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1、一、阅读理解 Volcanoesweremoredestructiveinancienthistory,notbecausetheywerebigger,butbecausethecarbondioxidetheyreleasedwipedoutlifewithgreaterease. PaulWignallfromtheUniversityofLeedswasinvestigat,ingthelinkbetweenvolcaniceruptionsandmassextinctions.Notallvolcaniceruptionskilled-of
2、flargenumbersofanimals,butallthemassextinctionsoverthepast300millionyearscoincidedwithhugeformationsofvolcanicrock.Tohissurprise,theolderthemassivevolcaniceruptionswere,themore.damagetheyseemedtodo.Hecalculatedthe"killingefficiency"forthesevolcanoesbycomparingtheproportionoflifet
3、heykilledoffwiththevolumeoflavathattheyproduced.Hefoundthatsizeforsize,oldereruptionswereatleast10timesaseffectiveatwipingoutlifeastheirmorerecentrivals. ThePermianextinction',forexample,whichhappened250millionyearsago,ismarkedbyfloodsofvolcanicrockinSiberiathatcoveranarearoughl
4、ythesizeofwesternEurope.Thosevolcanoesarethoughttohavepumpedoutabout10gigatonnesofcarbonascarbondioxide.Theglobalwarmingthatfollowedwipedout80percentofallmannegeneraatthetime,andittook5millionyearsfortheplanettorecover.Yet60millionyearsago,therewasanotherhugeamountofvolcanicactiv
5、ityandglobalwarmingbutnomassextinction.Someanimalsdiddisappearbutthingsreturnedtonormalwithintenthousandsofyears."Themostrecentoneshardlyhaveaneffectatall.”Wignallsays.Heignoredtheextinctionwhichwipedoutthedinosaurs65millionyearsago,becausemanyscientistsbelieveitwasprimarilycause
6、dbytheimpactofanasteroid.HethinksthatoldervolcanoeshadmorekillingpowerbecausemorerecentlifeformswerebetteradaptedtodealingwithincreasedlevelsofC02. VincentCourtillot,directoroftheParisGeophysicalInstituteinFrance,saysthatWignall'sideaisprovocative.Buthesaysitisincrediblyhardtodo
7、thesesortsofcalculations.Hepointsoutthatthekillingpowerofvolcaniceruptionsdependsonhowlongtheylasted.Anditisimpossibletotellwhetherthehugeblastslastedforthousandsormillionsofyears.Healsoaddsthatitisdifficulttoestimatehowmuchlavaprehistoricvolcanoesproduced,andthatlavavolumemaynot
8、necessarilycorrespondtocarbondioxideemis
