica), Laurentia (NorthAmer-ica and Greenland), Baltica(mostof Northern Europe and European Russia),Kazakhstania(Central Asia),China (China,Malaysia),and Siberia. (b)Paleocontinents in Early Carbonif-erous, about 340-360 million years ago.Gondwana has moved across the South Pole enter-ingthe oppositehemisphere;Baltica has collided with Laurentia to form a larger continentLaurussia. The continents are assemblingfor the collisionsthat formed the supercontinentPangea at the end ofthe Paleozoic.[AfterR. K.Bambach, C.R. Scotese,and A. M.Ziegler,American Scientist, January1980.]

ure 19-17a shows the world as i t looked in Per-mian t imes, a l it tle more than 200 mil lion yearsago.Pangaea was an irregularly shaped land masssurrounded by a universal ocean called Pantha-lassa , the ancestral Pacif ic . The Tethys Sea , be-tween Af rica and Eura si a, wa s the ance stor of

par t of the Mediter ranean. The f it of North andSouth America with Europe and Africa is verygood in detai l when taken at the outer edgeof thecontinental shelves , ins tead of a t the pre sentshore line s, which are some dis tance from theoriginal rift. It is the fit for which we have the


5/7

(a)

(b)

LithosphereHot spot\

(d)Figure 19-20 ,Possible driving mechanisms of plate tectonics.(a) The pla te is pushed by the weight of the r idgesat centers of spreading Oris pulled by cool, heavydowngoing slab or both. (b) The plate is draggedby convection current in mantle. (c) The plate is thecooled, brittle, boundary region of a convection cur-rent in the hot, plastic upper mantle. (d) Jetlike ther-mal plume rises from great depth, causes hQj spotsat mid-ocean ridges, and spreads laterally draggingthe plates. Downward return flow occurs throughoutthe rest of the mantle.

rate your fingernails grow. The lithosphere is bro-ken into rigid plates, somehow responsive in theirmotions to the flow in the underlying mantle,

Asis general ly the case when there is an abun-dance ofdata in search ofa theory, many hypoth-eses have been advanced. Some would haveplate s pushed by the we ight of the ridge s a t t hezones ofspreading or pulled by the heavy down-going slab at subduction zones. Others hold thatthe pl ate s a re dr agged al ong by curr ents i n theunderlying asthenosphere. Figure 19-20 showssome ofthese ideas. Inline with the discussion inChapter 13 ,we agre e wi th those who view theprocess not in piecemeal but as a highly complexconvective flow, involving rising, hot, partiallymolten materials and sinking, cool, solid materi-a ls, under a var ie ty of condi tions ranging frommelting to solidification and remelting. A signifi-cant part ofthe mantle must beinvolved, for slabsare known topenetrate to depths ofsome 700kilo-meters before being completely resorbed. Figure19-20c, shows one ofthe first computer models ofthe process-one that neglects some ofthe effectsjust mentioned, but that nevertheless accounts formany observa tions. A rising plume of hot mate-r ia l, hea ted from below, reaches the surface a t acenter of spreading. Itmoves away from the cen-ter, cools near the surface, and the cooled bound-ary becomes sol id , strong l ithosphere. Final lybecoming heavier after i t has cooled, the l itho-spher ic slab sinks back into the mantle in a sub-duction zone , where it is r ea ssimila ted, to beheated and to rise aga in in the fu ture . Anothertheory (Fig.19-20d) proposesthat hot, narrow, jet-l ike plumes r ise f rom the bot tom of the mantle,feed the growing plate, and drive itlaterally awayfrom spreading centers where the plumes mostlyoccur . These same plumes are evidenced at thesurface by hot spots. Among the problems left tbthe next generation ofEarth scientists isthe incor-porat ion of such impor tant detai ls as the shapesofplates, the history oftheir movements, and theformation and growth ofcontinents into an expla-nation of the distribution of convective currentsin t ime and space.

SUMMARY 1.According tothe theory ofplate tectonics, the lithosphere isbroken into about a dozenrigid, moving plates. Three types ofplate boundaries are defined bythe relative motionbetween plates: boundaries of divergence, boundaries of convergence, and transformfaults.

2.In addition to earthquake belts, many large-scale geological features are associatedwith plate boundaries, such asnarrow mountain belts and chains ofvolcanoes. Bound-aries of convergence are recognized by deep-sea trenches, inclined earthquake belts,

GLOBAL PLATE TECTONICS: THE UNIFYING MODEL 463

mountains and volcanoes, and pai red bel ts of melange and magmatism. The AndesMountains and the trenches ofthe west coast ofSouth America are modern examples.Divergent boundaries (for example, the mid-Atlantic ridge) typically show as seismic,volcanic, mid-ocean ridges. A characteristic deposit of this environment is the ophio-lite suite. Transform faults, along which plates slide past one another, can be recog-nized by their topography, seismicity, and offsets in magnetic anomaly bands. Ancientconvergences may show as old mountain belts, such as the Appalachians.

3.The age ofthe sea floor can bemeasured bymeans ofmagnetic-anomaly bands and thestratigraphy of magnetic reversals worked out on land. The procedure has been veri-f ied and extended by deep-sea dri ll ing. Isochrons can now be drawn for most of theAtlantic and for large sections of the Pacific, enabling geologists to reconstruct thehistory ofopening and closing ofthese oceans. Based onthis method and ongeologicaland paleomagnetic data, the fragmentation of Pangaea over the last 200million yearscan be sketched.

4.Although plate motions can now be described insome detail, the driving mechanism issti ll a puzzle. An att ract ive hypothesis proposes tha t the upper mantle isin a sta te ofconvection with hot material rising under divergence zones and cool material sinkingin subduction zones. The plates, according to this model, would b e the cooled, upperboundary region of the convection cell.

EXERCISES1. Summarize the principal geologic features of sub-

duction zones and divergence zones.2. Explain the following inthe context ofplate tecton-

ics: (a)Iceland. (b)San Andreas fault of California.(c) Ural Mountains. (d) Aleutian trench. (e) Earth-quakes in Italy and Turkey. (f) Andes Mountains.

3. How dowe know that spreading along the East Pa-cificrise isfaster than along the mid-Atlantic ridge?

4. What would an astronaut look for on Mars to f indou t i f p lat e t ectonics i s an act ive p rocess on theplanet?

5. How would one recognize the boundaries betweenancient plates no longer in existence?

6. Can you think ofa way not mentioned inthe text, bywhich to measure absolute motions of individualplates rather than relative motions between plates?

Marvin, U. B., Continental Drift . Washington, D.C.:Smithsonian Institution Press, 1973.

Meyerhoff, AA, and H.A. Meyerhoff, "The New Glo-bal Tectonics: Major Inconsistencies," AmericanAssociation of Petroleum Geologists Bulletin, v. 56,pp. 269-336, 1972.

Smith, A G., and J. C.Briden, Mesozoic and CenozoicPaleocontinental Maps. Cambridge: CambridgeUniversity Press, 1977.

Tarling, D., and M. Tarling, Continental Drift. GardenCity, N.Y.:Doubleday, 1971.

Vine, F. J., "The Continental Drift Debate," Nature,v. 266, pp. 19-22, 1972.Wilson, J. T.,ed., Continents Adrift: Readings from Sci-entific American. San Francisco: W. H. Freeman

and Company, 1970.Wylie, P.J., The Way the Earth Works. New York:John

Wiley & Sons, 1976.

BIBLIOGRAPHYBambach, R. K., C. R. Scotese, and A M. Ziegler, "Be-

fore Pangea: The Geographies of the PaleozoicWorld," American Scientist, v. 68, pp. 26-38, 1980.

Beloussou, V V, "Why Do I Not Accept Plate Tecton-ics?" EOS, v. 60, pp. 207-210, 1979. (See also com-ments on this paper by A M. S. Senger andK. Burke on same pages.)

Cox,A., ed., Plate Tectonics and Geomagnetic Rever-sals. San Francisco: W.H. Freeman and Company,1973.

Dewey, J. F., "Plate Tectonics," Scientific American,May 1972. (Offprint 900.)

Hallam, A, A Revolution in the Earth Sciences: FromContinental Drift to Plate Tectonics. Oxford: Clar-endon Press, 1973.

LePichon, S., J.Francheteau, and J.Bonnin, Plate Tec-tonics. New York: Elsevier Publishing Company,1973. .


6/7

610 INDEXMoon (continued)basal t of, 517breccia, 515*crust , 516differentiation of, 516Ear th t ides a nd , 151history, 519internal activity, 518*interna l s tructure, 518*lunar brecc ia , 517lunar provinces, 507magnet ism on, 517maria, 515-516, 516*mascons, 518meteori te impacts on, 510-513micrometeorites, 513origin, 517project Apollo, 507regolith, 513rock, 513*rocks on, 516-517seismic activity, 518-519seismographs on, 393-395stratigraphy of, 513-515surface, 508*surface processes, 510-513Morgan, J. , 368Morley, 1., 429Moulton, F. R., 7Mountain building, 454-456at convergent pla te margins, 454Mountainsdefinition, 119evolution of, 294*fault-block, 498,499*unwarped, 498var ia ti on i n fo rm and ori gi n, 498*Mt. Etna, Ita ly, 349Mt. Monadnock, New Hampshi re , 126Mt. Newberry, Oregon, 358Mt. St. Helens, 21,72, 134,351,359*,365-366, 366*Muav Limestone Formation, 35-36Mud, 274-275Mudflow, 109,110*Mudstone, 72Mylonite, 74,384

Natural gas, 544-545formation of, 538-539world distribution, 539-542Natural selec tion, 302Nazca pla te , 17Nebula, 7Nebular hypothesis , 7,7*Neptune, 6, 531-532Neutron, 42activation, 69Newton, Sir Isaac , 41, 243Noble gases, 57North AmericaCoastal Plain, 499-502Continental Shelf, 499-502orogenic belts, 489-499regional structures of, 485-503stable interior, 488-489tectonic features, 485*North American pla te , 25*Nuclear energy, 548-549, 548*breeder reactor, 549fusion, 549uranium reserves, 548-549Nucleus (of e lement ), 42

Obsidian, 51,55*,72Ocean basinsabyssal hil ls , 257age, 25 .aseismic ridges, 257Atlantic, 258*guyots, 257history of, 257Map of Nor th Atl an ti c, 254*profile, 253-257seamounts, 257Ocean, dissolved substances in, 150*Oceanic circulation, 257-263gyres, 250upwelling, 260-262vertical, 262-263water masses, 262Oceanic crust , composi tion, 413Oceanography, 231-268Oceansdep th a nd age of bas ins, 444*depth varia tion in, 445early history, 15measurement of depth, 247-248residence t ime of const ituents, 150-151steady-state composition, 149-152Ogallala formation, Texas and NewMexico, 142Oil. See PetroleumOil sha le , 547Old Fai thful Geyser, Wyoming, 362*Olivine, 66i n mantl e, 14sol id solut ion in, 61-62Olympus 'Mons, Mars, 361Oolite, 284*Opaque mineral s, 56

Oparin, A. E., 300Ophiolite suite, 452,452*,453Origina l continuity, principle of, 28Origina l horizontal ity, princ iple of,28Orogenic belts, 480, 490*Orogeny, 38Outcrop, 26Outgassing, 15,151ofEa rt h, 149Oxygen, 15-16cycle in a tmosphere and oceans, 306*evolution of in a tmosphere, 304-306,301*and evolution of l ife, 304*isotopes, and paleotemperature, 70in sediments, 288-289Ozark Dome, 489Ozark Mounta ins, Missouri and Arkan-sas,115

Pacific pla te , 19, 20*Paleocurrent map, 272Paleogeography, 39,270Paleogeology, 39Paleo-oceanography, 2q2-263Paleotemperatures, oxygen isotopes, 225Pal isades Sil l, New York, 337, 337*Pangaea, 17,441, 456-460, 18*breakup, 460Panthallassa, 457"Parama basalts, Brazil and Paraguay,353Parent e lement , 42Pasteur, Louis , 300Peat, 289

Pegmatite, 71Peneplain, 126Perched water table , 138Peridotite, 70Period (time unit ), 40Permafrost, 225Permeability, 136-137Petroleumon continental she lves, 542distribut ion in t ime, 541*formation of, 538-539oil fields; 439reserves, 54,2-544,541*t raps, 539world distribution, 539-542Petrology, 51Phanerozoic t ime sca le , 39-40Ph enocryst, 72, 3 84, .Photosynthesis, 15-16, 15*,90, 304-306Pil low lava, 452Pingo, 225Planetesimal, 8Planetology, 505-534Planetsorigin, 9vital statistics, 507Plate tectonics, 16-21, 441-462age of oce an bas ins, 451analyzing plate motions, 449-451Andes Mountains and, 456breakup of Pangaea , 458*-459*and continental his tory, 451continental margins, 443convergence zones, 443convergent boundaries, 453-454and displaced terra ins, 456divergent boundaries, 451-452divergent margin, 17*

driving mechanism, 461-462, 462*and earth history, 451-4,61geometry of pla te mot ion, 447-451and hea t f low, 445Himalayas and, 456history of concept , 441-443and magnetic anomalies, 447'..448mid-ocean ridges, 443and mounta in bui lding, 454-456and oce an dep th , 445opening of Atlantic, 448*pai red metamorphic bel ts , 454Paleozoic history, 457*and Phanerozoic earth history, 456-461pla te boundaries; 443plate collisions, 455*rate of pla te mot ion, 445-447, 446*and rock assemblages, 451,454structure and evolution of pla tes,443-445subduction, 445, 445*, 453*, 455*t ransform fault s, 443,447triple junctions, 448.-449,448*, 449*Pluto, 531-532Plutonism, 38,376*dike, 378*laccolith, 377*lapolith, 378*sill, 377*stock, 377*Plutons, 376-380batholith, 378-380dike, 378laccolith, 377-378lapolith, 378:"379

ring dike, 378sill, 377stock, 378.Porosity, 136-137Porphyrobl&sts, 384,384*Porphyry, 72Postglacial upl ift, 25Potholes, 161Powell, J. W., 27,26*Precambrian t ime, age of rocks, 46-48Products, 84Project FAMOUS, 255Proton, 42ProtO-Sun, 8Pumice, 72Pyrite, 86

ori gi n i n s ed imen ts 288sedimentary, 288* 'Pyroclastic deposits, 350-352ash, 350bombs, 350breccia, 351dust , 350ignimbrite, 352nuee ardente , 351-352tuffs, 351welded tuff, 352Pyroxene , 66cleavage of, 67granulite, 386i n mantl e, 14

L~ke Tanganyika, 122-123.RIver Jordan, 123Rmgwood, E. A., 415R!ver depOSi ts , a lluvia l fan, 167RIvers, 162-174

base level , 165-166 166*bra ided stream, 168channel deposi ts , 171*channelization, 170c~annel patterns, 168-171dIs~harge, 163,163*,164*, 174*dramage basin, 175*,176*flooding, 172-174, 173*~oodplain, 169*, 171-174mte~fac~ with lake or ocean, 179*10ngItudmal profile, 164, 165*, 166*meandering, 169*, 171*, 170*meanders, 168-171natural levee , 171,172*Oxbow lake, 171

point bar, 161*,170, 170*, 171*pools , 170, 170*riffles, 170, 170*sinuosity, 169-170splay deposit s, 171stage, 157suspended sediment, 164*thalweg, 170of t he U.S ., 174*River systemscontinental diVide, 174dendntto dra inage pat te rn, 176-177dramage basins, 174-175order, 175-176radia l dra inage pat te rn, 178rec tangu~ar dra inage pat te rn, 178stream piracy, 175superposed streams, 179t re ll is dra inage pat te rn, 178Rock fall, 110-111Rock glacier, 111Rocks, 52-53"ac idic" and "basic," 70aphanitic, 71classification, 70-78, 53extrusive, 31fel si c a nd maf ic 70!dentification fl~w chart, 76*Igneous, 70-72, 31intrusive, 31massive, 93metamorphic, 74,31-32phaneritic, 71pyroclastic, 71-72response t o s tr es s 467sedimentary, 72-74siliceous, 74ultramafic, 70Rockslide, 107,111*Rocky Mountains, 124

.,present topography, 498Rontgen, W., 41Ross Ice Shelf, Antarct ica, 216Rubey, W. H., 148-149Russian pla tform, 489Rutherford, E., 41

Quartz arenite, 99-100crystalline forms, 55*Quartzi te , 74

Radia tion, infrared, 312Radioactive decay, 10*, 42': ':44half-life, 43*Radioac tive waste disposa l 314Radioactivity, 4, 1 0 " ,of early Earth, 10-11Radiolaria, 287Radiolarian ooze, 266,287Radiolarite, 287Radiometric dat ing, 43,44HC, 43-44, 225calibrating the stratigraphic timesca le , 46of groundwater flow, 140Sources of error, 45-46Radius rat io, 60

Rain shadow, 132-134, 134*Reactants, 84Recharge, of groundwater 137Redwall Limestone Form~tion 36Reef, 280* 'Regression, 35,278~nd sprea di ng rat e, 489 .ReId, H. F., 396Relat ive humidi ty 130Relief, 114 'Remote sensing, 564Respiration, 86Rhine graben, 477,477*Rhyolite, 72origin, 367Rifting, 17*,18*Rift valley, 122-123Dead Sea , 123

Sabkha, 196-197Sal ton Sea , Cal ifornia , 375San Andre as Fau lt , 19-20 , 20* 25 25*406*, 484 ' , ,

INDEX 611Sand, 271-272

bd~~d!ng,271-272IVISIon by .frost' graIn size 271. mg,271 'I nf er en ce o f o n .so rtin g, 2 71 gm f rom sh ap e, 2 71S~~dkstone, 29, 72, 271-272ose, 74,272~ra:ywacke, 74,272ht!l1carenite, 272mmeralogy, 272quartz areni te , 272quartzose, 73,74San Fra nc is co e ar thquake f396*, 484 0 1906, 20,San Ioachin Valley Calif .Saturn, 6, 5 30-531 ' ornia 295moons, 531*rings, 530

wit h r ings a nd moons 530*Schist, 74, 383 'Schistosity, 382*, 383Sea-floor spreading, 17, 17*Sea level , eusta tic changes, 235Seamounts, 257Sediment, 15,27accu~ulat ion and buria l, 291-295chemical, 73,270, 278-291clastic, 72,270-275detrital, 72,270gravel and conglomerate, 272-274mud and shale , 274-275pelagic, 264sorting, 271*terrestrial, 36Sedimentary depositsa lluvia l fan, 276fining-upward alluvial cycle, 276loess, 276till, 276

Sedimentary environments, 275-278alluvial, 276bea ch and bar , 277deltaic, 276desert, 276glacial, 276pelagic, 277shallow marine, 277turbidite, 277Sed!mentary facies, 277

Sed~mentary particles, origin, 95-96SedImentary rocks, 78classification, 73*organic, 74phosphates, 74relat ive abundances, 74skeleton claSsification 77*Sedin;tentary structures, '158-160anhdunes, 160*assymetrical ripples, 159cross-bedding, 159,160*dunes, 159,160*oscillation ripples, 159-160, 161* 158159*, 160* ' ,tr?ugh cross-bedding, 161*SedImentation, 269-295on continental pla tforms, 293-294and current strength, 271deep-sea, 263-266eolian, 270 0/>j~'_'r-~physical, 269-270subSidence, 292and tec tonics , 295

Sedimenta tion rate, deep-sea, 264


7/7

612 INDEXSeismic stratigraphy, 278Seismic waves, 23density-depth relations in Earth, 412*in determination of thickness ofcrust, 414outermost 700km, 415*P waves, 409paths of P-waves and S-waves, 410*paths of through Earth's interior, 409*prospecting, 411refraction, 410

S waves, 409*surface, 410whole-Earth modes, 411*whole-Earth oscillations, 410Seismicity, 398-401deep earthquakes, 399of world, 399Seismogram, 395Seismograph, 394, 393-395, 395*Seismology, 393-417Earth's core, 408-410and Earth structure, 410-417seismic waves, 408-410Series (geologic), 4;0Shadow zone, seismic, 408-410Shale, 29, 72, 274-275Shapley, H., 307Shards (volcanic), 72Sheeting, in joints, 93Sheet silicates, 66Sheet wash, 112Shepard, F., 241Shields, 488Shiprock, New Mexico, 361Shorelines, 232. See also Beachesdrowned river valley, 236*landforms, 234-235Sierra Nevada, 124, 134, 379, 494, 496,498batholith, 379*Silica deposition, 287Silica mineralsclassification, 62oxygen and sil icon in, 61Silica tetrahedron, 61Silicosis, 310-311Sill, 37, 289*topographic, 288-289Sil len, L. G., 149Sinkhole, 123, 123*Skarn, 388, 389*Sklodowska-Curie, M., 41Slate, 74Slickensides, 478Slope wash, 111-113Slump, 109, 109*Smectite, 87-88, 87Smith, W., and strat igraphic correla-tion, 30-31Snider, A., 441Snowflakes, crystal form, 55*Soddy, F., 41Sodium chlorideionic bonding, 59structure, 59*Soil, 96-98A-horizon, 96B-horizon, 96C-horizon, 96humus, 96laterite, 98major types, 99*pedalfer, 98

pedocal, 98production of, 82-83relat ion of to c limate , 97-98Soil creep, 111Soil profile, 97*Solar energy, 549-550Solar system, 6age of, 5angular momentum in, 5chemical composition, 6chemical-condensation sequencemodel of origin, 9density of planets, 5-6origin of, 5-9planetary orbits, 5planetary rotation, 5planetary spacing, 5recent theories of origin, 8-9Solid solution, 61-62Solifluction, 111Sorby, H., 56Sorting (sedimentary), 72Space .composi tion of interstel la r particles , 8density of matter, 8Spect roscopy, of Sun, 6Spheroidal weathering. See WeatheringStalactite, 123, 286*Stalagmite, 123, 286, 286*St. Augustine, 40Steno, N., 28, 29-30, 53Stratification, 27-28Stratigrapher, 31Stratigraphic sequence, 30Stratigraphic time scale, 27-28, 39-40Streams, 155-174alluvium, 156bed load, 157, 157*, 160*braided, 168*capacity, 157, 157*competence, 157, 157*discharge, 157, 157*drainage patterns, 178*eddy, 156*erosion by, 160-162graded, 166laminar flow, 155*, 155measurement of discharge, 157order, 177*particle movement, 156-158relat ion between competence andvelocity, 158relat ion between gra in size andvelocity, 158*saltation, 117, 157, 160*settling velocity, 157shooting flow, 156, 156*streaming flow, 156, 156*streamlines, 155superposed, 179*suspended load, 156-157, 157*turbulent flow, 156, 156*Stress, 466Strike, 470, 470Stromatolites, 285*, 302-303, 303*Subduct ion, and Appalachian Moun-tains, 492Subduction zone, 19, 21*, 250*earthquakes at, 403Sublimation, 130Submarine canyons, 246Subsistence, 122, 292Suess, E., 441Sulfide, sedimentary, 288-289

Sun , e ar ly h is to ry , 9Supai Formation, 36Superior province, Canada, 47Supernova, 8Superposition, principle of, 28Surtsey, Iceland, 354*Swamp, 136*as natural reservoir, 134-135Symbiosis, 281Symmetry, in mineralogy, 56Syncline, 121System (time-rock.unit), 40

Talus slope, 110Tapeats Sandstone Formation, 35Tectonic valley, 122-123Tempera ture, his tory of in Eart~ , 11*Temple Butte Limestone Formation,35-36Terrestrial planets, 5-6Tethys Sea, 457Thermonuclear reaction, 8Thin sections, 56Tibetian plateau, 482Tides, 243-245, 243*, 244*a nd d is ta nc e betwe en Ear th a ndMoon, 244effec t of on Earth's rota tion rate,

244-245neap, 244spring, 244tidal currents, 245tidal flats, 245Titan, moon of Saturn, 531Titius-Bode rule, 5Topographic map, 113contours, 113Topography, 113-119effec t of on climate , 117effect of on weathering, 116features, 113*mat ure a nd o ld a ge , 126*worldwide distribution, 116-117Toroweap Formation, 36Transform fault, 19-20, 19*Transgression, 35, 278and spreading rate, 489Transition elements, 59Transition zone, 328Transpiration, 131Travertine, 148, 286Trilobite, 35*Tsunami, 365, 404*, 405Tufa, 286Tuff, 72Turbidites, 272, 293, 390, 453Turbidity current, 252*, 251-253, 453deposition by, 251-253,graded beds, 252turbidites, 252

Unconformity, 31, 37, 32*angular, 32Unconsolidated material, 109Uniformitarianism, principle of, 38United Statesmajor landforms, 124*physiographic provinces, 486*-487schematic section, 488*Uplift, and erosion, 116'Ural Mountains, 456

Uranium, radioac tive decay of, 43Uranus, 6, 531-532Ussher, Archbishop James, 40

Valence electrons, 57Valley and ridge topography, 122*origin, 123*Valley, V-shaped, 114-115Van der Waa ls bond, 61Varved clay, 28Venus, 5, 520-521atmosphere, 86, 520surface features, 520Vesicles, 72Vine, F.J . , 429, 442ViSCOSity,156, 322*Vishnu formation, 32-33Vitrinite, 289Volcanic ash, 72Volcanic dust, 72Volcanic phenomenabombs, 351*columnar jointing, 350*at convergence zones, 370dike, 355fissure eruptions, 353-355flood basalt, 352, 354*fumarole, 363gases, 362and geothermal energy, 373geyser, 362-363global pattern, 366-370global representation, 368*hot springs, 362-363ignimbrite, 355, 355*intraplate, 368-370lahar, 361-362, 373nues ardentes, 352*, 361a t ocean ridges, 368

phreatic explosion, 352, 353*, 373rift valley, 354Volcanic rocks, 345-352. See also Pyro-clastic depositsaa, 347, 348*columnar jOinting, 350l ava caves (tube), 350lava flows, 347-350lava fountain, 350pahoehoe, 347, 348*pillow lava, 349, 349*pumice, 350spatter cone, 350vesicles, 350welded tuff, 86*Volcanoescaldera, 359-360, 360*central vent (pipe), 356cinder cone, 357, 358*composite cone (strato-volcano), 359crater, 359diatreme, 361, 361*global distribution, 346* '0Kiluaea, 363-364Krakatoa, 365Mont Pelee, 352, 364-365Mt. St. Helens, 365-366, 366*OCcurrence, 345shield, 356, 356*volcanic dome, 356von Laue, M., 57, 64

Wadi, 195Wallace, A. R., 39Waterdistribution in crust, 146*distribution in natural reservoirs,143*hardness, 144infiltration, 131loss into space , 130magmatic, 147meteoric, 147origin, 148-152properties, 130quality, 143-149runoff, 131source on Earth, 15thermal expansion of, 93three states, 130*t ime of origin, 151uses of, 129*

Water table. See Groundwater, watertableWave-cut terrace, 234Waves, 236-239, 237*effec t of storms on, 239*e ff ec t o fwi nd on, 238*period, 236refraction, 237*, 239, 239*, 240*steepness, 238surf zone, 238swell, 236wavelength, 2~and winds, 238-239Weathering, 15, 38, 81-103, 83*of calcite, 90*carbon dioxide in, 86carbonic acid in, 86chemical, 81, 100-101climate and, 117-118controlling factors, 98*exfoliation, 95of feldspar, 83-88global scale, 100-101-importance of water, 83-84jointing and, 476of mafic minerals, 90-91mechanical, 81on Moon, 92physical, 92-96of pyroxene, 91*of quartz, 91

rate of mechanica l and chemica l,118-119as sediment source, 99-100and source rocks, 95-96spherOidal, 94*, 95topographic fac tors in, 100*, 116vegetation and, 117-118Wegener, A., 441Werner, A., 50

Wilson, J. T., 368, 442, 456Wind, 187-192deflation by, 191desert pavement, 191dunes, 191dust t ransport by, 189-190erosion by, 191-192frosted sand, 190-191global pattern, 188*particle saltation, 189particle transport, 187-192

INDEX 613sal ta tion t ransport by 189*sandblasting by, 191-192ventifacts, 191

XenOliths, 350Xenophanes, 40X-ray diffraction, 57, 64X-ray fluorescence, 69

Zagr?s Mountains, Iran, 122*Zeolrte, 386as water softener, 145*Zoned crystals, 336

extract from press and siever including some from index

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