Oceans

We know comparatively little about the oceans, yet they occupy a vast area of the Earth. In total the oceans cover 361,100,000 square kilometres (139,400,000 square miles) and occupy a volume of 1,370 million cubic kilometres (329 million cubic miles). The average depth of the oceans is 3,730 metres ( 12,238 feet). Water covers more than 70 per cent of the Earth's surface. Most of this water is contained within four interlinked oceans—the Pacific, which occupies an area larger than all of the world's continents combined, the Atlantic, which is about half the size of the Pacific, the Indian Ocean, and the small, icy Arctic Ocean surrounding the North Pole. The waters around Antarctica are sometimes described as the Southern Ocean, but most geographers regard this region as extensions of the three largest oceans. Despite their vast size, little was known about the oceans until comparatively recently. Before World War I, the ocean floors were believed to be featureless plains. They are now known to have features as irregular as those on land. Ocean Floor Topography Below sea level, most continents are surrounded by gently sloping continental shelf, which are effectively continuations of the continents. These shelves vary tremendously in width, and may extend as far as 1,500 kilometres (930 miles) out to sea. Many continental shelves are crossed by submarine canyons, some of which are larger than the Grand Canyon. Oceanographers believe these huge depressions were carved out by dense, sediment-loaded water flowing across the seafloor. The continental shelves end at depths of about 130 metres (430 feet). Beyond this point there is a marked change of gradient, where a steep continental slope plunges down to the ocean floor, or abyss. Where the continental slope meets the ocean floor is the continental rise. Sediments wash down from the continental shelf and slope and collect in the rise, which extends up to 1,000 kilometres (620 miles) across the abyss from the continental slope. The abyss proper, which reaches average depths of 4,000 to 6,000 metres (13,000 to 20,000 feet), consists of large abyssal plains broken by low hills. Features of the ocean floor include seamounts, and volcanoes such as the Hawaiian chain, which break the surface as islands. Some extinct, flat-topped volcanoes, such as the Pacific Guyots, are covered by coral deposits, including reefs and atolls.
The most prominent features rising from the abyss are mid-oceanic ridge. Formed when magma rises to the surface along separating underwater plate boundaries, these extensive mountain chains are present in all of the oceans. Mid-oceanic ridges cover nearly 23 per cent of the earth's surface, and generally rise about 1,500 metres (5,000 feet) above the ocean floor. Rift valley, bounded by fault zones, may appear along the crests of the ridges where new ocean crust has been formed by the oozing magma. The temperatures of the rocks in this area are higher than normal, demonstrating that the area is volcanically active. Parts of the Mid-Atlantic Ridge, for example, which stretches from the Norwegian Sea to the South Atlantic, emerge as volcanic islands in Iceland and the Azores. Hot springs were discovered in underwater rift valleys as recently as 1977. Minerals in the hot spring water are deposited around their vents, and build up to form chimneys. Because the water that gushes from these chimneys is dark, they have been named "black smokers". The mineral-rich water around the black smokers, which is often heated to 350C (662F) or more, provides a breeding place for strange bacteria and living creatures, such as blind crabs and tripod fish, formerly unknown to science. Their presence has led to speculation that life may first have evolved on earth in such hostile environments as these. The other main physical features in the oceans are deep trenches, alongside which are chains of volcanoes. Some volcanic chains, such as those in eastern Asia, form islands, while others intrude into adjacent land areas, as in Central and South America. The trenches and ocean ridges are active zones, where the Earth's crust is unstable. They are associated not only with vulcanicity, but also with intense seismic activity. The study of these features helped in the formulation of the theory of plate tectonics, and the recognition that mid-oceanic ridges are plate margins where new crustal rock is being formed as plates move apart, while the trenches mark where plates are being destroyed in subduction zones. The Marianas Trench, on the floor of the North Pacific Ocean, is the deepest known ocean trench, reaching a maximum depth of 11,033 metres (36,198 feet). Plate tectonics explains how the oceans were formed. Around 200 million years ago, all the continents were joined together in one supercontinent called Pangaea. During the last 180 million years, Pangaea has broken up and new oceans have formed through ocean-spreading along rift valleys in the mid-oceanic ridges between the continents. The theory is supported by the fact that none of the basaltic rocks which cover the ocean floor are more than 200 million years old. The oceans, therefore, are young features by comparison with the continents, where rocks have been found that date back around 3.8 billion years. Ocean Water
The oceans contain 97 per cent of the world's water. All of the Earth's natural elements are present in this water, the most common being sodium and chloride, which together form salt. The salinity of ocean water varies between about 3.3 and 3.7 per cent. Areas where the evaporation rate is high and there is little rain generally have a high salinity. Other areas, such as the Baltic Sea, which receive large amounts of fresh water from rivers, are much less saline. Variations in the salinity and temperature of the water in the oceans create density differences in the water, which in turn cause ocean currents. These currents flow through all of the oceans, redistributing the water, transferring heat, and modifying the climate. However, most of the familiar surface currents, such as the warm Gulf Stream which brings mild weather to northwestern Europe, are caused by prevailing winds. The effect of currents on climate were well demonstrated in 1997-1998, when a phenomenon known as El Niño, caused by currents in the Pacific Ocean, caused freak weather conditions in many parts of North and South America, eastern Asia, and Australia. Tides and waves influence the movement of the water in the oceans. Tides are caused by the gravitational pull of the Moon and Sun, which cause the waters in the oceans to rise and fall in a continuous cycle. Winds are normally responsible for the waves. Light winds create calm waters, while choppy waves and rough seas are the result of strong winds. Along the coasts, storm waves bombarding the shore cause erosion, but the most terrifying waves are generated by earthquakes or volcanic eruptions. Called tsunamis, these waves, which are 15 metres (50 feet) or more in height, most frequently occur in the Pacific Ocean. Ocean Life
The oceans are home to an incredible variety of living organisms, ranging from the world's largest animal, the blue whale, to microscopic algae. Around 160,000 ocean species have been named, and scientists believe there may be tens of millions more unclassified. The most important part of the ocean is the euphotic zone, where sunlight can penetrate easily. Beyond this zone, which extends to about 60 metres (200 feet) below the surface, the light becomes too weak to support plant life, and beyond 200 metres (650 feet) it fades away completely. Scientists divide marine life into four main groups: plankton, neuston, nekton, and benthos. Plankton, mostly visible only through a microscope, are found near the surface. They include tiny plants, called phytoplankton, which use sunlight to make their food, and many of which are single-celled, together with microscopic animals collectively known as zooplankton. Crustaceans, which also include many single-celled creatures, make up about 70 per cent of the zooplankton. The copepod, the most common animal in the ocean, is a zooplankton. Much plankton is actually the young of larger ocean species, such as crabs and starfish. Plankton provide the basis of the food chain in the ocean. Neuston are organisms that live at or within 10 to 20 centimetres (4 to 8 inches) of the surface, and include jellyfishes, Portuguese man-of-war, floating snails, and sargassum weed. Nekton, or free-swimming organisms, live mostly near the surface, although some inhabit the dark ocean depths. Among this group are fish, squids, and marine mammals such as whales. Close to the ocean floor dwell the marine organisms collectively known as benthos, among them crabs, lobsters, and starfish, together with fish such as halibut and sole. Other organisms, such as sea grass, become anchored to the ocean floor. Even the deepest ocean trenches, where water pressures are enormous, contain living creatures. Many are scavengers that feed on dead organisms which have drifted down from the top layers. Studying the Ocean
Oceanography began in the second half of the 19th century, and is therefore a relatively young science. One of the most significant early explorations in this field was the round-the-world expedition of a British research ship called Challenger. But while information accumulated about seawater and the myriad life forms it contained, measuring ocean depths was restricted. This was because the only means available before World War I involved lead-weighted ropes or wires, and such a time-consuming, laborious technique was suitable only for shallow coastal waters. From the 1920s, the use of echo-sounders enabled oceanographers to begin the huge task of mapping the ocean floor and, from the 1930s and 1940s, manned descents into the oceans took place in submersibles. Research accelerated after World War II, and the accumulation of information about the ocean floor in the 1950s and 1960s, using increasingly sophisticated techniques, led to the formulation of the theory of plate tectonics. Today the study of "inner space", as the oceans have been called, continues with the aid of research ships, submersibles, satellites, and computers. Overfishing and Pollution
The oceans are an important source of food, minerals, fossil fuels, and energy. Tidal power is already generated, and wave power has potential as an alternative energy supply for the future. However, many formerly rich fishing grounds, such as the Grand Banks off Newfoundland, have been overfished and the cod, for which this fishing ground was once well-known, are now rare. Other misuses of the oceans include oil spills, which are caused by accidents to oil tankers, or the deliberate emptying of oil into the sea when the tankers are cleaned. The dumping of poisonous factory wastes, untreated sewage, or other harmful pollutants, including radioactive wastes, adds to the pollution problem. Two important tropical oceanic environments, where fish and other organisms breed, have been seriously damaged. These are coastal mangrove swamps, which have been polluted and deforested, and coral reefs. A global survey in 1997 suggested that about 95 per cent of coral reefs have been damaged by overfishing, dynamiting, poisoning, pollution, or ships' anchors. The worst damage occurred in the Indo-Pacific region, where there is great demand for reef fish as a delicacy. Global warming could have major effects on the circulation of the oceans and, subsequently, on world climates. For example, computer models suggest that global warming could weaken the Gulf Stream and this would mean much colder winters for northwestern Europe. Global warming could also melt ice locked in the global ice packs and release it into the oceans, where it would cause sea levels to rise and low-lying islands, such as the Maldives, to be submerged within a century. The conservation of the oceans and their valuable resources is a matter for urgent coordinated international action.