The oceans cover 71 percent of the earth's surface and contain all of the saltwater found on earth. Oceanography, the study of the oceans, includes many sciences whose goal is to understand the oceans and the life within them. From the oceanographer's point of view, the planet Earth should be called the water planet, because only 29 percent of its surface is land and the rest is water. It is estimated that the volume of water in the oceans is 1.37 billion cubic kilometers—an amount that is difficult to comprehend. If the Earth were a smooth sphere, the water depth around the globe would be 2,686 meters (8,800 feet) deep or 1.7 miles. Adding water from the land, the level would rise another 56 meters (200 feet).

The average depth of the oceans as they exist today is 3,795 m; 75 percent of the ocean basins are between 3,000 and 6,000 m deep. The Pacific Ocean is the deepest, with an average depth of 4,188 m and contains 50.1 percent of the world's oceanic water; the Pacific covers one-third of the earth's surface.

The Southern Hemisphere contains 80.9 percent water and 19.1 percent land, while the Northern Hemisphere is 60.7 percent water and 39.3 percent land. The land is not as high as the oceans are deep—84 percent of the ocean floor exceeds 2,000 m in depth, while only 11 percent of the land surface is greater than 2,000 m above sea level. The Marianas

Trench adjacent to the Philippines is 11,035 m deep while Mount Everest is 8,848 m high.

The uneven distribution of the land and sea affects wind and ocean circulation systems. The most important force that drives the circulation of the oceans at the surface is the wind. Friction between the wind and the sea surface creates waves on the surface of the water that help move the water. Differences in atmospheric pressure produce wind patterns of easterly and westerly belts and, with the addition of the earth's rotation, produce water movements. As the water circulates, opposing currents bring water together from different directions causing the water to build several meters high. These convergence zones are located, for the most part, at 30° north and south of the equator. Water flowing away from these highs and influenced by the rotation of the earth forms a nearly closed circulation system called a gyre. Most of the currents move predominantly in east-west directions in the open sea and as a result remain approximately in the same climatic belt.

In the Northern Hemisphere the gyres circulate clockwise and in the Southern Hemisphere they circulate counterclockwise. This circulation pattern is obstructed by the imposition of continents, which interfere with the free movement of water.

Deflection by continents causes the currents to move in a north-south direction across climatic zones, with the result that cold or warm currents move through water of different surface temperatures. The Gulf Stream, for example, formed by the northward movement of a warm, low-latitude current into northern areas, moderates the temperatures along the Atlantic Coast of North America and Europe. Temperatures in the Gulf Stream may be more than 5°C higher than the surrounding water it is moving through. Currents transport large volumes of water, and in the case of the Gulf

The Pacific Ocean is the deepest and largest of the oceans, covering one-third of the earth's surface. (Tom Van Sant/Corbis)

Stream as much as 150 million cubic meters per second flow past Nova Scotia.

The average temperature of the ocean surface layer down to 200 m is about 15°C. Below the surface layer is the thermocline, a zone of rapidly decreasing temperature that extends from 200 to 1,000 m. Below 1,000 m the temperature decline is not as steep, but the bottom water at the seafloor can be close to freezing. This cold, deep layer originates largely in the polar regions where dense, cold water sinks and migrates toward the equator very slowly, taking up to 600 years to get there. Winds parallel to coasts cause warm surface waters to be blown offshore, creating a region of low pressure that causes upwelling of cold, deep water that replaces the surface water that has moved offshore. Containing dissolved nutrients, the water reaching the photic zone supports abundant plant life and the animals that feed on them. Many of the world's fishing grounds are located in zones of coastal upwelling.

Life on the seafloor is classified into zones; benthic plants and animals live on the seafloor and pelagic forms live in the water column as either swimmers or floaters. The neritic zone refers to water overlying the continental shelf, and oceanic water is located from the edge of the continental shelf seaward.

Where did all this water come from? Oceanographers turn to astronomy and geology for the answer. Ultimately, everything on earth came from the gaseous nebula that condensed to form the solar system. But as the earth began to evolve, it was too hot to contain water on its surface; huge volumes of water expelled by the large numbers of volcanoes remained as vapor. It was only after the earth cooled that vapor was able to condense and form liquid water. As early earth evolved, the differentiation between high granitic continents and lower basaltic ocean basins occurred, providing a place for liquid water to accumulate in large volumes in low places on the surface of the earth. In the traditional view it is thought that from the end of earth's formative period, about 3.9 billion years ago, the amount of water has remained the same— but its distribution between land and sea has been variable dependent on the hydrologic cycle, tectonic activity, and the changing shapes of the ocean basins and the subsequent rise and fall of the sea. However, all of this water might not have come only from within the earth; some scientists now believe that a large amount of water was added by impact of ice comets, which were much more numerous in the early days of the solar system

Modern technology utilizing satellites, sonar, and radar has allowed extensive mapping and sampling of the seafloor since the 1950s. Prior to that time, the shape of the seafloor remained largely unknown. Information came from activities such as dropping heavily weighted ropes to the bottom of the sea to determine depths. There were also tantalizing suggestions that the ocean floor had some relief as scientists contemplated the shapes of volcanic islands rising out of the sea. Today we know that the seafloor contains some of the most magnificent features of the earth's surface: continental shelves, continental slopes and rises that drop down to the abyss, and the very deep oceanic trenches. There are volcanic mountain ranges, and huge, solitary volcanoes, many of which form the underlying structure of atolls and others that are tall enough to break the ocean's surface and form oceanic islands.

A variety of sediments, either derived from land or from the accumulated remains of skeletons of plants and animals, cover vast areas of the seafloor. These sediments provide oceanog-raphers with the records about the rate of accumulation and the history recorded in their layers.

In addition to drawing academic interest, the oceans contain resources that are needed and removed by mining and drilling. Sand and gravel are pumped from shallow seas for construction, beach replenishment, and landfill. Many sands from the ocean contain large amounts of gold, diamonds, and tin. Phosphorite, for fertilizer, and sulfur and manganese nodules are removed from the seafloor. In addition, coal, oil, and gas are found below the seafloor where the continents are covered by water.

Salt is the resource with the longest history of being harvested from the oceans. On average, 1,000 g of seawater will contain up to 35 g of salt. The salinity at any given location is determined by the rate of evaporation, the amount of precipitation, freezing and thawing that may occur, and the amount of freshwater that enters the sea from the land. The original saltiness of the oceans came from the weathering of rocks and from the release of chemicals from volcanic eruptions.

Salt in seawater is more than sodium chloride; it is a complex mixture of inorganic salts, atmospheric gases, traces of organic matter, and a small amount of particulate matter. The most abundant dissolved ions by weight are chloride (55.04), sodium (30.61), sulfate (7.68), and magnesium (3.69).

Although the oceans have been receptacles for material created by natural processes, people have used them as dumping grounds. Most of the products of civilization have made their way to the oceans. Huge quantities of industrial wastes, sewage sludge, and garbage, not only from land-based communities but also from ships, have been dumped in the oceans. Studies are underway to determine the feasibility of placing toxic and radioactive materials in the seafloor, where they can remain for hundreds of thousands of years undisturbed, eventually becoming harmless. The containers for such wastes will consist of corrosion-proof material, and the burial in sediment will exert enough pressure to prevent leakage. The containers would be placed within plates, not along plate boundaries where volcanism and earthquakes could disturb them. Critics say that the dangers of leakage are real; that tectonic activity on the ocean floor cannot be predicted, and that there is great uncertainty that all nations would follow the proposed safety procedures.

—Sidney Horenstein

See also: Abyssal Floor; Continental Shelf; Continental Slope and Rise; Deep-Sea Hydrothermal Vent Faunas; Oceanic Trenches


Duxbury, Alison, and Keith Sverdrup. 2001. Fundamentals of Oceanography. New York: Mcgraw Hill Pub. Co.; Kennish, Michael J. 2001. Practical Handbook of Marine Science, 3rd ed. Boca Raton, FL: CRC Press; Montgomery, Carla. 1996. Fundamentals of Geology, 3rd ed. New York: McGraw Hill Professional Publishing.

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