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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.Thefirstmeteoritefromthisfallwasfoundi
2、nthevillageofPueblitodeAllende.Altogether,roughlytwotonsofmeteoritefragmentswererecovered,allofwhichbearthenameAllendeforthelocationofthefirstdiscovery.IndividualspecimensofAllendearecoveredwithablack,glassycrustthatformedwhentheirexteriorsmeltedastheywereslowedbyEarth'satmos
3、phere.Whenbrokenopen,Allendestonesarerevealedtocontainanassortmentofsmall,distinctiveobjects,sphericalorirregularinshapeandembeddedinadarkgraymatrix(bindingmaterial),whichwereonceconstituentsofthesolarnebula—theinterstellarcloudofgasanddustoutofwhichoursolarsystemwasformed.Th
4、eAllendemeteoriteisclassifiedasachondrite.ChondritestaketheirnamefromtheGreekwordchondros—meaning"seed"—anallusiontotheirappearanceasrockscontainingtinyseeds.Theseseedsareactuallychondrules:millimeter-sizedmelteddropletsofsilicatematerialthatwerecooledintospheresofglassandcry
5、stal.Afewchondrulescontaingrainsthatsurvivedthemeltingevent,sotheseenigmaticchondrulesmusthaveformedwhencompactmassesofnebulardustwerefusedathightemperatures—approaching1,700degreesCelsius—andthencooledbeforethesesurvivinggrainscouldmelt.Studyofthetexturesofchondrulesconfirms
6、thattheycooledratherquickly,intimesmeasuredinminutesorhours,sotheheatingeventsthatformedthemmusthavebeenlocalized.Itseemsveryunlikelythatlargeportionsofthenebulawereheatedtosuchextremetemperatures,andhugenebulaareascouldnotpossiblyhavelostheatsofast.Chondrulesmusthavebeenmelt
7、edinsmallpocketsofthenebulathatwereabletoloseheatrapidly.Theoriginofthesepeculiarglassyspheresremainsanenigma.EquallyperplexingconstituentsofAllendearetherefractoryinclusions:irregularwhitemassesthattendtobelargerthanchondrules.TheyarecomposedofmineralsuncommononEarth,allrich
8、incalcium,aluminum,andtitanium,themostrefractory(resistanttomelting)
