Plate Tectonics and Sea-floor Spreading

PlateTectonicsandSea-floorSpreadingThetheoryofplatetectonicsdescribesthemotionsofthelithosphere,thecomparativelyrigidouterlayeroftheEarththatincludesallthecrustandpartoftheunderlyingmantle.Thelithosphere(n.[地]岩石圈)isdividedintoafewdozenplatesofvarioussizesandshapes,ingeneraltheplatesareinmotionwithrespecttooneanother.Amid-oceanridgeisaboundarybetweenplateswherenewlithosphericmaterialisinjectedfrombelow.Astheplatesdivergefromamid-oceanridgetheyslideonamoreyieldinglayeratthebaseofthelithosphere.SincethesizeoftheEarthisessentiallyconstant,newlithospherecanbecreatedatthemid-oceanridgesonlyifanequalamountoflithosphericmaterialisconsumedelsewhere.Thesiteofthisdestructionisanotherkindofplateboundary:asubductionzone.Thereoneplatedivesundertheedgeofanotherandisreincorporatedintothemantle.Bothkindsofplateboundaryareassociatedwithfaultsystems,earthquakesandvolcanism,butthekindsofgeologicactivityobservedatthetwoboundariesarequitedifferent.Theideaofsea-floorspreadingactuallyprecededthetheoryofplatetectonics.Initsoriginalversion,intheearly1960`s,itdescribedthecreationanddestructionoftheoceanfloor,butitdidnotspecifyrigidlithosphericplates.ThehypothesiswassubstantiatedsoonafterwardbythediscoverythatperiodicreversalsoftheEarth`smagneticfieldarerecordedintheoceaniccrust.Asmagmarisesunderthemid-oceanridge,ferromagneticmineralsinthemagmabecomemagnetizedinthedirectionofthemagmabecomemagnetizedinthedirectionofthegeomagneticfield.Whenthemagmacoolsandsolidifies,thedirectionandthepolarityofthefieldarepreservedinthemagnetizedvolcanicrock.Reversalsofthefieldgiverisetoaseriesofmagneticstripesrunningparalleltotheaxisoftherift.Theoceaniccrustthusservesasamagnetictaperecordingofthehistoryofthegeomagneticfieldthatcanbedatedindependently;thewidthofthestripesindicatestherateofthesea-floorspreading.PlateTectonicsandSea-floorSpreading,此文章版权归原作者所有!