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the layer of rock that forms Earth's outer surface


the layer of hot, solid material between Earth's crust and core


the innermost layer of Earth; made mostly of the metals iron and nickel; consists of two parts - a liquid outer core and a solid inner core


the name of the single landmass that broke apart 200 million years ago and gave rise to today's continents

mid-ocean ridge

an undersea mountain chain where new ocean floor is produced; a divergent plate boundary

sea-floor spreading

the process by which molten material adds new oceanic crust to the ocean floor


the process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary

plate tectonics

the theory that pieces of Earth's lithosphere are in constant motion, driven by convection currents in the mantle

rift valley

a deep valley that forms where two plates move apart

convection current

the movement of a fluid, caused by differences in temperature, that transfers heat from one part of the fluid to another


the shaking that results from the movement beneath Earth's surface


the point beneath Earth's surface where rock breaks under stress and causes an earthquake


the point on Earth's surface directly above an earthquake's focus

P wave

a type of seismic wave that compresses and expands the ground; the first waves to arrive

S wave

a type of seismic wave that moves the ground up and down or side to side; these waves arrive after P waves

surface wave

a type of seismic wave that forms when P waves and S waves reach Earth's surface

Mercalli scale

a scale that rates earthquakes according to their intensity and how much damage they cause at a particular place


the measurement of an earthquake's strength based on seismic waves and movement along faults

Richter scale

a scale that rates seismic waves as measured by a particular type of mechanical seismograph

moment magnitude scale

a scale that rates earthquakes by estimating the total energy released by an earthquake


an earthquake that occurs after a larger earthquake in the same area


a large wave produced by an earthquake on the ocean floor


a weak spot in the Earth's crust where magma has come to the surface


the molten mixture of rock-forming substances, gases, and water from the mantle


liquid magma that reaches the surface; also the rock formed when liquid lava hardens

Ring of Fire

a major belt of volcanoes that rims the Pacific Ocean

island arc

a string of islands formed by the volcanoes along a deep-ocean trench

hot spot

an area where magma from deep within the mantle melts through the crust above it

physical property

any characteristic of a substance that can be observed or measured without changing the composition of the substance

chemical property

any property of a substance that produces a change in the composition of matter


the resistance of a liquid to flowing


a material found in magma that is formed from the elements oxygen and silicon


a hot, fast-moving type of lava that hardens to form smooth, ropelike coils


a slow-moving type of lava that hardens to form rough chunks; cooler than pahoehoe

magma chamber

the pocket beneath a volcano where magma collects

lava flow

the area covered by lava as it pours out of a volcano's vent

pyroclastic flow

the expulsion of ash, cinders, bombs, and gases during an explosive volcanic eruption


describes a volcano that is not currently active, but that may become active in the future


describes a volcano that is no longer active and is unlikely to erupt again

shield volcano

a wide, gently sloping mountain made of layers of lava and formed by quiet eruptions

cinder cone

a steep, cone-shaped hill or small mountain made of volcanic ash, cinders, and bombs piled up around a volcano's opening

composite volcano

a tall, cone-shaped mountain in which layers of lava alternate with layers of ash and other volcanic materials

rock cycle

a series of processes on the surface and inside Earth that slowly changes rocks from one kind to another

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