g.iv.1.a.c. Average angle of subduction=45 degrees g.iv.1.a.d. Different types of convergents g.iv.1.a.d.1. Oceanic-Continental g.iv.1.a.d.1.a. Denser oceanic slab sinks to asthenosphere g.iv.1.a.d.1.b. Partial melting of mantle rock generates magma g.iv.1.a.d.1.c. Resulting volcanic mountain chain is called a continental volcanic arc (ex. Andes, Cascades) g.iv.1.a.d.2. Oceanic-Oceanic g.iv.1.a.d.2.a. One descends (subducts) beneath the other g.iv.1.a.d.2.b. Often forms volcanoes on the ocean floor g.iv.1.a.d.2.c. When volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands) g.iv.1.a.d.3. Continental-Continental g.iv.1.a.d.3.a. Continued subduction can bring two continents together
g.iv.1.a.d.3.b. Less dense, buoyant continental lithosphere does not subduct g.iv.1.a.d.3.c. Collision produces mountains (obduction) (Ex. The Himilayas, Alps, Appalacians) g.iv.2. Transform boundary (conservative): no production or destruction g.iv.2.a. Plates slide past one another, no new lithosphere is created nor destroyed g.iv.2.b. Most join two segments of a mid-ocean ridge along breaks in the oceanic crust known as fracture zones g.iv.2.c. A few cute through continental crust (San Andreas Fault, Alpine Fault of New Zealand) g.iv.3. Divergent Boundary (constructive margins): plates move apart, hot mantle upwells to create new seafloor (more material is added) g.iv.3.a. Oceanic ridges (elevated seafloor w/ high heat flow + volcanism) g.iv.3.b. Form a Global Ridge system-Mid-Atlantic Ridge, East Pacific Ridge, Mid-Indian Ridge g.iv.3.c. Moves at 5 cm a year, takes 80 million years for material to cool, denser when cool and thins, newer hotter material is thicker g.iv.3.d. Continental rifting g.iv.3.d.a. Begins with mantle upwelling, then blocks sink creating continental rift valley, to narrow sea (ex. Red sea), to ocean basin and oceanic ridge creation g.v. Testing Plate Tectonics: Evidence from Ocean Drilling g.v.1. Samples of ocean floor from drilling (ex: Deep sea drilling project 1968-1983) g.v.2. Microorganisms date sediment: age increases w/ distance from ridge g.v.3. Supports seafloor spreading and young ocean basins (<180 million years) g.v.4. Project Mohole/recent drilling efforts g.v.4.a. Goal drill to Mohorovicic Discontinuity (Moho) g.v.4.b. Seafloor off the coast of Mexico-thin g.v.4.c. 11,700 water and 601 ft of core (.18 km) g.v.4.d. Failed…ran out of money Evidence from Hot spot and mantle plumes g.v.5. Mantle plume: upwelling of hot rock, causes decompression melting from reduction in P, hot spot on the surface (fixed)
g.v.6. Hot spot track: formed as plate moves (volcanic Island chain, ex. Hawaii) g.v.7. Measuring Plate Motion g.v.7.a.a. Mantle plumes-length of hotspot track and ages volcanic structures, also shows change in direction g.v.7.a.b. Paleomagnetism-width of magnetic stripes, rate of seafloor spreading g.v.7.a.c. From space-using GPS, shows rotation g.v.7.a.d. Forces driving plate motion g.v.7.a.d.1.Slab pull (most dominant) g.v.7.a.d.2.Ridge push-gravity driven force that results from the elevated position of the ridge g.v.7.a.d.3.Mantle drag-resists subduction or
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