#separator:tab #html:false #tags column:4 Plate tectonics The theory that Earth's outer layer is broken into moving plates earth-plate-tectonics Tectonic plate A large slab of Earth's lithosphere earth-plate-tectonics Lithosphere Earth's rigid outer layer: crust and top of the mantle earth-plate-tectonics Asthenosphere The soft, slowly flowing layer of mantle below the lithosphere earth-plate-tectonics Continental drift The idea that continents slowly move over time earth-plate-tectonics Who proposed continental drift in 1912? Alfred Wegener earth-plate-tectonics Pangaea The supercontinent that existed about 300 million years ago earth-plate-tectonics Seafloor spreading New ocean floor forming at mid-ocean ridges and moving outward earth-plate-tectonics Mid-ocean ridge An underwater mountain chain where new crust forms earth-plate-tectonics Convergent boundary Two plates moving toward each other earth-plate-tectonics Divergent boundary Two plates moving apart earth-plate-tectonics Transform boundary Two plates sliding past each other earth-plate-tectonics Subduction One plate sinking beneath another earth-plate-tectonics Subduction zone The place where one plate dives under another earth-plate-tectonics Ocean trench A deep, narrow valley in the seafloor at a subduction zone earth-plate-tectonics Mariana Trench The deepest known ocean trench earth-plate-tectonics Rift valley A valley formed where plates pull apart earth-plate-tectonics Fault A crack in Earth's crust where blocks of rock move earth-plate-tectonics San Andreas Fault A famous transform fault in California earth-plate-tectonics Earthquake Shaking of the ground from sudden release of energy in Earth's crust earth-plate-tectonics Focus (hypocenter) The point underground where an earthquake starts earth-plate-tectonics Epicenter The point on the surface directly above the focus earth-plate-tectonics Seismic waves Vibrations that travel through Earth during an earthquake earth-plate-tectonics P waves Fast seismic waves that arrive first earth-plate-tectonics S waves Slower seismic waves that arrive second earth-plate-tectonics Surface waves Seismic waves that move along the ground and cause the most damage earth-plate-tectonics Seismograph An instrument that records earthquake waves earth-plate-tectonics Seismologist A scientist who studies earthquakes earth-plate-tectonics Richter scale An older scale of earthquake size earth-plate-tectonics Moment magnitude scale The scale scientists now use to measure earthquake size earth-plate-tectonics Aftershock A smaller quake that follows a larger one earth-plate-tectonics Tsunami A giant sea wave usually caused by an undersea earthquake earth-plate-tectonics Ring of Fire A belt of volcanoes and earthquakes around the Pacific Ocean earth-plate-tectonics Volcano An opening in Earth's crust where lava, ash and gas escape earth-plate-tectonics Vent An opening where magma comes out of a volcano earth-plate-tectonics Crater A bowl-shaped dip at the top of a volcano earth-plate-tectonics Caldera A large crater formed when a volcano collapses earth-plate-tectonics Magma chamber An underground pool of magma earth-plate-tectonics Shield volcano A wide, gently sloping volcano like those in Hawaii earth-plate-tectonics Composite (stratovolcano) A steep cone built from layers of lava and ash earth-plate-tectonics Cinder cone A small, steep cone of volcanic fragments earth-plate-tectonics Active volcano One that is erupting or may erupt earth-plate-tectonics Dormant volcano A volcano that is quiet but may erupt again earth-plate-tectonics Extinct volcano A volcano that is not expected to erupt again earth-plate-tectonics Hot spot A place deep in the mantle that feeds volcanoes, like Hawaii earth-plate-tectonics Pyroclastic flow A fast cloud of hot gas and ash from a volcano earth-plate-tectonics Ash Tiny rock and glass pieces thrown out by a volcano earth-plate-tectonics Mount St. Helens A volcano in Washington State that erupted in 1980 earth-plate-tectonics Mauna Loa A huge shield volcano in Hawaii earth-plate-tectonics Mount Vesuvius The volcano that buried Pompeii in the year 79 earth-plate-tectonics Geyser A hot spring that shoots water into the air earth-plate-tectonics Old Faithful A famous geyser in Yellowstone National Park earth-plate-tectonics Fold mountains Mountains formed when plates push together and crumple rock earth-plate-tectonics Himalayas Mountains formed where India's plate pushes into Asia's earth-plate-tectonics Convection currents Slow movements of hot mantle rock that help move the plates earth-plate-tectonics Evidence for drift: fossils The same fossils appear on continents now far apart earth-plate-tectonics Evidence for drift: coastlines South America and Africa fit together like puzzle pieces earth-plate-tectonics How fast do plates move? A few centimeters a year, about as fast as fingernails grow earth-plate-tectonics Strike-slip fault A fault where blocks slide sideways past each other earth-plate-tectonics