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Thiswebsitewouldliketoremindyou:Yourbrowser(AppleSafari4)isoutofdate.Updateyourbrowserformore security,comfortandthebestexperienceonthissite. × EncyclopedicEntry mantle Forthecompleteencyclopedicentrywithmediaresources,visit: http://education.nationalgeographic.com/encyclopedia/mantle/ ThemantleisoneofthethreemainlayersoftheEarth.Itliesbetweentheinnermostlayer,thecore,andthethin outermostlayer,thecrust.Themantleconsistsofhot,dense,semisolidrockandisabout2,900kilometers(1,802 miles)thick. Themantleisdividedintoseverallayers. Layersofthemantle Lithosphere.Thethinoutermostshelloftheuppermantleissimilartothecrust,thoughcoolerandmorerigid.Together withthecrust,thislayeriscalledtheEarth’slithosphere. Asthenosphere.Thelithosphereisactuallybrokenupintoseverallargepieces,orplates.They“float”onasoftermantle layercalledtheasthenosphere.Theirveryslowmotionisthecauseofplatetectonics,aprocessassociatedwith continentaldrift,earthquakes,volcanoes,andtheformationofmountains. Uppermantle.Belowtheasthenosphereliesanotherlayer,strongerandmoresolidthantheasthenosphere.Alllayers belowthecrustdowntoadepthofabout670kilometers(416miles)areknownastheuppermantle. Lowermantle.Therestofthemantlebetweentheuppermantleandthecoreisknownasthelowermantle.Itisdenser andhotterthantheuppermantle. ExploringtheDeepEarth MostoftheEarth’sinteriorismuchtoodeepforustoexploredirectly.Instead,scientiststellthemantleapartfrom thecrustandcorebymeasuringthespreadofshockwavesfromearthquakes,calledseismicwaves.Twotypesof seismicwavespassthroughtheEarth’sinterior:P-waves,whichrepresentverticalmotion,andS-waves,which representhorizontalmotion.Instrumentsplacedaroundtheworldmeasurethesewavesastheyarriveatdifferent pointsontheEarth’ssurfaceafteranearthquake. Seismicwavestravelatdifferentspeedsandstrengthsthroughdifferentmaterial.Forexample,surfacewaves fromapowerfulearthquakenearNorthridge,California,in1994took30minutestoreachapointabout6,700 kilometers(4,163miles)away,butittookP-wavesonly10minutesandS-wavesjustunder20minutestotravelthe samedistance.Nearthesurface,P-wavestravelabout6kilometerspersecondthroughtheground.Belowa certainpoint—anaveragedepthofabout35kilometers—theytravelabout8km/s,indicatingthatthewaveshave reacheddensermaterialatthatpoint. Thisabruptdividebetweenslowerandfasterspeedsmarkstheboundarybetweenthecrustandthemantle.Itis 1of3 calledtheMohorovicicdiscontinuity,orsimplytheMoho.Atthebaseofthemantle,2,900kilometers(1,802miles) belowthesurface,S-waves,whichcan’tcontinueinliquid,suddenlydisappear,andP-wavesarestrongly refracted,orbent.Thispoint,calledtheGutenbergdiscontinuity,markstheendofthemantleandthebeginningof theEarth’sliquidcore. VOCABULARY Term PartofSpeech Definition asthenosphere noun layerinEarth'smantlebetweenthelithosphere(above)andtheuppermantle (below). boundary noun lineseparatinggeographicalareas. continentaldrift noun themovementofcontinentsresultingfromthemotionoftectonicplates. core noun theextremelyhotcenterofEarth,anotherplanet,orastar. crust noun rockyoutermostlayerofEarthorotherplanet. dense adjective havingpartsormoleculesthatarepackedcloselytogether. Earth noun ourplanet,thethirdfromtheSun.TheEarthistheonlyplaceintheknown universethatsupportslife. earthquake noun thesuddenshakingofEarth'scrustcausedbythereleaseofenergyalongfault linesorfromvolcanicactivity. Gutenberg discontinuity noun pointbetweentheEarth'smantleandthecorebelow. horizontal adjective left-rightdirectionorparalleltotheEarthandthehorizon. lithosphere noun outer,solidportionoftheEarth.Alsocalledthegeosphere. lowermantle noun bottomlayerinEarth'smantle,closesttothecore. Moho noun pointbetweenEarth'scrustandthemantlebelow.AlsocalledtheMohorovicic discontinuity. Mohorovicic discontinuity noun pointbetweenEarth'scrustandthemantlebelow.AlsocalledtheMoho. mountain noun landmassthatformsastectonicplatesinteractwitheachother. platetectonics noun movementandinteractionoftheEarth'splates. P-wave noun seismicshockwavethatrepresentsverticalmotion.Alsocalledaprimarywave. refract verb tobendorchangethepathofsomething. rigid adjective stiff. rock noun naturalsubstancecomposedofsolidmineralmatter. seismicshock wave noun vibrationthattravelsfromthefocusofanearthquakeinalldirections. seismicwave noun shockwaveofforceorpressurethattravelsthroughtheEarth. S-wave noun seismicshockwavethatrepresentshorizontalmotion.Alsocalledasecondary wave. 2of3 uppermantle noun layerinEarth'smantlebetweentheasthenosphere(above)andthelower mantle(below). vertical noun up-downdirection,oratarightangletoEarthandthehorizon. volcano noun anopeningintheEarth'scrust,throughwhichlava,ash,andgaseserupt,and alsotheconebuiltbyeruptions. ForFurtherExploration Articles&Profiles NationalGeographicNews:NewMagmaLayerFoundDeepinEarth'sMantle? Websites USGS:TheInterioroftheEarth ©1996–2015NationalGeographicSociety.Allrightsreserved. 3of3