:=:'i lvithin a percent or so. As more complete=: more precise daia become avai lable, these::,:, es should become more accuratc, perhapsi:€day convergjng to give us a perfect model of:: feal Earth. But what can we say now about the

armposition, Structur€,dd State of ihe Interior

.=-ocity and density models are important mainl,v:= a means to an endi the ultimate goal is to under-::rd the composiiion, structure, and siate of the:rih's interior. Laboralory experiments make the::nnect ion between seismology on the one hand= d petrology and geochemistry on the other.:::!h-pressure equipment and shock waves gener, :ed by explosives are used, as descr ibed in Box1: 2. io learn how velocity and density would

, ' . ' i d H e r e n , r o , k s . p r r h p r i r r h p s o l i d o r i n a:artially molten state, and eithcr near the surfacer in ihe deep interior. With this informalion the

:arth model shown in Figure 17-33 can be inter-lreted to give information about the materials:l1d their state.

The majof divisions, crust, mantle. and core,Fig. 17 3o), were discovered from the analysis ofreflected and reftacted P and S waves and havebeen known for more than 60 years. The bound-ary betwe€n the ffust and th€ mantle is called theMohorovieit discontinuity (M, or Moho, for short)after the Yugoslavian seismologist who discovefed it in :1909. It separates rocks in which Pwaves have velocities of about 6 to 7 kilometersper second (3.8 to 4.4 miles per second) from un-derlying mantle rocks, in which P waves have aveloci ty of about 8 ki lometers per second (5 mj lesper second). Thc f ic ld method of measuring theseveloci t ies is descr ibcd in Box 17 3. From geologi-cal sampling to find all possible crustal and man-tle materials and from labofatory measurementsof th€ properiies of ihese materials, we havel€arned to associate P- 7ave velocities wiih com-position, as indicated in Table 17 3. We concludefrom these measurem€nts that the continental

Table :17-3Corres!ondence beh{een compositionand P a'ave velo.jty in igneous rocks

SEISMOLOCY AND THE EARTH'S INTTRIOR

Tli€ lithosphere is topped by a relatively lightlveighiL r , - 1 . \ F i s r o o 8 y r F \ e d l s l l o l h F c a ' q v " r i p " i 1

thicknessi it is thin under o0eans, ihicker undd continents, and thickest under high mountains.

crust consists mosily of granitic rocks, withgabbro appearing near the bottom and that nogranite occurs on th€ floor of the deep ocean, thecrust there being entirely basalt and gabbro. Themantle below the M discontinuity is almost c€r-iain to be primarily the dense uliramafic rock pe-ridotite. The crust is a distillate of the mantle andtherefore differs chemically from its par€nt. Inthis sense, the Moho is a chemical boundary lo-cated by seismic waves.

Nowadays seismologists are excited by the flnerdetails, ihe variations within the crust, mantle,a n o r o r e T h p \ a r i e l i o n i n c r u . l a l . h . , l r e . s t n osection like the one shown in Figure 17-34 is oneo I l h F n o r . : m p o n J n l r e p n L s p i q r o 0 3 . . 1 1 F -sults. The thickness of the crust vari€s from about35 kilometers to 10 kilometers in a section extend-ing from continent io ocean. Under a high moun-iain the crust thickens to as much as 65 kilome-ters. l f Figure:17-34 suggests to you that thecontinental crust floats on ihe denser mantle likean iceberg on ih€ ocean. you have made a goodobsen ation. lcebergs float because they are lessdense than sea i\7at€ri flotation comes from thelarge volume of ice below the sea surface. WhenArchimedes principle of buoyanqv is applied tothe {lotation of coniinents and mountains, it be-comes th€ principle of isostasy, which holds thatthe relatively lighi continents float on a moredense mantlei most of a coniinent's volume liesbelow sea level for the same reason that most ofan iceberg lies below the oc€an surface. Naturehas contrived that large topographic loads such asmountains and continents are compensoted-thatis, supported primarily by buoyancy rather thanby the strength of the crust. Rocks, which weknow to be solid and strong over the short term(seconds or years), are, over ihe long term (thou-sands io millions of yea(s), weak and flow like aviscous fluidwhen loaded. When coniinenis growor mountains are pushed up. a supporting root

413

! l

i .5 r

a i

a

TER\AL TROCEssEs

?.,*

s !.ro!!-

! e r l r LkT l

Est imate of the var ia l ion in densl ty o.d in Pand S $ 'a!c vc loc i t ies in the Ea.Lh s n! . l le and.orc Uncerta iDty is p,obabl t : oDl ! a leN poccnt o lthe rc lu. r l la lue. [Pre!ared L] .A. I4 Dzlesonlk i ardD. L And.rson lor the Siandard Earth Commit tee o lthc l i l le fDat iondl I ln ion of Gcodes. ! and Geophysi (s l

change as ihe! pass through mater ials of di f ferent€last ic propclt ics. Thc ! , ibrat ion frequencies oithc Eurth d,"pend on the \ .elocj t ies of these r ,avesas r\/€ll as on the densiiy of difier€nt parts of th€int€r ior, just as thc tonc ot a b€l l depends on i ts€last ic i ty and densjty. Oncc al l lh-" scisnlologicaldata are accunulated, the ncxt stcp is to f indEarth modcls whosc P rva! 'c and S $'ave vcloci-ties and internal d€nsiti€s afe collsistent 1\,ith the

\ o \ ' 3 ' I - i n \ r r . r J o L . F r n . " \ i s . , 1 ' l o lmathematicians. is som€thing like b€ing told thata dr iver madc a tr ip from Los Angeles to SanFrancisco in scven hours, ln bad 1\,'eather ux aMondav, and h€,, , ing to make a b€st gu€ss of therouie he took. Thc mathematics of this proccsscannot b€ explained hcrc. but thc tcchnioLres arcpor.--" ul €nough to allorv us to make a b€sl csti-maie ol an Eafth model.

A plot of in lernal dcnsit jcs and P and S-lvavevcloci t ics fo. the Earih is given in Figure 17-33. I tis quj tc l jkcly that thes€ cu Jes represent th€ r-aal

BOX 17-2 HICH PRESSURE AND SHOCK EXPERIMENTS

nven if we knew in d€tail how d€DsitJ and scFmrc vclociiies chrnge ilith depth in the Ea h 1!. \forld stilli ! , rn1 to ide. t i fy the mrt€r ia ls and desc. ibe thei r phls i.a l .ondi l ions. To do th is we r lso need ln lofmat ion onthe densi t ies o l d i f terent maler ia ls and ihe yeloc i t iesa'ith which scjsnic Naves tfavcl through them !rderthe hiSh p.essures and ten !rfratures ihat cxist in thcin lef io f . l a p lanel . - l 'be

! .essu.e at the cent€r of theEarih is Dcarly 4 million iimes atmospheri. pfessure 1

and tem!eralurcs thcrc raDgc to sclcral thousand de-grees. LIslng a htdrrulic pr€ss, geophysicists caDsqueeze rocks in the labo.atory to pressures o l aborLt100 k i lob,r rs heat then to temp€ratures o l about1000"C. and aL the same r ine neisu.e m!n! o l thei rpropcr t ies. Ih is f .o .edure dr t l i .a tes (o. , l i i jons atdcpths of about 300 kilorneiers. ,A. re.enl te.ini,;albr€akthrolLgh now nakcs it !ossibl. to incrcasc lalrofa-tofy pressures to 17 meg{bdrs and tempcratrLres io3000'C, conditions similar to thosc in thc Eaith s cofe.l he ""xpe. inenl is s imi la. lo lha l depi . led in Lhe f igur€ex.cpt thai a d ianond anvj l is us.d. The sanple hsqueezed bet l leen ln 'o.ut d i i rmonds aDd heated by alaser beam Anorhe. e.hnique lor .ompressiDg mcksto lcry hish pressures happens to bc th€ roy sam€' F l L l ' l r ' | | I I r 8 8 " r " g " '

o \ ! o - \ ' J ' o . ' 6

such as dynamit€. is rvrapped around the rock. U/herthe dynarnitc js detonatcd, thc shock Nalc squeezes

Thc Bridgman squeeze., a device lor subjcctingmircrals to pressurcs ol a f""w tens ol thousands otatmoslhcrcs and tentefa l ! fes o l se i 'e .a l hundredd€grees. The low p.essure ol the h,tdraulic press tsamplified by .oncentrating the lotal forc€ on thcr l l " r ' " o \ " . u , n o ; n 8 |

d r \ . o F i \ . . 1 . 1 " r " . r . " i - r - " , F . " . \ r -roDneDts de€p in the Ea. th.s ,xusl .

ihe rx ik , .a is ing ihe pressure and lempcratufes to th€l i - : . r , . . - , . I o a , d 6 i , . . i . . . o ' . r e .

depths. The rock is d.s t ro led in rhe p joo€ss, bui D thefcw rn i l l i .Dihs o l a second belore i i fa l ls apar t . dataneeded to.a l .L l i re Lhe dcnsi t , ! , ! r .ssur-" , ard shock" . , J 1 \ ' . r , t - r i . r , o l - " n r i .

velocitics) ar. obtained electronicall-! from se.sol5 ont s " . o l r - p ^ o f e . ! r e , . . n t r . . o o 6 6 -

rninc such lhings as strcngth, thefmal. electrical, andelasij. prcpertics at high pressu.es and lempcratures.

-Pressme is measuFd in a lmospheres (a tn ) , baN, o r p . !ndspd sqlaL. i h (p s i ). 1 arn = 1.0:L brrs = 1.r7 I].s i. ce.to: i s ls lend t . us€ b ! r , l i l . l a rs ( l ( r r bad and negabaB ( rcN hdrs )

414 THE BODY OF 1'HE EARTH: h'TERNAL IRCCE5SES

, \ l d p \ . u o l . " " . " " o r o - c c t o o o i d -

F r o \ I n , \ . r d ( . p f h . n F . \ o d d f . o m , n t , . !

- . There is one variaDr to this gencrat mcchanism.

1r ror some rcason tor exampl€, regional heat_rng-pa ol lh€ uppcr mantl€ becomes tcss oensethar ih€ adja.rent mantle, i i $. i11 atso exert d buoy_, l o , " l a r , o i , r p r o r l - \ J ' . d t o p s d p , l, h o . " . r i , h u r l - . F - u " : . r \ , r t , u o t . r .s€ns€, the loIver densih, mantlc sen,-"s as I roor.This mode of isostar ic comp€Dsation scems to b€operal ing in the Basir and Range provinc€ (Utah.Arizona, Ne! 'ada) of th€ Unireci States. Th€ histrheat flo\\, in this regio. is c.rnsistent rv h th€ e_x_

planat ior. Seismological studics of crustat rhick-n€ss have provided quant i tar ive corroborat ion Iorthe mechanism of isoslasy.

Tn th€ years 1965 to 1970, geologists ara geoph\,sic ists lhe ,or ld ove. conc-"ntrated res€afchcf lor ls i rn the upper thousand ki lometers of lheEarih as part of the Intrmai ional Upp€r Manrt€lpfojecl . This concerted atack led to many exciring discoveries about a r€gion that had prevjoust_vDeen poorlv known. lrve can illusrrai€ the xnrfeimportani of thcse by discussjng the shcar-$raveveloci ly modei. According to rhis modet t l ic l ian_t ie is div id€d on the basis of shi l ts in veloci iy inro

BOi 17-3 SEISMOLOGICAL SOUNI)ING O! THE EARTH'S CRUST AND UPPER MANTLEscisnokJgists ha!'e del,cloped a fiekl !fo,;edrLrc r{i. r1e rive a lin€ s,ilh slo!c 1/v, and nrrercept r: values ofmeasurnrg rhe thickn-"ss of thc .r,st and the vetocity ot vr, vtr. and r obrain€d fronirhe sraph are usc.r ro .ialP wares iD Lhe crusl ..d at th. tof of ihc nanLte. SDla| culare ihe rhickness tron the fo;muraseismonetcrs: re p laced on thc suf la , ;e jn a the ext€ndine lsa l i lon a thot t )otnr

'Nhere an exptos ionis set ofl to gcnerate P 1va!es. 'Ihe Nales leavc ihe explos ion in a l l d i rect ions sohe t ra lc l in8 a iong rhe sulfa.e, others al.ng the top ol the rnanlle, ds shorvr ,. rtrcuppe. f igur . , { t f rvc l t ime .urve can h€ ptoted onwhi .h each poin l fepresents rhe r ravel L ime,cqu,reotof lhe wavcs t . r€ach a seismomcrer . fhe l to i o f\ {a! .s that t ravel a long the su. i ro€ is a s t r r ight t jneth.ough thc . r i8 in of the €Japh 1\ , i th d s lo lc o l 1 /V, .] ' h e s l o p c o l t h e l i n . i s n e r n t r € d r o o b L a i n t h c s p e e d V jo l P uaves i t1 ihe c(Et . The wdves r ra lers ing thc rn!n

- T V ' V_

r . / i t 1

A stude.l lamiliar !'iLh trigonometr\, ard Snell's lawmight t.y deriliDg this sintle but impofiant equarion.

Th. lower figure shl)r's scismj., tr,aves r€corded ai adisiaD.e oJ 163 kilofteters from an explosion oi TNT.Tlic ira.,es arc recofded tuon six setsmonerers Flaccd100 metcrs apar i . Th. waves that have r ravelcd rhroughlhe dust and n,rnt le are indicated by pr and p" respe, j

sE]SMOLOGY AND THE EANTH'S INIERIOR

s s/a!e ve D.lv (km 3ecl

ll'..,::.: lL|rcprr€re p ate $ d slrons

l l l09r par l ia y no Ien {e3k o0ve o . ly s .u r .e o l basa l l . nagm:

l+ s. dN

] " " * "u" - r , - rap d .c , .ase ir dens ly 3nC ve oc i r o !ne t sp j

lS . d de .s ty ano ve . . l y n . reas

ls radm

y , rh . . ,€as ns press l re

lI Phase r ransron break dow. lcJ Feo Nho. so , A ,o .

lSo ,d ders l l y and ve o . l y i . reasitradra y rr lh n.reas n! Pr€ssure

'l..i-r:'.

t \

;: a00t

A modern vied of the siNcturc of the out€rmost 700 km of thenarth is illustrated b,\j a ploi ol S wave velocity against depth.Note how changes in velocity mark the inporiant zones: litho-spher€, partially molten asthenosphe.e transitioDs to more densemolecular structLres.

zones a to g (Fig. 17-35). Zone a is the lithosphere,a slab aboul 70 kilom€ters (45 miles) thick inwhich the coniinents are embedded. Crust formsthe uppermost part of this outer shel l of the Earth.Iis 1o$'er boundary is marked by an abrupt decrease in shear-wave velocity. The lithosphere ischaracteriz€d by high velocity and efricient prop-agaiion of seismic rvaves. both of which implJsolidity and strength.

Zone b is the asthenosphere, or zone of weak-ness. h is also called thc low-veiocity zoile for theo h \ . o J s r p d - o n l h , l h c . F d - r " ' ' \ c l , ' i l l I F "

is lo$. Seismic waves are attenuaied mor€strongly in the asthenosph€r€ than any$'here elsein the Earth. Because laborator-v experimentsshow that seismic waves a!e slowed and absorbedin a cr-vstal l ine- l iquid mixture, a slush, ' mosigeophysicisis ancl petrologists lhink that thc asthenosphere is partidlly melted, perhaps 1 l. 1npercent. We har.e alrcady noted that th€ Earih'st i thosph€re is made up of abort ten dist inciplates, creaied alons mid-oc€an r idg€s and dcstroyed in subduct ion zones. A sol id slab underlain by a weak laver might be more easily movable. and perhaps this accounts for the mobilitr" ofihe l i ihospheric plaies.

Velocity and densilv in bolh the lithosphereand asthenosphere fit a p€ridoljtic composition.

These two zones, therefore, do not differ so muchchemically as they do in physical staier theboundar] 'at 70 ki lometers marks the sol idus(!vh€r€ melting begins), sepaiating the solid litho-sphere from the not so solid asthenosphere. InChapter 15 i t was proposed that ihe m€l l of theasthenospher€ is the primary soufce of basaliicmagma, which f i ts th€ picturc nic€ly.

The asihenosphere ends at a d€pth ol about 250ki lometers (155 mil€s), and the rocks become sol idagain in zone c. Th€ veloci ty increas€s sl ight lywilh d€pth in this region because of the cffcct ofincreasrng pressur€.

Zon€ d, about ,100 kilomet€rs (250 mil€s) b€lowthe surface. is thin but very lmportant. The rapidincrease in velocitv ih€re correlates with therapid increase in density in Figure:17 33. Thistransition is too abrupl io be accounted for b] acomposi i ion change. A change of phasc thai is. acloser repacking on the atomic level- is required.The theoretical explanation l,!'as beautifully ve.jfied in:196s (,hen E. A. Ring .ood and S. Akjmotosqueez€d ol iv ine in iheb laborator ies and foundthat at critical pressures and t€mperatures itsatoms iake up a more compact arrangemeni,changing inio the spinel structure (Fig. 17-36). Oli-v i r e r l h " p r i r c p d l m i r e r d l n D p r i d o i p .

" n d d r dd€pth of about.{00 ki lometers in the Earth, condi

II

476 THE BODI Of THE EART}! INIERNAL FROCEsSES

Ol-ivine (Ms,Sioa with Fe,SiOJ is a najor nineral in ihe Earin's mantle. Pa.t (a)shows oliviDe in its low'pressure forn. Th€ large pale-brown atoms are oxygen,the nedium-brown atons are silicon, and rhe dark-brown ones are magnesium (orironi. \/l/hen the pressure reaches a ditical value, codespondirs to a depth of about400km ir the Earth, ihe molecule collapses into a denser fom (b). Noie ine de-c.ease iD void space in the high-pressurc form, in which the oxygen atoms are morcclosely packed. SeismoloSists have found where this transition occu$ in the Earth,and petrologists have obseNed th€ transition in high-pressure laboratory exped-

612

Ii , '

Pr€ssure (105 atm)

ligtre a7-37Density iD the Eaith's fluid core plotied againstdepth below the surface and against pressurc (blackcuve). Comparison with the densities for ton,nickel, and iron-silicon nixiures measured in labora-tory studies enables seismologists to cbnclude thatthe core is nostly iron but slightly less dense thanpue iron, as iJ a snall amoutrt oI a !'lighteninC" ele-ment like silicon were preseni.

tions are just right for it to change phase. This isan excellent example of how the collaboration ofspecialists with different backgrounds (s€ismolo-gists and petrologists) can, little by little, removesome of the mystery of Earth's interior.

Zone e is one of small change with depth, butzone f, near 700 kilometers, shows another rapidtransition.In 1974 experiDenters in Japan and theU.S. reached the required pressures in the labora-tory and found that olivine breaks down intodense, simple oxides like FeO. SiOr, and MgO aithis depth. The entire region from 400 to 700 kilo-meters, containing zones d, e, and f, is sometimescalled the bansition zone.

The lower mantle, zone g, extending ftom 700kilometers (435 miles) to the core at a depth of2898 kilometers (1800 miles), is a region thatchanges little in composition and phase withdepth. Density and velocity inffease gradually.again due to increasing pressure.

The Earth's core is far away but not out of thercach of seismic waves. We know that its outerregion is fluid and its inner one solid (Fig. 17-30).To obtain its composition, we use the same ap-proach that has already proved so useful-compar-ison of laboratory experiments and seismologicaldata. Look at Figure 17-37 to see how this is done.

- : density in the f lu id core is plot ted. Also-.:oi!n are the densities of nick€l, iron, and a mix-: :€ of i ron and si l icon, deiermined b-v "shockjng"

-:.s€ materials in ihe laboratory with explosives,- \ras described in Box 17 2. Wc see Lhat nickel is::o dense. hon is better but nceds to be light€ned:i adding perhaps 15 percent silicon. Would:ier clements fit the data? P€rhaps. but our, ioice is l imited by the relat ive abundance of ele-:ents. B€cause the core accounts for one-third of:ie mass of the Earth, ii must contain relatively:bundant elemenLs. hon is ih€ only abundant e]€-=ent that approaches the required density underne pressure of millions of almospheres at these

.reat depths. l t is a l i t t le too dense, as Figure 17 37

.hows, so a plentiful "lighlening" element like sil

SEISMOLOGY AND THE EARTH'S N.{TERIOR 477

icon must be added. Oxygen or sulfur might alsobe possible lighiening elements.

In this way seismological observaiions and lab-oratory meas rements of ihe properties of mat€ri-als combine to give an incomplete bul neverthe-less good approximai ion ofth€ Earth's int€r ior ' Azoned. diflcrenLiated Earth is found in which themajor components are a metal l ic i ron core and arocky mantle consjsting primarily of iron-magnesium si l icales. The mani le includes a i ransi l ionzone in which atoms are forced into closer packing, a partially molten asihenosphere, and most ofh F o J l . ' i l h o s p h e F . q l h i r . l r g l - r $ r P i g l I L r J \ l

the end product of the difterentiaLion process-caps the manl le.

SUMMARY 1.MosL earthquak€s originaie in the vicinity of plate boundaries. The mechanism ofeafLhquakes is governed by th€ kind of plate boundary: fracture under iensile silessoccurs at borndaries of divergence, fracture undcr compressi\'€ stress at bounda es ofconvergence, and lateral slip along t.ansform faulis.

2. Great earthquak€s release ln a few minutes huge amounls of elastic sirain €nergy thathad been slowly stored in the rocks of the fault zone ov€r i€ns or hundreds of y€arsThe source of this strain is plate mDtions.

3. Richtcr magnitudes arc d€iermined from the siz€ of the ground motions. as measrrredwhen seismic ft'a1.es are recorded on seismographs Thc thr€e types of seismic wavesare P waves, S waves, and surface wavcs The entire Earth can b€ set into global

vibration by great carihquakes.

4. From a siudy of the travel times of scismic waves and the frequency, or pitch, of theglobal oscillations, seismologists have found that the Earth is divided into shells-rhatis, it is a zoned. differentiai€d planet. witha. A slablike, mostly ultramalic llthosphere, broken inio large, mobil€ plales.b. A partiallv molten asihenosphere, ihe primary source of basaliic magma. as evi

denc€d by reduced vcloci ty and high absorpt ion of seismic wavesc. A transition zone, where atoms are forced into a closer packing by the high pres-

d. A lolver mantle, mainly iron magnesium silicate.e. A fluid orter core, mostly iron but wilh one or more "lightening" €l€ments.f. A solid iron central core.

5. The continents with iheir lightweight fclsic crusts-the end producis ofthe difierentia-t ion process are embedded in the l i thosphere.

EXERCISEs

1. What is an e, r f lhqdak€? HoN is i ls magni tude m€asur€d? HoN many earthquakes cduse se.ious darnagc

- . H , . \ o ' \ l h F d i c o , ' ^ i o l " r l h q r J I " l o " : olatc Nilb the lhfee t-lpcs ol plate boundarics?

3. Seismograph stations repo.l the follorving 5-P timedilTerctrccs for an earihqrake: Dallas, S P = 3 min'utes i Los Angeles, S P = 2 minutes i S€n FrancFco.S-P = 2 minut€s. Use a map of the ILS. and t ra le llime .rrves (Fig. 17 1.1) 1o obLajn a rough epicenter.

Figure 18-22A plumb line ofdinaril"! hangs in a ve ical posiiion.Near a nounta in system we vould expcct ihe phmbbob to bc deflected torvard the mountains becauseof the gravitational attraction of their mass. The ol-scrv€d deflection is iypi.allJ less than expected, adiscrcpancy whose explanation led to an impo antdisco!ei-!. The diagam exaggerates the anount ofdeflection, which ls small but readil,v measLrri:ble.

THE BODY OF THE IARTH INTERNAT IROCEsSES

-{Q&tgif Manr e

Figure 18-23Examples of buoyanc!'. lcebugs and shiphulls floai because the volune s!bmerged islighter than ihe t,olume of Naier displa.ed.Simildrly the volume of relali!el-! lightcrustal rock projecting into thc dens€r nlant le prov ides a buoyant force that suppor lslhe nounta iD mass !bove,

Magn€tiz€d Moon Rocks-A Puzzle

Unl ike thc Eafth. the Moon has no planel lv idemagnetic f ie ld. Thcrc is no qu€st ion about ihis.Soviet and American spacecraft have b€en unableto det€ct such a fi€ld aft€r several cfforts. Yei magnetized rocks have been found lying on the lLlnarsurlace. Discordant data ar€ ihe stuff of great dis-coveries. and planetary scientists are vying toexplain these se€mingly coniradictor"v results.The leading hl"othesis at this time proposes thatthe Moon rocks in their r€manent magnetism''remember" an earli€r period of lunar histor-vsome 3 to 4 billjon -vears ago (the age range of therocks), when the Moon did hav€ a planetaq/ magnetic fiekL. This magnetic field implies in turn lheexistenc€ at this earlv t ime of a smal l l iquid i foncore that has since cooled and solidified. Is there abetter wa"v to manifest lh€ power of modcrn geo-logical and geophysical methods than to letch arock from the lunar sLrdac"". dale i i , mcasure i tsmagnetic f i€ ld. and then describe the physicalstate at the cent€r of the Moon billions of yearsa€o?

EXPLORINC THE EARTHWITH GRAVITY

The Indian Puzzle

Some 150 years ago during the great land surveyof lndia, € cur ious discrepancywas uncovered byBriLish sLr.vcl ,ors. Th€ distance between Kal iana,some 100 kilomeiers (60 mil€s) soLrth of the Himdla-va range, and Kal ianpur.600 ki lom€ters (375miles) farlher south, lvas delermined in two pf€-cise wa] 's-by measuremcnt over the s r face andbv rcfcr€nce to astronomical obscrvat ions-andthe rcsults disa$eed by some 150 mcters (s00 feet)in 600 kilometers. This mav se€m a small amount,but it rvas an iniolcrable surveying €rror €\'en byninete€nth-c€ntur l i s iandatd!. The astronomicalmethod of measuring distanc€ us€s ihe angles ofstars with r€sp€ct io thc vcrtical, rvhich is d-"finedby a plumb line (a lveight susp-"nded on a string).To account for the differ€nce, il l/1'6s proposed ihatthe plumb linc was tilted torvard the Himalayasbecaus€ ot thc gravitational attraction ot them o , r r d c o | h . o u m b b o h . . d . g J r , L u . i rthe dislance measurement. lvhen thc ef leol lvasactual ly calculated, i l was found that the moun-tains should have introduced an even largerer lor-one of abour 450 merers (1500 feer)-rhuscompoundng the prz,zlc (Fis. 1,8-22).

:: :805 no less a figure ihan the Astronom€r: . .1 Sir Gcorgc Airy. came forward . i th an. . rnat ion fof lh is discrepancy that contain-"d:: : rasis of the pr inciple of isostasi , discussed in, : . ! ier 17. Ai fy proposed that the enormousl)

- . r mourtaiDs arc not supportecl l ry a strong.- , l I ' p l o \ \ . l ' I l h , I . ! ' o . i ' . " s " "

: :cnser rock. Stated oth,"rwisc, the excess mass: hN mountains abo\.e s€a level is compensated: a dcf i . iency of mass in an underly ing root.: is root pfovidcs thc buoyEnl suppori , in the

- inn€r of al l f loat jng bodics, jusl as a ship with a-. .p hul l is buol. ,ed p (Fig. 18-23). Thc plumbr, i r " f€€ls both the excess mass on Lop and lhe

-rf ic i€nc! of mass b€low. henc€ the rcduccd de..ct ior l (Fig. 18-2a). Th€ resolut ion ol lhc Indian.Lrzzl€ nol only 1€d to the conccpt of isostas,v but=iso introdr{,cd gravity surveying as a method for

. i r r . - \ i - l P n , F r o L ) \ " ' f:orresponding gravi ty !ar iat ions.

Th€ local vdlue olgravi tv. g. canbc obLained fromthe period* of a s( ' inging penduium or from th€a.relerat ior of a fal l ing $,eight-an expef im€nlp.r formed in evcry elementarv ph-l 's ics course.P.ndulums have bccn us€d in $avity sur\.€ys, butLhev havc mosl ly been supplanl€d b-r ' the moderngf€vimet€r. ]his is a device no morc complexthan a wcight on a spring thal stretches or contracts as gravity increas€s or itccfeas€s trom plac€to place (Fig. 18-25). Al th.rugh the pr inciple issimple. ihe enginc ing of ihe gra\. imeter is most€lcgant: lh is dcvice, nol much larger than a ther-

I d F l e l g a i r r . r i r . " s " " l las 10 3 Earth's gravi tv (g). The standard uni i ofac{relerat ion us.d in gravi ty survNls is lhe mi l l i -gal . which js 0.001 cenl inet€r per seconi: l per s€c-ond. A modcrn gravimeter can easj ly m-"as'rre thcditr€r€nce in gfavi ty betrveen a tablc top and th€floor, €r'en thoush th.r tablc iop is onl-r or€ meterfa hcr from th€ center of thc Earthl

Graviiy Surveying

The millions of dollars r€quired to develop thesensit iv i ty achieved b"v the modern $avjmet€rl t r r ' l " t . . r i , ' n I r h " o i i r . l r .

- ' , n a 'thc direct r-"sul t of the r€cognit ion some fortyj ' , . . . o i h . h e I L P J g F o o g i , c u ' r ' . i l

TIII EARTH's MACNETISM AND CRAVIT"I

Obs.tued p nJib boi.lel e.l.nequa s .d c ! a lea aere . l . l

\

figure 18-24Thc discrepaocy between the observed and cxpecteddef lec l ion o l lhe pLdmb bob in F iguie 18-22 can berecon.iled if th€ cxccss mass of the nouniain Bcompensatcd b-v a deficiency ol nass in a "li8ht"oustal root belov The rooi prolid€s buoyant suP'pol t for the nounla in. \ {h ich othcn ' ise vould s ink

Figure 18-25The gravimetcr is simple in co.oepi but ele-gant in im! lemenlr l ion. A rnass at tached toa spf idg erper ienc.s a largcr or smal le. pul las grav i ty rar ies. Thc corr .spondinS exlen-sions or compressnrds ol lhe spfing aieneasured rerv pre.hel,v. so that small.hanges ]D gravi iy can le observed.

l\'hich oil is trapp€d (such as folds. faults, and salidc,nes) often prodLrce variations in th€ normalgravitational fiNld {hat ar€ detectable bv sensitiveinstruments. Such a gravity anomal:r is caused b"va .hange in subsurface mass due to a mounlain.oot, a sal t dome. or any othcr lateral geologicalchange (s€e Fig. 18-20). l he idea is to use i ts gfavity effect to find and describe any anomalousmass-anyihing more or lcss d€nse than thc aver_age rock and Lhus to explorc the subsurlacc gcologt'by naking $avity sun'cys. Before a profil€l ik-" that of l igur€ 18 26 can be produced, hoiv"Thre required loLin. ..n!l€1e os.illation.

ever. three important correciions must be appliedto the value oI g from each station at which thegravimeter is readi oiherwise, the most interestinganomalies would be obscured.

:1. If the Earth were spherical and nonrotating,the $avitalional attraction of ihe planeiary bodywould be the same eveq'whefe on the surlace. Be-cause oI centrifugal force. howcver, things tend tofly outward from a rotating body. H€nce, gravityis less at the equatori that is, things weigh lessthere than at the poles. The same centrifugal lorcemakes ihe Earth bulge outward at the equator and

436 IHE BoD\ or IHL rARlH I\TLRNAL pRoLl5srs

l i f i i - G dr n ed . tons d lF io! : - - - o i dFn r ! .ed men s t r r l r

---!j: - th.knesses 1 2 3 o,a

fisuie 18-26Schematic illustration of a sravitv anomaly. Thcvalue ol gravity changes a.ross the strucLDre shownbccause the less-dense sedinenls contain less massthan an equal volume of Sranite. The thicker the.Fd:mF r ld_v deFosi l nF Bre" " l " dp ' rp"" . ingravity. as the cuNes show.

flatten at the po1es. Remember ihat g is propor-tional to 1/R'z, in which B is the distarce to thecenter of the planet. Because R decreases by 21kilomeicft going from the equator to the pole ofour flattened or spheroidaL plNei. g increaseswith latitude. An int€rnational formula has beenadopted that b€si describes g everywhere on theEarth's surface, iaking into account the Earth'sshape and rotation." This formula value is sub-hacted from the gravimeter reading in the searchfor anomalies, which could otherwise be maskedby the big efiects of rotation and flali€ning.

2. Becaue of iopographic var iat ions, gravi tystations generally differ in elevation-that is, indistance R from the center of the Earth ln oursearch for anomalous subsurface masses, we mustr€move the obvious effect of elevation on thel o r d l g r d \ i l ] \ d l P r u s ' n p . h ' f d ' l \ " r s \ . r i " " a sI B r o 8 c l l h p c o r r F c l i o n ) . a , l a l r e a d i 3 s d Fcorrected as i f theywere made at sea l€vel. This iscalled a free-air corr€ction, and ii amounts to add-ing 0.31 milligals for each meter of elevationabove sea level.

3. Finally, in ord€r to highlight subsudace ef-fects, it is impo ant to account for all obviousnear-surface masses that affect gravity. The free-air correction reduces the reading to sea levelonly partially, because it allows for Lhe distanc€to the center ofthe Earth but not for the attractionof the mass oI rock between the station and sealevel. The correction ihat compl€tes the subtraction of the local topography is called the Bouguercorrection. It amounts to subtracting about 0.1milligals for each meter of rock between sea l€veland the point of ihe reading. For gravity suNeyson the ocean, where g is measured at sea level. theBouguer correction cofrects lor the low densityand, therefore, the low gravitational attraction ofwater by increasing the gravimeter reading by theamount necessary to "convert" the oc€an to rock.in this way, the obvious gravity deficienc-v of theocpar is re-oved In orJp lo Fmphasize annmdIous suboceanic masses. The free-air and Bouguer, o f l F , i o n s d r e d e p i c l " d o i a C n r r m d . i c d l l \ I n f : g -ve 18 27.

If there were no local variations in mass in theinterior, the sum of all these corrections and the

$avimet€f reading 'ould be close to zero-lherclvould be no gravity anomaly, because everything

--^

Ffee-a r cotre.l ons p . .es B : l same

j r&,.^ :iE*T:$**',

A BodqLer corecton^ ,z-/'q\\ removes gravlaliona

i / . < artraclon or mouniai.s

C;vitv is mcaiired at A and B to sce iJ there is adifferc;ce in subsurface mass. To emphasjze subsu!face elTects, coneclions are Inade to the value olgravity ai B, as if to bring B to the same elevalionas A and also to remove the obvious gravitational

" t r " c o I o l h e m o , r , " r . q r \ r d n d , r , 1 3 S r a v l \

difference beti{een A and B is asc.ibed to a changein subsudace 8eolo8y.

1By international a8tuenent the value ol s = s73 049(1 +o 1u52334 sin,,t o.oooooss sin'? 2+) milli€als, in whi.hdjs thelarittrde. Thns, gravily is alout0.5 percent slonger al lhe Nort!Fole whe.e d = 90' and sir d' = l than dt ltrc .quaior where

THE EARTH S MACNETISM AND CRAVITY 437

dnscontincntal gfavity survey tuom the Pacili. Ocean to the Atlantic Ocean The negaiiv€

s.ui;rv -.-.li"t-**ihe moiniainous regions, the near zero values at low elevations' and

ihe po'sitive values over cieep oceans, nino;ins the ioposraphv, demonstrate th€ role of iso-

static .ompensation in sha!ing Eadh's surface leatures

would have been taken into account. Howev€r.where the sum differs from zero, we have found

" n 6 r n a l o u s m a s s - a n o r p b o d y . a n I n ' - u ' i o n dsedimentary basin. or a mountain root The shaped n d r d g r i U d r o l ' n F

8 r a \ ' l j J n o m d l ! h e l D . d e -fine the dimensions and density of the anomalous

l m a g i n . d r i \ n g a c r o s \ l h e U n l e d s " r r s " o D -ping every mile to measure gravity You wouldthcrebyproduce a gravimetric profile ofth€ coun-try crossing many geotogical provinces, and itmight be interesting to see how the gravimerer

can aid us in interpreting th€ geological features.This has actually been done, and the resultantgravity-anomaly profil€ is shown in Figue :18 28,which includes an ext€nsion into the Ailanticocean made with a shipboard gravimeter. Thetop and middle sections of the figure show theg.avity profile and the topography. respectiv€ly.All the conections have been applied to the gra\-ity curve-that is, all the land mass above sealevel and all ofthe lock ofland mass in the oceanshave been accounLed for-so this wiggly curveshould refleci subsurface mass variations. Notethe n€gative values-thai is, the defici€ncy ingra\.ity-across the Sjerra Nevada. the RockyV o u n l d n s , J n d l h e B , . i r 3 r d R d n u p p r o \ n e .The anomaly is close to zero in the low plains.becomes slightly negative und€r ihe Appalachian

Mountains. approaches zero again in the coastalplains and continental shelf, and zooms to largepositive values over the deep Atlantic

The striking feature revealed by the graviiyanomaly and shown in the bottom s€ction of Figure 18-28 is lhe way the Mohoroviaia discontinuity minors the topography (except in the Basinand Range province). The gravity anomaly isstrongly negatjve where thc crusi thickens to pro-l,ide buoyant support {or mountains whai causesihe negative anomaly in these iopographicallyhigh places is the mass deficiency due to the dis-placement of denser mantle by lhe less densecrustal root. The high pDsitive gravity values overthe ocean basin signify the presence ol excessmass: dense Inantle rock is much closer to ihe surface here. This feature has b€en called an anti-root, and it mirrors the 'negative" topography(water instead of rock) of the ocean basin. TheAppalachians show a modest negative anomaly.which indicaies that they have a shallow rooiThis is appropriat€ for an old mountain systemiits root (and g anomaly) is disappearing as its to-pography erodes away. One might have expected' 1 F s t r r c l J r a l l t h r g h B " s i r " d R d n C P p o \ i n , " .with its average elevation of about one kilometer.to have a slightly thick€ned crust to go along withits negative anomaly. Actuall-\,, many geophysi-cists thought this lo be ihe case until experiments

_ g - !

89 JF ;S' - . . - - t

Bouguer gravily anomaly

THE BODT OF II{E EARTHI NTERNAL |IIOCESCTS

with seismic waves revealed the thin crust. Because thc seismic ioformation reveals that there is

t o r l h P D d \ n a n d R J n g F P o \ i r , P

u e d e d u c p l h b l l h e m d . . L l p f i i F n c \ l h p r e r - dresult of ihe area being underlain b-v mantlem a l e r , d l o t r p l d l ! a l y l o s d e n s . j " s - h o s r i nthe figur€.

Many ofthese results wcre anticipaied in Chap'ter 17, where we discLlssed isosiasy in lhe contextof the siructurc of the crusi and upper mantle asdetermined by seismic m€thods Actuallv, theconcepi of isostatic compensation wiih its notionsof "floaling" continents and still higher floatingmountains was discovered from gravity obserl'a-t ions such as these. Bul, s€ismological data con-tributed much to our undersianding by clearingup such quesiions as where ihe mass d€ficienciesare located and whether compcnsaiion involvingffustal roots or compensation via low densitymantle is the isosiatic mechanism responsible forthem. Low-densiiy mantle seems io go wilh a tec_l o n i c s p i 1 8 l h a l 1 ' u d p s - e c F n r ' o l ' a n i " m h ' 8 h

heat flow. and 1o$' seismic velocities which im-plies, perhaps, a partially molien mantl€ directlybelow ih€ Moho. Some geologists suggest thatthese features. as they occur in the Basin andRange province, indicate ihat tension-producingforces, perhaps due to a spreading or divergencezone, are activ€ within a continent Compensationinvoiving crustal roots is th€ predominant iso-siatic mechanism for continenis as a whole. aswell as for high mountains.

In this wa-r', gravimetry and s€ismology combin€ to r€veal the importance of isosiasy to ther l u o v o l g F o , o g \ o n a r p g i o n d l a r d ' o n i r e r l a ls ' o ' e . t h " " o l ' o u : n g F \ a n p l a \ s h o t { t \ h a r L i r d s

of insight the combination can ofier. Eroding

mountains wiII b€ pushed up by the excess buov-ancy of ihe root until both rool and mouniainrange neariy disappear. Such loads as ice caps or

sediments filling a basin can depress the crust'Wher€ gravity studies indicaie thal a positive or

n€gative toad (a depression. for example) is not

comp€nsated, some force must be present thal

helps support ii and keeps it from subsiding or

risins. Continents are unlikely to be destroved bv

subductionr the ability to float allows them to"ride o t ' repeaied episodes of splitting and colli-sion.

The Fennoscandian Uplift-Nature's Experiment with Isostasv

If you depress a cork floating in water wrth vourfinger and then r€lease it, the cork pops up almost

Th-e nechanism of lostslacial upLift (a) A continent- l l lauFr sross, l^da ns fF c-u. lh) " r " r r ' l

" o e J " . o i ' o o d c . e ' o p . t 6 . x 6 p ' , d l i a d i q o ' r l _i r ; l l \ . r . r

- l e s l d c r " ' d i q J p p " " r . h u r l l F r u n l r " _

- . , i t " , o u . . " o , \ . c o q t t , t n a 1 . F I r , u Fevjdenccd by ncsative sravily anomallr' (d) Buo"lancvof the root leads to slow uDlift The root dlsappcaN,L l p ! J r l o , p d s s u m F ! i s u , E i n d l l F r F l d 1 d r h p c r d " l J

aoom"\ oisrop'a" qrrous d"pi ' l ' " d _c o I o 'forees riue to ice load and rool. FennoscaDdia todavis betwee. stases c and d FigLtrc not to scaler the. ,st is aboul 40 km thi.ki a 3-km thick Slacier$ o r l . E r o d , , F a r o u r " b o r r \ n r 1 r ' l l d s c I r \ p

" e , F l h o l h , c u q D r p o o , r d o o d _ o l h " s p i S l

instantly. A cork floating in molass€s would rise

more stowly; ihe drag oI lhe viscous fluid wouldslow down the process. lf rl'e could perform a

similar experiment on the Earth, we cofld ]e'rn

much aboui the viscosity of the mantle and how itaffects rates of uplilt and subsidence How con-venieni it would be if w€ could push the crustilown somewhere, remove the {orce. and ih€n sitback and watch it rise.

N a t L r F h d \ h a e r g u o o e ' l o . g \ l o d o l - ' . e \ o P f i -

m e n t l o u . . T h P l o d o . d , o n i ' " n r a l S l " " P - " 1

ice she€t 2 to 3 kilometers thick that can appearwilh the onset of an ice age in the geologicall-v

shori per iod of a few thousand years The crust is

depressed by the ice load, and a downward bulgedevelops on its underside to provide buovant support. At the onset of a warming trend, the glaci€rmelts !apidly. Wiih the rcmoval of the load. uplift

: : : ie depr€ssed crust begins (Fig. 18 29). ' Ihcrate:: rplift can be cLocumented by dating anci€nt

, p r r \ b d , . 1 . t r s , . l d r , , . , , , a . " , . , G F " t g .

: - ; ) . Such raised b€aches can t€I] us ho ' Iong

:-r a part icular s iretch of land was at sea ]eve]. Ar-:at ive $avitv anomaly tel ls ho$.much of a. : l ic rooi r€malns and hou much more upl i f t wi l l

:cur before lh€ rooi disappea]s.Such depression and upl i f t has occurred in

l , - . vay, Sweden, and Finland. as wcl l as else-;rere in glacialed regions. Thc ice cap rctr- .ated:Dm these regions some 10,000 years ago. and therd has be€n r is ing since. Figufe 18-30 shows

:r1r much up\{arping Look place ln ihe past 5000.=.rs, Thc mosl in lcnse uplvarping has occurredi:ar w€st-central Srveden, rvhich is believed to-: \ € been overlain bv the thickesi ice. Some 200reiers of uplift has occurred in 10,000 years, an.rerage rate of 2 cent imeters per year. The re-:raining negaii'.,e gaviiy anomaly of 50 milli-.als impl ies that part of ihe root st i l l remains andhai about 200 mcters more upl i f i must occur be-:ore isostat ic comp€nsal ion is complete. A fewrinor car(hquakes occur in th€ region, addit ional:1idence, perhaps, that stresses due to too much!uoyancy are st i l l present. Geophysicis ls havers-"d th€se data to show ihat the l\reak zonellhich "flo1\,s' so Lhal Lhe crustal root col d de

r '€lop) coincid€s with the pa ial ly molten asthenosphere. This r€gion is important not onl-v as afactor in ih€ mobility of ihe lithospheric plat€sand ds ihe soulce of basaltic rnagmai its abilityio -vicld makes it th€ key factor ol isostatic com-p€nsat ion.

THE TARTH'S MAGNETISM AND CRAVITY

Figure 13-30- r " u p ' | ( i ' | ' - t ) | . F ' n o - . r d "

i , \ , D d '5000 years, according to M. Sauramo The crust, de-prcssed b- ! the aeight of the ice cap of the last iceage. is s l i l l rebounding somc 1o,oo0 years a l ter the

\ o h r m a n . o . l J h b \ . J s g - ' d d , ' r r F r F \ p e r i -

m€nl to demonstrate the isostatic mechanism thannature s demonstration with the Fennoscanoratrpostg lac ia l upl i f t .

439

SUMMARY 1 .

2.

Motions in thc f lu id i ron core someho , set up a dynamo act ion ihat generates theEarth's magn€tjc f ie ld. Thc f ie ld can be fair ly lvei l dcscr ibcd by a hypothei ical barmagnei local€d n€ar lhe center of the Earth and approximat€Iy aligned rvith its axis of

Manvrocks became magnetizedin lhe dir€ct ion of the gcomagnci ic f ie ldthai prevai l€d'!vh€n the_r wer€ formed. If the rocks are dated radiometrically, the histor,v of the mag-n-" i ic l i "" ld can be recovered from this remanent magnetizal ion.

Th€ remanent magnetizal ion of old rocks sugg€sis that the Ea h's magnetic pole occu-pied dlfferent positions in the past. Actually, th"" magnetic pole probably did not wan-der but stay€d fairly close to the g€ographic pole. The appar€nt polar wandering Lsprobably an indicalion thai the lithospheric plaies have been moving, changing th€geograph-! of ih€ surfac€. Evidence such as coal in Antarctica and glaclal deposits nearth€ equator support this idea.

Remanent magn€tization has also led io ih€ discovery ofreversals in th€ magnetic field.Th€ history of rev€rsa1s sinc€ Cr€taceous time has been lvorked oui, Although unex-plaincd, these revcrsals have become a very impoftant tool in dat ing the sea f loor.Wh€n th€ sea f loor crusi is formed at mid oc€an r i f ts, i i becomes magnetized Thrs

44n THT BOD\ Of THE EARTH INIERNAT PRO([95E9

5.

magnetic imprinting sLays with the crust as il spreads away ftom the rift. The sequenceof reversals sholvs as positive and ncgativ€ magnetic anomalics. which a suNeyingship can readily detect. Using the history of reversals. we can determine thc age ol theundeflying sea floor and the rate of sea+loor spreading.

cravitaiional chanses over the surface of the Earth are due io the planct's oblateness,its rolation, its topo$aphy, and differences in ils subsurface mass The first three fac-tors can be allowed for. so that the rcmaining gravitational anomalies indicate subsur-face geological ditrerences.

Gravity anomalies associated u'ith continents, oc€ans, and mounlains show that theEarth's crust is not strong enough to support topographic loads ovcr long periods.Roots, or dorvnward bulges of the crust, develop and provide buoyanl supporl This isan examplc oI isostatic adjustment. Another exampl€ is the depresrion of thc Fenno-scandian usi bythe weight of a continental glacier. Alihough the ice cap disappearedsome 10.000 years ago, uplifi is still continuing in that srea and will conlinuc until therel ic root disaBDears.

EXERCISES

I What evidence supporis the hypothesis Lhat theEadh s mdgnetic field oriSinales in a fluld iron core?The renaneni magnctization in some mctcorites andMoon rockshas y€t to be ex! la ined. Would yo! hazard a guess ds to its oriSin?

2. In a region ('herc the geothermal gradient that is,the temperature incrcase with depth is 3"C pe.hundred meteN, al Nhat depth Nould you cxpectthe rocks to lose theif magnetism?

3. A sedimentary formation is found to have remanentmagtretism, ivith an inclination of 45' measufedfrom the bedding plane. What was thc laiitude ofthe formation when it became magnetized? Whatwds its longitude? World you answer.hange if thebed had bccn tihed ailer ii was magneiized?

4. What is the connection betweeD thc sequences ofmagneiic revcmals worked out on land and the

bands of posiiive and negative magnetic anomalicsfound on lhe sea floor? Therc are regions ol the seafloor known as 'magnetic quiel zoncs" Nhcre noreversals in magnetic anomalies oc.ur. Can youguess lbe age of the crusl in lhese rcgions tuom thcmagnetic feve$al time scale in Figure 18 20?

5. Would the gravily anomaly (after making ftee-di.and Bousuer corrections) show larse nesatilc val-u€s, values near zcro, or large positive values at ea.hol the followins llaces? (al Rocky Mountains.(b) East coasi of the U.S. (.) Middle of an o.eaDbasin. nxFlain yoitr answers.

6. You obse.ve a near-zero gravity anomaly atop amouniain alter maklngthc free air and Boaguer correclions. What a'ould you conclude?

BIBLIOGRAPHY

Carr igan, C. R. , aDd D. Gabbins. ' lhe Source of theEarth 's Magnet ic F ie ld, ' S. ieni i f i .Ane. icon, Ieb '

Cox, A., cd., Plote Tectonics ond Geondgneli. Reve.sok. San Franciscorw' H. Freeman and Company,1973,

Garland, al D.. The Eorih's Sfiope dnd Grdvity.New York: Pergamon P.ess, 1965.

Hejdzler,l. R., "Sea Floor Spreadins," Scientilic Amer-icdn, December 1968. (Ofrpdnt 875.)

LePichon, X., J. Francheteau, and J. Bonnin, Plote Tectonics. Nei! York: Elscvie. PublishinS Company,7973.

Strangway, D. W', Historl, of the Eorti's MogneticField. Ncw Yorkr Mccraw Hill, 1970.

Takeuchi, S., S. Uyeda, and I-I. Kanamori Dcbote oDoullie Edrtfi, rcv. ed., San ftanoisco: Ff€enan,Cooper & Company 1970.