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

How ocean basins formed

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 Boundaries Fig. 2.5 Transform Divergent Convergent

Plate boundaries Divergent Convergent Strike-Slip

Divergent Plate Boundary Usually start within continents— grows to become ocean basin Fig. 2.6

Divergent Plate Boundaries Fig. 2.8

Plate boundaries Divergent Convergent Strike-Slip

Ocean-Continent ConvergenceOcean-Ocean Convergence AndesUnimak Island, AK

Convergent Boundaries Three types: ocean–oceanJapan ocean–continentAndes continent–continentHimalaya

Ocean–Ocean Island arcs: Tectonic belts of high seismicity High heat flow arc of active volcanoes Bordered by a submarine trench

Convergent plate boundary Fig. 2.9

Ocean–Continent Continental arcs: Active volcanoes Often accompanied by compression of upper crust

Convergent 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 Boundary Fig. 2.10

Measured Plate Velocities correspond well with velocities inferred from magnetic stripes. Plate Movement is an observed fact.