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