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David AllisonGY305
Physics of Tectonic Plate MotionsGlobal Positioning SystemsEarth’s Magnetic Field Radiometric Decay & Age DeterminationsUplift RatesEarth’s Gravity FieldEarth’s Heat FlowKinematic Physics of Deformation
Tectonic Plates
2 Criteria Composition (i.e. mineralogy and geochemistry) Seismic (mechanical behavior)
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)
Depth Thick LayerLithosphere70-150km Brittle
Asthenosphere550km700km
Ductile
2900km
5100km
Mesosphere2200km Brittle
6400km
Outer core2200km Liquid (very ductile)
Inner core1300km Brittle
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
• Vectors are proportional to plate spreading velocity• Note that the Pacific is spreading much faster than the Atlantic
GPS-determined Absolute Plate Motion Rates
• Convection Cells in the Mantle• Note that “Slab Pull” is due to older denser ocean
lithosphere
Seismic Activity is confined mainly to plate boundaries
Paleomagnetic “stripes” generated by sea floor spreading
Phanerozoic Plate Motions
Plate Motion on the Earth Spheroid
The 3 possible plate boundary configurations
Divergent Convergent Transform
Plate Motion on the Earth Spheroid
The 6 possible dextral transform configurations
2 Plates on a Flat Earth
A 3-Plate Configuration
• Plate velocity vectors are additive 1st order tensors
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
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
• Transform boundaries are small circle paths relative to rotation pole
• On a Mercator projection the transforms are parallel to latitude lines
• Stereographic Projections may be used to track plate motions relative to rotation pole
Implications of Plate Boundary Interactions
Absolute Plate Motions
• Hot Spots: assumed to have constant position in asthenosphere
• GPS: Satellite geometry calculates absolute latitude-longitude position to within millimeters
Emperor Seamounts & Hawaiian Islands