Earth seen from space; the Pacific Ocean is in view The oceans cover more than 360 million square kilometres (139 million square miles) of the Earth’s surface, approximately 71% of its total area. More than 1350 million cubic kilometres (324 million cubic miles) of water is contained within it, representing nearly 97% of the Earth’s entire supply. The oceans hold enough salt in them to cover Europe to a depth of 5 kilometres (3 miles). There are five oceans: in order of size, the Pacific, Atlantic, Indian, Arctic and Southern. We often use the word "sea" to mean ocean, although, strictly, seas are smaller parts of the ocean partly enclosed by land, for example, Mediterranean Sea or North Sea.
The ocean waters are not still, but move in tides and currents. Tides shift the water daily under the gravitational pull of the Moon. Currents, great flowing bands of water, swirl around the globe, swept along by the winds. Some are cool and and some are warm currents: together they have a significant effect on the climates of lands bordering the ocean.
The ocean floor
The ocean floor is mostly a level plain, called the abyssal plain. It also features mountain ranges, volcanic peaks, long ridges, deep trenches and high ledges where it borders the continents. The abyssal plain lies at an average depth of about 4500 metres (15,000 feet). Most of it is covered by a thick layer of sediment called ooze, the remains of dead organisms that have sunk to the bottom.A cross-section through the ocean floor
The floor of the Atlantic OceanThe Mid-Oceanic Ridge runs all the way around the Earth. The ridge is made by magma (molten rock), rising up from beneath the Earth’s crust. As the magma spreads outwards, cools and solidifies, the ocean floor becomes wider. Meanwhile, other parts of the ocean floor are gradually sinking down beneath the continents in deep-ocean trenches.
The continental shelf, the ledge that surrounds the abyssal plain, plunges steeply down to it at the continental slope. The water above the continental shelf, really a part of the continent that lies under the ocean, is never deeper than 200 metres (650 feet). It is here that most ocean life is found.
Consultant: Ian Fairchild
See also in Life
See also in Geography