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1、智课网TOEFL备考资料托福阅读TPO22阅读P3详细解析SometimeaftermidnightonFebruary8,1969,alarge,brightmeteorenteredEarth'satmosphereandbrokeintothousandsofpieces,plummetedtotheground,andscatteredoveranarea50mileslongand10mileswideinthestateofChihuahuainMexico.Thefirstmeteoritefromthisfallwasfoundinthe
2、villageofPueblitodeAllende.Altogether,roughlytwotonsofmeteoritefragmentswererecovered,allofwhichbearthenameAllendeforthelocationofthefirstdiscovery.IndividualspecimensofAllendearecoveredwithablack,glassycrustthatformedwhentheirexteriorsmeltedastheywereslowedbyEarth'satmosphere.Wh
3、enbrokenopen,Allendestonesarerevealedtocontainanassortmentofsmall,distinctiveobjects,sphericalorirregularinshapeandembeddedinadarkgraymatrix(bindingmaterial),whichwereonceconstituentsofthesolarnebula—theinterstellarcloudofgasanddustoutofwhichoursolarsystemwasformed.TheAllendemete
4、oriteisclassifiedasachondrite.ChondritestaketheirnamefromtheGreekwordchondros—meaning"seed"—anallusiontotheirappearanceasrockscontainingtinyseeds.Theseseedsareactuallychondrules:millimeter-sizedmelteddropletsofsilicatematerialthatwerecooledintospheresofglassandcrystal.Afewchondru
5、lescontaingrainsthatsurvivedthemeltingevent,sotheseenigmaticchondrulesmusthaveformedwhencompactmassesofnebulardustwerefusedathightemperatures—approaching1,700degreesCelsius—andthencooledbeforethesesurvivinggrainscouldmelt.Studyofthetexturesofchondrulesconfirmsthattheycooledrather
6、quickly,intimesmeasuredinminutesorhours,sotheheatingeventsthatformedthemmusthavebeenlocalized.Itseemsveryunlikelythatlargeportionsofthenebulawereheatedtosuchextremetemperatures,andhugenebulaareascouldnotpossiblyhavelostheatsofast.Chondrulesmusthavebeenmeltedinsmallpocketsoftheneb
7、ulathatwereabletoloseheatrapidly.Theoriginofthesepeculiarglassyspheresremainsanenigma.EquallyperplexingconstituentsofAllendearetherefractoryinclusions:irregularwhitemassesthattendtobelargerthanchondrules.TheyarecomposedofmineralsuncommononEarth,allrichincalcium,aluminum,andtitani
8、um,themostrefractory(resistanttomelting)
