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.
continental drift — The first hypothesis proposing large horizontal motions of continents. This idea has been replaced by the theory of plate tectonics.
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.
Curie point — the temperature (about 580 degrees C) above which a rock loses its magnetism.
deep-sea trenches — long, narrow, and very deep (up to 11 km) basins 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 crust and uppermost mantle and is about 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.
Pangaea or Pangea — the proposed "supercontinent" that began to break apart 200 million years ago to form the present continents.
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.
sea-floor 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.