Continental drift:the hypothesis that states that the continents once formd a single landmars broke up,and drift to their present location.
Lithosphere:the solid,outer layer of Earth that consists of the crust and the rigid upper part of the mantle.
Asthnosphere:the solid,plastic layer of the mantle beneath the lithosphere,made of mantle rock that flows very slowly, which allows tectonic plates to move on top of it.
Panegea:the hypothetical landmass that existed when all continents were joined,from about 300 to 200 milion years ago.
Plate tectonics:the theory that explains how large pieces of the lithosphere , called plates move and change shape.
Mid-ocean ridge : a long ,undersea mountain chain that has a steep, naarrow valley as it center,that forms as magma rises from the asthnosphere, and that creates new oceanic lithosphere (sea floor) as tectonic plates move apart.
Rift valley:a large elongated depression with steep walls formed by the downward displacement of a block of the earth's surface between nearly parallel faults or fault systems.
Subduction:the sideways and downward movement of the edge of a plate of the earth's crust into the mantle beneath another plate.