托福阅读――地质地貌类(二)

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1、智课网托福备考资料托福阅读――地质地貌类（二）Earth’ssurfaceisnotmadeupofasinglesheetofrockthatformsacrustbutratheranumberof“tectonicplates”thatfitclosely,likethepiecesofagiantjigsawpuzzle.Someplatescarryislandsorcontinentsothersformtheseafloor.Allareslowlymovingbecausetheplatesfloatonadensersemi-liquidm

2、antle,thelayerbetweenthecrustandEarth’score.Theplateshaveedgesthatarespreadingridges(wheretwoplatesaremovingapartandnewseafloorisbeingcreated),subductionzones(wheretwoplatescollideandoneplungesbeneaththeother),ortransformfaults(wheretwoplatesneitherconvergenordivergebutmerelymovepa

3、stoneanother).ItisattheboundariesbetweenplatesthatmostofEarth’svolcanismandearthquakeactivityoccur.Generallyspeaking,theinteriorsofplatesaregeologicallyuneventful.However,thereareexceptions.AglanceatamapofthePacificOceanrevealsthattherearemanyislandsfaroutatseathatareactuallyvolcan

4、oes----manynolongeractive,someovergrownwithcoral----thatoriginatedfromactivityatpointsintheinteriorofthePacificPlatethatformsthePacificseafloor.Howcanvolcanicactivityoccursofarfromaplateboundary?TheHawaiianIslandsprovideaveryinstructiveanswer.Likemanyotherislandgroups,theyformachai

5、n.TheHawaiianIslandsChainextendsnorthwestfromtheislandofHawaii.Inthe1840sAmericangeologistJamesDalyobservedthatthedifferentHawaiiislandsseemtoshareasimilargeologicevolutionbutareprogressivelymoreeroded,andthereforeprobableolder,towardthenorthwest.Thenin1963,intheearlydaysofthedevel

6、opmentofthetheoryofplatetectonics.CanadiangeophysicistTuzoWilsonrealizedthatthisageprogressioncouldresultiftheislandswereformedonasurfaceplatemovingoverafixedvolcanicsourceintheinterior.WilsonsuggestedthatthelongchainofvolcanoesstretchingnorthwestfromHawaiiissimplythesurfaceexpress

7、ionofalong-livedvolcanicsourcelocatedbeneaththetectonicplateinthemantle.Today’smostnorthwestislandwouldhavebeenthefirsttoform.Theyastheplatemovedslowlynorthwest,newvolcanicislandswouldhaveformsastheplatemovedoverthevolcanicsource.Themostrecentisland,Hawaii,wouldbeattheendofthechain

8、andisnowoverthevolcanicsou