新SAT-2016年3月真题

11Unauthorizedcopyingorreuseofanypartofthispageisillegal.Ifyouseethisline,youarenotansweringthequestions.()CONTINUE18Questions43-52arebasedonthefollowingpassageandsupplementarymaterial.ThispassageisadaptedfromAngusChen,“TheDawnofPlateTectonics.”©2014byAmericanAssociationfortheAdvancementofscience.AjourneytotheMarianaTrench,thedeepestcreviceonEarth’ssurface,revealsthegreatPacifictectonicplatedescendingdeepintotheplanetwhereitrecyclesbackintomantlerock.Thisrecyclingofoldtectonicplate,calledsubduction,drivesplatetectonicsandisnothingnewtoscientists,butexactlywhentheprocessgotstartedisahotdebate.Anewstudymayputthattorestbyunmaskingasequenceof4.4-billion-year-oldlavasastheremnantsofthefirstsubductionzoneonEarth.Ifcorrect,thediscoverymarksthedawnofplatetectonicsandthusseveralgeologicalprocessescriticaltoEarth’senvironmentandperhapsevenitslife.In2008,scientistsstudyingancientlavasinnorthernQuebec,knowntogeologistsastheNuvvuagittuqgreenstonebelt,sawthattheyhadthesamegeochemicalsignatureaslavasfrommodernsubductionzonesliketheMariana.Thismeantthattheymusthavemixedwithbrinyfluidssqueezedupthroughsubductionzonesandonlythere.Thegeochemistryofthoserockscouldbeusedasasortoffingerprinttohelpidentifysubductionzonelavas.GeologistsTracyRushmerandSimonTurnerofMacquarieUniversityinSydney,Australia,decidedtotakeacloserlook.Theyandtheircolleaguesnoticedadistinctchemicalpatterntothelayersinthelava,creatingauniquesequenceofrocks.TheteamthoughtthissequencecouldbesimilartolavasequencesmadebymodernsubductionzonesliketheMarianaTrench.MarkReagan,ageologistattheUniversityofIowawhohastakensubmersiblesintothetrenchfivetimesasdeepas6500meters,confirmedRushmerandTurner’ssuspicions.“Thewholesequence,”Rushmersays,“linkedinwithwhatMarkwasseeingintheMarianaTrench.”TheteamsayseachrocklayerinthesequenceattheMarianalavasandtheNuvvuagittuqlavasdescribesastepinthebirthofasubductionzone.Thekeyisinhowrocksandtheirchemistrychangewitheachsuccessivelayer.Astheoceanicslabdescends,magmabeginsrisingupanderuptsonthesurfaceinlayersatoponeanother,creatingarisingsequenceofigneousrocks.Withincreasingdepth,heatandpressurebeginsqueezingdifferentelementsoutoftheslabinfluids.Overtime,thesefluidsLine51015202530354045changethechemicalcompositionofthelavassothattheybecomerichinrareearthelementslikeytterbium,butpoorintheelementniobium.Thefirstlayerinthesequenceeruptsbeforethefluidscanescapetheslab,butthenextlayerinthesequencegetsjustenoughfluidtomakeapartialsignature.Thefinallayercarrieshugeamountsofrareearthelementsandverylittleniobium,togethermakingtheclarionmarkofsubductionzonelava.Theteamrealizednotonlydobothrockscarrythesamegeochemicalsignature,butincomparingtheMarianaandNuvvuagittuq,theyalsodiscoveredtherocksandthegeochemistryofbothsequenceschangeintheexactsameway.ThisfindingbolsteredthetheorythattheNuvvuagittuqsequenceisanancientsubductionzone.“Seeingtheevolvingchemicalsignature,”Turnersays,“wasmuchmorerobustthanjustsayingthereisorisn’tniobium.”GeochemistJulianPearceofCardiffUniversityintheUnitedKindomstillisn’tcompletelyconvinced,though.HesaystheNuvvuagittuqgreenstonebeltmightjustbetoooldandwarpedtohaveareliablesignalfrom4.4billionyearsago.“Theevidencewouldbecompellingiftherockswereyoung,undeformed,andfresh,”Pearcesays.Astheyarenow,theNuvvuagittuqrockshavebeenmodifiedbyintenseheatandpressure“whichcanmaskandmodifygeochemicalsignal”thoughcontaminationfromnearbyrocks.Furthermore,whilePearcebelievesasubductionzoneisoneplacethesegeochemicalsignaturescanbemade,“itisnottheonlylocation.”Whilethosearelegitimateconcerns,geoscientistNormanSleepofStanfordUniversityinPaloAlto,California,isn’ttoobothered.“It’snotfullysortedoutyet,”Sleepsays,buttheNuvvuagittuqrocks“reallyseemlikemodernarclikelavas”foundattheMariana.Whenitcomestothegeochemicalsignatureofsubduction,Turnerandthemembersoftheteamsaythatheatandpressuredon’talterthegeochemicalfingerprintmuch,andSleepsaysthisisreasonable.Despiteallofthedifficultiesofstudyingsuchtimewornrocks,hesays“theworkdoneinthispaperisveryvaluable.”5055606570758085AdaptedfromE.J.DionneJr.,ACallforNationalService©2013bytheWashingtonPostOriginallypublishedJuly3rd,2013HereisthesentenceintheDeclarationofIndependencewealwaysremember:“Weholdthesetruthstobeself-evident,thatallmenarecreatedequal,thattheyareendowedbytheirCreatorwithcertainunalienableRights,thatamongtheseareLife,LibertyandthepursuitofHappiness.”Andhereisthesentenceweoftenforget:“AndforthesupportofthisDeclaration,withafirmrelianceontheprotectionofDivineProvidence,wemutuallypledgetoeachotherourLives,ourFortunesandourSacredHonor.”This,theverylastsentenceofthedocument,iswhatmakesthebetter-rememberedsentencepossible.Onespeaksofourrights.Theotheraddressesourobligations.Thefreedomswecherishareself-evidentbutnotself-executing.TheFounderspledgesomething“toeachother,”thecommonlyoverlookedclauseintheDeclaration’sfinalpronouncement.Wefindourselves,237yearsaftertheFoundersdeclaredusanewnation,inaseasonofdiscontent,evensurliness,abouttheexperimenttheylaunched.Wearesharplydividedovertheverymeaningofourfoun