The Pacific Ocean, the world's largest water body, occupies a third of the Earth's surface. Extending approximately 15,500 km (9,600 mi) from the BERING SEA in the Arctic north to the icy margins of Antarctica's Ross Sea in the south, the Pacific reaches its greatest width at about 5 deg north latitude, where it stretches approximately 19,800 km (12,300 mi) from Indonesia to the coast of Colombia. The western limit of the ocean is often placed at the Strait of Malacca. The Pacific contains about 25,000 islands (more than the total number in the rest of the world's oceans combined), almost all of which are found south of the equator. The Pacific covers an area of 179.7 million sq km (69.4 million sq mi). The lowest known point on Earth, in the MARIANAS TRENCH, lies within the Pacific.
Along the Pacific Ocean's irregular margins lie many seas, the largest of which are the CELEBES SEA, CORAL SEA, East China Sea, Sea of Japan, SULU SEA, and YELLOW SEA. The Strait of Malacca joins the Pacific and the Indian oceans on the west, and the Strait of Magellan links the Pacific with the Atlantic Ocean on the east.
The ocean floor of the central Pacific basin is relatively uniform, with a mean depth of about 4,270 m (14,000 ft). The major irregularities in the area are the extremely steep-sided, flat-topped submarine peaks known as SEAMOUNTS. The western part of the floor consists of mountain arcs that rise above the sea as island groups, such as the Solomon Islands and New Zealand, and deep trenches, such as the Marianas Trench, the Philippine Trench, and the Tonga Trench. Most of the deep trenches lie adjacent to the outer margins of the wide western Pacific continental shelf.
Along the eastern margin of the Pacific basin is the East Pacific Rise, which is a part of the worldwide mid-oceanic ridge. About 3,000 km (1,800 mi) across, the rise stands about 3 km (2 mi) above the adjacent ocean floor. Because a relatively small land area drains into the Pacific, and because of the ocean's immense size, most sediments are authigenic or pelagic in origin. Authigenic sediments include montmorillonite and phillipsite. Pelagic sediments derived from seawater include pelagic red clays and the skeletal remains of sea life. Terrigenous sediments are confined to narrow marginal bands close to land.
Water temperatures in the Pacific vary from freezing in the poleward areas to about 29 deg C (84 deg F) near the equator. Salinity also varies latitudinally. Water near the equator is less salty than that found in the mid-latitudes because of abundant equatorial precipitation throughout the year. Poleward of the temperate latitudes salinity is also low, because little evaporation of seawater takes place in these areas.
The surface circulation of Pacific waters is generally clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The North Equatorial Current, driven westward along latitude 15 deg north by the trade winds, turns north near the Philippines to become the warm Kuroshio, or Japan, Current. Turning eastward at about 45 deg north, the Kuroshio forks and some waters move northward as the ALEUTIAN CURRENT, while the rest turn southward to rejoin the North Equatorial Current. The Aleutian Current branches as it approaches North America and forms the base of a counterclockwise circulation in the Bering Sea. Its southern arm becomes the slow, south-flowing California Current.
The South Equatorial Current, flowing west along the equator, swings southward east of New Guinea, turns east at about 50 deg south latitude, and joins the main westerly circulation of the Southern Pacific, which includes the Earth-circling Antarctic Circumpolar Current. As it approaches the Chilean coast, the South Equatorial Current divides; one branch flows around Cape Horn and the other turns north to form the Peru, or Humboldt, Current.
Only the interiors of the large land masses of Australia, New Guinea, and New Zealand escape the pervasive climatic influence of the Pacific. Within the area of the Pacific, five distinctively different climatic regions exist: the mid-latitude westerlies, the trades, the monsoon region, the typhoon region, and the doldrums. Mid-latitude westerly air streams occur in both northerly and southerly latitudes, bringing marked seasonal differences in temperature. Closer to the equator, where most of the islands lie, steadily blowing trade winds allow for relatively constant temperatures throughout the year of 21-27 deg C (70-81 deg F).
The monsoon region lies in the far western Pacific between Japan and Australia. Characteristic of this climatic region are winds that blow from the continental interior to the ocean in winter and in the opposite direction in summer. Consequently, a marked seasonality of cloudiness and rainfall occurs. Typhoons often cause extensive damage in the west and southwest Pacific. The greatest typhoon frequency exists within the triangle from southern Japan to the central Philippines to eastern Micronesia. Although more poorly defined than the other climatic regions, two major doldrum areas lie within the ocean, one located off the western shores of Central America and the other within the equatorial waters of the western Pacific. Both areas are noted for their high humidity, considerable cloudiness, light fluctuating winds, and frequent calms.
The Andesite Line is the most significant regional distinction in the Pacific. It separates the deeper, basic igneous rock of the Central Pacific Basin from the partially submerged continental areas of acidic igneous rock on its margins. The Andesite Line follows the western edge of the islands off California and passes south of the Aleutian arc, along the eastern edge of the Kamchatka Peninsula, the Kuril Islands, Japan, the Mariana Islands, the Solomon Islands, and New Zealand. The dissimilarity continues northeastward along the western edge of the Albatross Cordillera along South America to Mexico, returning then to the islands off California. Indonesia, the Philippines, Japan, New Guinea, and New Zealand--all eastward extensions of the continental blocks of Australia and Asia--lie outside the Andesite Line.
Within the closed loop of the Andesite Line are most of the deep troughs, submerged volcanic mountains, and oceanic volcanic islands that characterize the Central Pacific Basin. It is here that basaltic lavas gently flow out of rifts to build huge dome-shaped volcanic mountains whose eroded summits form island arcs, chains, and clusters. Outside the Andesite Line, volcanism is of the explosive type, and the so-called Pacific rim of fire is the world's foremost belt of explosive volcanism.
The largest landmass in the Pacific is the continent of Australia, which is approximately equal in size to the 48 contiguous U.S. states. About 3,200 km (2,000 mi) southeast of Australia is the large island group of New Zealand. Almost all of the smaller islands of the Pacific lie between 30 deg north and 30 deg south latitude, extending from Southeast Asia to Easter Island; the rest of the Pacific Basin is almost devoid of land. The great triangle of Polynesia connecting Hawaii, Easter Island, and New Zealand encompasses the island arcs and clusters of the Cook, Marquesas, Samoa, Society, Tokelau, Tonga, and Tuamotu islands. North of the equator and west of the international date line are the numerous small islands of Micronesia, including the Caroline Islands, the Marshall Islands, and the Mariana Islands. In the southwestern corner of the Pacific lie the islands of Melanesia, dominated by New Guinea. Other important island groups of Melanesia include the Bismarck Archipelago, Fiji, New Caledonia, the Solomon Islands, and Vanuatu. Islands in the Pacific Ocean are of four basic types: continental islands, high islands, coral reefs, and uplifted coral platforms. Continental islands lie outside the Andesite Line and include New Guinea, the islands of New Zealand, and the Philippines. These islands are structurally associated with the nearby continents. High islands are of volcanic origin, and many contain active volcanoes. Among these are Bougainville, Hawaii, and the Solomon Islands.
The third and fourth types of islands are both the result of coralline island building. Coral reefs are low-lying structures that have built up on basaltic lava flows under the ocean's surface. One of the most dramatic is the GREAT BARRIER REEF off northeastern Australia. A second island type formed of coral is the uplifted coral platform, which is usually slightly larger than the low coral islands. Examples include Banaba (formerly Ocean Island) and Makatea in the Tuamotu group of French Polynesia.
Important human migrations occurred in the Pacific in prehistoric times, most notably those of Polynesians from Tahiti to Hawaii and New Zealand. The ocean was sighted by Europeans early in the 16th century, first by Vasco Nunez de Balboa (1513) and then by Ferdinand Magellan, who crossed the Pacific during his circumnavigation (1519-22). For the remainder of the 16th century Spanish influence was paramount, with ships sailing from Spain to the Philippines, New Guinea, and the Solomons. During the 17th century the Dutch, sailing around southern Africa, dominated discovery and trade; Abel Janszoon Tasman discovered (1642) Tasmania and New Zealand. The 18th century marked a burst of exploration by the Russians in Alaska and the Aleutian Islands, the French in Polynesia, and the British in the three voyages of James Cook--to the South Pacific and Australia, Hawaii, and the North American Pacific Northwest.
Growing imperialism during the 19th century resulted in the occupation of much of the Pacific by the Western powers. Significant contributions to oceanographic knowledge were made by the voyages of the H.M.S. Beagle in the 1830s, with Charles Darwin aboard; the H.M.S. Challenger during the 1870s; the U.S.S. Tuscarora (1873-76); and the German Gazelle (1874-76). Although the United States took the Philippines in 1898, Japan controlled the western Pacific by 1914. By the end of World War II the U.S. Pacific Fleet was the virtual master of the ocean.
Fourteen independent nations are located in the Pacific: Kiribati, Nauru, Papua New Guinea, Taiwan, Tuvalu, Western Samoa, Australia, Fiji, Japan, New Zealand, the Philippines, the Solomon Islands, Tonga, and Vanuatu. Eight of these nations have achieved full independence since 1960, and the islands formerly within the Trust Territory of the Pacific Islands have redefined their relationships with the United States. Also within the Pacific are the U.S. state of Hawaii and several island territories and possessions of Australia, Chile, France, Japan, New Zealand, the United Kingdom, and the United States.
The exploitation of the Pacific's mineral wealth is hampered by the ocean's great depths. In shallow waters off the coast of Australia, petroleum and natural gas are extracted, and pearls are harvested along the coasts of Australia, Japan, Papua New Guinea, Nicaragua, Panama, and the Philippines, although in sharply declining volume. The Pacific's greatest asset is its fish. The shoreline waters of the continents and the more temperate islands yield herring, salmon, sardines, snapper, swordfish, and tuna, as well as shellfish. In 1986, the member nations of the South Pacific Forum declared the area a nuclear-free zone in an effort to halt nuclear testing and prevent the dumping of nuclear waste there.
Bibliography: Barkley, R.A., Oceanographic Atlas of the Pacific Ocean (1969); Cameron, I., Lost Paradise (1987); Couper, A., Development in the Pacific Islands (1988); Crump, D.J., ed., Blue Horizons (1980); Gilbert, John, Charting the Vast Pacific (1971); Lower, J. Arthur, Ocean of Destiny: A Concise History of the North Pacific, 1500-1978 (1978); Oliver, D.L., The Pacific Islands, 3d ed. (1989); Ridgell, R., Pacific Nations and Territories, 2d ed. (1988); Soule, Gardner, The Greatest Depths (1970); Spate, O.H., Paradise Found and Lost (1988); Terrell, J.E., Prehistory in the Pacific Islands (1986).