Tectonics Planets, Moons & Rings 9/11/13 movements of the planet s crust
Planetary History Planets formed HOT Denser materials fall to center Planet cools by conduction, convection, radiation to space Radioactive heating declines Crust solidifies, moves on currents below: breaks, bends Volcanoes and impacts break surface Erosion from meteorites, wind, water
Planetary Processes Tectonics Volcanism Impact cratering Weathering and erosion
Simplest Tectonics: As planet cools Early - global volcanism Global expansion causes crust to crack lava leaks through Later - global contraction Mantle and core cool, compress the crust Compressional tectonics Mercury and Moon
Compression Extension Topography caused by mantle flows
Convection heats upper mantle -> raising lava towards surface - stresses crack the lithosphere - tectonism
Faults are where the crust fails, causing deformation Rock acts like silly putty flows slowly cracks when stressed quickly strongly effected by temperature Faults Normal (extension) Thrust (compression) Strike-slip (shearing)
Mercury shrinking as it cools
Horizontal Stresses
Graben Extension stress Rift valley Mars
7000 km long lava river - Baltis Vallis - longest river in the solar system Extensive cracking from: loading by mass of lava shrinking on cooling stresses from underlying convection
Stresses from underlying plume pushing up crust from below
Plate Tectonics = Extreme Tectonism Strong convection drives recycling of crust on time scale of ~100 MY
Colliding Sinking Spreading Compressing Spreading
Plate boundaries: Convergence Ocean-continent convergence
Shearing
Continents - Blocks of lighter crustal rock ("granite scum") moved around by convective motions Ocean ridges - volcanism making new oceanic crust (basalt) Plates collide -> mountain chains Plates subduct -> (ocean) trenches
Seafloor spreading
Mid-ocean spreading Rate measured from magnetic field reversal pattern
Colliding plates -> mountain ranges
Andes - Pacific ocean plate sinks under south american plate
Plate motions from GPS measurements
Plate motions measured with accurate GPS Typically cm / year
Plate tectonics shaped the Earth Seafloor recycling Keeps the seafloor young Ocean ridges and trenches Built and shaped the continents Mountain ranges Tectonic features (e.g. faults) Volcanoes Earthquakes
Are the plates pushed - or do they pull? PUSH - away from expanding mid-ocean ridge PULL - sinking, dense plate material "Eclogite engine": basalt + water -> eclogite denser, sinks, pulling plate lower melting point -> more volcanism
Breakup of Pangea 180 MY ago
Gondwanaland -
Saudi Arabian oil made here
Summary Tectonic movements are preserved on surface Contraction/compression early Mantle motions crack, move surface Ridges, faults, graben formed Earth geology shaped by plate tectonics Young seafloor, mid-ocean ridges, trenches, marginal mountains, subduction
Venus - Isolated mountain blocks, extensive volcanic plains. Plate tectonics? Why resurfaced ~600 million years ago?
Earth Mars Earth Continents Venus Little spread around average Sea floor
Is Earth just have more vigorous convection or is something else involved? Could liquid water be involved? Earth has lots, Venus has none
Differences in surface geology due to their atmospheres? Watery Earth: vigorous recycling of crust / upper mantle lubricated by water in the rocks Dry, hot Venus: static crustal lid holds in heat generated by radioactivity in mantle until erupts in (periodic?) episode(s?) of volcanic resurfacing
Venus completely re-surfaced by volcanism ~600 MY ago Mars - some volcanism, south heavily cratered Mercury - lots of craters, little volcanism Terrestrial Planets Earth - heavily re-worked surface Moon - volcanism stopped 3.2 BYago Lots of craters