PlateTectonicsandSea-floorSpreading
板块结构与海床扩展
Theideaofsea-floorspreadingactuallyprecededthetheoryofplatetectonics.Initsoriginalversion,intheearly1960’s,itdescribedthecreationanddestructionoftheoceanfloor,butitdidnotspecifyrigidlithosphericplates.ThehypothesiswassubstantiatedsoonafterwardbythediscoverythatperiodicreversalsoftheEarth’smagneticfieldarerecordedintheoceaniccrust.Asmagmarisesunderthemid-oceanridge,ferromagneticmineralsinthemagmabecomemagnetizedinthedirectionofthegeomagneticfield.Whenthemagmacoolsandsolidifies,thedirectionandthepolarityofthefieldarepreservedinthemagnetizedvolcanicrock.Reversalsofthefieldgiverisetoaseriesofmagneticstripesrunningparalleltotheaxisoftherift.Theoceaniccrustthusservesasamagnetictaperecordingofthehistoryofthegeomagneticfieldthatcanbedatedindependently;thewidthofthestripesindicatestherateofthesea-floorspreading.
Thetheoryofplatetectonicsdescribesthemotionsofthelithosphere,thecomparativelyrigidouterlayeroftheEarththatincludesallthecrustandpartoftheunderlyingmantle.Thelithosphereisdividedintoafewdozenplatesofvarioussizesandshapes,ingeneraltheplatesareinmotionwithrespecttooneanother.Amid-oceanridgeisaboundarybetweenplateswherenewlithosphericmaterialisinjectedfrombelow.Astheplatesdivergefromamid-oceanridgetheyslideonamoreyieldinglayeratthebaseofthelithosphere.SincethesizeoftheEarthisessentiallyconstant,newlithospherecanbecreatedatthemid-oceanridgesonlyifanequalamountoflithosphericmaterialisconsumedelsewhere.Thesiteofthisdestructionisanotherkindofplateboundary:asubductionzone.Thereoneplatedivesundertheedgeofanotherandisreincorporatedintothemantle.Bothkindsofplateboundaryareassociatedwithfaultsystems,earthquakesandvolcanism,butthekindsofgeologicactivityobservedatthetwoboundariesarequitedifferent.
板块结构理论描述岩石圈的运动。岩石圈是相对坚硬的地球外层，包括全部地壳和部分地幔。岩石圈被划分为几十个大小不同形状各异的板块，一般而言这些板块都处于相对运动之中。一道中海脊是板块之间的边界，在那里新的岩石圈的物质从下部注入。当板块从中海脊脱离时，它们滑向在岩石圈基部较易变形的地层上。因为地球的大小本质上是不变的，只有同等数量的岩石圈物质在其它地方被吞没，新的岩石圈才能生成。销毁旧岩石圈的地方形成另外一种板块边界一块潜没的区域。在这里，一块板块潜没到另一板块的边缘之下并结合入地幔之中。两种板块边界均与地层系统、地震以及火山活动有关，但在两种边界处观察到的诸般地质活动却迥然不同。