Many other large bodies of water have been designated as seas, but all are marginal to the great oceans. The largest of these are the Bering Sea, the Coral Sea, the East and South China seas (see China Sea, East, and China Sea, South), the Sea of Okhotsk), the Sea of Japan, the Yellow Sea, and the Philippine Sea, bordering the Pacific; the Arabian Sea, the Red Sea, and the Bay of Bengal), bordering the Indian Ocean; the Scotia Sea, the North Sea, the Labrador Sea, the Weddell Sea, the Norwegian Sea, and the Greenland Sea, bordering the Atlantic Ocean. Marginal seas differ from the major oceans primarily in size, but also in depth and bottom topography (see gulf and bay).

The boundaries between the oceans are based on geographic criteria and have little to do with physical water-mass boundaries. The Atlantic is separated from the Indian Ocean by the 20 degrees E meridian, and from the Pacific Ocean (in the south) by a line extending from Cape Horn at the tip of South America to the South Shetland Islands off Antarctica's tip and (in the north) by the narrowest part of the Bering Strait. The dividing line between the Pacific and Indian oceans extends along an arc through the Malay Peninsula, Sumatra, Java, and Timor to Cape Londonderry in Australia, to Tasmania, and then along the 147 degrees E meridian to Antarctica.

Reference is often made to the Antarctic, or Southern, Ocean, which encircles the Antarctic continent and consists of the southernmost sectors of the three principal oceans. In spite of the lack of definitive geographic boundaries, the meteorological and oceanographic conditions in the high southern latitudes combine to produce a well-defined circumpolar current called the West Wind Drift. This current distinguishes the Antarctic Ocean as a physical entity, but the ocean's geographic borders are less easily defined.

Oceanic regions constitute a much larger percentage of the Earth's surface in the Southern Hemisphere (81%) than in the Northern Hemisphere (61%). This factor is reflected by major differences in oceanic circulation and weather patterns between the two hemispheres.

ORIGIN
An explanation of the origin of the world's oceans must account for both the great ocean basins as well as the source of the water filling them. Perhaps surprisingly, neither the basins nor the volume of water in them has remained constant over the history of the Earth.

The Water
As the cosmic dust collapsed billions of years ago to form the planet Earth, water was probably locked into rock-forming compounds. These compounds (hydrated silicates) would have slowly released the trapped water during the first billion years or so of Earth history and formed the primordial ocean. The duration and time of initiation of this process are not exactly known, because rocks containing the record of that time span have been destroyed in the succeeding 3 billion years. Water would not have been released at the earliest stage of the Earth's development, however, because a molecule of water is lighter than a molecule of any of the lighter elements, or "volatiles," such as neon, that would have escaped into space during the intense heat that accompanied the formation of the planet. On the other hand, the origin of the primordial ocean must have occurred during the first billion years, because some of the earliest rocks found on Earth show evidence of deposition in a large body of water.

The time required to accumulate the volume of water in the present oceans is unknown. The water was released during the cooling of the Earth and attained, early after its initiation, a volume not drastically different from that of the modern oceans. That the volume of water in the oceans has not changed drastically during the last few hundred million years is inferred from evidence indicating that the interiors of the continental land masses have never been inundated by deep oceans. Any incursions of seawater upon the continents that did occur were in response to tectonic changes (that is, changes due to deformations of the Earth's crust) rather than changes in the volume of water in the oceans.

The most recent changes in ocean volume have accompanied the ice ages during the last two to three million years. The northern polar icecap has expanded and contracted with great regularity, and the volume of the oceans has fluctuated correspondingly as water is alternately locked into or released from the ice cap. These fluctuations, however, have accounted for changes in sea level of no more than 200 m (660 ft).

The Basins
The events of formation of the initial ocean basins that held the primordial ocean are unknown, because they have been destroyed by subsequent geologic events. The history of the present ocean basins, however, is well known. All of the continental land masses as they are now known began to break up about 200 million years ago from two great supercontinents called Gondwanaland and Pangea. As the continents drifted to their present positions, the great ocean basins were left in their wake. This concept has been refined within the framework of the theories of continental drift and plate tectonics.
Each ocean basin has evolved in a slightly separate manner, but the overall history of each basin has a similar series of events. The origin of the Atlantic Ocean basin is best known and serves as a good example of the processes involved. About 150 million years ago forces at work beneath the crust of the Earth split the supercontinents into large fragments. One such fragment contained North America joined to Eurasia, and another contained South America joined to Africa. By 135 million years ago North America had separated from Eurasia, leaving a small, restricted ocean basin with no open ocean circulation. At the same time, the early South Atlantic basin was formed by the separation of South America and Africa. The two parts of the Atlantic were not connected with a pathway for deep circulation until about 65 million years ago, after the South Atlantic had widened to approximately 3,000 km (1,800 mi) and the North Atlantic had widened as a result of a separation between Greenland and Europe. The process continues today as new (basaltic) ocean crust is added at the Mid-Atlantic Ridge, widening the Atlantic Ocean.

The same process has formed the Pacific Ocean basin, but the relatively simple pattern outlined for the Atlantic Ocean has been complicated by other factors in the Pacific. The Pacific Ocean basin is surrounded by the great marginal trench systems, which are accompanied by volcanic island arcs and violent earthquake activity. These trenches mark the location where great slabs of oceanic crust are being reabsorbed back into the Earth by a process called subduction (see subduction zone), which generates the seismic activity along the boundaries of the Pacific Ocean basin. This process is active along the Aleutian Trench to the north, the Kuril-Japan-Marianas Trench System and the Mindanao Trench to the west, the New Hebrides, Tonga, and Kermadec trenches to the south, and the Middle American and Peru-Chile trenches to the east. New oceanic crust is being produced in the Pacific Ocean basin along the East Pacific Rise in the southern and eastern parts of the basin.

The history of the Indian Ocean basin has not been completely determined. India and Australia separated from Antarctica about 80 million years ago. India slid past Australia about 45 million years ago and rammed into Asia, leaving behind a deep ocean basin that widened in an east-west direction beginning about 35 million years ago. The pattern of seafloor formation, however, in this relatively small ocean basin has not been completely determined, due to the complexities found there.

PHYSIOGRAPHY OF THE SEAFLOOR
The seafloor is shaped into a host of volcanic features that grade from long submarine mountain chains, which are larger than their continental equivalents, to deep trenches that are thousands of times larger than those found on land. The shape of all these physiographic features is related to the origin of the slice of ocean floor on which they are found. The shape of the seafloor, in turn, affects the origin and distribution of some of the great oceanic circulation systems (see ocean currents). These systems play a role in the distribution of the oceanic nutrients that control biogenic productivity in the oceans.

Ridges, Plains, and Trenches
Perhaps the most striking feature on the seafloor is the mid-oceanic ridge system that, through branches, extends across all the major ocean basins. Typically, the central axis of this system is marked by a steep- walled valley, usually about 40 km (25 mi) wide and 2 km (1 mi) deep. Small segments of the ridge extend above sea level to form islands (for example, Iceland, the Azores, and the Galapagos Islands), but most of the ridge crest is at a depth of approximately 2.5 km (1.5 mi).
The seafloor continues to deepen away from the crests of the mid-oceanic ridge system out to the extensive, flat abyssal plains, which constitute the largest segment of the seafloor. The depths of the individual plains are roughly uniform--the deepest of them occur in the Pacific Ocean (6,000 m/20,000 ft), and the shallowest occur in the Atlantic Ocean (5,000 m/ 16,000 ft).

In selected areas, usually at the margins of the ocean basins, the abyssal plains descend into steep oceanic trenches, where the greatest depths in the oceans are found. Examples of these trenches include the Peru-Chile Trench (Pacific), 8,055 m (26,428 ft); the Puerto Rico Trench (Atlantic), 9,200 m (30,200 ft); the Tonga Trench (Pacific), 10,880 m (35,700 ft); and the deepest hole in the ocean, the Marianas Trench (Pacific), 11,022 m (36,163 ft). Associated with most of these major trench systems are volcanic island arcs found on the landward side of the trenches.

Fracture Zones and Seamounts
Superimposed on the features of the seafloor previously mentioned are many other smaller-scale, but very important, physiographic features. Segments of the mid-oceanic ridges are commonly offset laterally by parallel, linear fracture zones thousands of kilometers long. These oceanic fracture zones are characterized by ridges and valleys separated by steep rock cliffs that are hundreds to several thousands of meters high. These fracture zones can be traced out into the abyssal plains, where the traces of the cliffs are lost beneath the sediment.

The abyssal plains are also dotted with numerous isolated mountains called seamounts that extend in some cases above sea level to become islands. Characteristically, these seamounts belong to large groups of such features, which may be randomly dispersed over a large area or arranged in a line.
 
Seafloor Spreading
The origins of many of these strikingly different physiographic features are linked together by the concept of seafloor spreading. The seafloor is produced by the cooling of upwelling molten material at spreading centers characterized by the mid-oceanic ridges. The ridges are broken and discontinuous due to fracture zones along which transform faulting occurs. Newly formed material moves away from the ridge crest, and tens of millions of years later it is subducted or downwarped in an oceanic trench.

Continental Shelves, Slopes, and Rises
The margins of all the continents extend seaward as a broad, flat, shallow shelf. These continental shelves dip gently seaward and are usually less than 200 m (660 ft) below sea level. The continental shelves comprise only a small portion (7.6%) of the seafloor, but their importance is far greater, because the shallow depths make it possible to exploit their natural resources.

The shelves abruptly end at the shelf breaks, where the seafloors rapidly descend along the continental slopes to the abyssal depths (see continental shelf and slope). At the bases of the continental slopes, which cover about 15.3% of the oceanic area, are the continental rises, which represent a series of sediment slumps from the slopes above that have spilled down onto the deep seafloor or ocean basins. The basins cover about 75.9% of the oceanic area. Only 1.2% of the ocean is greater than 6,000 m (19,686 ft) in depth.

IMPORTANCE OF THE OCEANS AND SEAS
Oceans and seas are now understood to be integral parts of the entire geologic process of continental weathering, runoff, and deposition, followed by either uplift and subaerial exposure or subduction into the depths of the Earth during the process of plate tectonics and seafloor spreading.

The oceans and seas, also known as the hydrosphere, are responsible for the regulation of many major processes that occur on the surface of the Earth. Much of the precipitation that falls upon land areas is derived from oceanic evaporation. The hydrosphere acts as a tremendous heat reservoir, exerting a dominant effect on temperature extremes over large land areas. The movement of ocean currents also creates moderating effects in some areas at latitudes where weather extremes might otherwise make life unpleasant (see ocean-atmosphere interaction). In addition, the oceans act as reservoirs for numerous other substances that provide a buffering effect on the levels of various gases in the atmosphere and, in some cases, a dilution of otherwise toxic materials that humans have introduced into salt and fresh waters.

The oceans and seas represent a place of recreation, a means of transportation, and a storehouse of food, mineral resources, and energy. Their potential as a source of immeasurable resources is just beginning to be realized.

Transportation
The ocean provides tremendous potential as a means of transportation. The major portion of goods distributed worldwide are shipped by water--the least expensive method of transport, and one that provides a livelihood for a significant percentage of the world's population.

Food
As a biological entity, the oceans represent a highly productive environment. The basic biological habitats of the ocean can be divided into pelagic (water region) and benthic (seafloor region) environments. The pelagic zone can be divided into the neritic zone (down to 200 m/660 ft and over the continental shelf) and the oceanic zone (below 200 m). The neritic and the upper oceanic (epipelagic zone) regions correspond to the photic zone, in which photosynthesis is possible. The benthic habitat (see benthonic zone) is also divided into the intertidal zone (between high and low tide), the sublittoral zone (down to 200 m; see littoral zone); the bathyal zone (200 to 4,000 m), the abyssal zone (4,000 to 5,000 m/13,000 to 16,000 ft), and the hadal zone (deeper than 5,000 m). The photic zone is the most highly productive area, containing numerous benthic, planktonic (free-floating), and nektonic (free-swimming) marine creatures. Below the photic zone the biomass decreases considerably, with the only food source for marine life being the constant rain of organic matter from above. The lack of light and food has produced numerous adaptations in deep-sea life, resulting in some very strange forms at these depths.
Throughout recorded history people have used the ocean as a source of food. But even at present rates of removal, the food-resource potential of the oceans has barely been touched. At present, most countries remove only certain choice species of fish from the ocean (see fishing industry). This practice has led to the depletion of fish populations of these few species, whereas at the same time other species have remained almost untouched. The depletion of fish populations in once-choice fishing grounds has caused considerable international dispute between the governments and fishing fleets of many countries--ultimately leading to expansion of offshore fishing limits in many parts of the world--and has also forced the development of techniques for fish farming and the cultivation and seeding of many bottom areas in order to enhance shellfish production.
A change in attitudes about the consumption of various species of fish may also help alleviate fishing pressures on certain species and encourage a more balanced exploitation of fisheries resources. The infancy of research in fisheries biology is such that the behavior, food requirements, and breeding habits of most fish are not well known. Investigation may lead to a situation where human beings can depend more heavily on the ocean as a food source without creating a drastic impact on the overall ecology of the ocean.

Water
The ocean is a source of fresh water in many highly arid, nearshore areas where the cost of transporting water from regions where naturally occurring fresh water is abundant is greater than the cost of desalinization. Most current desalinization methods resemble a distilling process. Other methods that are available but not in widespread use are freezing, reverse osmosis, and ionic processes. Current costs of desalinization are not so high as to be prohibitive to domestic and some industrial users; but whether new developments will bring costs down to a level where agricultural and other industrial users will find the use of desalinized water practical remains uncertain.

Energy
Theoretically, the ocean represents a tremendous source of energy. Current use of the oceans by the energy industry is restricted for the most part, however, to the use of seawater as a coolant in nearshore nuclear power plants. This particular use has produced considerable response from groups concerned with the environmental impact of the discharge of heated water from these reactors.

Some energy can be extracted from the ocean by making use of the change in sea level caused by tidal cycles. The use of this method, however, is currently limited. Numerous other methods are mostly in experimental stages at present. Several of these methods make use of the temperature differential that exists across the thermocline. Other methods would make use of wave and current energy. Most of these designs suffer from one or more disadvantages, such as high generation costs, great distances between suitable generating sites and the power market, or a number of engineering problems (see ocean thermal energy conversion).

The Sun replenishes the oceanic energy reserves at a much greater rate than human beings could ever remove energy from the oceans. The problems of economically removing this energy, most of which is diffused over the entire ocean, are, however, at present so insurmountable as to make widespread dependence on this energy resource impractical.

Oil
Since the 1960s the production of oil from wells on the continental shelves has increased drastically, to the point where it represents a substantial percentage of the world's production (see petroleum). More recent evidence indicates that oil deposits also reside on the continental slope. Current drilling and maintenance technology, however, has not yet advanced to a state where tapping of these additional reserves is practical in a routine manner. Current estimates of offshore reserves far surpass presently known onshore reserves. A need for additional energy supplies will no doubt stimulate the technological advancements necessary to take advantage of these reserves and to minimize damage to the environment (see oil spill).

Minerals
Economically important minerals are constantly introduced into the ocean from a variety of sources, and most of the material accumulates on the ocean bottom. Rivers dump vast quantities of particulate mineral materials into the oceans each year. Volcanic eruptions and hydrothermal solutions introduce many metals into solution and in solid form.
Heavy minerals have a tendency to accumulate in superficial placer deposits, which can be located using a variety of geophysical techniques and then can be mined by dredging. Phosphate rocks are also mined from nearshore areas. Salt domes, which are subbottom bedrock deposits, are mined by pumping hot water down to melt the sulfur and to force the molten material back up the drill string.

The deep sea, as well as nearshore environments, stores vast amounts of economically important mineral resources (see oceanic mineral resources). Large areas of the ocean bottom, mostly in temperate and tropical regions, are covered with calcareous oozes (see ooze, deep-sea), which are the product of deposition of calcium-carbonate skeletons accreted by planktonic and benthonic microorganisms. This material could be useful in the manufacture of various building supplies, most notably cement. In higher latitudes the ocean-bottom sediments are dominated by siliceous oozes. These oozes, the product of silica-secreting microorganisms, serve as an efficient filtering and insulating material.

OCEAN POLLUTION
Oceans clearly play an essential role in life on Earth, yet because of their vastness humans have tended to use their waters as dumping grounds for many waste materials. This practice has increased as land areas for such wastes diminish. Oceans also receive all of the pollutants that are fed to them by the rivers of the world. Even when ships are not actively engaged in dumping wastes, they are themselves sources of pollution--most notably, the giant tankers that have caused numerous massive oil spills.
As a result, by the late 20th century ocean studies indicate that what had once been thought impossible is becoming a reality. The oceans as a whole are showing signs of pollution (see pollution, environmental). Even the surface waters of the oceans are increasingly plagued by obvious litter. Some of this litter washes ashore to render beaches unsightly, while other such debris entangles and kills many sea birds and mammals every year.

More insidious than these litter problems are the effects of toxic contaminants from wastes that are dumped in the ocean. These chemicals can upset delicate marine ecosystems as they are absorbed by organisms all along the food chain. Even the paints that are being used on many ships can be hazardous (see tributyl tin).

The need to address the matter of ocean pollution has been recognized at national and international levels. The U.S. Congress, for example, passed an act in 1988 that would prohibit ocean dumping by 1991, and in that same year 65 nations agreed to cease burning toxic wastes at sea by 1994 should acceptable alternative practices be found. In the late 1990s the latter action remains under debate, however, as several nations continue the practice of ocean burning of toxic wastes. The U.S. act may prove as unenforceable as a prior one in 1977 that attempted the same prohibition. Worldwide, the problem of ocean pollution remains.

Craig Amerigian and Michael T. Ledbetter

Bibliography: Bendavid-Val, L., Realms of the Sea (1991); Borgese, E. M., et al., eds., Ocean Yearbook (annual); Cousteau, Jacques, and Dugan, James, The Living Sea (1988); Duxbury, A. B. and A. C.,  An Introduction to the World's Oceans, 4th ed. (1994); Fine, J. C., Exploring the Sea (1982); Geyer, R. A., Handbook of Geophysical Exploration at Sea, 2d ed. (1991); Gwynn, R., Way of the Sea: The Use and Abuse of the Oceans (1992); Lamson, C., ed., The Sea Has Many Voices: Oceans Policy for a Complex World (1994); Neal, P., Oceans under Threat (1990); Thorne-Miller, B., Ocean: Photographs from the World's Greatest Underwater Photographers (1993); World Conservation Union Staff, Random House Atlas of the Oceans (1991).