The ocean floor The continents continue beneath the sea as shelves and slopes. Beyond them lie sediment-covered plains, volcanic mountains and deep trenches. Plate motion creates ocean crust at ridges and recycles it at subduction zones.
The continental shelf is the submerged edge of a continent. Its gently sloping seabed receives sediment from rivers and coasts; sunlight reaches much of the shallow water and supports productive ecosystems.
Beyond the shelf break, the continental slope descends toward the deep basin. Submarine canyons cut through the margin, and sediment-laden currents can carry sand and mud downslope.
Abyssal plains commonly lie 3–6 km below the surface. Fine sediment settles over the oceanic crust, smoothing its relief. Food arriving from upper waters sustains life beyond the reach of sunlight.
Ocean trenches mark places where one tectonic plate bends beneath another. These long, narrow depressions reach the hadal zone; the deepest parts approach 11 km below sea level.
At mid-ocean ridges, plates move apart and magma rises to form new oceanic crust. Seawater circulates through fractured rock, carrying heat and dissolved minerals back to the seabed.
A seamount is an underwater mountain, usually built by volcanic eruptions. Currents flowing around its slopes can concentrate food, making these isolated peaks rich habitats.
A submarine volcano becomes an island when its summit rises above sea level. Waves erode the exposed rock while eruptions may add new lava; some island chains trace the motion of a plate over a hotspot.
The ocean floor Continental shelf Continental slope Abyssal plain Ocean trench Mid-ocean ridge Seamount Volcanic island Return to the coast Ocean geology · NOAA 