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1、一TheEarthcomprisesthreeprincipallayers:thedense,iron-richcore,themantlemadeofsilicate(硅酸盐)thataresemi-moltenatdepth,andthethin,solid-surfacecrustTherearetwokindsofcrust,aloweranddenseroceaniccrustandanupper，lightercontinentalcrustfoundoveronlyabout40percentoftheEarth'ssurface.Therocks
2、ofthecrustareofverydifferentages.Somecontinentalrocksareover3,000millionyearsold,whilethoseoftheoceanfloorarelessthan200millionyearsold.Thecrustsandthetop,solidpartofthemantle,totalingabout70to100kilometersinthickness,atpresentappeartoconsistofabout15rigidplates,7ofwhichareverylarge.T
3、heseplatesmoveoverthesemi-moltenlowermantletoproduceallofthemajortopographical(地形学的)featuresoftheEarth.Activezoneswhereintensedeformationoccurareconfinedtothenarrow,interconnectingboundariesofcontactoftheplates. Therearethreemaintypesofzonesofcontact:spreadingcontactswhereplatesmovea
4、part,convergingcontactswhereplatesmovetowardseachother,andtransformcontactswhereplatesslidepasteachother.NewoceaniccrustisformedalongoneormoremarginsofeachplatebymaterialissuingfromdeeperlayersoftheEarth'scrust,forexample,byvolcaniceruptions(爆发)oflava(火山熔岩)atmid-oceanridges.Ifatsuchas
5、preadingcontactthetwoplatessupportcontinents，arift(裂缝)isformedthatwillgraduallywidenandbecomefloodedbythetea.TheAtlanticOceanformedlikethisastheAmericanandAfro-Europeanplatesmovedinoppositedirections.Whentwoplatescarryingcontinentscollide,thecontinentalblocks,toolighttobedrawndown,con
6、tinuetofloatandthereforebuckle(起褶皱)toformamountainchainalongthelengthofthemarginoftheplates. 练习题： Choosecorrectanswerstothequestion: 1.TheEarth’scrust______. A.canbeclassifiedintotwotypes B.isformedalongthemarginsoftheplates C.consistsofsemi-moltenrocks D.isabout70to100kilomete
7、rsthick 2.The15platesoftheEarthareformedfrom___. A.theoceaniccrustsandcontinentalcrusts B.thecrustsandthemantle C.thecrustsandthetopandsolidpartofthemantle D.thecontinentalcrustsandthesolidpartofthemantle 3.Seriously-deformedzonesappear_______ A.wheneverthecrustsmoveovermantle
8、 B.wh
