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9.3 & 4: Theory of Plate Tectonics lithosphere forms the thin outer shell of Earth and is broken into several pieces called plates asthenosphere: solid, plastic layer of the mantle beneath the lithosphere made of mantle rock that flows very slowly which allows plates to move on top of it plate tectonics: theory that explains how large pieces of the lithosphere (plates) move and change shape plates ride on the asthenosphere like blocks of wood float on water continents and oceans are carried along on the moving plates like passengers are carried by a bus plates can include continental crust, oceanic crust, or both scientists have identified about 15 major plates boundaries of the plates do not always match the outlines of continents plate boundaries may be in the middle of the ocean floor, around the edges or within continents three types of plate boundaries are divergent, convergent, & transform fault each plate boundary has a characteristic type of geologic activity Chapter 10 divergent: where plates move apart area where new crust forms usually found on the ocean floor can also create volcanoes Chapter 10 convergent: where plates run into one another (come together) three types of convergent boundaries based upon types of plates that are colliding Chapter 10 ocean/continental = ocean crust (is denser) is pushed down into the mantle, creating a trench can also make a range of volcanic mountains called a continental volcanic arc due to melting of the ocean plate in the mantle Chapter 10 ocean/ocean = one is pushed down into the mantle creating a trench can also make a range of volcanic mountains that are in the ocean so are called a volcanic island arc Chapter 10 continental/continental = both are pushed up creating a mountain range Chapter 10 transform fault: plates grind past each other causes earthquakes How Does This Happen? convention currents in the mantle are the driving force behind plate movements