Transcript Document
Plate Tectonics Geological and geophysical evidence of plates and plate dynamics Earth’s lithosphere consist of relatively rigid plates that move independently Plates consist of crust and lithospheric mantle Any individual plate contains oceanic and/or continental crust Each plate relatively rigid, little internal deformation Plate boundaries defined by active seismic and volcanic activity Dynamic: new ones form, boundaries change character, etc. GPS data allow us to track plate motions How fast do plates move? Typically ~50 mm/yr (2 in/year), but up to 240 mm/yr (10 in/yr). Geologic evidence shows that rates and directions change over time What is the geologic record of plate tectonics? Patterns of magnetic anomalies on seafloor Locations of earthquakes, many volcanoes, and mountains Evidence of ‘continental drift’ Magnetic Field Electric current/field: QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. orientation of compasses 3 properties: magnitude, inclination, declination Electrical dynamo - Dipolar field: poles near geographic poles (= spin axis) Results from movement in Iron-rich core Rotation of the Earth affects orientation and strength Poles move (Polar wander) Magnetic Reversals A rich geologic record of plate motions over time Magnetic stripes QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Geologic evidence of past movement is in the continents Paleomag record of movement of plates through different latitudes (inclination) Age and character (internal structure, metamorphic grade, etc.) of margins (mountain belts, subduction zones, etc.) Patterns of sedimentation, sed provenance Fossil record of times when different continents shared flora and fauna; also of movement of plates through different latitudes (climate) Plate Tectonics • All rocks owe their origin to plate tectonics • Plate motions record inevitable and irreversible cooling of the planet PLATE DRIVING FORCES Pull of sinking lithospheric plate (slab) at subduction zones: SLAB PULL. This is thought to be the dominant mechanism Push of intrusion, gravity at mid-oceanic ridges: RIDGE PUSH. Traction caused by motion of asthenosphere below plate: BASAL TRACTION