ch chpt10

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10

Putting It All lbgether

The goal of this book wd5 to sho$ lou how Io do fie brsicoperationsof plate lectonics.Hoselcr it would be amazing

(and disappoinliqq) il at this sta€le.our curiosity hadnt

carricd \,ou befond the techniqucs ofplate l€clonics to deeper

scicntificquesrions In rhis rst ch?pter {e \lill addresssome

of rhese, beginning $'1th the question of what drives fie

plaresand going on to a lew other topics ofcurrcnt interesl

What Drives the Plates?

Ahhough almost ever-r'oneagrees hat the processrespon-sible 1orplalenolion is somcsort of rernul convection,

there are rwo schoolsof thoughtabout he nature of the

co ecdon Onc schoolholds har be plales idc aspassne

passcrycrs n manlle convection cllssith dimensions f

scvcraldrousand ilomet€rs. hc locationand ife q'cle of

thesecells s determincdbv processesn the nrxndc as a

whole and not by re geometryof thin plates iding on the

surfaces f rhe cells.The odrerschoolof fioughl holds h.t

rheplatesarenot p3ssive assengersut are hemsehcs n

xctivepart ol the convectbn pfocess v'e e ill .onsider each

\N$,,r\r.\r\ in r tn.ln!t \hrn .h,,q $h\ n,u{ geuth 'i Ft:

l-avot \e sccont\olrt331

7/27/2019 CH chpt10


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Figu.e 10.2.

;::i'jJ:,::^-,-".",.,,\e..,is,rb!,ed

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tcr \ t i f \ r Lnn\rder hecu. n,1e, , un. ei l . . ,^ ; ; ; ' " ' . ; : " ; l : l :h"

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WhatDriLesthe Plates 339

laterxlly y mande los (ligure 10J).A-longie des.rnding

limb oi a cell,snyalonpiheJapan rcnch, he ithospheres

draggeddo$n bv mande culrenLs,causingsubduction This

was he basirconvecrionmodel edvancedrior to platc ec

onics by cading cientistsikeArthurHolmcs,l)avidGriggs,

and arry Hess,The siruauon s not changed er-v much if the flo\r of

material hrough he whole nantle ls blockedbv densitv

laledng, :rssomeg!'ophvsici$! hevcsuggested.ou :n mcxlel

this t,!pc of convedion b,vpourin!! some wxter and oil inlo

a pan,allowing iem to separatento t\lo la)e.s.and then

hearinghepans'ith a lameat hc center ithc pan APlunre

of hor water moles up lhfough dre centcr ot ure \| ater la]er

and then larerall,voulward dong tlle oilwatcr interface A

plLrme f fising hor oil lormsabo!'e hc fisingplumeoihot

sater andnn)vesaterxlltout along he top of the oil la,vcr

In the mantle, hermalplumcsmav similaflv ise drrough

fie enftc manlles'ithout he exchanllc f material etseen

the upperand owcr nantle.

In bo|h of thesepassivemodcls. the lo.ation of rid€lesandtrenchcs s determinedb,v he size and locationof deep

nlantle conveclion cells lddgcs form along thc up goiqq

limbs of conveclioncells,sub.luaion .rkng descendingimbs

ln othcr $ords, convection €llsarecauses ndridgesand

rrenches are effects.The dislrnce between xdjxccnt ridges

andtrenchess determined -vhe ch3raderisticcngthscale

of the convedioncell.This distance 'ould be expecteclo

remainnearl-v onstant uring he lilt me ofa cell

Now lefs considera secondconvectionheor! in which

the plate is an ifirinsic part of lhe convectioncell and not

aparsi\'eprsscnger.Recall hat the lowcr boundarv of r plate

is sidplyan sother.ral urfaceThrceproperlies istinguish

rhe lithosphere nbove iis isotherm from the asthenosphere

beneath:The lithosphere s colder, t is stifttsr n(l more

viscous, nd t is denserhan he asthenosphcre ecause i

itsgreatcr ensiry.he ithosphereends o sink ike hecold'

upper laver of xn-vconleding fluid Subduc )n occurs not

because slab s pulled downb-v ie deme,sinking imb of

a convection cll bul becausehe slab s fie dense, inking

limb of the celltsecausehe ifiosphcre s somuchmorevis.orG han ne

asthenosphere,convectioo in plate tcctonics resembles he

motion of platesof icc o\'er a pond more dnn ir doescon-

vedion in a uniform fluid like watef In fad, if ice were

dcnser fian water, Ihe analogv would bc al'rrosl perfect

Figurc 1O-3.

Mamles'id. conrecLion rodu.i.8 ri.lScs

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vhat Drftr\ tlx Plates 311

rlv$su8 tnar$. They f,fe fed l)Y nraAm?coming not from

i ser oi 1)ffser onvcctive drrcnls irising from deep in the

manrle bur rether from shallolv soufces in nearb]' parts ol

fhc irvhaDospherc.

Second rest: Jut tptng and Pro,agatlng Ridges

lddgessometimesunp lo new posid)ns or rcasons txt

arenot n'ell undersroocLut whichappear o efleci hxngesin rhe dircctionof tlate moaon. SonFtimes he drstance

Jumpeds short,sometimest is long. Oitcn I riclge ith a

Ieu ufrcntati.r'prup4Jr( LAe d u$ine . ra.k rrr ' IFFinr

near r dying ridge, replxcing he old ridgc Generill,v h(

orientxtion d re propagating ricige is more nearlYperpen

dicular() a nes diredion of sprc'adinghan s'asdre old

ridge.Thc first model of convecrionequifcsdlat each ridge

junrp reflecrs a clraoge n fie pirttern ot coN€ction: A con

vcctioncurrent, ising from deep n drc mantle,dies n xn

old locadonand a ncw convection ell iorms in dre nes'

l(rltion. The obsention that ridgescommonly ump at timeinter\elsoionh a a fe*'million years ould require3 phls

icallv mplausibleerk-v rodcof convection

The secondmodel of convec )n, in $'hicha ridge ssim

ply a cfackbeMeen s'o di\c{ling plalesnlo $'hichmagna

floss, offersan enplarationhat sph,vsicall,voreplausible.

Vhen a fidge jumps, he crackbctweendivergingplates

simpl_vumps o a newposition.Propagatng idgesaresim

plv growing cracks s'hich, Js the,vpropagate, tap nearb-v

magma ourccsn the lhenosphere.

Tbtrd Test: Ridge Meets Zrett tr vhat happenss'hen a

ridge mee1sa lrench? Thar ridllcs do, in facl, nigrxte into

trenchesss'ellestxblishcd theplale ectonic ecord.For

exlrmple, 0 milln ),ears go he }arallonplatewas subl

ductingbeneathCatiforniaInd hc PaciJic-Farallonidge a'

on the far side of rhe larallon plateaboul500kilometers

from drc subduction zone. The o.ean floor record shoB'ri

rat cluring the nel1 20 lnillion .1,earsre Pacific arallon

ridge noved eists ard rapidly utuil iI wrs fairl-vclosc to the

n-ench, t $hich time ocean ]oor spreading asstill going

on at e viltorous ate When he ridgc n1ct lte trench.sea-

floor spreadng xnd subdu.li()nboth stopped.wh,l was going

for the irsl modcl ofconlecion (t-igurc10'i), dris boils

d()sn to the queslion, r(h.rthappens\hen dre fising con-

Firsr mo<lel:A-r idge oigmres ro rferch. hotasce.ding nd cold descendinSnbs of

deep uantle odvectlon ur.cnr cnrerge

7/27/2019 CH chpt10


342 ?uftnry ft Au n)gett)t

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Vhat IriEs the Plates 343

la)cr floil''As rp]atemovcsover he viscous sthenospherc,

a bound y leler of eslhenosphereornrsnext () h€ plalel

lvithin dris a)ef,velocit-vlecreasesrom tbatofthe plate tr

tharof the asftenosphere cneathdre a\€r Next ets con

sider vertjcal low lmaginea Rorid encjrclinghorizont2l

surfacc<ratcclatadepth f100kilomete.sustbeneathhe

l)asc or rl'L li,ln,rpl,r,c ard $k -yoursclf Vhere does flow

rakeplace across his surface?One place s subduc'tlon ones,shere large olumes of lithosphere rfeobtiouslt flosing

doqnward,eccompanied v a boundar)1arerof asfieno-

spherecarried ong b-v iscous rag.A secondplace s ai

.ises:r\sdrenosphereust loF up$'ffd acrosshe m.ginary

lrr i /nnr,, l .ur ld'{ i r I l ,e c8i-n $hcre uldr( J ( i rosir ' j lnexrriscs. or example,lte volumeoflhc cntireplate$at

underlies he Atlxnticbasin hd5 losed up$ard from $e

lower asthenosphc.ct the Mid AtlanticRidgeThird, here

is rerurn flow through dre ,sthcnosphere,\'hich carics

aslh€nosphere isplacedbv subdudrng idrosphereaI renc:hes

all over the world to ridges wherevcr the-rare located. The

resultng loB pdtern is not fie simpleonc of\€ll delinedconvecliol cells ike those n lriSure102 but morc like ihe

c.omplex o\l'ln xn aquifcr 'hich is beingpumpedat manv

{ells andreplenished t Dranl' ources.

Drlt)ing Forces

l\vo appfoxcheshavc heen Lrsedo anal]".e mantle convec

don. The first is to developcomputercodes o solve dle

differenrial quationshaidescribe on\ecdonn afluidwilh

disrribured heat sources and \\'ith a rheologr that varies as

.r functbn of tempefaturc. The second appfoach,s'hich is

more aruned o the ntuitiveapproach f rbepfcscntbook.

rcllerdsa plate ir! a mech1rnicd bjcctsubjecl o sressesr, r i ,q dl^rg I . "rrfu, . r l i tJ '( l " ' ' ' ) . The.rre*e'J , r r ts , l

dilferenr pffts of rhe platc are then anallzecl separatel-vn

order to assesshe fclative mportance f ridge push,slabpull, and other potentialdriving foroes.This.pproach, drough

less rigorous than m:rfiemadcal xna\'scs of the tou.l con-

!€dion process,noneth€le$sfers some ntere$ing nsigh6.

Mantlc Drag force FD. The forc.c(or more properht

the tractjon)actingon lhe bottom of a plate, ermed hc

nrantle dtag force or4. is due o viscous ouplingbe6,"een

rheplateanclhe asthenosphere.hc magnitude fdre man

de drag brce sproportbnil to the areaofthe plate.nd its

7/27/2019 CH chpt10


!!L!!::!t! AI tuselhe

Fiaue 10-6.Ibrcesa.rtngon ptares j;l':::

fi iJ ;i:::: :jhen-.phcrc,,,J,c.,e p,r..,,

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7/27/2019 CH chpt10


vb.lt Dtues bePldtes 345

x plate s proporlonal to tlte sine of thc slopeofdlc ridge

at thit dislanceThusridgepush orcesaregen€retcd ver

thc cntire n'idth of e rise, bciru greatest ear he crestof

thc rise rnddecrcasingaterall) o zero vhcrc herisemeets

SL.b PuvForceFtp Tlt slab pull force Fi? arises ecause

subdudingslabsare denser han fie r5lhenosphere nddrcreforesink through t. The expression pull does not

mean ral lhe lidr()sphere djacenlo the trench s under

rcnsion n an absolutc cffe, bul ralheftluta!thc slabsnrks,

rhe srr€ss n dre adjacent late s less han he lithostad(-

srressharsoukl existh a suric horizontalplatc due to ils

o\{n mass.The slabpull force is produccdb the densirv

contrilsr betsecn a slab and ndiacent .sthenosphere. 'his

densit_vonrrasts smallaI shallo$'depths ccauselle tem-

peraturediffercncc s smallbeN'een he slah rodadiacenl

mxnder I is greatest t deprhs f 200 o 300kmi and l dies

our xt depths of 700 to 1000 km ll5 the slab hcats to the

remperatufe fthe surroundingmllntle.The slabpull forceis greaier of older hao or youngerifiospherebut appears

ro be independcnt f the €locirl ofsubduction

Sl4b Drag Fotce FsD In addition to F.r?.which is inde

pendetuof thc r3tcof co €rgence,Fe candefinca sccond

velocilv-dependentcsistantbrce,l57,. tich is due o \.is

cous drJg a! rhe slirb descendsnto fte mantle ll as seems

likeh,, he riscosiryof the asrhcnosphcr€s much ess ian

that of the underl-vingmesosphefc. sD ould be greaiest

near hebouomof slabs 'here hc) buft rgainst r peneirate

i o re mesosphere.

Tr..flsjrrrm Fault Resltttrrcefr. Seismicadivit-valong

transf$m faulrsshows hat iiction is presenl ls plates ryto slidcpasteachother.producinga resisti\c orce. n most

sdck-slipmodels or lhe lcncrltbn ofearthquakes,aLllting

ocrursBhen a critic.rl vieldlresss reached.'l'hisressandtherefore fie magnitde of $e transform fault rests-

tance is independent of .he relatnc vclocit) of the plates

on eithersideofrhe transforn.

CoUAtng Reststal,nceFcF Shallor' sebmic acivit-v xlonSzonesof plateconvefgencendi(ares hat fri.tion bctwecn, or^efgrng lJr<. c rs . ' r ' i \ r rheni ' e-. ol . .nlersence

,dsnas truc for ransibrm faults, n most stickslip moclcls

7/27/2019 CH chpt10


346 pudiflatuAI Tbgaher

ik?if:r,xt,:i*:l:{SfiH

-N*r*$,ffij*i+fl+I,f*,lffi

*l*a**NfjrlottonRek fia,e to tbe Mar. e

**+***rWfi'n;,'*ffiif#*,fi,';

.ffiffi

7/27/2019 CH chpt10


what Dnues be Plates 347

100

ASEA

/0" knz

Ftgule lO-7.Toul Jrea ot plate !€rsus absolurcveloclry

Otlocities are basedon 1918eni.le by

Clement hare istedar end ofchapte.)

50

oa /40

nh/t /

rhe mande thirt interac't$ ith the lithosphere is zero. Differ'ent wa]'s of definng such a reference frame are discussed

in the ne){t sec.ion.Thc one used in re present annlysis s

essentiall_v ne in which it is assumed hat hotspots risiqg

firough the mantle dcfine areference rame fiat movesv{ith

the melln ve)ocit-vof thc mande Valuescited for meanplate

!€locniesh the follosing discussion ere found by aver-

aging over rhe surface of individual platcsthe scalarvelocity

of pointson theplare elarive o hismantle eferenccramc,

Vel4rcltJt Versr6 Plate,,l/eka Plo(ing the areasof plates

againsrherrmeanvelocitiesFigure 0 7),wesee hal here

is no correlaLion erween hem. what conclusion irnwedraw from this? one that can (and ha.s)been drawn is that

thc passi\c cot]lvedion model discussedearlier is correct.The reasoning is as folloqr. The Prcific, Cocos, and Nazcapl:rtes re similar n the followingwafs: They are all com-plctel_voccinic, their boundaries all bave aboul lhe sameproponion of ridges, renches, and transforms,and they areall moving at aboul the samevelocity. In fact,$c only way

in which .her-are signifiontly ciiffercnt is in their areas, he

ratiosof which afe 50:5:1or Pacific:Nezca:Cocos.ince hemantle drag fbrce F;r ispropordonal Io plate area, he dr.rgon the boftom of rhe Pacificplate is 10 times greater thanthat on the Nazcaplate and 50times that on the Cocospiate.

Therefore$e driving forcesofthc threeplatesmust be in

7/27/2019 CH chpt10


34A puftin? t A/t jb4aher

rhr. farr, , 05 I tr,L henlatrs , . I h

il*liiil{ii."#}.r{i,:;it$;];:'i:J"'ili_'t,::l:it"1li:11l:i,l

#::,TjtijJ1l1:-1":l*. ;lilil;;i;ilJ'i;:::,;.,f,,,,* ,*:,,:,i ii:iti:;,9,11,11!.11{'1';::1fiilHttjr.:isfi :,r],,::til...i;":;i;;1;11;i__mi:i,iril.rr.#"lrnrufi

i,il.,;-ili:r:it'f:..:;'li"ll"-: ::' ;-,,,;1J,1",'"i,1r."*,",, . ;";,;l;,1;.J)ll,#i|":;:j:rel:r#L;.11*rr.ft; :;ii:iH,'r,il;fi* il.:rx;fi#t ii"$,;li: l;i ,Y::r?:,.{i::2:: : qi,,uush,^

Iiin ^"r,n:f'*.;#*tii*:l'i'il,,,,ililJii{:,;lj*n];::mr:!-rl**rjr# i;s::n;h:iliiJ':fr:r,*l;ni*t,:;:

7/27/2019 CH chpt10


wbat biLes the Pkdes 3i49

/002

Fo7"

o7"5A - loo

! nn/y,

In Figure 10'9 $e see no correlation aI all berlveenvel(xit_\'and toral ridge lengrh and at most a weak conelationbeNeen velocirv and effecdve ridge length. If we pick 50km/my as he boundarv between slow and fastplates,mostof the fastplateshave orsofridge. The one exceprion s rhe

taoz

5az

Ftgure 1O{.Tolal lengrh of rransform boundfy

expressedaspercentageof total cifcumfer

FtSure 10-9.l-ength of.idge d percenrageof pbre circumference ersus absolutevellxly. Dshedline is total lenath,solid li.e is effecrivelenalh.

a

o@ 5a - loo

6t

i

i iii

ii

rii i ti l !

HIi ; T

7/27/2019 CH chpt10


35O hning I AU tAetbq

Phi^Lp-pheslare..which, buunderlld.rtv b, uen(nes. \FnInough r mighr e ;rgued har heveltr r\ . , f rhr.pldtc ,

n: ' ] ld", , .* *1,"9,hdr ba.k.ar. prerornsrn rheNJFrana sland5 rovrde\ hr\ pldreq irh .r ;gmen, ,,l dae, hr .

l laresul l el l .u\ lhJrpl /re.do norner j r, ,nate one rdgeDoundaries n order to be fasr

ue!:!A yers6 lenstb oJf ubduc ngsbb rhi, colteralron,(Figurel0- t0, i , r - renl jns dr;m the ptr le\ ofrne e.r f lh hl l in lo. qo gruut\ . or i . l | ra.,co ru \un tu! nBsi.rD,oler d targc rJ(.rior, t ,n.i. t .rn.t".i., rt," ",t ., .,^i

,'lT F,lI -"-:,r::'p ub6 namer'hc (n, \.yLJ,

i,:''.:it:'i,i #i,'t,;lil.;{:t:.Jilff:],rateson earth The other platesare nor connccreo o do$.n-8o^mg-slabsnd havevelociriesof less han.10 mhv lhescresurs eaveno doubtrhatslabplrll is an mpoftafi drivinla

UeLritJt-Uers-rtt

orrrtnenlal Area qfpr..ra pt,)te-\th

:T]TnTareasfea,erhan s mr ion quarebJomere^,

liil".::i-.,f^,lIg,n i,eri{:.sou,hmer,.,jnlrri(J,

lnc|lie. Ie\s hrn 25 km m\ ( figufe,u r I t i \ r ptarc\ r lhose wirh \eluci t re. SreJlcr t , . rn 5n

loaz

5O7.

ozFlaure to-ro.PercenBgeof ci.cumference of DlarerftaLhedo doqnao,ngst,rboaitre6Ln. i. ,rorai engrh,old trneb effecuteen$h

o 5 0 - 100ntu/!F

d

i isi

:5

7/27/2019 CH chpt10


vhat DriL6 tbe Plates 357

ta0

la6 kaL

rtgure 10-11.CioDrnrcntal rea ofplares versusabsolute

50

oa 5 a - too

km/my)are completel,v ceanicSo in general, conrinentality appearso be associaled ith slowness. ur et's ooka ittle more closely The India and Antarcticaplateshave hcsamecontinental rca of 15 million square ilomere$,yetAnlarcticas slo$.and ndia asr.Moreover heoceanicCxr,ibbeanplate sslow So rhe correlaton s oot perfect

Se\reralnterp.etations a!€ been madeof this correla-tion. One is lhirt rhe manlle drag forcc is a resistive forcethat is s'eak benerth oceansand strong beneirrh conrinensAn alternative nterpreradon s rhepassivcmodel of conlection disd$sed earlier:Plates rc coupledstrongly o deep

convection urrentrand aredrivenby rhcmiconvecdonurrenls beneath ontinen.al lates impll,happen o be slowat the presenr ime.A third irrerpretaiions rlur piarcswithlargccontinenml rersareslowbccausehe_v.1oor happcnto be attachcdo subdudingslabsat rhe presenr ime.vetend o ialor this ast nterprefiron for re;Lsonshat aredis,

A Model jor Wbat Dr.loes the Pl.ates

The followmg modcl fits most of dre abore obscrr,nions.Staft wi.h a plareresring on an asrhenospherewifi viscositvso low that mandedrag, he producrofviscositvl]nd plate

area, s small; his fce$re of the model e'plains why largc

{

7/27/2019 CH chpt10


J52 ruftina h AI Taaethel

,[i,":itl:if.:l,l;; :l* "friill*^':*^',,;ffii;^iitJ:l;;:'.i:l:;"{:ilIfi] ll;j_,i;;x1,,l *-l.,*-,,.,,rr

,ro",.,n;,;:;;j,,**,*l,i;;i:;:ul*ffi?r#:l'i,r.,:,#1;r,$ljn*

f illllliii;i;1pl'nlii:if):i:{lr.tl,,.;:.#**tjy; [:il ;.j:''g:",i.::ll:"j;J;*j:;jili,:i:,..;l"i"1i::l, ::

l'""^n'rph.e

L,h

ililffit,lt1''##i**i-'il:{:fl}tti*;;i'

't#illti.l

;l?*Lu*rtnl',#ii[,H

Id [n#?f,'ji, ,:;r',,:,1r1',y;, 'J;ilT,r#: liti:: .1onuro' nr,r,er11,L'. ra;c

*,';*;:'l'"*iJli.,"n"-iil::l'j lll f; ';',;:ri';ili'::l: fl+]iiiil :i'ti:i:;i:::.,;#;",:;';:t:T:";thi;';:f l:l;T;lli:;*:".:t; ?*; ;.1",,,',"

:ilit ;:1i:iJrll1,:"r:* :* jli*i

7/27/2019 CH chpt10


\xlbdt Dtn€s $e Ptat^ 353

dcscending limb d a coN'edion cell. a processanalogous

to slabpull is mkingplace.

To complete he model,put e dozcnplaleson the globe

end attach ubducting labs o half of them Thesebecome

fastplaies.The othcrs are sloF or stationar,v

lhis model doesnot explajn$'h-v latesike NorthAmcr

icanot aftachedo subductinliplatesremovingatxli.,\ppar'cnrlyslrb pull isnt thc $hole story Anotherdrivingforce

thar may be signilicant is ridge push. Recall hal e ridge is

an enormousmJssof rock risingseveral ilometers bove

the ab,\'ssalcean loor and recallalso he earlieranaloav-

with an iceghcier ofsinrilardimensions:he orceex€rled

on an obstruction at the base of such a flowing body is

subsrantial. here ridgessufound most of a plate,as in

Anrarcicaand Africa, he forcesof ridgepushon opposite

sides f the platearenearl-v alanced nd end to compress

thc continent The efiecive lengdr of tidgc. $hich is the

unbalanced omponentof fidge push,pfobabiy mpartsa

velocirl albeita low one, o the plate.

Mostofrhe fastplates,ncludingPacific, xzca, ocos,and

India,e\Txrriencexlth pulls ftom suMucting slabsandpushes

fronr loig ridges.Moreover dre direcion of the loft:luevec_

tor produced b-v idge push in each of theseplates s nearlv

par3llel o the rorque cctor producedbv slabpull Since t

is difTicrh o determinehc conribudons o plalemotion of

forcesacting n the samedircctlon, he relati\'emagnitudes

of fidgc pushdnd slabpull arepoorlvdetermined

A processakln to ridlle plrsh occurs in other tedonic sel_

iings. Thc ridge force rcsultsfrom uplift of fie ridge caused

br rhcrmaleipansi()n.which s accompaniedy an uPwarp

of the isothenn aI the top of lhe asthenosphere.n essence

thc platesli.lesdosn this sotherm. his conditionappearsraeu,r Inorhu Fr. ruc l r rhcBr!n JnLli rn8( rn\ Ince

ofscs|ern Nofth America,he Fift valleyof EastAfrica, nd

ma€inal searbehjndman-vslandarcsThe resuh s rifting

like ihat et ridges.oftenaccompanied v olctnism, but spread

ofer a broader a.clt, reflecdng a broadcr upwarping of the

isodrenr Jt the top ofthe asthenospheren theseareashan

at ridgcs. The ralc o1 clivergcnceacrossthese dtu is less

than thal acrossmosl spreading idges ln E!.st frica, lbe

rate is slow becauseridge push is exerted on Africa from

both hc AtlanlicOcean ideand he IndiaOcean ide lf in

rhe future one of the oceanicplatesxttached o either side

ofAfrica begins o subduct, he EastAfrican dft will undoubt

cdl.v become a fulffledged ri.lge spreadingat a mpid rate

7/27/2019 CH chpt10


354 Puttina t AI ,tbgcth.f

Fip$tilhfl,,#*I*rr"

*fl;lut*$n[#il::ffii#:ffi{+ilr**rn*ffi*l*ffi{ililj

r'l'ril:ilJffi[t,il*;h-X=+,*,;,,.mAbsolute plate

Motion

$1[i[9,**fuilri:;ilf,ili ,:t,,iffi;i*a;qu:;:;' : : #;"

*"'' ;'ftJ'jil*'j:

i,f,fi"t,J,;:l::::las n, s,Lre.osDir

,l-fr,t+:,'ffih#tf

7/27/2019 CH chpt10


Absolute LaeMonorz 355

platevelocities measuredusing the grid systempainredonand moving with the mesosphere.

What do absolureplare velocires do for us on planet A?First, some of the forces acdngon plates,for example, thedrag orce, require that we know the velocity relarive ro rhemesosphere;his is given by the absoluteplatevelocitl,rSec-ond, let's?-ssumeharon PlanetA m(xt ridges

havevery highabsolutevelocides whereasmost rencheshave ow absolulevelocities or are $arionary. Relarionships ike these, whichalso happen to b€ valid for PlanerEarth, are obvious if seuse absolLrte elocities but obscure if we use relative plare

Planet B This is like PlanerA e{cepr rhat the mesosphereis deformable like tar Subductingslabs can penetrare inroit, but the ra.e of deformation is sluggish, so afrer a timegrid lines painted on the mesospherewill be disroned butstill recognizable.Le.s define our fixed referenceframe onthis planet by a perfecr set of laritude and longirude lines

which align themselves o fit the old, deforming grid sysremas well a5possible in a leastsquaressense,Absolute veloc,ities defined in this reference frame would be almost a5useful ason PlanerA, The effectofdeformation ofthe meso-sphere would be to intrcduce noise nto relarionshipsbasedon the fixed referenc€ fmme and absolute velocities. Forexrmple, if two trenches remainecl ixed relativeto the localparts of the mesosphere nto which they srere suMucringand if these wo partsof the mesospherewere moving rel,a \e to grch othet then the posirions of the rwo rrencheswould mo!€ in the fixed reference tame. However since onPlanetB the rareof deformation of the mesosphere s much

slo\l€r than the rate of plaremotion, we would still be ableto clraw rhe usefui conclusion rhat he rate at which trenchesand heir subductingslabsmove aterallyrela.ive ro rhe man,

P/4ret C The mesosphereon rhis planet is underyoinglivelv defofmation. Is rhe concept of absoluteplate morionstill useful? How can we define a fixed reference fmme?Ilere s an alremativedefinition to the one we usedon planet

B. Imaginedifferent setsof referenceaxesoriginaring ar thecenter of the planet,eachser oraringwirh a different angularvelocity. When we calculaterhe insrantaneous elociries atdifferent points in rhe mesosphere,he value we getclepends

upon which set of reference axes we use. ln Planet Ae or

7/27/2019 CH chpt10


356 putting h A Togetbq

Flgufe 1O-12.PlanerA: Mesospherc s

dgid, eDLireirhospne.e s oDesphe.icdplaLe

elemple. rf\{.e selectasolrr referenceaxesrhe fixecl refer-enc,e amethatmoves rv,iththe rnesosphcre,he velocityatc!, 'r loinr n rheme,u\D1.,, . r .^, arn fr* , C ". , rn". , ihnd-anl, refe.enre exesfor which this wiil be true becauseorrrcrenllvfr.ol lhe me.u,pherrarc ,^Lts erJrr\(rocalh

f indon..ct , , releren,exi . rorwnrcn he sumof the velocitieswill be minimum n a lcastsquaressense This reference frame moveswrth the mean.rnlguur.elo \ oI \nmF ,jpprnpnJe pJn .t rhr me5upncn Ler r'Jonr ii\a-urtacd -ejcle;LehJmcr,.I,iand

L -rr( : (q( rc ink re\rcdn nrer.r(r iun.erq{ | plalc, nu

:: , '?i , . j .""j I f l , , f rheme.u.nhere e \pe(L^ hJrourr \eo re,cn.n,c tum.hJ, t t)L.mcJn,i1gul.rrcl , \1r] ot r l ,c

l l l1T""p," ui rhe me.o,rhcreun | l rner ( . Ab,urure

f rarcmdmn h mcaufed elalr\ ru rhi . "(rL]e1,e ftJmF

_ Lu".n,g se n;rvF re ge erat drJ.Lel . ( . r q( .dn rppt\wn.I qe know.ruJ neqJ) dr .cn\r rcLl r lcs,ralhod1. , tanc,r.-wni. h wc $ rll \uo haved ( h.rn(r ro ri\fl. ue , rcijr(Llrhe.r f rwn,pLners oursqtvc, .! wi Ines In,n rnr brgjnnint

eI&]J) l ruq rher. f nte- io, \ u, , rked. pt jncl L ^ mure, , t , r.nJtcngc uc $r l l hJvc o trgurcour huw ir . Inrcnor$urhJlr m ub\crvJInn\ mad( oI rhe ur t r (c atrcrwe tanLt,here.lvc hopF rhJl planc{t j , \ imple l j tc phnsr \ I ,ut u, l rc l r lmay De complcx like ptanet C. How.lo we proceed? Todclermjnc b,oiLrre elq re.wc r( .ed o$,.rbt i ,h J ,urrable

frdm( mo\, | |Rw h rhe n,F! .pt ,cre H(. fe$-e nlt a major problem: we cr:lnl see the mcsosplrere, so vrec. ln dcrermine , \c lo( \ di .e( lh. Hun ,an qs ns/5x, .romelrrlng q e crni \ec? tl s rrme to \ rs plrn(t Fjrflh, !\ h(,repeopn$i hs I'a\e bren q rsrtrng s irh rtu, p,uLt<m tor \er_..

;: l :: i l ]*,_ . * " i f l \ce.umrorrhcJpp,uJ!'hes,hc\

No Net Torqae

::lTl: ,,":,.\ rra5 ,'h unr nrar( n ,hcrormorr .insrc\ t r r ( ' " rcr t het j ( f jgure t0 t2 r .

thr . t l i rc i , , ^uptc<jb! ! r ..ou'_urJA hrough rhe:*rh<norphire ru rhe,rgnt ne,o.\DncrF I \uu knuu lt ,L.mor iun t rhc plJre qnrr ( r r v.Lsry Jhour rhc moriur oldre mc5nspher(/

- leDegin.b] , r1, r t . I rnp hF rorquc crc eJ h\ rhe t l lospnere,,n rhenre\osnhere. t v is rhe vet,rrR.na rae cr,r" ro i,c .,".il;:.;# li.':,":.:;lX;

per unf l r rea ewf l(d un lhe mc,,5pher<Jr | l rarfor l ^

7/27/2019 CH chpt10


AbsotutePlate Motion 357

where, is e drag coefticienr. he correspondingorqueT

srefted on the mesosphere"siivenbv he vi:cor crosspro,Cuct

T = r x F

rvhere is the radiusp.rsitionvecror Tbc total torque exeted

br a plate s found bv integrating over he suffaccoi theplare.For a spherical hell ikc that on Planet , his ntcgra

tion giresa orquevcdor thal sparallel o lhe angular ota'

tion vector.'/ra of the sllell relative .o the mesosphere.

If the angular velocily of dre mesosphere s differenr from

that ofrhe sphcrical hell,viscous rxg hrough he as|hen-

osphere$ill exert orquc on dre mesosphere,ausingI to

accelerateuntil it hrs the sameangulif velocity a.she plate

Only then sould there be no net torque exerredon the

mesosphere y rhe plate and none on the plate bv thc

mesosphere.his s hc geoeral ondition ought n theno-

net-torque approxch to ffnding absolute motionr No nct

torque is exened on thcmesosphere b-v ie set of platcs

that comprise he lithosphefcand none is exertecl n the

tolalshcll of lifiosphereby the mcsosphere

Now suppose Planet A has two plates labeled N rnd S(Figure 0 l3). eacha hemispheric?l hell.The boundarv

berlveen he plates s a transform along the equator Motion

across he rrnnsform s at the rate of 111krr/mv, correspond

ing to a rolation vector vjth magniludeJrt- - 1'/my aligned

sith the rotarion axis. What (!n )ou say aboul fie motion

of the mcsosphereelative o eitherplate?

The orquccxertedby he ithosphefe n themesosphere

is rhe sum of rhc ts. torquesexertedby lhe two shclls. n

ordcr iofthis sum o be zero, he wo torqucsmustbeequal

and opposite. .ronr he srmmetryof the rwoplales, r follows har his conditionwill be met if the aogul.rvclocity

oi the mesosphere relarive to Plxte S is half that of Plale N

relative o PlaleS, hxt s, f

ro,tr= !or/2

viewed rom the xbsoluteeierence ramenrovingwith t\e

mesospherc, he angulxr !€locit-v \eclors of the two shells

are equal and opF)site n nxtrExtudeand are clrecled bward

the two poles

If PlareS s largeandPlateN small,which plte will have

the higher rate d rotalon in an absolute eierence iam€nroving ith dre mesosphere?incedrag orce s the prod

Itgure 1O-13.I,lanetAs ith rwo equalhemispherielplates.

^Dr= "1^l

e,"rtrlf

"r^tt s

7/27/2019 CH chpt10


358 hltttng It AI Togetbq

\=iz,EN

^{rr,Fiaue lo-t4.Planer q irh tso unequalptares n nrantlcrercrencerame,angul.rvetocir!of smallplareN is grcxrer rhan rharof large platc S.

rrr ufvclvLur me. rreJ lhe plalcs h rhe mal lcrareJm.l\l hdvehe hrghervelo.rrrrel.Irvc ,, rhr meso,phrfe n.rLrer .r rhe q^ lurquc. u hahn(c I . rsurr u I r l

P?:: !Eartb 01 ou, uqn ptrner rr lour dolcn rtrrvror-otnetcnl trc\ rre mu\rng buut JrnrrentE{t lerfolc..racft exertsa brqueon rhc mcsosphere [ bich,for e|)€il,crlrererencetrame, cen be caicula.ed if we know .he ple.et

arylular velocirv and the locadon of its boundaries.Lisingastaigtdorward madrematicJr€clDique, t is pcrssibic o 6;da uniqus no-net-torque reference frame nr,h th.I rh(

l :r j l lur. :ue\e:| I r)-

l | | ur rheprJe: un rr ,e e,u,pher,

l :?,:we,

ln,hrn_rl rh,s cfer., ,e rJme , m.nn8 $,,h

rnem€n \et,( i t \ ul rhc tJ\erur rhe manl lr rrmcJr.rr ihDer.q heJ,rheno,pher. ametr .hcmcrspherc. rrmu.r.onrF.nporur){Lh.e. ut gtubJ phre mu,rotu. b.ul.n.phlcmol,r'll dc gtrrn rclJljle u,j nu-nclorqu(. etcrel( trumcround n lhis wr\

Ho'qro,s

::^::!:!ttb,Ho'5eobAs before, planet^ hasa risid

mesu--,pher(n. l a. i IRhpldlr lJ,rrhi \ rme.n. lcJdurpJrnl-inggru lrncion rhemeso.phcreo repre,cnrne ctcren.errumc ter huf\ rlx .phcre. uf hishtv r.r(fio.r.̂ .i materidlIn rne m$orphcre o mrrk rhe n(.I jon.of lhc 1x. y..L' ld-, 'z \e\ o[ rhc rcrefen.e r. lem Frgufe0 lc, Noqroo Ine Fl e,tu.nhrrend hc mo\ ngpl.r l r .a b<lnrr. herrojorr tr!e rrerl our(c\ lruduce plurDe,uf magma r hor-<porl l rhdri( rhr uah-l(. m$o:there. hearne||o.t tre"e,an-. l ] rc r .vrnq lJle.urmrng hain, t ot(arr,es p.n r t r<

PrarF r l mu\e.over h( plumc\ ! i /c,uulJ mJae rncrAr3l:ri1:.11:. rJ.qe",rrdnible rhcrN,npprume.ur masnJr ' rme ro, rrun\ l t rcterencc\$,,0. rpaf lbLr,n. leid rrrereJ ncm ,x,duml\ lrruughnurhe lgid ne\o.phcre .t

h(rc<L soul(]be he .] |nc ot,dnn r:tand. " in.soutd orm

.l f , plr lel i \or. r rourd .rbour Eutrrpule har e.nirn!

r ) ,eu ct jr i \e u rheme,o.pt,e.c he okJnr, , l ,J in r.rke!rne, urmot . r m.dl ln le ccntercd n lhc Fuler ntc. hj .r ( n r u q n l r : i g L r r cu t r , r , r r h < . n c r r J ti r u r r r r r w n e r ( . r h c

l l i - : : i : i i : i,.,h{ pr/rc,nerhe o,.po, ,u, mov,na

rerr r \ e In rh( mc,o.nher( . . , , l l r r r hF .or \pol rJ, anJ thcrr ; rn(rofm\ rc . .n(enrrr , gererr th

,hr , i , nor rhc cr . (

{DI

7/27/2019 CH chpt10


Ahelute rfute M.tion 359

Plate cconstrucrions re easyon PlanetA. The circular

form of a hotspor mck givesus thc locationof lhe Euler

pole nccded br a plate rcconstruction. gesof volcanoes

aloog he treckgiveus rhe anglesO needed or reconstruc

tion polcs ,\s a check, f there are tracks fiom different hot

spotson the sameplate, hcy shouldgive the sane Eulerpolc aod the sameangles.Moreor€r because he meso

PlaneiA Radioactile heat sources are buried

in rigid mev)sphere, roduci.ShoGpot(.aci$on 'novinaphtes

Ftgure 10-16.Hotspor track forms along snzll .ir.le cen

re.edon EulerpoleE dcsc.ibingmoibn olplate fclarive to de&xphefe

rrt.

7/27/2019 CH chpt10


360 t,utting h AU jba.,tha.

)*i : .1,,1, u,,PrJnerrhe L'

;:;,;it:l;;:t:;.;:J:l*:"1;|'.'-uin,'- ,ne

,,,"i;;i:lT;,,.."'''"":t':.ir,ii'.: : *,r,," ,,,..,".,,,;;:; ;1.:i:Jj,,il:;:ilpla.ne.s-B and Cbttb f'otslroi Dt

niTi:*ir#,::r:timrrui:r

illi;ilrJr*-$-its,*l ̂ .,1r, i:i:5" :,,.;li;;l;:i,;il ;:!::!

Ei,!! u,trbHoqpo6Do.,,,

li:;,ii[ $jlj:**#r#;:iL{.1i:}:[il] r;

n;l;il'i-:l ,.:':j]lTili. li,ln:;.Hi.,,.,;_;;,,.',lif ;5lil;ii:T,jT;;T?;F ;:#'.;-ffilj;:i:"_#

;;;i1i"1:::r;:,:i;l[lhlri rl, ;J] ;;:1.?l::i :nL,n.,h,.,emJ, ahrc oLrxrin

jJiii"jr; r*rjihiixilrir,iiirl#il: ;,:jl:i,i.,,I:,";; ,- ;::,",,,",,",.,lrlrrl';*i,d: ';.li:*:lr;ii,.:r,lij::.r;r;::.r;,:;i*ril*{.}#ti{J:, .t,$,:5 ;.1,x :.r(.Lrj\

. u u6p.,,Fletun.ft .re

,'u. uo.,"".."l;;i iill; l!,li .: ii,,ll.

7/27/2019 CH chpt10


HNhne Pbte )rknian 367

FtAu.e 10-17.Hawalian-Emperorezmountnam

bt merhods escribedn earlierchapters.o thesecond rep,

the mande is €livena velocily that producesasgood a fit as

possible to the azimulh of all knovn hotspot iracks and theprogression of agesalong them. This lit is made under the

constrainthal lhe motionsbetwccnplatesdetermined n

the first steparenot changed. bsolute latemotioosdetermined n tbiswayaregiven n mostcontemporrry nalyses

ofglobal platcmotions.The absolute elocities sed n thcdiscussionot' whar drives he plateswcre determined in this

A global mapof absolute velocilv ve.tors found from hot

spots by this meftod is gi!'en in Figure 10 1il. Let t'our e!€

roam over and let _yourmagination soar $ramples of almost

c\crlthin!! discussed n this book leap .o the el'€ The enormous Pacilicplare is racing toward its trenchcs. asare the

smaller Nazcaand Cocosplirtes.The India Austmliaplate is

being pulled norths'rrd inro the garknd of trenches around

souther.stA.sia,riving India into Asiaand ma_vbevengiving

Eurasia small nonh*'ard shove. ts fun to run.l'our e]€

along plate boundaries anclmentally add vedors tail Io tail

to gct rclativeplatemorions. iy this xlongwesternNorth

$ t

HAlr 'AI IAN

, . . , . , . . : , . ry-o"Po'i?XC'ilirs

]D

7/27/2019 CH chpt10


32--:!:y.1rt zbsethq

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Figur.elo-la.

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b::l:nsm$,hcarftr$,s** nl,o t.,(,*,*. u, -,'i"""'n"l,s

t li- t.,... wha. .i ls -n . " r r t , c , , . . , . . ] l 1 r cn f : ru i , ,u$ :

piarcrc.runr.s.r-"_",."1i"1:y.ip..!,n cunrempo&fi.

ft c/r,hJnr ^.r, ri,,,,1.,ii"l,'",r.,. ,,r,,.,,",,,,...qu€s,,'n\b(ina,r".r il,_ --'_llf""ill's re , neof heDasmr

t luD,e, osi , ,ghi- ,' ' - " !nI . hr i rA pn,ct l rccdbvJiJr',iq,r'r-"Fh,r";,,r,^.,r,,.i '"

l,l l l ',I-:,

t .,,,, L r,.,p,'vrur'e\i Ir h1 prr'*, "i,"..',,1'L.'-"mi,,8 Jo.Jlu,asmaasthe,ru\fh!ft,,r,'. ,.,ra.l,,i"i,-." ,1'.,r

,{,s,n.lte,rn rhe,l \,u,,.1,,,,,^...**;.*ll,i_,JiiJ,.i,..,i,.o*,.,,:"",,.,iii,L,,ii,liatir.e " ..act ,,trrc,rir.r,

-:',:'. pelnuLinD ,,fpklles re]_,t. r,"r', r,,"".i. , "jril::.:.:'*r..r rh,ou;h",,taI ,,rot'rest"'e''e.ri"r.,n,.' ffi:;i.:-',,:'

'hr rseorrhe

A CoB.

r*'ffflf:i-fl: ttururem.',,,nse,erm,nedr,onl r . - ,cr_rcF, rtume\rh(,,m(,. ,1rn, , r ,

7/27/2019 CH chpt10


Al\tulue Pllxte votion 363

one mightquesion hc usefulnessfrhc concepr fabsoluteplate motion Lets a.sk he question irst for our cartoonplanets.On PlanetA, the ansv,'ers cr$tal clear:Since hemesospheres rigid, the |$'o epproaches ould give Lhesame esultsOn PlanetB, resulLshoulcl e similarbut notidendcirl.On PlanetC, he,v ould be eider similaror vast\'diiicrent.dependingon tire rateof deformarLonera6'e o

the rateofplaic motion.How aboutPlanetEarrh?Firsr hc bad news. ror slosly noling plaresikc Antrc-

tic!. .he two approichcsdo nor agree crv nell. Howererdris snt d.mstaring ecausehe urcedainriesn borh nethods areabouras argeasare he low velociricshemselves.\or' lirr the good ne$s. or |lpidlv movingplates ike rhePacific, hc tso methods give esscntiall_vhe samc resuksUsing no inlirrmationabour horspots, he no-nerbrqueapproaclr ields absolutc plate modolrs tharagreequirewell\\ilh the observed azimurhs of hotrpor rracks. Ir predidscorrecllr the zimuth of lhe Haiaiian sgamoum hainonthe Paciic plare Ir predicrs he rrendsof rhe calapagos

Archipelago n the Nazca lare. r predicts he direcrionofthe Yello$stone otspor ack on the Nor r ,{mcricaplarelairty E'ell. It is dris inrernal consisrency hat suggesG halboth approaches re \'ieldiogplatemorions n a referencetrame that is more-orless lockcd to x slugllishl movingsublithosphcricmxntle.

Slngle-Platu forque Dte to S,.zb Pull

Suppose har for a singlephte, you kno$' rhe locarionof ilsridges, r.msfoms. end trenchesbur vou know norhing aboutits motion relative o other platesor to hotspors. an ousa_vn\thingabout he xbsoluremotionofthat plate?

Thisquestions eresring ecause,swc go fartherandfarther back nro rhegeologic pasi.our kno\| ledlle abour dre

ltloMl geometrr of plares becomes more and more fragmentafi: The no-ner-brque approach requircs knowledgeof the locations i the boundaries nd relariYemotionsofall plates fia1 existcd xt a gi\.en lime. If s-e are willing romekceducrledguesscs boutplaregeomcrryprior to rheearlv Tertiar\', \l'e can use the no-ner-torquc approach rocalculate bsolute elocitiesr ut it tur[s out that he resultsarequitc sensidve (]unceftaintiesn plate econstructions.Hence the interest in rryiryl to estimate absolure velGiit_vfrom information about the boundar,vof a single plare.

Antppfoach lhat has_vicided uryrisingl.vgood resuhshasbeen o simply&lculare he torquc hxt would be exe(ed

7/27/2019 CH chpt10


3&l I'u6ng L A/t ToAetbs

, l : ,: . f , j : .

n , nds rpu . t r, r . t r l , p r r r ra fc< \^ rh .dpu .ho'J lrr l t F Jpr. l jeJ , J drf. , l , ,n _r . r th c.pe Jrc lar o'| l : -r l : l r o | |er(| b,,und!f\ _of sr\ r, rAmc||r rb, und

:.: j , : : l : l : lrurc\",nnr_J ' iJsr n, r - i l5r, F\ , j , l !n.jil; :.til'Tl::,1,i::til;j::;,;,:T,:,,:,ll

r J ' u n u t

,

d r . . D p , o , , f l i , , 1 , 1. , h e , n C , n , t i n . h . , r ,c - n

i:::::Jlt,i:l'rl TT,:"?ii 'i:i,;i,;:;;il"i'l:r"i,'llTj.,l:'H : in:nn:-1i;l;,il'; lf ,:Jli:,:iilffi ; :,:,i:

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-o " ' ^ t " ,ou

nq ,orquc Jnd hur.pnr m(1trud, An r(| bFDernr ,, ob,/rni.rl

.i:it,l.,"::.,Til;;::lt[: : .'j :J::,.:, ' : , , : le, Jr, .n(. l rhpaturreruAec:,,r t ,4s hc,, heJ,rc.. ,un

,llil5,:l';1, ;:::l:,"'runiqura"^"'r'u,,,,'cn(he.

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. .Jnc.- rnbr1. t , ion Jt r jdger, . .h Jn, l r iJ; f , r t l

ir i:!fijitl.i.Tn:.*r*:n;:;:;i l,;:t:,1;.[::,i:1il:::t;,::

l;,]'i,?

"'*,,-,^,

:..,irf;:$::f ;r'il:.ilH .;:"J fJ:e,rmounrc| i rn. me(r .uhr(h oclur \ qhe,"

;"m,",,1.,1,,:;m*j i:ii:rl#:ffi-:i::"iil:TiJhd'rection or heprale 'ust ave

". l l : . . '- ',1",:,, , , r, , , , i ,"" ,^ hr( ,,h, J gr.brj .o,

j'.:i:; "?ilirir.iffi:; f;llT ilrrtJI'zrLu',. n,plr<J.n $^ (\nl.L.uuu,I i. , !"cu( tv JrfineJ etobJlr ' .n lc in n"nr lF. . r^e(r .n cl .nsnl ! the lngF phre rurqJF

7/27/2019 CH chpt10


AbsoluteHate Lloti.rl 365

,ffiilrj{r.+:' i" r..; -€ ,,:?;i|

t

"i

* r^rroiiPLATE

modcl providesa sinpler and more explicitexplanadon.

Toda) sLrbdu.dngslabsare xtt"chedto the Pacilicplate :ion!!

thc leudanTrenchLr the north,along heJapan rench nthc $€sL and alongthe TongaKernudecTrench n the south

Slabpull roward dre north largely cirncels hal lonard dre

south.As a result,when the tot?lslabPuil torque s calcu

lated, thc net result is to move the Pacificplatc toward fie

Jepan rcnch elong ie trend of the Hawarian €-mount charn

At 25 I'la rhe c:oofigurationi trenches nd he direcdooof

motion $as about he same Fi€lLrre019) The boundar,v

of $e Pacilicpide ias differentprior Io the time ol the

bend.Platemodels or lhe north Pacilic asin n the earl-v

Teriirry sho$ the Pacificplatesubducting nto renches only

along ls nordrern boundaryl he slabpull lorque ior this

plaremodel(rigure 10 19)mol'es he Paclficlateparallel

ro the irend ofthc 16 to 43 MaEmperorsexmount hain.

Thesucessof thesingleplate orquemodel n expldnin!!

the modon of the Plcificplatesugges$he following lter

native o globalchanllcsn manllecon\ectionc!ffentsasan

explanationof x lllobal reorllanizalion i plate modons:

simuhaneous changes n plate nodon are sinplt .lue to

simultmeous hxngesn the configuration f plntebound

ariesThc co redionbehreen he |so ismain\'a kinemadc

one lor example, change n the motion of PlateA might

co$'eft a boundar-vetnecnplalesA andB rom arransbrm

to a rrench: iis chxngcn boundar-v ould causehemodon

of PlatcB to chllngc.

Figue 10-19.Ar 65IlJ. the nrnion of the Paciflc latc*s

northerh rowxrd treD.hes along irs northernboundarr Ar 25 \la. morio

erll ri the .esultol lecrof sum ol slabpuu

7/27/2019 CH chpt10


-t66 r)ukinah A// TbAaha

Pal.eomagneflc Euler poles

il;;J:l'l:,3:.lil1iJ;:j:lli, " Rr, hd,rc

,i:l#ryrrylj.lrrjTr,xlfli:.;

fr#**li:'fl*Hl1'.*i,r.*nil:r.,*l:"1;::ljiri;;l;: :i:lTil r;::i;:il,:;llii:i.''I;5t;Jfii.tx:;l r:\:,.,( ",o,\.

,li::lt,''"r''t."""sn' ,l;i'll',"'i:#illlf1llffjji+fi#lln ;r:jr:;:f:T:,;".a,,.,,,f i..-'"1,::,#:J :

..,nrv l,r,r,r.,r

;?:1:,;Hi',,'". :; ;l';,,,I;:".,;j.i:''i::$.'.::,";,;:f"';,::lJ'",lJ"i'"i:;,t:,.",lll,iil,J.15!"''A morea,,h,,ke

f,,i:ti:1i;?;:';';-"J1ii;'ili:iJfll:

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'.;il""::Ti,,T:;:it:'llne,r,, ,,,ulr'

J#n,{lll:}!11,,1*:;rJiiu"t.' 0".,I ri, ,j,"1',r"tc

' ' 'tt"a naleonasneric

; li:fffrllti::,]fi:,il''iT:i:]:,:l:::.lr.*. ,r.r..,.a.'.";illffi:l;;::;i,T,:J

7/27/2019 CH chpt10


Ahrolute Plate Motiofl 367

,t-.(i,*r{Pl

Flgure lO-20.Bodr he holspoLmck and rhepaleomag

nericpole prdr are concentricbour rhelulcf poleB fd rnorionol thetlzte reli{i(

passover the llc'ographic pole. So hotspor r?cks are rarcr

than AP\ri'padN.F!:w horspot racks older rhan_lurassic avebeen recogniTcd. second eason s fiar in older focks, tis diflicult 1o use drc no nei orque xpproachbecausehisrequires nfbrm.unnrabout plxte boundaries or availablcprior to re Tertiaf!:On theother hand,paleomagneticolepahs like the onc shosn in ligure 9-9 havebeen derermnred firr mostcontinenal plrtes back no dre lrrcclmbrian.

Sorne Concludtng 7houghts

A fiml andperhapsmorc philoi)thicd rerson or concluding fiat absoiutc cfcfcnce rames re useful s rhe follow

ing The large and compler scr of relarionshipshat conrprise hc globd serof rehive platenxxbns Frl1ntoa simplepatern wher considcfed n the nbsolure eferencc rxme,and fiis parern gener.rllvmakes ht'sical ense.or cxamplc, it tums out drat le velocitrof t.enches s verv low inlhe absoluter:Inre.lfa referenceiamc thatsupposedlv\!asnroving ith ihe sub ithosphericmanrle xve hc rcsuh hxrrmnv rcnches arc movng f:Lst, nc would immediarelv mella rat.Trcnches re housrnds f kilomcrcrs ong Theymarkthe upper edgcof slabsenending asmuch as700km inrothe mandc.Rapjd noton through trc mandeoi sucha seaanchor"clisplacingargevolumesof asdrenosphereouldbe phvsicall\mpl:rusible.

7/27/2019 CH chpt10


36a pn h, tt 4t/ ugetucr

,"),?j::l ;::T"ii:1,,T[::i,,:,-:,hch\er\,io,

fi ;[:llip:':H,,ri;j{illrHfr,il,lr

,l***i*#i*It,"r;;djll i:.J;'iH;: ;;; ";::iii ilJ:*:#,:l;:/:;u::l ;x;f;; il;l : '.tj,:t:f : l'lil.';:ii il\l J;Jil:l;"iil;l.'i:,T';;..,il:

:: :,j,liiY;;#;,T:,:lj,l,;l;:;:*_,;"il:[#:i,1];.:,'"l,l.il,,il;,:,^,r,r.".,..);:liJli[:':'il]';il;"fi',11":::'ii;J n d r h cJ h \ o l u r cm o t i u r , l i

r ' _ ' _ ' r r r ) e r r c r f h . o n . r J n l

;5.i^fflit::i*;:Hl;l'# i;ii.J;ili llillii:l.""";;r:i:':"'l;"i':i li'

*i#ilti:l:.'llxhi,t,rffit :i"' ill litxi: l[$:lr*:;,:l:':;:,,..,,

;i;lr;:;!",""'*';iJ':il:'"".,ilili

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i. i ; ; "';:',**'"''''nil.','ffi#:,|ry1t

;*;[.i;f

7/27/2019 CH chpt10


Ttle[blLlrVAtuld 369

True Polar Wander

The .listinctionberween rlre polar $ander and apparcnl

polarwander s anelogouso thatbe$'een eladve nd abso

lute plate motion. To an observeron aphle moving tos ard

the polc, i! seems ike lhc pole is movhg to$'arcl hc plale.

Thc movementof poles alonSa paleomagoelic ole pathroward or eva-vfrom an observer on the plate simllarly

reflects $e motion of the plnte To emphasize he relaiive

nature of motbn betweenplates andpoles.pal€omagnetists

describc such pole paths as4ppdet r polar lrander paths

Sinceall of our observalions fpoles arcmadeon moving

plates, snl all polir wander apprcnt? Hos' could one cven

dcfine truc polar wancler?

A Thought E4tertrnent

Lets return to our cartoonplane6 and seehow x'e might

define truc polar vander there PlanctA llas an equatorialbulge ike thc earlhanda rigid mesosphcre,'hich ogether

rule out rhepossibility fpolaf wande. PlanelB h3sa man'

rle capable f undergoing lastic eformation this is more

promising. Again leti paint a Srid of longitude and latitude

lines on the surfaceof the mcsosphereof Planet l. centering

the gricl s-vstem n thc North Pole,asdefinedby the rotadon

axisaI imezero.Lets assumehatPlanelR: manlle s veak

and \i'ill deform undcr the stress mposed b,v some nev,r

gcologic eature ike a mountain angeor rhe Greenland

ic€cap. lndcr the.e condtions, the entire n1antle 'ill dclorm

in such aE eyas 1()movc an excessmass,sa-vhe Greenland

icecap, oward the equalor'Deformational trainnced notbe large. PoinLs hal are close togdher beforc the defor

matiorl eill be close ogetherafterwards, o thal the gfid

s-vstem aintedon the mesosphcre, hile it mav chanlle

shape littlc,will stjllbe easily ecognizablc o2n obscrver

in space, he equxtorial bulge does nol scem ro move bul

remanrsnclinedat r lxed angle o the planeoflhe ecliptic

However, he cntire paintcdgrid s-vstemeemso move ovcr

the surfu(cof theplanet. n obscrversandingn Grecnland,

moving l'ifi the grid s]stem,sees he geographic olecof

responding o lhe cuffenl rotationaxismove fitrther and

lurther a$,ayas G.eenlanclmoves owrd the equator The

mesospheref Phnel B s undergoingrue polar wander'

7/27/2019 CH chpt10


31o puriaph AI h7e tu

,;[****',*rt*l:,,r.l:.,*

*f*rtffitirtiitryIii:1iP'"qiiii};nu;::r:

#ril#i'Fffiilffi;'[ijHr:li1fi ili{,flI;;j

-tr-a[*ff

7/27/2019 CH chpt10


rrue Ibklr wah.lq 371

Fiaure rO-22.'lrue

polarwmder hs o.curf.d Horsfotshar€allshifEd abour 0" retrrne to the rotatronaxrs, nd dre paleomalrretc rclinarlonsoI seamountschangedat rlie time of the

I

Obseraatlons ofi planet Earth

On PlanetEardr dlis cperimenr h:$ been done rn r,!ro 1i lfercnt \ra\r, the t-irsrusing a single hotrpor :md rhe sccondusing the global serof hot5pors.The singlc i()LsPorexpeflncnr \,as done on the longest hoLspor rack,rhe Hawaiian,Emperof chain.The experimcnt colsisrcd ol rrieasunng nepaleomxgncdc rrclinationarSL ko Seamoum.a volcanowithrn agcoi 65 \{a. usinga tong coredrilted from the scamoun(b_vrc Decp SeaDrilling pfoject. The results lere sinritarto .hosc shosr in exatgeratcd form for

severar norspors lnl-itlure 10 22: rhe magnerizadonofrhe seanrounts, sshownbv dre arrow ahovee;)chscanrount,has he same nctinfiionin suc.essile seanrounGunrit rhc hotspor shitrs relatnc rotne rot.rion axis. The p2leoftlgnetic inclnEtion olthe coretronr Suiko scamounr is s.eepcr fiin rhe inclination tod.rv/ r , 1 , ( . h r i \ F r , , l ! d rt . . r t t r u r r r r r r r r , n c r r n r c r r rh o \ \ \tbat.lulng the past65 milh.rnr€ars, fic HaNaii horrpot hi$\hihro -

Jeercr . /sd\ | lom rhc qFu8r. ,nt , r ,notr

r . r n r , , n r n l - r e / \ e r , i r , ! , b F J u \ e $ ckno$ thxt holspots shifr relad\,ero erch other One mighrJr . r re rJ{ J- rhe I /q Jr i hor . t \u $ J, .hi t r i r lsr$.J! r ru,n nsI n r ( u i l h n r . p u r . r f e , h r l | r n S I n r h c r i f _ , r . , n . n , t r a r

me mcan motions ofatl hotsporscancellcd. n this case,no

7/27/2019 CH chpt10


372 puftinEb Alt TogeM

I'PJI N%

I

FiAu.e t0-2J.

**ms:

***Hffi\ .

( \ \L(nrLi . shr tr , r , rc . , , . .1, , " " , , ^

i , , , , , r " , , .J , , , " , u , _n s $ r , - l c r , . l J , , \ er , . . e o l . l

, ff:;, i,;:),:,]!ll;,t:;,,'r'.:'.^,,,su,ruunL,,,,\o * , , , " * u . , . r " - , l - j - . ' " - K r l n , , u r , f , , r , IJ ,A , r , , r h c i l

0,,'*,"'"...:,"."1','jl,I,:r]:il,Ji:ill:*

7/27/2019 CH chpt10


True tblatUander 373

the assumption hathorspotsha.t nor nro\,.eoeFtlve t.:r achother Paleomagnetic :ttaare not Llsccln rhis firsr stepandno assumptrcn s ftrde abour possible shitis berween polesand nolspots Ncxt,withrhe plates cconstructed n a hotspoaretcr-ncc rJme.u.cr . lc, ,nrrBr e,r . d I , r i runr r t rcl lJrc. rul in l i Julc, , r , . rgrer i ( ,u l - . \ hout r . , . r pulJ- qJ . l r r rhrp . r $ q r t ' b F t , , U n c J J r - u n J t h e p f , { n t R ( . r - J p h r p ,

( .' : " ' | | : " . , l fu ie. r (ne.r ins cfr r \ r . r t r r n, t - ,mrsncrr .daraandshifisbcfir'ecnrhc hotspotsused of thc reconstruc,rion. Ifrhe nranrlc15a wholc tr:r^shifrcd relati\e ro thc polc,' . , f i \ 'nSr,hcslul , r l \1 nr ho{\ tuL5 i ,h r rhef . r te, ,T,sncrr .|Ule\ s r l1 c dr.nl . (cL felJ ve ro r t rorr . ru| rxb

ceoph,rsicistsha\e done rhisexperimenr,resornrg plarc,sIo trrer posirions in the lare Creh(eous rn a ho$pol referencerrame 'Itre

.esult is ttlx.latc Creraceous aleomxgndicpolcs arc slstemaric lv displa.ed 5 to t0 degrces tonarclr I ( ,en, tul t . rcdr . bJ.r I I ,LreT, l , i r q, i ' lJr r r1 l t . \en.c(rscus5eor,jrtjcr,rnprrr\ ru Ld\e orcurred

,,,The picrure-rhar emergcs froD aI rhis is rhe lbllowing.

Piumes rising from rhe mantle movc \,r,jrh hc pirrtsof th;m J n r l ( q n ( n , r h F \ ( , , " 1 c r r J r t , r u , . s h h i , h r h ( \ p J *Duf, q r ; rn( ! ^f nu l r r r fJ l , j r s. , r d(r rhc r , , rJ| |on \ 5 bin a suble position reladve to the densj.\, srrudufe ot themanllc During rhese irnes, he mrntle slowlv deforms anddrmes move rclatilc lo cadl o.her in ir p{rern lhat is mndom rctarivcto the orienration ofthc roration txis. Then, inrcsponscro some nrasspernrrbation in thc mantle.perhepsthe inlli.uion of e rev! ptunrc or a new sub.tucuonzone, rhenmtle lls a whole rohrcs relirtvc to rhe rotaton axif untilr. ffrives ar l new posirxD tlut is srablc ehtile t() rhc newdensirvstrudure in rc m.rnile.An observcr h space\r^uldsee the cnrirc glob:i pat.ern of holspot5mor.e, ike

rhe gri.1rn ou. uoughr etrpcfimcnl, r() some nci, posilion relati\,cto dte romton axis

Paleomaagnetic and Hory)ot Euler /:otes

Vhar nrpiications oes ruc pol.rr r.an.ler ave or Iherctx-r r n n ' h r nc r u . ! r I ' u r , f J r h k . / n t r r - p ; r r r r r t , , t a r u a rq rn:rrn., , r pturn.JnLt ] , , . r . .Br.rf ,hrI ,ulet,orn,rd\ l ,uv n r e N , " r . { i r ' , j , J r , o { r m i n g , , r r t ( r ' , . . a 1 , , r r .ei e! hotsp()trxcks ndpaleomgnedcpolesu.ilt ie atongthes.rme mallcifclc.Howelerthis will norbe rrLre .ner aplulrr.hin.^ J\ um rhc 4r! [ i r d\r\ r is: lJte* Jr h.rhFrr1e q r r l l

, \ JJr ru r -uc p i , l j f \ . le r u r t , . r rnuom l l foc( , .

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374 Putt tg lt AI Tbgethq

in r lo(rJpaftofrhc mJnrles,, .- . . , -

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life Cycle ofa plate

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Lalej.le oJa t,tate 377

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Problems ru&qestudRea.lings379

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381

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ch chpt10

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