文档介绍：--------------------------校验:_____________-----------------------日期:_____________英语六级冲刺阅读训练及答案(20篇)prisesthreeprincipallayers:thedense,iron-richcore,themantlemadeofsilicate(硅酸盐)thataresemi-moltenatdepth,andthethin,solid-surfacecrustTherearetwokindsofcrust,rustandanupper,lightercontinentalcrustfoundoveronlyabout40percentoftheEarth',000millionyearsold,,solidpartofthemantle,totalingabout70to100kilometersinthickness,atpresentappeartoconsistofabout15rigidplates,-moltenlowermantletoproduceallofthemajortopographical(地形学的),interconnectingboundariesofcontactoftheplates. Therearethreemaintypesofzonesofcontact:spreadingcontactswhereplatesmoveapart,convergingcontactswhereplatesmovetowardseachother,'scrust,forexample,byvolcaniceruptions(爆发)oflava(火山熔岩)atmid-,arift(裂缝)-,thecontinentalblocks,toolighttobedrawndown,continuetofloatandthereforebuckle(起褶皱)toformamountainchainalongthelengthofthemarginoftheplates. 练习题: Choosecorrectanswerstothequestion: ’scrust______. -moltenrocks . -deformedzonesappear_______