| A | B |
|---|
| continental drift | a hypothesis that originally proposed that the continents had once been joined to form a single supercontinent; The supercontinent broke into pieces, which drifted into their present-day positions. |
| continental volcanic arc | mountains formed in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent |
| convective flow | the motion of matter resulting from changes in temperature; The convective flow of material in the mantle is due to Earth’s unequal heating and causes the tectonic plates to move. |
| convergent boundary | a boundary in which two plates move together |
| divergent boundary | a region where the rigid plates are moving apart, typified by the oceanic ridges |
| hot spot | a concentration of heat in the mantle capable of producing magma, which rises to Earth’s surface; The Pacific plate moves over a hot spot, producing the Hawaiian Islands. |
| mantle plume | a mass of hotter-than-normal mantle material that ascends toward the surface, where it may lead to igneous activity |
| normal polarity | a magnetic field that is the same as that which exists at present |
| oceanic ridge | a continuous elevated zone on the floor of all the major ocean basins and varying in width from 1000 to 4000 kilometers; The rifts at the crests of ridges represent divergent plate boundaries. |
| paleomagnetism | the natural remnant magnetism in rock bodies; the permanent magnetization acquired by rock that can be used to determine the location of the magnetic poles at the time it became magnetized |
| Pangaea | the proposed supercontinent that 200 million years ago began to break apart and form the present landmasses |
| plate | one of numerous rigid sections of the lithosphere that moves as a unit over the material of the asthenosphere |
| plate tectonics | the theory that proposes that Earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself |
| reverse polarity | a magnetic field opposite to that which exists at present |
| ridge-push | a mechanism that may contribute to plate motion; It involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity. |
| rift valley | deep faulted structure found along the axes of divergent plate boundaries; Rift valleys can develop on the seafloor or on land. |
| seafloor spreading | the process by which plate tectonics produces new oceanic lithosphere at ocean ridges |
| slab-pull | a mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along |
| subduction zone | a destructive plate margin where oceanic crust is being pushed down into the mantle beneath a second plate |
| transform fault boundary | a boundary in which two plates slide past each other without creating or destroying lithosphere |
| trench | a surface feature in the seafloor produced by the descending plate during subduction |
| volcanic island arc | a chain of volcanic islands generally located a few hundred kilometers from a trench where subduction of one oceanic slab beneath another is occurring |
