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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.
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.
The continental margin near an area of lithospheric plate convergence; also called Pacific-type margin.
The submerged outer edge of a continent, made of granitic crust; includes the continental shelf and continental slope. Compare ocean basin.
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.
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.
The sloping transition between the granite of the continent and the basalt of the seabed; the true edge of a continent.
Area of irregular, seismically inactive topography marking the position of a once-active transform fault.
A spring of hot, mineral- and gas-rich seawater found on some oceanic ridges in zones of active seafloor spreading.
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.")
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.
Curving chain of volcanic islands and seamounts almost always found paralleling the concave edge of a trench.
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.
The continental margin near an area of lithospheric plate divergence; also called Atlantic-type margin.
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°.
The abrupt increase in slope at the junction between continental shelf and continental slope.
A deep, V-shaped valley running roughly perpendicular to the shoreline and cutting across the edge of the continental shelf and slope.
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.
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