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1、一TheEarthcomprisesthreeprincipallayers:thedenseironrichcethemantlemadeofsilicate(硅酸鹽)thataresemimoltenatdepththethinsolidsurfacecrustTherearetwokindsofcrustalowerdenseroceaniccrustanupper，lightercontinentalcrustfoundover

2、onlyabout40percentoftheEarthssurface.Therocksofthecrustareofverydifferentages.Somecontinentalrocksareover3000millionyearsoldwhilethoseoftheoceanfloarelessthan200millionyearsold.Thecruststhetopsolidpartofthemantletotaling

3、about70to100kilometersinthicknessatpresentappeartoconsistofabout15rigidplates7ofwhichareverylarge.Theseplatesmoveoverthesemimoltenlowermantletoproduceallofthemajtopographical(地形學(xué)的)featuresoftheEarth.Activezoneswhereinten

4、sedefmationoccurareconfinedtothenarrowinterconnectingboundariesofcontactoftheplates.Therearethreemaintypesofzonesofcontact:spreadingcontactswhereplatesmoveapartconvergingcontactswhereplatesmovetowardseachothertransfmcont

5、actswhereplatesslidepasteachother.NewoceaniccrustisfmedalongonememarginsofeachplatebymaterialissuingfromdeeperlayersoftheEarthscrustfexamplebyvolcaniceruptions(爆發(fā))oflava(火山熔巖)atoceanridges.Ifatsuchaspreadingcontactthetwo

6、platessupptcontinents，arift(裂縫)isfmedthatwillgraduallywidenbecomefloodedbythetea.TheAtlanticOceanfmedlikethisastheAmericanAfroEuropeanplatesmovedinoppositedirections.Whentwoplatescarryingcontinentscollidethecontinentalbl

7、ockstoolighttobedrawndowncontinuetofloattherefebuckle(起褶皺)tofmamountainchainalongthelengthofthemarginoftheplates.練習(xí)題：Choosecrectanswerstothequestion:1.TheEarth’scrust______.A.canbeclassifiedintotwotypesB.isfmedalongthema

8、rginsoftheplatesTheycomeinmanydifferentcolsvariousshapes.Insectsareextremelyusefultohumanspollinating(授粉)ourcropsaswellasflowersinmeadowsfestsdesertsotherareas.Butlickssomeinsectssuchasmosquitoesfleascantransmitdisease.T

9、herearemanyreasonswhyinsectsaresosuccessfulatsurviving.Theiramazingabilitytoadaptpermitsthemtoliveinextremerangesoftemperaturesenvironments.Theoneplacetheyhavenotyetbeenfoundtoanymajextentisintheopenoceans.Insectscansurv

10、iveonawiderangeofnaturalartificialfoods—paintpeppergluebooksgraincotton，otherinsectsplantsanimalsBecausetheyaresmalltheycanhideintinyspaces.Astronghardbutflexibleshellcoverstheirsoftgansisresistanttochemicalswaterphysica

11、limpact.Theirwingsgivethemtheoptionofflyingawayfromdangeroussituationstowardfoodmales.Alsoinsectshaveanenmousreproductivecapacity:AnAfricanantqueencanlayasmanyas43000eggsaday.Anotherreasonftheirsuccessisthestrategyofprot

12、ectivecol.Aninsectmayberightbefeoureyesbutnearlyinvisiblebecauseitiscleverlydisguisedlikeagreenleaflumpofbrownsoilgraylichen(青苔)aseedsomeothernaturalobjectSomeinsectsusebrightboldcolstosendwarningsignalsthattheytastebads

13、tingarepoison.Othershavewingpatternsthatlookliketheeyesofahugepredatbittertastinginsectshungryenemiesarefooledintoavoidingthem.練習(xí)題：Choosecrectanswerstothequestion:1.InsectscanbefoundinlargeamountsinthefollowingplacesEXCE

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