plate tectonics geological and geophysical evidence of plates and plate dynamics
TRANSCRIPT
Plate Tectonics
Geological and geophysical evidence of plates and plate dynamics
Earth’s lithosphere consist of relatively Earth’s lithosphere consist of relatively rigid plates that move independentlyrigid 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 FieldMagnetic Field
Electric current/field: 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
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A rich geologic record of plate motions over time
Magnetic stripes
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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 FORCESPLATE 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