ocean

The Earth's oceans and continents

A continuous body of salt water that surrounds the continents and fills the Earth's great depressions. The ocean's cover about 71% of the Earth's surface (more than 80% of the Southern Hemisphere) and comprise about 98% of the water of the planet (see hydrosphere).

There are five main oceans: the Atlantic, Pacific, Indian, Arctic, and Antarctic. They may be described by distinct region (littoral, benthos, pelagic, and abyssal), or by depth (continental shelf or margin, deep sea plain, and ocean trench). The seafloor has a varied topography. See also mid-ocean ridge.

Seawater contains salt and other mineral deposits; the salt content, between 3.3% and 3.7%, is the result of washout from the land and interchange with the atmosphere over the ages. Light penetrates seawater to a maximum depth of about 300 meters (1,000 ft), below which plant life cannot grow.

The oceans are constantly moving in currents, tides, and waves. With the atmosphere, they form an integral part of the Earth's hydrological cycle and climate. By acting as a reservoir of solar heat energy, they ameliorate the effects of seasonal and diurnal (day-night) temperature extremes for much of the world. The provide minerals such as manganese nodules, petroleum, and natural gas. The continental shelf yields sand and gravel. Marine fauna, such as fish and plankton, are a vital part of the world's food chain.

Ocean facts
Total area	300 million sq km (138 million sq mi)
Total volume	c. 1.4 billion cu km (322 million cu mi)
Average depth	3,500 m (12,000 ft)
Average temperature	3.9°C (39°F)

History of the oceans

Through volcanic activity the oceans have been forming over the last 200 million years. The theory of continental drift (and associated seafloor spreading) has revealed that the ancient supercontinent Pangea was surrounded by a vast ocean, Panthallasia. As Pangaea began to split, a smaller and shallower ocean, the Tethys Sea, formed between the continents. By about 65 million years ago, the Atlantic and Indian oceans appeared. The Pacific became separated from the Atlantic and Indian oceans when the North and South American continents joined. The separation of Greenland from North America, and the widening of the North Atlantic, completed the encirclement of the Arctic Ocean. See also origin of the oceans.

Related category

• GEOLOGY AND PLANETARY SCIENCE

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