Adapted from 'Earth: Portrait of a Planet' by Stephen Marshak, 2005, W. W. Norton & Company, Inc.
Absolute plate velocity
The movement of a plate relative to a fixed point in the mantle.
A wedge-shaped mass of sediment and rock scraped off the top of a downgoing plate and accreted onto the overriding plate at a convergent plate boundary.
Active continental margin
A continental margin that coincides with a plate boundary.
The layer of the mantle that lies between 100-150km and 350km deep; the asthenosphere is relatively soft and can flow when acted on by force.
The cloud of suspended minerals formed where hot water spews out of a vent along a mid-ocean ridge; the dissolved sulphide components of the hot water instantly precipitate when the water mixes with seawater and cools.
The upward force acting on a less dense object immersed or floating in denser material.
The process of two buoyant pieces of lithosphere converging and squashing together.
A linear belt along which continental lithosphere stretches and pulls apart.
A broad, shallowly submerged region of a continent along a passive margin.
A boundary at which two plates move toward each other so that one plate sinks (subducts) beneath the other; only oceanic lithosphere can subduct.
A tabular (wall-shaped) intrusion of rock that cuts across the layering of country rock.
A boundary at which two lithosphere plates move apart from each other; they are marked by mid-ocean ridges.
Downgoing plate (or slab)
A lithosphere plate that has been subducted at a convergent margin.
The point on the surface of the Earth directly above the focus of an earthquake.
A narrow band of vertical fractures in the ocean floor; fracture zones lie roughly at right-angles to a mid-ocean ridge, and the actively slipping part of a fracture zone is a transform fault.
Global positioning system (GPS)
A satellite system people can use to measure rates of movement of the Earth's crust relative to one another, or simply to locate their position on the Earth's surface.
A location at the base of the lithosphere, at the top of a mantle plume, where temperatures can cause melting.
Another term for the focus of an earthquake.
The relatively rigid, nonflowable, outer 100 to 150km thick layer of the Earth; constituting the crust and the top part of the mantle.
A space below ground filled with magma.
A column of very hot rock rising up through the mantle.
A small ocean basin created when sea-floor spreading occurs behind an island arc.
A 2km high submarine mountain belt that forms along a divergent oceanic plate boundary.
Overriding plate (or slab)
The plate at a subduction zone that overrides the downgoing plate.
A continental margin that is not a plate boundary.
Glass-encrusted basalt blobs that form when magma extrudes on the sea floor and cools very quickly.
One of about twenty distinct pieces of the relatively rigid lithosphere.
The border between two adjacent lithosphere plates.
A region away from the plate boundaries that consequently experiences few earthquakes.
The theory that the outer layer of the Earth (the lithosphere) consists of separate plates that move with respect to one another.
Relative plate velocity
The movement of one lithosphere plate with respect to another.
A proposed mechanism for plate motion; because mid-ocean ridges lie at a higher elevation than abyssal plains, gravity causes the ridge to push on the lithosphere that lies farther from the ridge.
The formation of a divergent boundary through the splitting in two of a continent.
The force that downgoing plates (or slabs) apply to oceanic lithosphere at a convergent margin.
The process by which one oceanic plate bends and sinks down into the asthenosphere beneath another plate.
A boundary at which one lithosphere plate slips laterally past another.
A fault marking a transform plate boundary; along mid-ocean ridges, transform faults are the actively slipping segment of a fracture zone between two ridge segments.
A deep elongate trough bordering a volcanic arc; a trench defines the trace of a convergent plate boundary.
A point where three lithosphere plate boundaries intersect.
A curving chain of active volcanoes formed adjacent to a convergent plate boundary.
A sloping band of seismicity defined by intermediate and deep focus earthquakes that occur in the downgoing slab of a convergent plate boundary.