CHAPTER 4 PLATE TECTONICS Plate Tectonics Theory The theory that the outer layer of the Earth the lithosphere consists of separate plates that move with respect to one another Driving Mechanism Two forces ridge push force and slab pull force strongly influence the motion of individual plates Ridge Push Force develops because the lithosphere of mid ocean ridges lies at a higher elevation than that of the adjacent abyssal plains Develops because the region of a lift is elevated The mass of the ridge pushes sideways Slab Pull force the force that subducting downgoing plates apply to oceanic lithosphere at a convergent margin arises simply because lithosphere that was formed more than 10 million years ago is denser than asthenosphere so it can sink into the asthenosphere Thus once an oceanic plate starts to sink it gradually pulls the rest of the plate along behind it like an anchor pulling down the anchor line This pull is the slab pull force Three Types of Margins 1 Divergent 2 Convergent 3 Transform Divergent boundaries is where new crust is formed developed from rifting Convergent boundaries are places where plates crash or crunch together are called convergent boundaries CRUST IS CONSUMED Plates only move a few centimeters each year so collisions are very slow and last millions of years Transform boundaries where plates move past each other How does crust from Ocean crust mafic basalt gabbro Ophiolite slice of oceanic crust that has been thrust onto continental crust Pillowed basalt fine grained mafic igneous rock Sheeted diked composed of gabbro coarse grained igneous rock Layered gabbro coarse grained igneous rock How rifting works Extension is initially distributed over a broad region Extension not highly focused until sometime after rifting begins once rifting focused it rapidly progresses to breakup Accretionary Prism A wedge shaped body of faulted and folded material added to a continental margin in a subaqueous thrust zone Volcanic Arc Develops behind the accretionary prism The magma that feeds these volcanoes forms just above the surface of the downgoing plate where the plate reaches a depth of about 150 km below the Earth s surface Continental Arc A continental arc is what happens when you have a continental oceanic collision rather than an oceanic oceanic collision The oceanic crust is subducted because it s cooler and denser you get a deep trench Island Arc A type of archipelago often composed of a chain of volcanoes with arc shaped alignment situated parallel and close to a boundary between two converging tectonic plates Microcontinental Accretion Land Mass Approaches impact and accretion subduction continues Hot Spots A location at the base of the lithosphere at the top of a mantle plume where temperatures can cause melting Mantle Convection Cold Downwelling Hot Upwelling Rodinia A proposed Precambrian supercontinent that existed around 1 billion years ago Ocean Crust Age The oldest is rarely more than 200 million years old varies with location The youngest crust lies along a mid ocean range and the oldest along the coasts of continents Pillow lava bulbous spherical or tubular lobes of lava They form during eruptions with relatively low effusion rates Slow extrusion gives enough time for a thick crust to form on all sides of a pillow lobe and prevents individual pillows from coalescing into a sheet Active Margin A continental margin that coincides with a plate boundary Passive Margin A continental margin that is not a plate boundaries Continental crust is thinner than normal