TheEarthcomprisesthreeprincipallayers:thedense,iron-richcore,themantlemadeofsilicate(硅酸盐)thataresemi-moltenatdepth,andthethin,solid-surfacecrustTherearetwokindsofcrust,aloweranddenseroceaniccrustandanupper,lightercontinentalcrustfoundoveronlyabout40percentoftheEarth’ssurface.Therocksofthecrustareofverydifferentages.Somecontinentalrocksareover3,000millionyearsold,whilethoseoftheoceanfloorarelessthan200millionyearsold.Thecrustsandthetop,solidpartofthemantle,totalingabout70to100kilometersinthickness,atpresentappeartoconsistofabout15rigidplates,7ofwhichareverylarge.Theseplatesmoveoverthesemi-moltenlowermantletoproduceallofthemajortopographical(地形学的)featuresoftheEarth.Activezoneswhereintensedeformationoccurareconfinedtothenarrow,interconnectingboundariesofcontactoftheplates.
Therearethreemaintypesofzonesofcontact:spreadingcontactswhereplatesmoveapart,convergingcontactswhereplatesmovetowardseachother,andtransformcontactswhereplatesslidepasteachother.NewoceaniccrustisformedalongoneormoremarginsofeachplatebymaterialissuingfromdeeperlayersoftheEarth’scrust,forexample,byvolcaniceruptions(爆发)oflava(火山熔岩)atmid-oceanridges.Ifatsuchaspreadingcontactthetwoplatessupportcontinents,arift(裂缝)isformedthatwillgraduallywidenandbecomefloodedbythetea.TheAtlanticOceanformedlikethisastheAmericanandAfro-Europeanplatesmovedinoppositedirections.Whentwoplatescarryingcontinentscollide,thecontinentalblocks,toolighttobedrawndown,continuetofloatandthereforebuckle(起褶皱)toformamountainchainalongthelengthofthemarginoftheplates.
练习题:
Choosecorrectanswerstothequestion:
1.TheEarth’scrust______.
A.canbeclassifiedintotwotypes
B.isformedalongthemarginsoftheplates
C.consistsofsemi-moltenrocks
D.isabout70to100kilometersthick
2.The15platesoftheEarthareformedfrom___.
A.theoceaniccrustsandcontinentalcrusts
C.thecrustsandthetopandsolidpartofthemantle
D.thecontinentalcrustsandthesolidpartofthemantle
3.Seriously-deformedzonesappear_______
A.wheneverthecrustsmoveovermantle
B.whentheplatesmovetowardseachother
C.inthenarrowboundarieswheretwoplatesmeet
D.tobethemajortopographicalfeatureoftheEarth
4.Accordingtothesecondparagraph,theformationoftheAtlanticOceanistheexampleof_______.
B.theinfluenceofvolcaniceruptions
5.Thispassageisprobably_______.
A.anewspaperadvertisement
D.ascientificreportofnewfindings