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Ocean
A large body of salt water that
is not enclosed by land. Parts of oceans that are partly cut off from the open ocean by fingers of land or islands are called seas.
Ocean water contains about 35g of dissolved salts per liter (see: Seawater). This is too high an amount for people to drink or for most plants to use.
The oceans occupy 71% of the surface of the Earth. They have
a total volume of 1,370 million
cu km. The average depth of the world’s oceans is 3,800m. Only about 1% of the ocean floor lies below 6,000m, and it is confined to narrow arc-shaped trenches that lie
mainly on the edges of the Pacific Ocean. They are places where the ocean floor is sinking below the surrounding continent as part of the process called plate tectonics.
Elsewhere the seafloor
begins with a gentle slope. This flat shallow area is called the continental shelf and is up to 150m deep. Beyond the continental
shelf is a steeper slope, called the continental slope, that leads down to the true ocean floor, called the abyssal plain. In the center of some oceans are long, mainly submerged mountain chains. The largest of them is the Mid-Atlantic Ridge, the longest mountain range in
the world. It is made entirely of
volcanoes, very few of which rise to the surface of the sea. Those that do rise to the surface form volcanic islands.
The ocean ridges play an important role in shaping the pattern of ocean currents.
The sediments carried by rivers on land drift out to sea and settle on the ocean floor, where they build up to form layers of sediment that will eventually compress into new rock.
The continual movement of the world’s land surface causes oceans to form and close. It is likely that no current ocean is older than
100 million or so years. (See also: Ocean life; Tide; Water cycle.)
Ocean
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