托福阅读真题补充(4套)

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1、第七篇MeteoriteImpactandDinosaurExtinctionThereisincreasingevidencethattheimpactsofmeteoriteshavehadimportanteffectsonEarth,particularlyinthefieldofbiologicalevolution.SuchimpactscontinuetoposeanaturalhazardtolifeonEarth.Twiceinthetwentiethcentury,largemeteorite

2、objectsareknowntohavecollidedwithEarth.Ifanimpactislargeenough,itcandisturbtheenvironmentoftheentireEarthandcauseanecologicalcatastrophe.Thebest-documentedsuchimpacttookplace65millionyearsagoattheendoftheCretaceousperiodofgeologicalhistory.ThisbreakinEarth'sh

3、istoryismarkedbyamassextinction,whenasmanyashalfthespeciesontheplanetbecameextinct.Whilethereareadozenormoremassextinctionsinthegeologicalrecord,theCretaceousmassextinctionhasalwaysintriguedpaleontologistsbecauseitmarkstheendoftheageofthedinosaurs・Fortensofmi

4、llionsofyears,thosegreatcreatureshadflourished.Then,suddenly,theydisappeared.ThebodythatimpactedEarthattheendoftheCretaceousperiodwasameteoritewithamassofmorethanatrilliontonsandadiameterofatleast10kilometers.Scientistsfirstidentifiedthisimpactin1980fromthewo

5、rldwidelayerofsedimentdepositedfromthedustcloudthatenvelopedtheplanetaftertheimpact.ThissedimentlayerisenrichedintheraremetaliridiumandotherelementsthatarerelativelyabundantinameteoritebutveryrareinthecrustofEarth.Evendilutedbytheterrestrialmaterialexcavatedf

6、romthecrater,thiscomponentofmeteoritesiseasilyidentified.By1990geologistshadlocatedtheimpactsiteitselfintheYucatregionofMexico.Thecrater,nowdeeplyburiedinsediment,wasoriginallyabout200kilometersindiameter.Thisimpactreleasedanenormousamountofenergy,excavatinga

7、craterabouttwiceaslargeasthelunarcraterTycho.Theexplosionliftedabout100trilliontonsofdustintotheatmosphere,ascanbedeterminedbymeasuringthethicknessofthesedimentlayerformedwhenthisdustsettledtothesurface.Suchaquantityofmaterialwouldhaveblockedthesunlightcomple

8、telyfromreachingthesurface,plungingEarthintoaperiodofcoldanddarknessthatlastedatleastseveralmonths・Theexplosionisalsocalculatedtohaveproducedvastquantitiesofnitricacidandmeltedrockthatspr