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Plate Tectonics and the Ocean Floor Continental Drift • Theory proposed by Alfred Wegener in 1912 • Continents once formed a single landmass Early Evidence Age of Oceanic Crust Courtesy of www.ngdc.noaa.gov Paleomagnetism Plate Tectonics – 1960’s • Explains HOW the plates moved The Crust Continental Crust Oceanic Crust - thick (10-70km) - buoyant (less dense than oceanic crust) - mostly old - thin (~7 km) - dense (sinks under continental crust) - young Plate Movement • “Plates” of lithosphere are moved around by the underlying hot mantle convection cells Three types of plate boundary • Divergent • Convergent • Transform Divergent Boundaries • Spreading ridges – As plates move apart new material is erupted to fill the gap Mid-Ocean Ridge • Underwater mountain ranges • Due to plates pulling away from each other • A Rift Valley forms in the middle where magma comes out • Have many fracture zones which break the ridge up • Black smoker at a midocean ridge hydrothermal vent in the Atlantic. Close up at the mid-ocean ridge Iceland: An example of continental rifting • Iceland has a divergent plate boundary running through its middle Convergent Boundaries • There are three styles of convergent plate boundaries – Continent-continent collision – Continent-oceanic crust collision – Ocean-ocean collision Continent-Continent Collision • Forms mountains, e.g. European Alps, Himalayas Himalayas Continent-Oceanic Crust Collision • Called SUBDUCTION Subduction • Oceanic lithosphere subducts underneath the continental lithosphere • Oceanic lithosphere heats and dehydrates as it subsides • The melt rises forming volcanism • E.g. The Andes Trenches • Due to one plate subducting (going below) another plate • Earthquakes • Many volcanoes and volcanic island arcs form here Ocean-Ocean Plate Collision • When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. • The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. • The worlds deepest parts of the ocean are found along trenches. – E.g. The Mariana Trench is 11 km deep! Transform Boundaries • Where plates slide past each other Above: View of the San Andreas transform fault Pacific Ring of Fire Volcanism is mostly focused at plate margins Other Ocean Features Continental Shelf • Part of a continent covered by water • Gentle slope • Average depth is 60 m • Amount exposed changes with sea level Continental Slope • Steep • May be cut by submarine canyons • Sediment piles up at the bottom and forms continental rise Abyssal Plains • Flattest area on earth • Cover ½ of deep ocean • Covered with fine sediment Seamounts • Submerged volcanic mountains • Called oceanic island if they rise above water Guyot • Seamounts that have been eroded and are now flat on top