A broad, relatively fl at region of the ocean that lies at least 4.5 km below sea level.
apparent polar-wander path
A path on the globe along which a magnetic pole appears to have wandered over time; in fact, the continents drift, while the magnetic pole stays fairly fixed.
The layer of the mantle that lies between 100-150 km and 350 km 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 sulfide components of the hot water instantly precipitate when the water mixes with seawater and cools.
continental drift hypothesis
The idea that continents have moved and are still moving slowly across the Earth's surface.
convergent plate boundary
A boundary at which two plates move toward each other so that one plate sinks (subducts) beneath the other; only oceanic lithosphere can subduct.
divergent plate boundary
A boundary at which two lithosphere plates move apart from each other; they are marked by mid-ocean ridges.
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.
A chain of now-dead volcanoes transported off the hot spot by the movement of a lithosphere plate.
The relatively rigid, nonflowable, outer 100- to 150-km-thick layer of the Earth; constituting the crust and the top part of the mantle.
A portion of the outer, relatively rigid layer of the Earth; most seismic activity happens at the boundaries of plates, while the interior of a plate is relatively stable.
The difference between the expected strength of the Earth's magnetic field at a certain location and the actual measured strength of the field at that location.
The angle between the direction a compass needle points at a given location and the direction of true north.
The angle between a magnetic needle free to pivot on a horizontal axis and a horizontal plane parallel to the Earth's surface.
The change of the Earth's magnetic polarity; when a reversal occurs, the field flips from normal to reversed polarity, or vice versa.
marine magnetic anomaly
An unusually strong or unusually weak magnetic field, as measured over the sea floor; in map view, they look like stripes that are parallel to the mid-ocean ridge.
A 2-km-high submarine mountain belt that forms along a divergent oceanic plate boundary.
The supposed position of the Earth's magnetic pole in the past, with respect to a particular continent.
passive margin basin
A thick accumulation of sediment along a tectonically inactive coast, formed over crust that stretched and thinned when the margin first began.
The force that drives plates away from a mid-ocean ridge; it is caused by the fact that the ridge is elevated relative to the regions of oceanic plate away from the ridge.
The process by which continental lithosphere stretches and breaks apart; rifting produces normal faults and, commonly, volcanism.
The gradual widening of an ocean basin as oceanic crust forms at a mid-ocean ridge axis and then moves away from the axis.
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 deep elongate trough bordering a volcanic arc; a trench defines the trace of a convergent plate boundary.