ContinEntAlDriFt
ContinEntAl DriFt, AnD thE suBsEquEnt thEory oF plAtE tECtoniCs, Forms thE moDErn FrAmEwork For gEologiCAl stuDy oF thEEArth.ContinEntAl DriFt stEms From sCiEntiFiC notions As EArly As thE 16th CEntury thAt thEEArth’s ContinEnts wErE onCE A singlE lAnD mAss, whiCh sCiEntists hAvE tErmED pAngAEA, mEAning "All lAnDs" in grEEk. in 1912, gErmAn mEtEorologistAlFrED wEgEnEr proposED thAt nAturAl ForCEs suCh As EArthquAkEs AnD FlooDs BrokE pAngAEA ApArt BEtwEEn 225 AnD 200 million yEArs Ago, AnD it EvEntuAlly FrAgmEntED into thE ContinEnts As wE know thEm toDAy. wEgEnEr BAsED his thEory on Four mAjor oBsErvAtions: First, thE AmAzing Fit oF thEAFriCAn AnD southAmEriCAn ContinEnts; sEConD, thE similAritiEs oF unusuAlly struCturED plAnt AnD AnimAl Fossils DisCovErED Along thE CoAstlinEs oF southAmEriCA AnDAFriCA, EvEn though thEy ArE sEpArAtED By thEAtlAntiC oCEAn; thirD, similAritiEs in roCks AnD iCE shEEts on thEsE two ContinEnts; AnD Fourth, EviDEnCE thAt somE AnCiEnt ClimAtEs wErE DiAmEtriCAlly oppositE oF moDErn onEs.
wEgEnEr’s thEory oF ContinEntAl DriFt woulD EvEntuAlly spArk A nEw wAy oF viEwing thEEArth. initiAlly, howEvEr, CollEAguEs gEnErAlly DismissED it As ABsurD, DEspitE thE FACt thAt it sEEmED to ComplEmEnt AvAilABlE sCiEntiFiC inFormAtion. thEir primAry CritiCism wAs wEgEnEr’s inABility to ADEquAtEly ExplAin how thE lArgE mAssEs oF lAnD CoulD DriFt suCh grEAt DistAnCEs. ■
A、wEgEnEr spECulAtED thAt thEy simply plowED through thE oCEAn Floor, ButEnglish gEophysiCist hArolD jEFFrEys rEFutED thAt ContEntion, noting thAt it is physiCAlly impossiBlE For A gArgAntuAn mAss oF soliD roCk to Do so without BrEAking up. ■
B、it wAs not until thE 1950s--two DECADEs AFtEr wEgEnEr’s DEAth--thAt tEChnologiCAl ADvAnCEs EnABlED sCiEntists to ConFirm wEgEnEr’s hypothEsEs. improvED tEChniquEs in oCEAn Floor mApping rEvEAlED thAt thE Bottom oF thE oCEAn wAs not mostly FlAt AnD FEAturElEss As most sCiEntists hAD spECulAtED, AnD thAt gEologiC proCEssEs on lAnD, suCh As EArthquAkEs AnD volCAniC Eruptions, wErE linkED to DynAmiCs on thE oCEAn Floor. ■
C、sCiEntists DisCovErED thAt hot roCk Boils up From thEEArth’s mAntlE¹ nEAr riDgEs in thE miDDlE oF thE oCEAn, sprEADing ovEr thE sEA Floor AnD Forming nEw oCEAniC Crust. ■
D、As this Crust sprEADs, it movEs ContinEnts An AvErAgE oF onE or two CEntimEtErs EACh yEAr. sCiEntists Also FounD thAt As nEw Crust Forms At oCEAn riDgEs, olDEr Crust DEsCEnDs into trEnChEs² At thE rim oF thE pACiFiC oCEAnBAsin, EFFECtivEly rECyCling thE oCEAn BAsins AnD mAintAining thEEArth At A stEADy sizE. sEismiC ACtivity is most prominEnt in EArthquAkE zonEs pArAllEl to thEsE trEnChEs At thE EDgE oF ContinEnts, likEly DuE to thE FACt thAt sinking Crust mElts, CAusing hot roCk to BuBBlE up, lEADing to Eruptions AnD violEnt quAking.
thEsE FinDings on thE oCEAn Floor lED to thE thEory oF plAtE tECtoniCs, whiCh holDs thAt thEEArth’s outErmost lAyEr Consists oF morE thAn A DozEn lArgE AnD smAll plAtEs--mAssivE slABs oF soliD roCk, Both ABovE AnD BEnEAth thE wAtEr--thAt ArE DriFting ABout thE surFACE oF thEEArth, oCCAsionAlly ColliDing, ruBBing AgAinst EACh othEr, ClustEring AnD sEpArAting, AnD EvEn DisAppEAring ComplEtEly unDEr onE AnothEr. thEsE intErACtions ArE ConstAntly shAping thE FACE oF thE plAnEt, signiFiED By thE tErm "tECtoniCs" From thE grEEk worD mEAning "to Buil
D、" thE rEsults oF plAtE-tECtoniC ForCEs ArE most EviDEnt At plAtE BounDAriEs--nArrow zonEs BEtwEEn plAtEs.At DivErgEnt BounDAriEs, plAtEs pull AwAy From EACh othEr, CrEAting nEw Crust.A、prominEnt ExAmplE CAn BE sEEn in iCElAnD, whiCh is splitting As thE northAmEriCAn plAtE movEs wEstwArD rElAtivEto thEEurAsiAn plAtE.At ConvErgEnt BounDAriEs, Crust is DEstroyED As onE plAtE DivEs unDEr AnothEr. this tEnDs to Form mountAin rAngEs on ContinEntAl, or surFACE, plAtEs, suCh As thE southAmEriCAnAnDEs AnD thEAsiAn him