Plate tectonics II

The oceans are where most of the action takes place

Theory of Plate Tectonics

• Surface of the Earth is covered by a series of plates

• Creation of seafloor occurs at mid-ocean ridges, where volcanism creates the seafloor.

• Consumption of the seafloor occurs at convergent margins, where plates are subducted back into the mantle.

• Continents never subduct; they are passengers riding on the plates

Symmetric stripes of “normal” and “reverse” magnetism in ocean crust

on either side of the ridge

Howocean basinsformed

Sediment thickness increases away from ridge crest

Confirmation of seafloor spreading: Age of oceanic crust

Seafloor is created at ridges; it is destroyed at subduction

zones

Plate Tectonics

Plates of the Earth’s crust

Plate Structure: The crust is not the plate! The plate is the “lithosphere”

Fig. 1.11

Plate Tectonics

• LithosphereLithosphere: the outer rigid shell of the earth (~ 100 km). The plates are composed of this material

• AsthenosphereAsthenosphere: part of mantle beneath lithosphere

• The lithosphere rides on top of the asthenosphere

Plates

• Group of rocks all moving in the same direction

• Can have both oceanic and continental crust or just one kind.

Three Types of Plate BoundariesThree Types of Plate Boundaries

Fig. 2.5

Transform Divergent Convergent

Plate boundaries

• Divergent

• Convergent

• Strike-Slip

Divergent Plate BoundaryDivergent Plate Boundary

Usually start within continents—grows to become ocean basin

Fig. 2.6

Divergent Plate BoundariesDivergent Plate Boundaries

Fig. 2.8

Plate boundaries

• Divergent

• Convergent

• Strike-Slip

Ocean-Continent Convergence Ocean-Ocean Convergence

Andes Unimak Island, AK

Convergent Boundaries

Three types:

ocean–ocean Japan

ocean–continent Andes

continent–continent Himalaya

Ocean–Ocean

Island arcs:Island arcs:

• Tectonic belts of high seismicity

• High heat flow arc of active volcanoes

• Bordered by a submarine trench

Convergent plate boundaryConvergent plate boundary

Fig. 2.9

Ocean–Continent

Continental arcs:Continental arcs:

• Active volcanoes

• Often accompanied by compression of upper crust

Convergent Plate BoundaryConvergent Plate Boundary

Fig. 2.9

Continent–Continent

• In ocean–continent boundaries, collision convergence is taken up by subduction

• In continent–continent boundaries, convergence is accommodated by deformation of the crust without subduction (both plates are too buoyant to be subducted)

Transform Faults

Transform Boundaries

The San Andreas Fault

Transform Plate BoundaryTransform Plate Boundary

Fig. 2.10

Measured Plate Velocities correspond well with velocities inferred from magnetic stripes.

Plate Movement is an observed fact.