Tectonic plates “float” on the higher density mantle.

mantle convection The driving mechanism of plate tectonics is mantle convection. Hot mantle material rises at ridges and cooler mantle material sinks at subduction zones.

There are several large tectonic plates and a number of much smaller plates. The Earth’s continents sit on plates composed of both oceanic and continental crust. The huge Pacific plate is composed almost entirely of oceanic crust, and is being subducted around almost its entire western boundary. PacificPacific NorthAmericanNorthAmerican NazcaNazca SouthAmericanSouthAmerican AntarcticAntarctic AfricanAfrican EurasianEurasian Australian-IndianAustralian-Indian PhillipinePhillipine CaribbeanCaribbean PacificPacific Major Tectonic Plates

Plate Boundaries convergent divergent transform boundaries The contacts along which plates meet are called boundaries. Individual plates move relative to each other in one of three ways: Convergent Convergent - moving toward each other. Divergent Divergent - moving away from each other Transform Transform - moving past each other

convergent divergent transform The different kinds of boundaries have different properties: Convergent Convergent – old oceanic crust destroyed through subduction Divergent Divergent – new oceanic crust accreted at mid-ocean ridges Transform Transform – crust neither created or destroyed

Oceanic-Continental Convergent Boundaries Subduction leads to orogeny partially melt A sinking slab of oceanic crust will partially melt as it heats up, creating pockets of magma that rise through the crust, forming volcanoes.

Convergent Boundaries When oceanic crust converges with oceanic crust, the denser plate (usually the oldest and coldest) generally subducts. An arc of volcanic islands forms from upwelling magma. Oceanic-Oceanic

Convergent Boundaries Since continental crust is buoyant, it does not subduct easily. While the edge of a plate can be dragged under by the weight of attached oceanic crust, it does not go far. Continental-Continental

Divergent Boundaries

Mid-ocean ridges are long mountains formed parallel to oceanic rifts. spreading center. The plates on either side of the ridge grow as new crustal material is added at the spreading center. Rifts are characterized by relatively shallow earthquake foci along the length of the rift. Mid-Ocean Ridges

Divergent Boundaries Spreading Centers upwelling magma causes bulge

Divergent Boundaries Spreading Centers upwelling magma causes bulge rift valley forms as plates pull apart

Divergent Boundaries Spreading Centers upwelling magma causes bulge rift valley forms as plates pull apart oceanic crust accretes to continental crust as ridge forms

Divergent Boundaries Spreading Centers upwelling magma causes bulge rift valley forms as plates pull apart oceanic crust accretes to continental crust as ridge forms continents continue to move apart, opening ocean

Divergent Boundaries

Transform Boundaries Most transform boundaries are associated with mid-ocean ridges (they form perpendicular to the rifts). This movement resolves stresses caused by different rates of spreading along the divergent boundary.

Transform Boundaries

San Andreas fault system The San Andreas fault system is part of a system of strike-slip faults caused by the relative motion of the North American and Pacific plates

Boundary Type ConvergentDivergentTransformVolcanoesyesyesnoMountainsyesyesnoEarthquakesshallow-deepshallowvaries