Gravity, Isostasy & Heat flow .Figure 4.16 Non-uniform body: divide it into...

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Transcript of Gravity, Isostasy & Heat flow .Figure 4.16 Non-uniform body: divide it into...

  • Gravity, Isostasy & Heat flow

    Chapter 4 pp. 119-126

  • m

    M

    Gravity: force which is caused by mass.

    g =GMm

    r2r

    g is much bigger for smaller rg is bigger for bigger M and m

    G is a universal constantIt never changes. We can ignore it.

  • uniform sphere

    g

    gg

    Uniform sphere: same gravity at the same distance from center of mass

    but no planets or moons are uniform spheres...

  • Figure 4.16

    Non-uniform body: divide it into pieces, calculate gravity from each piece, and add them up

  • Gravity low expected over large depressions...

  • ... and gravity high expected over large areas of high elevation

  • Fig. 4.19

  • Fig. 4.20

  • mars.jpl.nasa.gov

    Mars: big gravity anomalies that correlate with large-scale topography, as wed expect...

  • Philippine Plate

    Pacific PlateEurasian Plate

    Australian-Indian Plate

    Arabian Plate

    Average Surface Elevation Oceanic: - 4.5 km Continental: + 0.5 km

    Earth topography

  • Earth gravity: small anomalies that do not correlate with topography!

    Why?

  • short video clip - the Grace mission

  • Fig. 4.11 (kinda sorta)

    Isostasy.

  • Density of ice = 0.9 g/cmDensity of water = 1.0 g/cmDensity of air = 0.0 g/cm

    Just the tip of the iceberg

    Ice heavier than air -- sinks Ice lighter than water -- floats

    Reaches a balance determined by the densities

    3

    3

    3

    Isostasy (a.k.a. Isostatic Compensation

  • Isostatic CompensationBimodal Topography & Crustal Thickness

    Average Crustal Thickness Oceanic: 5 km Continental: 35 km

    Average Surface Elevation Oceanic: - 4.5 km Continental: + 0.5 km

    Surface elevations: coloursCrustal thickness: contour lines

    Mooney et al., 1998

    Average Crustal Density Oceanic: 3.0 g/cm Continental: 2.7 g/cm

    3

    3

  • low density

  • Fig. 4.18

    low density root

  • Fig. 4.12 Erosion and uplift

    How isostasy is maintained (though imperfectly) - 1

  • Fig. 4.13 Crustal rebound

    How isostasy is maintained (though imperfectly) - 2

  • Fig. 4.15

    How isostasy is maintained (though imperfectly) - 3

    adding bouyant plutons / thickening the continent

  • Fig. 4.17

    Small-scale gravity anomalies

  • Local (small area) anomalies local variations in rock density

    (mass = density times volume) can be used to find metal ores never isostatically compensated because they are

    small: crust is strong enough to prevent sinking or rising

    Regional (large area) anomalies none would exist if isostasy were perfect tectonic forces cause anomalies

    surface pulled down at subduction zone trenches (-)

    rapidly building mountains lack buoyant roots (+)

    Gravity anomalies are still present...

  • Heat flow

    The inside of the Earth is HOT

    High temperatures make asthenosphere gooey, allowing plate motion

    Heat is needed for convection of the

    core (which makes the magnetic field)

    mantle (which starts plate tectonics)

  • Biggest sources of heat:

    stored heat from when the Earth formed

    heat from decay of radioactive elements

  • Fig. 4.29

    True, except at mid-ocean ridges.

  • Fig. 4.28

    A couple of causes of high heat flow

  • Football field = about 3 light bulbs worth of energy flux

  • Geothermal gradient Temperature increases with depth Geothermal gradient is (change in T) /

    (change in depth) Deep mines are very hot (need AC) On average 25 degrees per km Heat can be used for geothermal energy Temp at drill bit can be > 200 degrees

    Steep gradients do not persist temperatures in convecting layers increase

    just slightly with depth (pressure): 0.4 degrees/km in mantle

  • Fig. 4.27

    convection

    conduction

    convection

  • The Earth is gradually coolingHeat loss exceeds heat production from radioactivity, exothermic reactions, etc.Heat loss rate is 44 x 10^12 Watts, but...

    Rate of temperature drop: 0.000000046 per year

    (F. Stacey, 1980)