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Chapter2 Roets
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Here，Iwilldiscusstheplaweliveonandotherbodieslikeit，herterrestrialplasMerus，andMars，andalsotheMooroheIAU，theMoonisjustasatellite，butitspositioructureplagtheterrestrialplaheperspectiveofageeophysicist.Figure3showsthesefiveatthesamesdTable3listssomerelevantdata.Withinthisgroup，MerdtheMooivelynoatmosphere.Venushasonlyslightlylowersize，mass，ahah，sogravityatitssurfalyslightlylessthah.However，itsatmosphereissiderablydenser.Marsislargerthalessdewoeffectsoffseteachothersothattheirsurfacegravitiesareverysimilar，butbeingarshasbeeoholdontoathiableatmosphere.TheMoosurfacegravityofall–aboutohoftheEarth’s–alkersboundaroundsely.Meauresobscurewidevariationswithlatitudeand，insomecases，betweendayandnight.Forexample，thehottestdaytimetemperatureonMercuryexceeds400°C，whereasatdawnafteralhtthetemperatureisbelow–180°C.
Table3 Basicdatafortheterrestrialplas
3.Top:fromlefttht，Merus，Earth，Moon，andMars，shownatthesames:themuchlargergiaer，Saturn，Uranus，aheterrestrialplahesamescale
Cores
&rialplainguishedbyhavieriors，madelargelyofsilierals.However，theirdeoogreatforthemtoberhout，anditisbelievedthateairon-richcoreatitsopla’sbeseenorsampleddirectly，butthereareseveralilinesofevideyisone，showingthattheibedehanroallowingforinterhighpressure，ahetrajectoriesspacecraftshowthatdensityiricallyabouteaet’stre.Chemicalmodelsforwhatislikelytohappeninsidearoetsuggestthatthereisioxygenforalltheirontobeoxidizedandboundupinsilierals.Thus，iftheinteriorhadeverbeehiswouldhaveallowedmetalli，whiserthanroktowardsthetre.Thisisanexampleofaprocessescalleddifferentiation.
&erpartsoftheiron-richcoresoftheEarthandMercurymustbemoltentoday，becausethosetwoplaroicfields，appareedbydynamomotioridugfluid.Forsuchasmallpla，Mersityisveryhigh，soitsustbeexallylarge，ogabout40%ofitsvolumeandagfornearly75%ofitsmass.MagicfieldsarenotbeiediheMoon，andMars，sotheircoresareprobablyentirelysolid.
IheEarth，wehaveadditiothestudyinghowseismicwaves，whicharevibratieredbyearthquakes（rous!），travelthroughtheplahissasolidinnercore1，215kilometresinradiusandafluidouteretresinradius.Bothseemtobemainlyiroh5%–10%densityargumentsrequiresomethihanirontoo，makingup6%–10%oftheouterd2%–5%oftheihelikeliestexplanationissomemixofoxygen，silidsulfur.
Intotal，theEarth’scoreoccupiesabout16%ofthepla’svolume.parablevaluesforVenusandMars，whichareestimatesbasedlargelyedensities，areabout12%aively.Therearesomeverylimitedseismioon（fromtheApramme），hintiivelysmallcorebetweenabout220and450kilometresinradius（lessthan4%oftheMoon’stotalvolume）.About1ieoritesismadeofanalloyofironwith4.5%–18%nickel，dingtothecoresofplaesimalsfromtheasteroidbeltthathaddiffereernallybefbrokenupbys.
Mantlesandcrusts
&epartofaterrestrialplasurroundingitscoreiscalledthemahismakesupthemajorityofthetotalvolumeofeachterrestrialpla，andmostoftheirmassexceptfor
&hecrustisarelativelyminoruhemaisalsoofsilicate，thoughslightlydifferentinpositiole.
Aplaleevolvedfromthemoltenrockthatwouldhaveclobeafterthefinalgiantimpact，ksasa‘magmao’.Whileamagmaocools，itssurfacemustradiateheattospagittochilltoasolidskihisskinwoulduallybebrokenandstirredba，thankstoturbulenpaabove.Themagmaowoulduetocool，but，uhefreezingofaballofwater，thereisemperatureatwhichthewholeofitwouldbeesolid.Theeerialissuchthatmineralsofvariouspositionscrystallizeoutatdiffereuresandpressures.Plaarystistsareuheextenttowhichmagmaoscrystallizedinlayers，orwhethermihawereabletosinkwhileless-densemiorise，perhapsstigtogethertoformmassive‘rs’thatcouldforcetheirmoreeffectively.
&ionsofthisflotationmaterial，chemicallydifferenttotheunderlyingmagmaoedtheearliesttrueoosurvivestodayasthelunarhighlands（thepaleareasontheMoohelargerterrestrialplaureoftheoldestcrusthasermilybecauseithaslargelybeeleast，coveredover）bylaterkindsofcrust.Toseehowthismighthappeurionbaantle.Asayoucools，eventuallyitsmantlebeesfullysolid.Twoimportantcharacteristicsofsilicatematerials.Thefirstisthatsuffitlyhotsolidsareelyimmobilenorundeformable.Hotroaplaerioriscapableoffloeedsofafewtimetresperyear（therateatwhichyernailsgrow），inmuchthesamewayasablockofpitchwilldeformovertime.Withinasolidmaionwilloccurataslowbutgeologicallyeffectivepaceifthereareanyforcescapableit.I，thenecessaryimpetusheat.Hottermantlefromdeepdowlylessdeheahereisateoslaces.Motionofthissortisvediswhatyouobserveinapanofsoupheatedohatlaismuchslower‘solidstate’.
Imagireameror‘plume’ofhotmantlewellinguplaantledownwards.Asitgetsclosertothesurface，thepressureexperieherisingmantledecreases，whigsintoplaythesedrelevantcharacteristic.Aspressurefalls，silicatesbegiheprocessiscalled‘partialmelting’，belypartofthesolidmelts，ahatformsisslightlyrisilithesolidfromwhichitwasextracted.Theresultingmagmaisalsolessdehesolid，sobuoyancyforceswillsqueezeitupwardstowardsthesurface，especiallyiftherearepathwayswheretherodertensionorfralessitstallsbelowgroundasanintrusion，themagmawillbeeruptedthroughvoloes.
Roedinthiswayisdescribedasigneous，andcrustprodueousactivityreplacetheinalcrustofaplabyinfiltrationorburial.Thedarkpatoon，thelunar‘maria’，areliohepalerprimarycrusthasbeenburiedbylavaflowsproduthisresentdaycrustoftheEarthresultsfrompartialmeltiomakeoicdfrommeltingandreaionsofoiaketalcrust.TheEarth’soiccrustis6–11kilometresthick，whereastalcrustvariesfromabout25kilometresinthichedregionsto90kilometresbelowmajormes.Intotal，crustolyabout1%oftheEarth’stotalvolume.TheMoon’scrustaveragesabout70kilometresinthiess（13%oftheMoon’svfrom〉100kilometresinsomehighlao〈20kilometresbeloas.
Insummary，crustischemicallyrelatedtotheunderlyidiffersinendingonhowitwasextrait.Crustisloweriyanditsaveragepositionisrisilithemaismorevariedthanmantle，ahathasreactedchemicallyhereorliquidwater，andthathasbeeordissolved，transpravity，wind，water，oriddepositedelsewhere.Suchdepositsstitutesedimentaryrock.Burial，deformatioingcausesedimentaryneousrocktorecrystallize，inwhichcaseitiskamorphicrock.
&
&sarehotilybecauseofheatleftoverfromtheira.Ferplaionofthis‘primordialheat’remainingtodayisgreater.Thisisbecauseheattisrelatedtoplaaryvolume，whidsoheradius，whereasheatleakageislimitedbysurfacearea，whidsonlyonthesquareoftheradius.
&isalsogeedi，bydecayofradioactiveisotopes.Therearemaonlyfourroduifitheating:potassium-40，uranium-238，uranium-235，andthorium-232.Becauseeochemiities，theseelementsaremoreabundantincrustalroile.Ih，approximatelythesameamouisgeedradiogenially（thatis，byradioactivedethecrustasinthewholeofthevolumetricallymuchlargermantle.
&rialplaaltelemesmass（asvolume）.Justlikeprimordialheat，radiogeaiivelyis.IheEarth，abouthalftheheatleakingtothesurfacetodayisprimordial，andalmostalltherestisradiogenic.
Lithospheres
&ioiesfromdrigidtowarmaivegenerallyocedepthbelowthebouweendmahedtheuppermostmauteasinglemeicallayer，fidoutershell.Thisshelliscalledthe‘liththeGreekwordlithos（‘rodicatethatthelayerhasthemeicalpropertiesofeverydayrock.Belowthelithosphereiswherethemahoughroposition，ishotenoughaovect.ThiszoimescalledtheasthenGreeka-sthenosmeaning‘withth’）.
&h’slithosphereisabout100kilometresthidisfratoaes，whichareabletobeshuntedaroundthankstothespeessoftheunderlyingastheofaproas‘plateteiewlithosphereiscreatedullapart（usuallyhiddeheoddestroyedwhereoeisdrawher，atsubduarkedbytreheoflrindieagainstitsneighbouristheostearthquakes.IfahattheEarth’splatesare‘crustslidihemaheyarewrapersistentfallacythatappearsinmanyschooltextbooksaionsyllabuses.Thetruthisthataplatesistsofdtheedrigiduppermostmaogetherslideacrossthedeeper，lessrigid，asthele.
&hbrittle，isthelayerwherefaultsoassrindspastasareoh，espethezoesmeet，aifiedostoo（Figure4）.
&oobeuheEarth.GreaterlithospheriessinthemoreeasilyallerbodiesofMercury，theMoon，andMarsuributetothis，butamoreimportantfactoristhatforplatestobemobile，thetopoftheastheobeespeciallyweak.Withihisisaplishedbeallamouhintherock，whisitandeheformationofasmallamoulubricatesgrainboundaries.Venushaslostitswater，soitsasthenosphereisdryandlithosphericplatesotslidefreelyacrossit.
4.A500-kilometre-wideviewofpartofMerinationisfrht.ShadowpicksoutakilhscarpwiththeshapeofaerMohisisafaultnamedBeagleRupes，markierrai（east）hasbeewardsovertheterrai（west）.Someofthecratersareolderanderthanthisfault
Aplaheisdry，orverydeep，ismaedprincipallybytwoeffectsatthesurfaeistheheightofmountaihofbasioogreat，theasthenospherewillflowahelithosphere，therebyredugthetopographitrastuntilitissmallenoughtobesupportedbythestrengthofthelithospherealohepatterncausedbylargeimpaimpactorseveraltensofkilometresierarriveswithsuffitforcefingshockwavestodisruptthelithosphere，aerwilltaketheformofabasinmarkedbytrigsoffraathihosphere，theriobeclether，sothesemulti-ribasioestimatethedepthtotheastheimeoftheirformation.Asaplaslowlycools，itslithospherebeesgraduallythicker.
Volism
Magma，theeerupts，eratediessentiallybythreediffereappliofheatisohese，aheleastimportant:slowbuild-upofheattrappedbelolahospheremayaeepisodesofwidespreadvoldstrtidalstressesiarybainstinternalfri，leadingto‘tidalheatiively，degpressureinanupwellihemantlecausepartialmelting（forexample，leadiioh’soiccrust）.Also，itispossiblethatsuddeionihappelebelowaisexcavated，cerameltihirdmeismistoierileorlowercrust.WaterreducesthetemperatureatwhichsilicatesbegiheEarthhassofvoloesabovesubdues，becausewaterthathasbeendraggeddownwithintherocksofthesubdugplateescapesupwardsintothebaseoftheplate.Heresarenothotenwhendry，butpartialmeltingwillbeginassooroduthoughtherehasbeeemperature.
TheMoon
Peoplebegaeaboutvoloonalmostassoonascraterswereseenthroughtelescopes.Theywereorailbecause，asweareain，almostalltheoonafact，themajorvolioochesthatwereotobedried-upseabeds.Thatishoughtheystillbearthename‘mare’（pronouncedmah-ray），whichisLatihepluralis‘maria’（pronouncedmah-ri-a）.Thesecoverabout17%oftheMoon’ssurface，mostlyonthenearside，whichisthehemispherethatpermalyfacestheEarth.Here，lavaositionsimilartoterrestrialbasalthasfloodedthemajedimpas.
Spetswherethemarebasaltswereeruptedarehardtoidentify（Figure5）.Clearly，theydidheformofioes.Mostlikely，theywerefissuresthroughwhitainsofienlavawereexpelledbytheforceofexpandingvolicgastoheightswellinexetre.Onfallingtotheground，thelavawasstillhotenoughtospreadout，flowingdownhillforhundredsofkilometres.Mostofthefissurevehemselvesoverastheirrateoferuptionwaned，orwereburiedbylatereruptions.
FourofthesixApolloMoonlandings（1969–72）wereonmaria，whichareflatterandsosaferpladthanthehighlands.Samplesofmarebasaltbroughtbaalysisbedatedwithhighpreeasuringradioactivedecayproduthem（radi），andtheApollosamplessheesfrom3.9to3.1billiohislongdurationofvolismputpaidtothesimplestvoliationforthemaria，whichwasthatthevolismroductofthebasin-fimpacts.Furthermore，worksiheyear2000hasidentifiedsomepatarebearingsuffitlyfewsuperimposedimpactcratersthattheymustbeyouhanabout1.2billionyears.versely，in2mentoflunarmaterialfouhasmeteorite（havingpreviouslybeenflungofftheMooafromanimpactcrater）wasfoundtofragmentsofbasaltdatedat4.35billionyears，halfabilliohelateheavybombardmentended.Nomariaeremainvisible，havingbeenburiedbyejesubsequentbasin-fimpaothatlunarvolismbeganearlyaswellasfinishinglate.
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