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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
