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EncyclopedicEntry
crust
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“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
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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
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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.
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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.
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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
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Websites
NationalGeographicScience:InsidetheEarth
USGS:EarthquakeHazardsProgram—CrustalDeformationMonitoring
©1996–2015NationalGeographicSociety.Allrightsreserved.
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