Andrija Mohorovicic´ The first velocity discontinuity discovered was named for its discoverer. Most people now use a shortened form of the name and call this boundary the Moho.

discontinuities Each discontinuity marks the boundary between two layers with different physiochemical conditions.

Thickness (km) Density (g/cm 3 ) silica rocks andesite, basalt at base peridodite, eclogite, olivine, spinel, garnet, pryoxene magnesium and silicon oxides iron+oxygen, sulfur, nickel alloy (liquid) iron+oxygen, sulfur, nickel alloy (solid) Typical Rocks Crust Crust Upper Mantle Upper Mantle Lower Mantle Lower Mantle2, Outer Core Outer Core2, Inner Core Inner Core1, Center Center Moho Discontinuity zone including uppermost mantle and lowermost crust is called the aesthenosphere

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

The plates “float” on the astenosphere, which is a thin boundary zone where the mantle and crust meet. The asthenosphere is “mooshy” (can be sheared relatively easily) allowing the plates above the slide along.

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.

Fig 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 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 created 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.

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 Island Arcs

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 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

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