David Allison GY305 - University of South Alabama › geography › allison › ...David Allison GY305 Physics of Tectonic Plate Motions Global Positioning Systems Earth’s Magnetic

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