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mantle
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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
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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.
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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
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