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 37PTYS 554Evolution of Planetary SurfacesWeathering and Fate of SedimentsWeathering and Fate of SedimentsPYTS 554 – Weathering and Fate of Sediments2Production of sediments on terrestrial planetsSource rocksPhysical and chemical weatheringProduction of clays and oxidesTransport and deposition of sedimentsLandscape evolution and sedimentary basinsSize-sorting and Desert pavementStratigraphy of sediments – ripple laminae, cross-bedding etc…Burial and metamorphism of sedimentsDiagenesisKinds of metamorphismThe following 5 lecturesPYTS 554 – Weathering and Fate of Sediments3Most sediment comes from weathering initially solid rocks Ultimately, almost all terrestrial planet sediments were volcanically producedIntrusive volcanismSurface flowsAsh flows/falls1. Rocks need to be broken up2. Sediment needs to be transported and lithified3. Buried sediment can be metamorphosedPYTS 554 – Weathering and Fate of Sediments4Tephra/pyroclastic depositsBombs/blocks > 64mmCinders/Lapilli 2-64mmAsh < 2mmEscaping volatiles drive eruptions of different strengthsMeasured by % fragments < 1mm in sizeMeasured by area covered by tephraHigh-silica magmas drive the most explosive eruptionsSchmincke 2004PYTS 554 – Weathering and Fate of Sediments5Volcanic ash can be unconsolidated or easily remobilizedPYTS 554 – Weathering and Fate of Sediments6Crust of airless bodies suffers many impactsRepeated impacts create a layer of pulverized rockOld craters get filled in by ejecta blankets of new onesRegolith grows when crater breccia lenses coalesceAssume breccia (regolith) thickness of D/4Maximum thickness of regolith is Deq/4 , but not in all locationsSmaller craters are more numerous and have interlocking breccia lenses < Deq/4Shoemaker et al., 1969Growth of RegolithPYTS 554 – Weathering and Fate of Sediments7Minimum regolith thickness:Figure out the fractional area (fc) covered by craters D→Deq where (D < Deq)Choose some Dmin where you’re sure that every point on the surface has been hit at least once Typical to pick Dmin so that f(Dmin,Deq) = 2hmin of regolith ~ Dmin/4General caseProbability that the regolith has a depth h is: P(h) = f(4h→Deq) / fminMedian regolith depth <h> when: P(<h>) = 0.5Time dependence in heq or rather Deq α time1/(b-2) hmin=heq4 b- 2( )fminpbceq+1é ë ê ù û ú - 1b- 2( ) h =heq2heqhminæ è ç ö ø ÷ b- 2+1æ è ç ç ö ø ÷ ÷ é ë ê ê ù û ú ú - 1b- 2( )PYTS 554 – Weathering and Fate of Sediments8Impact brecciaUnsorted angular fragmentsPYTS 554 – Weathering and Fate of Sediments9All rocky airless bodies covered with regolith (‘rock blanket’)Moon - Helfenstein and Shepard 1999Itokawa – Miyamoto et al. 2007Eros – NEAR spacecraft (12m across)Miyamoto et al. 2007PYTS 554 – Weathering and Fate of Sediments10Thermal weatheringRocks expand and contract with changing temperatureThermal response timeStresses increase with steeper thermal gradientsRate of change of the surface temperature is a measurable proxy for rock thermal gradient t »pkd2where k =kr cDayNightSurfacecompressionSurfacetensionROCK ROCKPYTS 554 – Weathering and Fate of Sediments11Temperature gradients of 2 K/minute…Thermal fatigue at lower rates can accumulate over timeAided by heterogeneous nature of rocksMolaro and Byrne 2012Eros, Dombard et al. 2010PYTS 554 – Weathering and Fate of Sediments12Sediments have much lower conductivity than rocksConductivity variations Material differencesInduration (compaction, cementing agents etc…)Radiative heat transfer in regolithsRadiation across pore spaces is important in very hot regolithOnly really important for MercuryRatio of radiative to solid conduction varies widely ~0.1 for densely packed grains~1.5 for fluffy surface layerConductivity from gas in pore spacesNot a factor on the Moon/asteroids etc…Can assume a conductivity of solid material and calculate pore sizePore size ~~~~ grain size, so thermal inertia can be used to estimate grain sizeOnly works because pore size ~ mean free path of atmospheric moleculesi.e. only works on MarsPYTS 554 – Weathering and Fate of Sediments13Mike Mellon, U. ColoradoComposition varies little on MarsThermal inertia variations from induration and grain-size variationsThermal inertia on MarsLow – dust coveredHigh – rocky or IcyMost Geologic materials have value of (ρ.c)0.5 = 900-1400 (factor of ~1.5)Differences in thermal inertia mostly measure differences in thermal conductivity I = kr cPYTS 554 – Weathering and Fate of Sediments14Other physical weathering mechanismsFrost weatheringNot driven by the 9% volume increaseThin films of liquid water exist below the freezing point – surface energy effectWater migrates to form ice lens(generates all sorts of periglacial activity)Thermomolecular forces can generate kilobar pressuresPressure from the surrounding rock increases which lower the melting point and stops the processCan easily propagate cracksRempel 2007Taber 1930PYTS 554 – Weathering and Fate of Sediments15Other physical weathering mechanismsSalt weatheringBrine can fill rock poresEvaporation leaves salt crystalsHydration of crystals or thermal expansion forces rock cracks openGranular disintegration of graniteRocks are a heterogeneous messHydration of some minerals more effect than othersIn Granite, biotite becomes hydrated, expands and forces other crystals apartGruss = disintegrated graniteSand-sized debris is mostly quartz with less feldsparMost continental crust on Earth is granitic so most sand on the Earth is quartzPYTS 554 – Weathering and Fate of Sediments16Pre-existing cracks control weatheringJoints – often from tectonic unloadingSpallation/exfoliationCracks parallel to the free surfaceSurface-parallel compressive forces and surface curvature•Produces near-surface normal tensionMartel 2006PYTS 554 – Weathering and Fate of Sediments17Chemical