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1、YELLOWSTONEPARKtheworld’sfirstnationalpark.Thearea,ahugecraterlikevolcanicbasin,isageological“hotspot”andthesiteofseveralmassiveeruptions,themostrecentoccurring600,000yearsago.Theplateauismostlyformedfromonce-moltenlava.Volcanicactivityisevidencedbynearl
2、y10,000hotsprings,200geysers,andmanyventsandmudpots.Themoreprominentgeysersareunequaledinsize,power,andvariety.Theparkalsohaspetrifiedforests,lavaformations,andthe“blackglass”ObsidianCliff.Theparkhasawidevarietyofflowersandotherplantlife.Bears,mountainsh
3、eep,elk,bison,moose,manysmalleranimals,andmorethan200kindsofbirdsinhabitYellowstone,whichisoneoftheworld’slargestwildlifesanctuaries.Firesin1988burnedabout36%ofthepark,butanimalandplantlifereboundedquickly,asthenutrientinfluxintheashnourishedthesoil.****
4、***************************************************************************ALLOYAlloysareusedmoreextensivelythanpuremetalsbecausetheycanbeengineeredtohavespecificproperties.Forexample,theymaybepoorerconductorsofheatandelectricity,harder,ormoreresistantto
5、corrosion.Alloysofironandcarbonincludecastironandsteels;brassandbronzeareimportantalloysofcopper;amalgamsarealloysthatcontainmercury;andchromiumisanimportantadditiveinstainlesssteel.Becausepuregoldandsilveraresoft,theyareoftenalloyedwithoneanotherorwitho
6、thermetals.Newalloysarebeingengineeredforuseinnewtechnology,includingmaterialsforthespaceprogram.Metallicglassesandcrystallinealloyshavealsobeendeveloped,andmetalalloysaresometimesbondedwithceramics,graphites,andorganicmaterialsascomposites.*************
7、******************************************************************SOILEROSIONIntheUnitedStates30%isnaturalerosion,while70%isbecauseofhumanintervention.Suspendedsedimentfromerosionisoneoftheworld’sgreatestpollutants.Sedimentcanfillreservoirsandnavigablewa
8、terways,impairwildlifehabitats,increasefloodingandwatertreatmentcosts,anddepletevaluabletopsoil.Itcanalsoconcentrateharmfulchemicalsandbacteria.Amongthemethodsofpreventingsoilerosionarereforestation,maintenanceoffallowstri
