PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40PTYS 554Evolution of Planetary SurfacesForming Planetary Crusts IIIForming Planetary Crusts IIIPYTS 554 – Forming Planetary Crusts III2More detail on planetary structureMostly known from seismic velocity dataPREM – preliminary reference earth modelMixed terminology for composition and mechanical distinctionsMohoTransition zone400-600kmlowerupperD’’Not to scalePYTS 554 – Forming Planetary Crusts III3Original planetary crusts from silicate differentiationCalcium-rich plagioclase feldspar (anorthosite)Floats in an anhydrous melt – moon, mercury?Sinks in a hydrated melt – Earth, Mars, VenusUnstable at high pressures – so sinking anorthosite is doomedOlivineSinks in shallow magma oceanStable to depths of ~400km on Earth – becomes spinelAt 600km on Earth spinel becomes perovskitePyroxeneSinks in shallow magma oceanStable similar depths as OlivinePYTS 554 – Forming Planetary Crusts III4Lunar anorthosite crust forms with specific REE abundance patternMantle is depleted in the opposite pattern Later sampled by mare basaltsPYTS 554 – Forming Planetary Crusts III5A basaltic initial crustPartial melting of bulk silicate materialMelts are not the same composition as the bulk solidMelts rich in pyroxene and plagioclase feldsparBasalt is a broad term (to be expounded upon in the volcanism lectures!)Variations in water contentVariations in alkali metal contentVariations in silica contentMantles became depleted in pyroxene and feldsparAnd enriched in OlivineMars/Venus retain basaltic crustsEarth took a different pathBowen’s reaction seriesDiscontinuousContinuousPYTS 554 – Forming Planetary Crusts III6Venus rock compositionSampled in only 7 locations by Soviet landersComposition consistent with low-silica basaltExposed crust is <1 Gyr old thoughVenera 14PYTS 554 – Forming Planetary Crusts III7Martian in-situ and orbital measurementsCrust dominated by basalt (GRS + MER)With a thin weathered coating (TES)McSween et al., 2009PYTS 554 – Forming Planetary Crusts III8Earth has two types of crustOceanic crust – low-lying Continental crust – high-standing Oceans cover oceanic crust and some of the continental crustOceanic ContinentalDensity 3000 kg m-32700 kg m-3Thickness 5km 10-80kmComposition Basalt GraniteAge <0.1 billion years > 1 billion yearsOcean rigid mantle materialOceanic crustContinental crustOceaniccrustContinental crustAsthenospherePYTS 554 – Forming Planetary Crusts III9Water sequesterization in pargasite may in part control lithospheric thicknessOceanic lithosphere <100kmCrust ~5kmContinental lithosphere 40-200km10-80 kmPYTS 554 – Forming Planetary Crusts III10Most plate move at centimeters/yearMeasured with GPS and Very Long Baseline InterferometeryPlate motionPYTS 554 – Forming Planetary Crusts III11Past plate motions can be reconstructedUsing sea-mount traces as plate moves over hotspotsMagnetization direction of rocks vs. timePress & Siever, 2nd editionPYTS 554 – Forming Planetary Crusts III12Oceanic spreadingSpreading ridge formsHot material is bouyantNew oceanic lithosphere is addedContinental spreadingRift-valley formsNew oceanic lithosphere is added Continental crust soon becomes a passive marginSpreading and new crust formationPress & Siever, 2nd editionPYTS 554 – Forming Planetary Crusts III13We can calibrate the spreading rate of mid-ocean ridges by looking at their symmetric magnetic lineations.Compare with known magnetic reversals such as the Gilbert and Matuyama periodsPress & Siever, 2nd editionPYTS 554 – Forming Planetary Crusts III14Creation of new oceanic crustCharacteristic stratigraphic sequence:Gabbro(large grained basalt)Sheeted dikesEach sheet was the wall of the inner ridgePillow basaltsBlobs of basalt that are quickly quenchedOcean sedimentsFine-grained mudsCalled an ophiolite sequenceCan be obducted onto a continental settingE.g. Southwest GreenlandIsua supracrustal belt 3.8 GaPillow BasaltPYTS 554 – Forming Planetary Crusts III15Crust moves away from spreading centersCrust near the spreading centers is still youngPYTS 554 – Forming Planetary Crusts III16We could assume the followingTemperature at the spreading center is that of the asthenosphere (Ta)No heat production in crust and temperature field is stableAssume lateral heat diffusion is slower than plate motion, and u is constant:Solution is the error function:(half space cooling model)Thermal evolution of oceanic crustxTuxTzTCkP2222tTzTCkP22T z, t( )- TwaterTa- Twater=erfz2 ktæèçöø÷»T z, t( )TaA couple of useful applicationsOcean heat flux (Q): 21210 xttkTzTkxortQazPYTS 554 – Forming Planetary Crusts III17A couple of useful applications (cont.)Lithospheric thickness (L):Base of lithosphere is determined by temperature•usually ~1100 KAsthenosphere temperature (Ta) ~ 1300 KTake thermal diffusivity to be about 10-6 m2 s-11100 =1300erfL2 ktæèçöø÷L =2 k erf- 111001300æèçöø÷t12L =2.016 ´10- 3t12L1kmæèçöø÷=11t1Myræèçöø÷12 tzerfTtzTa2,Oceanic lithosphere thickens over time (distance from spreading ridge)Thickening causes it to sink – isostatic responseAdding cold mantle material to the lithosphere cause density to increase (thermal contraction)Oceanic lithosphere tends to end up being negatively buoyant by the time it reaches another plate – it’s eager to be subducted…PYTS 554 – Forming Planetary Crusts III18Plate Tectonic control of sea-levelAbout 10% of Earth’s historyMore young oceanic lithosphere means seawater is displaced onto landPYTS 554 – Forming Planetary Crusts III19Continents can be rifted apartUsually split along old suturesAfrica currently breaking upFlood basalts Horst and graben from extensionNew oceanic crust being formedSpreading within