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asthenosphere a subdivision of the mantle situated below the lithosphere continental drift a hypothesis, credited largely to Alfred Wegener, that suggested all present continents once existed as a single supercontinent continental rift a linear zone along which continental lithosphere stretches and pulls apart continental volcanic arc mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent convergent plate boundary a boundary in which two plates move together, resulting in oceanic lithosphere being thrust beneath an overriding plate, eventually to be reabsorbed into the mantle Curie point the temperature above which a material loses its magnetization divergent plate boundary a boundary in which two plates move apart, resulting in upwelling of material from the mantle to create new seafloor fracture zone linear zone of irregular topography on the deep-ocean floor; follows transform faults and their inactive extensions hot-spot a proposed concentration of heat in the mantle capable of introducing magma that in turn extrudes onto Earth's surface lithosphere the rigid outer layer of Earth, including the crust and upper mantle lithospheric plate a coherent unit of Earth's rigid outer layer that includes the crust and upper mantle magnetometer a sensitive instrument used to measure the intensity of Earth's magnetic field at various points magnetic reversal a change in Earth's magnetic field from normal to reverse or vice versa magnetic time scale the detailed history of Earth's magnetic reversals developed by establishing the magnetic polarity of lava flows of known age 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 the same as that which presently exists oceanic ridge a continuous mountainous ridge on the floor of all the major ocean basins paleomagnetism the natural remnant magnetism in rock bodies Pangaea the proposed supercontinent that 200 million years ago began to break apart and form the present landmasses partial melting the process by which most igneous rocks melt plate tectonics the theory that proposes Earth's outer shell consists of individual plates, which interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself reverse polarity a magnetic filed opposite to that which presently exists ridge push a mechanism that may contribute to plate motion; involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity rift a region of Earth's crust along which divergence (separation) is taking place seafloor spreading the hypothesis first proposed in the 1960s by Harry Hess, suggesting that new oceanic crust is produced at the crests of mid-ocean ridges, which are the sites of divergence 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 zones a long, narrow zone where one lithospheric plate descends beneath another supercontinent a huge landmass that consists of all, or nearly all, of the existing continents combined into one transform fault boundary a boundary in which two plates slide past one another without creating or destroying lithosphere volcanic island arc a chain of volcanic islands generally located a few hundred kilometers from a trench where there is active subduction of one oceanic plate beneath another