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Thiswebsitewouldliketoremindyou:Yourbrowser(AppleSafari4)isoutofdate.Updateyourbrowserformore security,comfortandthebestexperienceonthissite. × EncyclopedicEntry crust Forthecompleteencyclopedicentrywithmediaresources,visit: http://education.nationalgeographic.com/encyclopedia/crust/ “Crust”describestheoutermostshellofaterrestrialplanet.Ourplanet’sthin,40-kilometer(25-mile)deepcrust—just1%of Earth’smass—containsallknownlifeintheuniverse. Earthhasthreelayers:thecrust,themantle,andthecore.Thecrustismadeofsolidrocksandminerals.Beneaththecrust isthemantle,whichisalsomostlysolidrocksandminerals,butpunctuatedbymalleableareasofsemi-solidmagma.Atthe centeroftheEarthisahot,densemetalcore. Earth’slayersconstantlyinteractwitheachother,andthecrustandupperportionofthemantlearepartofasinglegeologic unitcalledthelithosphere.Thelithosphere’sdepthvaries,andtheMohorovicicdiscontinuity(theMoho)—theboundary betweenthemantleandcrust—doesnotexistatauniformdepth.Isostasydescribesthephysical,chemical,andmechanical differencesbetweenthemantleandcrustthatallowthecrustto“float”onthemoremalleablemantle.NotallregionsofEarth arebalancedinisostaticequilibrium.Isostaticequilibriumdependsonthedensityandthicknessofthecrust,andthe dynamicforcesatworkinthemantle. Justasthedepthofthecrustvaries,sodoesitstemperature.Theuppercrustwithstandstheambienttemperatureofthe atmosphereorocean—hotinariddesertsandfreezinginoceantrenches.NeartheMoho,thetemperatureofthecrustranges from200°Celsius(392°Fahrenheit)to400°Celsius(752°Fahrenheit). CraftingtheCrust Billionsofyearsago,theplanetaryblobthatwouldbecometheEarthstartedoutasahot,viscousballofrock.Theheaviest material,mostlyironandnickel,sanktothecenterofthenewplanetandbecameitscore.Overmillionsofyears,thesurface oftheEarthslowlycooledandhardened.Surfacerocksbecamethecrust. Frommudandclaytodiamondsandcoal,Earth’scrustiscomposedofigneous,metamorphic,andsedimentaryrocks.The mostabundantrocksinthecrustareigneous,whichareformedbythecoolingofmagma.Earth’scrustisrichinigneous rockssuchasgraniteandbasalt.Metamorphicrockshaveundergonedrasticchangesduetoheatandpressure.Slateand marblearefamiliarmetamorphicrocks.SedimentaryrocksareformedbytheaccumulationofmaterialatEarth’ssurface. Sandstoneandshalearesedimentaryrocks. DynamicgeologicforcescreatedEarth’scrust,andthecrustcontinuestobeshapedbytheplanet’smovementandenergy. Today,tectonicactivityisresponsiblefortheformation(anddestruction)ofcrustalmaterials. Earth’scrustisdividedintotwotypes:oceaniccrustandcontinentalcrust.Thetransitionzonebetweenthesetwotypesof crustissometimescalledtheConraddiscontinuity.Silicates(mostlycompoundsmadeofsiliconandoxygen)arethemost abundantrocksandmineralsinbothoceanicandcontinentalcrust. OceanicCrust Oceaniccrust,extending5-10kilometers(3-6kilometers)beneaththeoceanfloor,ismostlycomposedofdifferenttypesof basalts.Geologistsoftenrefertotherocksoftheoceaniccrustas“sima.”Simastandsforsilicateandmagnesium,themost 1of6 abundantmineralsinoceaniccrust.(Basaltsareasimarocks.)Oceaniccrustisdense,almost3gramspercubiccentimeter (1.7ouncespercubicinch). Oceaniccrustisconstantlyformedatmid-oceanridges,wheretectonicplatesaretearingapartfromeachother.Asmagma thatwellsupfromtheseriftsinEarth’ssurfacecools,itbecomesyoungoceaniccrust.Theageanddensityofoceaniccrust increaseswithdistancefrommid-oceanridges. Justasoceaniccrustisformedatmid-oceanridges,itisdestroyedinsubductionzones.Subductionistheimportant geologicprocessinwhichatectonicplatemadeofdenselithosphericmaterialmeltsorfallsbelowaplatemadeoflessdenselithosphereataconvergentplateboundary. Atconvergentplateboundariesbetweencontinentalandoceaniclithosphere,thedenseoceaniclithosphere(includingthe crust)alwayssubductsbeneaththecontinental.InthenorthwesternUnitedStates,forexample,theoceanicJuandeFuca platesubductsbeneaththecontinentalNorthAmericanplate.Atconvergentboundariesbetweentwoplatescarryingoceanic lithosphere,thedenser(usuallythelargeranddeeperoceanbasin)subducts.IntheJapanTrench,thedensePacificplate subductsbeneaththeless-denseOkhotskplate. Asthelithospheresubducts,itsinksintothemantle,becomingmoreplasticandductile.Throughmantleconvection,therich mineralsofthemantlemaybeultimately“recycled”astheysurfaceascrust-makinglavaatmid-oceanridgesandvolcanoes. Largelyduetosubduction,oceaniccrustismuch,muchyoungerthancontinentalcrust.Theoldestexistingoceaniccrustis intheIonianSea,partoftheeasternMediterraneanbasin.TheseaflooroftheIonianSeaisabout270millionyearsold.(The oldestpartsofcontinentalcrust,ontheotherhand,aremorethan4billionyearsold.) Geologistscollectsamplesofoceaniccrustthroughdrillingattheoceanfloor,usingsubmersibles,andstudyingophiolites. Ophiolitesaresectionsofoceaniccrustthathavebeenforcedabovesealevelthroughtectonicactivity,sometimesemerging asdikesincontinentalcrust.Ophiolitesareoftenmoreaccessibletoscientiststhanoceaniccrustatthebottomofthe ocean. ContinentalCrust Continentalcrustismostlycomposedofdifferenttypesofgranites.Geologistsoftenrefertotherocksofthecontinental crustas“sial.”Sialstandsforsilicateandaluminum,themostabundantmineralsincontinentalcrust.Sialcanbemuch thickerthansima(asthickas70kilometerskilometers(44miles)),butalsoslightlylessdense(about2.7gramspercubic centimeter(1.6ouncespercubicinch)). Aswithoceaniccrust,continentalcrustiscreatedbyplatetectonics.Atconvergentplateboundaries,wheretectonicplates crashintoeachother,continentalcrustisthrustupintheprocessoforogeny,ormountain-building.Forthisreason,the thickestpartsofcontinentalcrustareattheworld’stallestmountainranges.Likeicebergs,thetallpeaksoftheHimalayas andtheAndesareonlypartoftheregion’scontinentalcrust—thecrustextendsunevenlybelowtheEarthaswellassoaring intotheatmosphere. Cratonsaretheoldestandmoststablepartofthecontinentallithosphere.Thesepartsofthecontinentalcrustareusually founddeepintheinteriorofmostcontinents.Cratonsaredividedintotwocategories.Shieldsarecratonsinwhichtheancient basementrockcropsoutintotheatmosphere.Platformsarecratonsinwhichthebasementrockisburiedbeneathoverlying sediment.BothshieldsandplatformsprovidecrucialinformationtogeologistsaboutEarth’searlyhistoryandformation. Continentalcrustisalmostalwaysmucholderthanoceaniccrust.Becausecontinentalcrustisrarelydestroyedandrecycled intheprocessofsubduction,somesectionsofcontinentalcrustarenearlyasoldastheEarthitself. ExtraterrestrialCrust Oursolarsystem’sotherterrestrialplanets(Mercury,Venus,andMars)andevenourownMoonhavecrusts.LikeEarth, theseextraterrestrialcrustsareformedmostlybysilicateminerals.UnlikeEarth,however,thecrustsofthesecelestial bodiesarenotshapedbytheinteractiontectonicplates. DespitetheMoon’ssmallersize,lunarcrustisthickerthancrustonEarth.Lunarcrustisnotauniformthicknessandin 2of6 generaltendstobethickeronthe“farside,”whichalwaysfacesawayfromEarth. AlthoughMercury,Venus,andMarsarenotthoughttohavetectonicplates,theydohavedynamicgeology.Venus,for instance,hasatpartly-moltenmantle,buttheVenusiancrustlacksenoughtrappedwatertomakeitasdynamicasEarth’s crust. ThecrustofMars,meanwhile,featuresthetallestmountainsinthesolarsystem.Thesemountainsareactuallyextinct volcanoesformedasmoltenrockeruptedinthesamespotontheMartiansurfaceovermillionsofyears.Eruptionsbuiltup enormousmountainsofiron-richigneousrocksthatgivetheMartiancrustitscharacteristicredhue. OneofthemostvolcaniccrustsinthesolarsystemisthatofJupiter’smoonIo.TherichsulfiderocksintheIoniancrust paintthemoonadappledcollectionofyellows,greens,reds,blacks,andwhites. VOCABULARY Term PartofSpeech Definition abundant adjective inlargeamounts. accessible adjective relativelyeasytoapproach,use,orobtain. accumulation noun abuildupofsomething. ambient adjective havingtodowiththesurroundingareaorenvironment. arid adjective dry. atmosphere noun layersofgasessurroundingaplanetorothercelestialbody. basalt noun typeofdarkvolcanicrock. basementrock noun oldestunderlyingrockformationinanyregion. boundary noun lineseparatinggeographicalareas. characteristic noun physical,cultural,orpsychologicalfeatureofanorganism,place,orobject. Conrad discontinuity noun seismicboundarybetweenthecontinentalcrustandoceaniccrust. continental crust noun thicklayerofEarththatsitsbeneathcontinents. convergent plateboundary noun areawheretwoormoretectonicplatesbumpintoeachother.Alsocalledacollision zone. core noun theextremelyhotcenterofEarth,anotherplanet,orastar. craton noun old,stablepartofcontinentalcrust,madeupofshieldsandplatforms. crucial adjective veryimportant. crust noun rockyoutermostlayerofEarthorotherplanet. dappled adjective spotted,orhavingareasofdifferentlycoloredshadesortones. dense adjective havingpartsormoleculesthatarepackedcloselytogether. desert noun areaoflandthatreceivesnomorethan25centimeters(10inches)ofprecipitationa year. dike noun abarrier,usuallyanaturalorartificialwallusedtoregulatewaterlevels. ductile adjective capableofwithstandingacertainamountofforcebychangingformbefore fracturingorbreaking. 3of6 dynamic adjective alwayschangingorinmotion. Earth noun ourplanet,thethirdfromtheSun.TheEarthistheonlyplaceintheknownuniverse thatsupportslife. erupt verb toexplodeorsuddenlyejectmaterial. eventually adverb atsomepointinthefuture. extinctvolcano noun volcanothatwillnolongererupt. extraterrestrial adjective locatedorformedoutsideEarth'satmosphere. geologic adjective havingtodowiththephysicalformationsoftheEarth. granite noun typeofhard,igneousrock. granite noun typeofhard,igneousrock. hue noun tintorgeneralvarietyofcolor. iceberg noun largechunksoficethatbreakofffromglaciersandfloatintheocean. igneousrock noun rockformedbythecoolingofmagmaorlava. iron noun chemicalelementwiththesymbolFe. isostasy noun equilibriumofEarth'scrust,wheretheforcestendingtoelevatelandmasses balancethosetendingtodepressthem.Alsocalledisostaticequilibrium. lava noun moltenrock,ormagma,thateruptsfromvolcanoesorfissuresintheEarth'ssurface. lithosphere noun outer,solidportionoftheEarth.Alsocalledthegeosphere. lunar adjective havingtodowithEarth'smoonorthemoonsofotherplanets. magma noun molten,orpartiallymelted,rockbeneaththeEarth'ssurface. magnesium noun chemicalelementwiththesymbolMg. malleable adjective flexibleandcapableofreformingitselfwithoutbreakingwhenunderstress. mantle noun middlelayeroftheEarth,madeofmostlysolidrock. mantle convection noun slowmovementofEarth'ssolidmantlecausedbyconvectioncurrentstransferring heatfromtheinterioroftheEarthtothesurface. metal noun categoryofelementsthatareusuallysolidandshinyatroomtemperature. metamorphic rock noun rockthathastransformeditschemicalqualitiesfromigneousorsedimentary. mid-ocean ridge noun underwatermountainrange. mineral noun inorganicmaterialthathasacharacteristicchemicalcompositionandspecific crystalstructure. Mohorovicic discontinuity noun pointbetweenEarth'scrustandthemantlebelow.AlsocalledtheMoho. molten adjective solidmaterialturnedtoliquidbyheat. oceanbasin noun depressionintheEarth'ssurfacelocatedentirelybeneaththeocean. oceaniccrust noun thinlayeroftheEarththatsitsbeneathoceanbasins. 4of6 oceantrench noun along,deepdepressionintheoceanfloor. ophiolite noun remnantofoceaniccrust(certainigneousrocks)embeddedincontinentalcrust. orogeny noun thewaymountainsareformed. planet noun large,sphericalcelestialbodythatregularlyrotatesaroundastar. platform noun ancientrocksthatformedaspartofcontinentalcrust,nowoverlainwithsediment andsedimentaryrock,locatedintheinteriorofcontinents. rock noun naturalsubstancecomposedofsolidmineralmatter. sediment noun solidmaterialtransportedanddepositedbywater,ice,andwind. sedimentary rock noun rockformedfromfragmentsofotherrocksortheremainsofplantsoranimals. shield noun ancientrocksthatformedaspartofcontinentalcrustandarelocatedintheinterior ofcontinents. sial noun rocks,mostlysilicatesandaluminum,makingupmostofEarth'scontinentalcrust. silica noun chemicalcompound(SiO2)thatmakesupmostoftheEarth'srocks. silicate noun mostcommongroupofminerals,allofwhichincludetheelementsilicon(Si). sima noun rocks,mostlysilicatesandmagnesium,makingupmostofEarth'soceaniccrust. subduction noun processofonetectonicplatemeltingorgoingbeneathanother. subduction zone noun areawhereonetectonicplateslidesunderanother. submersible noun smallsubmarineusedforresearchandexploration. sulfide noun negativelychargedionofsulfur,orachemicalcompoundcontainingsuchanion. tectonic activity noun movementoftectonicplatesresultingingeologicactivitysuchasvolcaniceruptions andearthquakes. tectonicplate noun large,moveablesegmentoftheEarth'scrust. temperature noun degreeofhotnessorcoldnessmeasuredbyathermometerwithanumericalscale. terrestrial planet noun oneofthefourplanetsclosesttothesun:Mercury,Venus,Earth,orMars. transitionzone noun areasintheEarth'sinteriorbetweentheuppermantle,neartheEarth'scrust,and thelowermantle,neartheEarth'score. uniform adjective exactlythesameinsomeway. universe noun allknownmatter,energy,andspace. viscous adjective liquidthatisthickandsticky. volcano noun anopeningintheEarth'scrust,throughwhichlava,ash,andgaseserupt,andalso theconebuiltbyeruptions. ForFurtherExploration Audio&Video NationalGeographicChannel:HowtheEarthChangedHistory—BeneaththeCrust 5of6 Websites NationalGeographicScience:InsidetheEarth USGS:EarthquakeHazardsProgram—CrustalDeformationMonitoring ©1996–2015NationalGeographicSociety.Allrightsreserved. 6of6