Plate Tectonics II: How it works Continental Crust versus Oceanic Crust Mantle Convection The Asthenosphere Boundaries Convergent Divergent Transform Continental Crust versus Oceanic Crust Mantle Convection The Asthenosphere Boundaries Convergent Divergent Transform

Upwelling Magma new crust added felsic and low density “light and fluffy” mafic and high density “dark and dense” Mid-Ocean Ridge Continental Crust Oceanic Crust Subducting Slab old crust destroyed Mantle Aesthenosphere thick and buoyant does not subduct thin and sinks does subduct Tectonic plates “float” on the higher density mantle.

The plates “float” on the asthenosphere, which is a thin boundary layer incorporating parts of the uppermost mantle and the base of the crust. The asthenosphere is “mooshy” (can be sheared relatively easily) allowing the plates above the slide along.

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.

Plate Movement

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

Convergent Boundaries subductiontrenches It is impossible for plates to move toward each other unless crust is “moved out of the way” - usually by subduction and destruction of oceanic crust at trenches.

Convergent Boundaries subductiontrenches It is impossible for plates to move toward each other unless crust is “moved out of the way” - usually by subduction and destruction of oceanic crust at trenches.

Convergent Boundaries Trenches Trenches form where oceanic crust is subducted down into the mantle. Benioff Zones Benioff Zones - zones of earthquake foci that increase in depth with distance from the trench in the direction of subduction Western Pacific Benioff Zone

Convergent Boundaries Descending Slab Western Pacific

Convergent Boundaries “Ring of Fire.” The western Pacific Ocean is surrounded almost completely by trenches, which subduct the giant Pacific Plate and several smaller plates. The volcanic activity along the Pacific Rim inspired the nickname “Ring of Fire.”

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 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. Edge of North American Plate

October 1,

October 1,

October 4, 2004 Volcanologists install new equipment in the crater.

October 4, 2004 Mount St. Helens lets out some more steam

Mount St. Helens

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 Oceanic-Oceanic

Convergent Boundaries Island Arcs

Convergent Boundaries Montserrat is a very small island with a very big problem. © Montserrat Volcano Observatory Video Interlude Uninhabitable zone Uninhabitable zone.

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

Convergent Boundaries The massive compressive stresses caused by the collision causes huge low-angle reverse faults to form, stacking crust on crust. The crust also thickenes through ductile compression. The Himalayas

Divergent Boundaries rift mid-ocean ridges When plates move away from each other, mantle material wells up into the space (called a rift), generally creating new oceanic crust. The great oceanic mid-ocean ridges are formed of this upwelling, basaltic magma.

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 Pillow Basalts Pillow Basalts - lava extruded into ice-cold ocean water forms weird-looking structures that resemble pillows.

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). One of the few major continental crust- continental crust transform boundaries runs up the west coast of North America.

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