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abyssal hill

Small sediment-covered inactive volcano or intrusion of molten rock less than 200 meters (650 feet) high, thought to be associated with seafloor spreading. Abyssal hills punctuate the otherwise flat abyssal plain.

abyssal plain

Flat, cold, sediment-covered ocean floor between the continental rise and the oceanic ridge at a depth of 3,700 to 5,500 meters (12,000 to 18,000 feet). Abyssal plains are more extensive in the Atlantic and Indian Oceans than in the Pacific.

active margin

The continental margin near an area of lithospheric plate convergence; also called Pacific-type margin.

angel of incidence

In meteorology, the angle of the sun above the horizon.


The discovery and study of submerged contours.

continental margin

The submerged outer edge of a continent, made of granitic crust; includes the continental shelf and continental slope. Compare ocean basin.

continental rise

The wedge of sediment forming the gentle transition from the outer (lower) edge of the continental slope to the abyssal plain; usually associated with passive margins.

continental shelf

The gradually sloping submerged extension of a continent, composed of granitic rock overlain by sediments; has features similar to the edge of the nearby continent.

continental slope

The sloping transition between the granite of the continent and the basalt of the seabed; the true edge of a continent.


The point on Earth's surface directly above the focus of an earthquake.

fracture zone

Area of irregular, seismically inactive topography marking the position of a once-active transform fault.


A flat-topped, submerged inactive volcano.

hydrothermal vent

A spring of hot, mineral- and gas-rich seawater found on some oceanic ridges in zones of active seafloor spreading.

ice age

One of several periods (lasting several thousand years each) of low temperature during the last million years. Glaciers and polar ice were derived from ocean water, lowering sea level at least 100 meters (328 feet). (See Appendix II, "Geological Time.")

ice cap

Permanent cover of ice; formally limited to ice atop land, but informally applied also to floating ice in the Arctic Ocean.


A large mass of ice floating in the ocean that was formed on or adjacent to land. Tabular icebergs are tablelike or flat; pinnacled icebergs are castellated, or jagged. Southern icebergs are often tabular; northern icebergs are often pinnacled.

island arc

Curving chain of volcanic islands and seamounts almost always found paralleling the concave edge of a trench.

ocean basin

Deep-ocean floor made of basaltic crust. Compare continental margin.

oceanic ridge

Young seabed at the active spreading center of an ocean, often unmasked by sediment, bulging above the abyssal plain. The boundary between diverging plates. Often called a mid-ocean ridge, though less than 60% of the length exists at mid-ocean.

passive margin

The continental margin near an area of lithospheric plate divergence; also called Atlantic-type margin.


A gap in polar pack ice at which liquid water contacts the atmosphere.


A circular or elliptical projection from the seafloor, more than 1 kilometer (0.6 mile) in height, with a relatively steep slope of 20° to 25°.

shelf break

The abrupt increase in slope at the junction between continental shelf and continental slope.

submarine canyon

A deep, V-shaped valley running roughly perpendicular to the shoreline and cutting across the edge of the continental shelf and slope.

transform fault

A plane along which rock masses slide horizontally past one another.


An arc-shaped depression in the deep-ocean floor with very steep sides and a flat sediment-filled bottom coinciding with a subduction zone. Most trenches occur in the Pacific.

turbidity current

An underwater "avalanche" of abrasive sediments thought responsible for the deep sculpturing of submarine canyons and a means of transport for sediments accumulating on abyssal plains.

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