Devoir de Philosophie

Pacific Ocean - Geography.

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Pacific Ocean - Geography. I INTRODUCTION Pacific Ocean, largest and deepest of the world's five oceans, covering more than a third of the earth's surface and containing more than half of its free water. It is sometimes divided into two nominal sections: the part north of the equator is called the North Pacific; the part south of the equator, the South Pacific. The name Pacific, which means peaceful, was given to it by the Portuguese navigator Ferdinand Magellan in 1520. II BOUNDARIES AND SIZE The Pacific Ocean is bounded on the east by the North and South American continents; on the north by the Bering Strait; on the west by Asia, the Malay Archipelago, and Australia; and on the south by the Southern Ocean. In the southeast it is arbitrarily divided from the Atlantic Ocean by the Drake Passage along 68° west longitude; in the southwest, its separation from the Indian Ocean is not officially designated. Apart from the marginal seas along its irregular western rim, it has an area of 155.6 million sq km (60.1 million sq mi), substantially larger than the entire land surface of the globe. Its length north to south is 13,900 km (8,637 mi) and its greatest width is 17,700 km (11,000 mi) from Panama to the Malay Peninsula. Its average depth is 4,280 m (14,040 ft). The greatest known depth in any of the world's oceans is 11,033 m (36,198 ft) in the Mariana Trench off Guam. III GEOLOGIC FORMATION AND STRUCTURAL FEATURES The Pacific is the oldest of the existing ocean basins, its oldest rocks having been dated at 200 million years. The major features of the basin and rim have been shaped by the phenomena associated with plate tectonics. The coastal shelf, which extends to depths of 180 m (600 ft), is narrow along North and South America but is relatively wide along Asia and Australia. The East Pacific Rise, a midocean ridge system, extends 8,700 km (5,400 mi) from the Gulf of California to a point 3,600 km (2,240 mi) west of the southern tip of South America, and rises an average of 2.1 m (1.3 ft) above the ocean floor. Along the East Pacific Rise molten rock upwells from the earth's mantle, adding crust to the plates on both sides of the rise. These plates, which are huge segments of the earth's surface, are thus forced apart, causing them to collide with the continental plates adjacent to their outer edges. Under this tremendous pressure, the continental plates fold into mountains, and the oceanic plates downbuckle, forming deep trenches, called subduction zones, from which crust is carried back into the mantle (see Earth: Plate Tectonics). The stresses at the areas of folding and subduction are responsible for the earthquakes and volcanoes that give the rim of the Pacific basin the name "ring of fire" (see Ocean and Oceanography). IV ISLANDS The Pacific Ocean contains more than 30,000 islands; their total land area, however, amounts to only one-quarter of one percent of the ocean's surface area. The largest islands, in the western region, form volcanic island arcs that rise from the broad continental shelf along the eastern edge of the Eurasian Plate. They include Japan, Taiwan, the Philippines, Indonesia, New Guinea, and New Zealand. The oceanic islands, collectively called Oceania, are the tops of mountains built up from the ocean basin by extruding molten rock. The mountains that remain submerged are called seamounts. In many areas, particularly the South Pacific, the land features above the sea surface are accretions of shell material (see Coral Reef). Along the eastern edge of the Pacific, the continental shelf is narrow and steep, with few island areas. The major groups are the Galápagos at the equator, which rise from the Nazca Plate, and the Aleutians in the north, which are part of the North American continental shelf. V CURRENTS The driving forces for ocean currents are the earth's rotation, wind friction at the surface of the water, and variations in seawater density due to differences in temperature and salinity. The interaction between wind and current has a major effect on climate and is studied for long-range weather prediction and for sea travel. The surface currents of the North Pacific consist of two gyres, or circular systems. In the extreme north the counterclockwise Subarctic Gyre encompasses the westward-flowing Alaska Current and the eastward-flowing Subarctic Current. The main body of the North Pacific, however, is dominated by the huge North Central Gyre, which circulates clockwise. It encompasses the North Pacific Current, flowing east; the California Current, flowing southeast; and the Kuroshio Current (or Japan Current), flowing north up the coast of Japan. The California Current is cold, broad, and slow-moving; the Kuroshio is warm, narrow, and rapid, similar to the Gulf Stream. Close to the equator at 5° north latitude, the eastward-flowing Equatorial Countercurrent separates the North and South Pacific systems but sends most of its waters into the North Equatorial Current. The South Pacific is dominated by the counterclockwise-moving South Central Gyre, which encompasses the South Equatorial Current flowing east and south, the South Pacific Current flowing west, and the Mentor Current flowing north, parallel to South America. Located in the extreme south is the Antarctic Circumpolar Current (West Wind Drift), which encircles the globe, merging the waters of the Pacific, Atlantic, and Indian oceans. It is the most important source of deep-sea circulation. From it flows the broad, cold Peru, or Humboldt, Current, which travels north along the coast of South America and sends its waters into the South Equatorial Current. VI WIND SYSTEMS The outstanding wind systems of the Pacific Ocean are the twin belts of westerlies, which blow from west to east between 30° and 60° latitude, one in the northern hemisphere and one in the southern hemisphere. These winds vary in seasonal patterns. The stormy and unpredictable westerly of the North Central Pacific is being studied for its possible controlling effect on global weather patterns. Between the westerlies are the much more steady trade winds, which move from the east in the northern hemisphere and from the west in the southern hemisphere. Violent tropical storms, called typhoons in the western Pacific and hurricanes in the southern and eastern Pacific, originate in the trade wind belt in late summer and early autumn. At the equator are the equatorial doldrums, light winds with seasonal cyclonic activity. At the highest latitudes of the Pacific, the winds have little direct effect on climate and water currents. VII RESOURCES Much of the plant and animal life of the Pacific Ocean is concentrated along its margins. Nutrient-rich waters from the deep Antarctic Circumpolar Current upwell to the surface in the Peru Current along the coast of Chile and Peru, and the area sustains a large population of anchovetas that is of great importance as a world food resource. A large guano industry has been established from droppings of the seabirds that feed upon the anchovetas. The northwestern Pacific, including the Sea of Japan (East Sea) and the Sea of Okhotsk, is another major world fishery. Coral reefs rich with sea life reach their peak in the Great Barrier Reef, which extends for 2,010 km (1,250 mi) along the northeastern coast of Australia. Tuna is another important Pacific resource, bringing fleets of many nations in search of the schools that migrate over much of the ocean. The Pacific has also begun to be exploited for its vast mineral resources. The continental shelves off the coasts of California, Alaska, China, and the Indonesian area are known to contain large reserves of petroleum. Patches of the ocean floor are covered with "manganese nodules," potato-sized concretions of iron and manganese oxides that sometimes also contain copper, cobalt, and nickel. Programs are under way to examine the feasibility of mining these deposits. See also Deep-Sea Exploration. Contributed By: Fred N. Spiess Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

« concretions of iron and manganese oxides that sometimes also contain copper, cobalt, and nickel. Programs are under way to examine the feasibility of mining thesedeposits. See also Deep-Sea Exploration. Contributed By:Fred N. SpiessMicrosoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved. »

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