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tectonic plates

blocks of rock that ride on top of the asthenosphere

island arc

a chain of volcanic islands that forms near the collision of two oceanic plates


a piece of lithosphere that has a unique geologic history and that may be part of a larger piece of lithosphere, such as a continent

Pacific Ring of Fire

a zone of active volcanoes that encircles the Pacific Ocean

convergent boundary

the boundary between tectonic plates that are colliding

divergent boundary

the boundary between tectonic plates that are moving away from each other

continental drift

the hypothesis that states that the continents once formed a single landmass, broke up, and drifted to their present locations


the large northern landmass that separated after the break up of Pangaea including North America, Europe, and Asia


the large southern landmass that separated after the break up of Pangaea including South America, Africa, India, Antarctica, and Australia


the process by which Earth's crust breaks apart; can occur within continental crust or oceanic crust.

sea-floor spreading

the process by which new oceanic lithosphere (sea floor) forms as magma rises to Earth's surface and solidifies at a mid-ocean ridge


the single, large ocean that covered Earth's surface during the time the supercontinent Pangaea existed


the study of the alignment of magnetic minerals in rock, specifically as it relates to the reversal of Earth's magnetic poles; also the magnetic properties that rock acquires during formation


the supercontinent that formed 300 million years ago and that began to break up beginning 250 million years ago

plate tectonics

the theory that explains how large pieces of the lithosphere, called plates, move and change shape

deep-ocean trench

a deep valley that forms along the boundary where an ocean plate subducts

transform boundary

a boundary between tectonic plates that are sliding past each other horizontally

mid-ocean ridge

a long, undersea mountain chain that has a steep, narrow valley at its center, that forms as magma rises from the asthenosphere, and that creates new oceanic lithosphere (sea floor) as tectonic plates move apart

rift valley

a narrow valley that forms where the plates separate


the movement of heated material due to differences in density created by temperature differences

supercontinent cycle

The process by which supercontinents form and break apart over millions of years

geomagnetic reversal time scale

alternating periods of normal and reversed polarity in oceanic crust

subduction zone

the region along a plate boundary where one plate moves underneath another


The solid, outer layer of Earth that consists of the crust and the rigid upper part of the mantle


The solid, plastic-like layer of the mantle that flows very slowly, which allows tectonic plates to move on top of it


a crack in Earth's crust


molten rock inside the Earth


a large area of stable rock that are older than 540 million years


the process in which a terrane becomes part of a continent


an underwater mountain


a small coral island

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