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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


a proposed concentration of heat in the mantle capable of introducing magma that in turn extrudes onto Earth's surface


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


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


the natural remnant magnetism in rock bodies


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


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


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

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