Gravity Tectonics Volcanism Atmosphere Water Winds Chemistry. Planetary Surfaces

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1 Gravity Tectonics Volcanism Atmosphere Water Winds Chemistry Planetary Surfaces

2 Gravity & Rotation Polar flattening caused by rotation is the largest deviation from a sphere for a planet sized object (as opposed to non-spheric objects that miss the planetary cut-off due to insufficient self gravity). Sometimes will be described in terms of inverse flattening, i.e. 1/f to get ratios for planets: Earth 1:298, Saturn 1:10, Jupiter 1:16, Moon 1:900, Sun < 1:1000 f = a b a a is the equatorial radius, b is the polar radius

3 Gravity & Rotation Polar flattening caused by rotation is the largest deviation from a sphere for a planet sized object (as opposed to non-spheric objects that miss the planetary cut-off due to insufficient self gravity). The amount of flattening will depend on the relation between gravity and centrifugal force, as well as on the size, density and elasticity of the body. f = a b a a is the equatorial radius, b is the polar radius EAS 4803/ CP 18:3

4 Gravity & Rotation The equipotential surface created by the rotation of an ellispe about its minor axis is an ellipsoid, whereas the geoid is the equipotential surface that best matches the mean sea level of the Earth. Topography and tides of a planet are measured with respect to its geoid

5 Earth Mars Earth Continents Venus Little spread around average Sea floor

6 Gravity & Rotation Gravity also shapes the surface of planets as it partially determines the angle of repose of given materials, and drives mass movements such as landslides/avalanches (fast particle laden gravity currents) as well as creep motions such as glacier motion and lava flows.

7 Gravity Tectonics Volcanism Atmosphere Water Winds Chemistry Planetary Surfaces

8 Tectonic Activity Any crustal deformation caused by motions of the surface. Deformation of a material due to an applied stress (force per unit area) is characterized by the strain (dimensionless): Melosh (2011) longitudinal strain ε l = Δl / l shear strain ε x = Δx / b θ volume strain ε V = ΔV / V

9 Tectonic Activity Any crustal deformation caused by motions of the surface. Deformation of a material due to an applied stress (force per unit area) is characterized by the strain (dimensionless) Elastic materials will respond to stress, but regain original properties when stress is removed Hooke s law: σ l = E ε l E is Young s modulus (like a spring constant) σ s = 2µ ε s µ is shear modulus p = -K ε V K is bulk modulus

10 Tectonic Activity Any crustal deformation caused by motions of the surface. Deformation of a material due to an applied stress (force per unit area) is characterized by the strain (dimensionless) Elastic materials will respond to stress, but regain original properties when stress is removed ε ij = 1 2 $ u i + u j & % x x j i ' ) ( u i is the displacement or deformation component

11 Tectonic Activity Any crustal deformation caused by motions of the surface. Deformation of a material due to an applied stress (force per unit area) is characterized by the strain (dimensionless) Viscous materials will deform or flow in a slow smooth way when stress is exerted Newtonian viscosity: σ s = 2η dε s /dt η is viscosity

12 Tectonic Activity Any crustal deformation caused by motions of the surface. Deformation of a material due to an applied stress (force per unit area) is characterized by the strain (dimensionless) Viscous materials will deform or flow in a slow smooth way when stress is exerted ε ij = 1 2 $ u i + u j & % x x j i ' ) ( u i is the displacement or deformation component

13 Tectonic Activity Materials can behave both elastically and viscously; viscoelastic materials may behave elastically on short time periods but viscouly on longer (geologic) timescales silly putty! Usually at low temperatures materials tend to be brittle, and at high they tend to be ductile (much deformation before fracturing) τ rx ν vρ µ rg Where τ rx is the viscoelastic relaxation time, or the ration between the rigidity (µ rg ) and the dynamic viscosity (ν v ρ)

14 Elastic vs. plastic deformation

15 Faults Faults are where the crust fails, causing deformation Rock acts like silly putty Flows slowly Cracks when stressed quickly Strongly effected by temperature Normal (extension) Thrust (compression) Strike-slip (shearing)

16 SIMPLEST Tectonics - As planet cools Early - global volcanism Global expansion caused crust to crack lava leaked through Later - global contraction Mantle and core cooled, compressed the crust Compressional tectonics Mercury

17 Mercury: Shrinking as it cools

18 Horizontal Stresses

19 Graben Extension stress Rift valley Scarps Mars

20 Vertical Stresses

21 Stresses from underlying plume pushing up crust from below

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23 Plate Tectonics Strong convection drives recycling of crust on time scale of ~100 MY

24 Colliding Sinking Spreading Compressing Spreading

25 Mid-Ocean Ridge Seafloor spreading

26 Mid-Ocean Ridge Mid-ocean spreading Rate measured from magnetic field reversal pattern

27 Plate motions measured with accurate GPS Typically cm / year

28 Plate boundaries: Convergence Ocean-continent convergence

29 Colliding plates == mountain ranges

30 Andes - Pacific ocean plate sinks under South American plate

31 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

32 Venus - Plate tectonics? Volcanism? Tectonics? Resurfaced ~600/700 MY ago - How?

33 Why does Earth have Plate Tectonics, but not Venus Is Earth just have more vigorous convection or is something else involved? Could liquid water be involved? Earth has lots, Venus has none

34 Why does Earth have Plate Tectonics, but not Venus Could liquid water be involved? What does this provide? *Water lowers the melting temperature of rocks (consider that the subducting oceanic plate is rich in both sediments and water) *Water in the Earth s interior also reduces the strength of rocks leading to breakup of the lithosphere

35 Differences in surface geology due to their atmospheres? Watery Earth: vigorous recycling of crust / upper mantle - driven by "eclogite engine" Dry, hot Venus: static crustal lid holds in heat generated by radioactivity in mantle until erupts in (periodic?) episode(s?) of volcanic resurfacing

36 Plate tectonics on Mercury or the Moon? While both show evidence of tectonic activity through observable faults, neither demonstrate evidence for plate tectonics The likeliest explanation is that they cooled too fast, resulting in a thick lithospheric plate.

37 Plate tectonics on Mars? It is within reason based on composition, rheology, and geology that Mars may have had plate tectonics very early on But the only remaining evidence is highly contentious EAS 4803/ CP 18:33

38 Tectonics on Mars Anderson et al. (2001)

39 Plate tectonics on Europa? While Europa reads like a densely populated tectonic map, the evidence is not that of glacial plate tectonics, but rather a system governed by tidal forces and polar wander. Schenk et al., 2008

40 The Big Picture -- Careful not to confuse volcanism associated with plate convergence wit hot spots