asthenosphere — a portion of the mantle which underlies the lithosphere. This zone consists of easily deformed rock, and in some regions reaches a depth of 700 km.
convergent plate boundary — a boundary between two lithospheric plates that move towards each other. Such boundaries are marked by subduction, earthquakes, volcanoes, and mountain-building.
deep-sea trenches — long, narrow, and very deep basins (up to 11 km) oriented parallel to continents and associated with subduction of oceanic lithosphere.
divergent plate boundary — a boundary between two plates that move away from one another; new lithosphere is created between the spreading plates.
lithosphere — the rigid, outermost layer of the Earth; includes the Earth’s crust and uppermost mantle and is approximately 100 km thick.
mid-ocean ridge — a continuous mountain chain on the floor of all major ocean basins which marks the site where new ocean floor is created as two lithospheric plates move away from one another.
normal polarity — a magnetic field that has the same direction as the Earth's present one.
paleomagnetism — the permanent magnetization recorded in rocks that allows reconstruction of the Earth's ancient magnetic field.
plate tectonics — the theory that proposes that the Earth's lithosphere is broken into plates that move over a plastic layer in the mantle. Plate interactions produce earthquakes, volcanoes, and mountains.
reversed polarity — a magnetic field with direction opposite to that of the Earth's present field.
transform plate boundary — a boundary between lithosphere plates that slide past one another.
seafloor spreading — a hypothesis, proposed in the early 1960s, that new ocean floor is created where two plates move away from one another at mid-ocean ridges.
subduction zone — a long, narrow zone where one lithospheric plate descends beneath another.
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