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