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David AllisonGY305
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Physics of Tectonic Plate MotionsGlobal Positioning SystemsEarth’s Magnetic Field Radiometric Decay & Age DeterminationsUplift RatesEarth’s Gravity FieldEarth’s Heat FlowKinematic Physics of Deformation
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Tectonic Plates
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2 Criteria Composition (i.e. mineralogy and geochemistry) Seismic (mechanical behavior)
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Depth Thick LayerCrust7-50km Basalt (3.0) Diorite (2.7)
Upper mantle650km700km
Peridotite (3.3)
2900km
5100km
Lower mantle2200km Garnet Lherzolite (3.5-5.5)
6400km
Outer core2200km Ni-Fe-S Alloy (10-12)
Inner core1300km Ni-Fe Alloy (13)
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Depth Thick LayerLithosphere70-150km Brittle
Asthenosphere550km700km
Ductile
2900km
5100km
Mesosphere2200km Brittle
6400km
Outer core2200km Liquid (very ductile)
Inner core1300km Brittle
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Primary platesThese seven plates comprise the bulk of the Earth’s Lithosphere:Pacific Ocean.African PlateAntarctic PlateEurasian PlateIndo-Australian PlateNorth American PlatePacific PlateSouth American Plate
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• Vectors are proportional to plate spreading velocity• Note that the Pacific is spreading much faster than the Atlantic
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GPS-determined Absolute Plate Motion Rates
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• Convection Cells in the Mantle• Note that “Slab Pull” is due to older denser ocean
lithosphere
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Seismic Activity is confined mainly to plate boundaries
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Paleomagnetic “stripes” generated by sea floor spreading
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Phanerozoic Plate Motions
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Plate Motion on the Earth Spheroid
The 3 possible plate boundary configurations
Divergent Convergent Transform
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Plate Motion on the Earth Spheroid
The 6 possible dextral transform configurations
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2 Plates on a Flat Earth
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A 3-Plate Configuration
• Plate velocity vectors are additive 1st order tensors
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A 3-Plate Circuit Diagram
• Start with any plate• Calculate relative velocity in a specific direction (C or CC)• Continue to calculate velocity in C or CC direction
BVC
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Plate Motion on the Sphere of the Earth• 2 plates always move relative to each other relative to a pole of rotation• Transform boundaries are the “small circles” of the plate motion• The angular velocity (w) is constant but the linear velocity of relative plate
motion increases from 0 at pole to a max. at 90 degrees to the pole of rotation
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• Transform boundaries are small circle paths relative to rotation pole
• On a Mercator projection the transforms are parallel to latitude lines
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• Stereographic Projections may be used to track plate motions relative to rotation pole
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Implications of Plate Boundary Interactions
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Absolute Plate Motions
• Hot Spots: assumed to have constant position in asthenosphere
• GPS: Satellite geometry calculates absolute latitude-longitude position to within millimeters
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Emperor Seamounts & Hawaiian Islands