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一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.canbeclassifiedintotwotypesB.isformedalongthemarginsoftheplatesC.consistsofsemi-moltenrocksD.isabout70to100kilometersthick2.The15platesoftheEarthareformedfrom___.A.theoceaniccrustsandcontinentalcrustsB.thecrustsandthemantleC.thecrustsandthetopandsolidpartofthemantleD.thecontinentalcrustsandthesolidpartofthemantle3.Seriously-deformedzonesappear_______A.wheneverthecrustsmoveovermantleB.whentheplatesmovetowardseachotherC.inthenarrowboundarieswheretwoplatesmeetD.tobethemajortopographicalfeatureoftheEarth4.Accordingtothesecondparagraph,theformationoftheAtlanticOceanistheexampleof_______.A.spreadingcontactsB.theinfluenceofvolcaniceruptionsC.convergingcontactsD.transformcontacts5.Thispassageisprobably_______.A.anewspaperadvertisementB.achapterofanovelC.anexcerptfromatextbookD.ascientificreportofnewfindings二AremarkablevarietyofinsectsliveinthisplanetMorespeciesofinsectsexistthanallotheranimalspeciestogether.Insectshavesurvivedonearthformorethan300millionyears,andmaypossesstheabilitytosurviveformillionsmore.Insectscanbefoundalmosteverywhere--onthehighestmountainsandonthebottomofrushingstreams,inthecoldSouthPoleandinbubblinghotsprings.Theydigthroughtheground,jumpandsinginthetrees,andrunanddanceintheair.Theycomeinmanydifferentcolorsandvariousshapes.Insectsareextremelyusefultohumans,pollinating(授粉)ourcropsaswellasflowersinmeadows,forests,desertsandotherareas.Butlicksandsomeinsects,suchasmosquitoesandfleas,cantransmitdisease.Therearemanyreasonswhyinsectsaresosuccessfulatsurviving.Theiramazingabilitytoadaptpermitsthemtoliveinextremerangesoftemperaturesandenvironments.Theoneplacetheyhavenotyetbeenfoundtoanymajorextentisintheopenoceans.Insectscansurviveonawiderange,ofnaturalandartificialfoods—paint,pepper,glue,books,grain,cotton，otherinsects,plantsandanimalsBecausetheyaresmalltheycanhideintinyspaces.Astrong,hardbutflexibleshellcoverstheirsoftorgansandisresistanttochemicals,waterandphysicalimpact.Theirwingsgivethemtheoptionofflyingawayfromdangeroussituationsortowardfoodormales.Also,insectshaveanenormousreproductivecapacity:AnAfricanantqueencanlayasmanyas43,000eggsaday.Anotherreasonfortheirsuccessisthestrategyofprotectivecolor.Aninsectmayberightbeforeoureyes,butnearlyinvisiblebecauseitiscleverlydisguisedlikeagreenleaf,lumpofbrownsoil,graylichen(青苔),aseedorsomeothernaturalobjectSomeinsectsusebright,boldcolorstosendwarningsignalsthattheytastebad,stingorarepoison.Othershavewingpatternsthatlookliketheeyesofahugepredator,bitter-tastinginsects;hungryenemiesarefooledintoavoidingthem.练习题：Choosecorrectanswerstothequestion:1.InsectscanbefoundinlargeamountsinthefollowingplacesEXCEPT_____.A.onthemountainswithlittleairB.inthecoldpolarareasC.inthehotdesertareasD.intheopenoceans2.Insectsprotectthemselvesfromchemicalsby_______A.hidingintinyspacesB.havingastrongshellC.flyingawaywhennecessaryD.changingcolorsorshapes3.Someinsectsdisguiselikenaturalobjectssoasto______A.frightenawaytheirenemiesB.avoidbeingdiscoveredC.sendwarningsignalsD.lookbitter-tasting4.Thepassagementionsthatinsects______.A.canbefoundinanyextremeenvironmentsB.havesurvivedlongerthananyothercreaturesC.canbefedonanynaturalorman-madefoodsD.areimportantforthegrowthofcropsandflowers5.Thepassageismainlyabout______A.howinsectssurviveindifferentplacesB.whyinsectscansurvivesosuccessfullyC.whatinsectscandototheenvironmentD.whereinsectscanbefoundinquantity三Thefridgeisconsiderednecessary.Ithasbeensosincethe1960swhenpackagedfoodlistappearedw