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Living on the Edge Plate Tectonics 2 Convection in the mantle drives the plate motion 3 Types of Plate Boundaries • A) Divergent = moving away from each other • B) Transform = sliding past each other • C) Convergent = moving toward each other A) Divergent plate boundaries • Continental plates break, move apart New oceanic crust forms at MOR B) Transform Boundaries • Plates move past each other • San Andreas fault Most transform boundaries are parts of MORs Displacement along Transform Boundaries C)Convergent Plate Boundaries • Plates collide • This is where MOST of the world’s earthquake and volcanic activity occurs • “Pacific Ring of Fire” Three Types of Convergent Boundaries • 1) Oceanic-Oceanic • 2) Oceanic-Continental • 3) Continental-Continental 1) Oceanic – Oceanic Convergence • As the oceanic crust moves further from the MOR, it increases in density because of the weight of water and overlying sediments. • Therefore, the older the Oceanic crust is – the more dense it is. When the edges of two oceanic crustal plates converge: • The more dense plate will subduct. • As the subducting plate moves into the mantle, it heats and melts. • Melting decreases the density, so it rises, forming a row of volcanoes in the ocean parallel to the convergent boundary. • This chain of volcanoes is known as a “Volcanic Island Arc”. Japan is the classic example: Mt. Fuji 2) Oceanic-Continental Convergence • Oceanic Crust is more dense than Continental Crust because: • Many of the minerals in Oceanic Crust are inherently more dense. • Erosion, weathering, and other surficial processes leave pore space (filled with gasses or fluids) in the rocks, decreasing their density. When an oceanic plate converges with a continental plate: • The more dense plate (oceanic) will subduct. • As the subducting plate moves into the mantle, it heats and melts. • Melting decreases the density, so it rises, forming a row of volcanic mountains parallel to the convergent boundary, therefore parallel to the continental coast. • This chain of volcanoes is known as a “Volcanic Arc”. Classic Examples are the Cascades and the Andes: Mt. St. Helens 3) Continental -Continental Convergence • Both crusts are low-density, so neither will automatically subduct. • The two continents will crash together, crumpling both edges in the process. • This forms a non-volcanic mountain range at the convergence, and intense heat and pressure cause metamorphism in the rocks. The Himalayas are the classic example: The Appalachians are another example The Rock Cycle So that raps up Plate Tectonics This Webinar will be repeated on Sunday evening – please spread the word. No Webinars Next Week Have a Happy Thanksgiving!