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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. Andrija Mohorovicic´ discontinuities Each discontinuity marks the boundary between two layers with different physiochemical conditions. Moho Discontinuity Thickness Density (km) (g/cm3) Crust 30 2.2 2.9 Upper Mantle 720 3.4 http://www.glossary.oilfield.slb.com zone including uppermost mantle and lowermost crust is called the aesthenosphere Lower Mantle 2,171 4.4 Outer Core 2,259 9.9 Inner Core 1,221 12.8 Typical Rocks 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) Center 13.1 http://pubs.usgs.gov/gip/interior/ Major Tectonic Plates 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. Eurasian North American Pacific Phillipine Caribbean African Nazca South American Antarctic Pacific Australian-Indian 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. Continental Crust Oceanic Crust felsic and low density “light and fluffy” thick and buoyant does not subduct mafic and high density “dark and dense” thin and sinks does subduct Mid-Ocean Ridge Aesthenosphere Subducting Slab old crust destroyed Mantle Upwelling Magma new crust added Tectonic plates “float” on the higher density mantle. Fig. 19.12 The driving mechanism of plate tectonics is mantle convection. Hot mantle material rises at ridges and cooler mantle material sinks at subduction zones. The contacts along which plates meet are called boundaries. Individual plates move relative to each other in one of three ways: Convergent - moving toward each other. Divergent - moving away from each other Transform - moving past each other Plate Boundaries convergent divergent transform The different kinds of boundaries have different properties: Convergent – old oceanic crust destroyed through subduction Divergent – new oceanic crust created at mid-ocean ridges Transform – crust neither created or destroyed convergent divergent transform Convergent Boundaries 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 Oceanic-Continental http://geo.lsa.umich.edu/~crlb/COURSES/270/ Subduction leads to orogeny 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 Oceanic-Oceanic http://geo.lsa.umich.edu/~crlb/COURSES/270/ 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. Convergent Boundaries Island Arcs Lesser Antilles http://www.geology.pomona.edu/PETRO/IAMAP.GIF http://www.sabapark.com/research_activities/volcano_monitoring/volcanic_info.htm Convergent Boundaries Continental-Continental http://geo.lsa.umich.edu/~crlb/COURSES/270/ 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. 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. http://pubs.usgs.gov http://www.high-altitude-medicine.com/photos-mountain.html Divergent Boundaries Mid-Ocean Ridges Mid-ocean ridges are long mountains formed parallel to oceanic rifts. 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. http://wwwneic.cr.usgs.gov/ Transform Boundaries 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. http://pubs.usgs.gov http://www.ingv.it/~roma/cultura/ingescuola/terremotopagina/SanAndreas.html http://wwwneic.cr.usgs.gov/ Boundary Type Volcanoes Mountains Earthquakes Convergent yes yes shallow-deep Divergent yes yes shallow Transform no no varies