Plate Tectonics Geological and geophysical evidence of plates

Plate Tectonics Geological and geophysical evidence of plates and plate dynamics

Earth’s lithosphere consist of relatively rigid plates that move independently n n 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 n 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? n n n Patterns of magnetic anomalies on seafloor Locations of earthquakes, many volcanoes, and mountains Evidence of ‘continental drift’

Magnetic Field n Electric current/field: n n n orientation of compasses 3 properties: magnitude, inclination, declination Electrical dynamo - Dipolar field: poles near geographic poles (= spin axis) n Results from movement in Iron-rich core n Rotation of the Earth affects orientation and strength Poles move (Polar wander) n Magnetic Reversals n

A rich geologic record of plate motions over time n Magnetic stripes

Geologic evidence of past movement is in the continents n n 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 n n n 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