INDIAN OCEAN

body of salt water, covering approximately one-fifth of the total ocean area of the world. It is the smallest, youngest, and physically most complex of the world's three major oceans. It stretches for more than 6,200 miles (10,000 kilometres) between the southern tips of Africa and Australia and, without its marginal seas, has an area of about 28,360,000 square miles (73,440,000 square kilometres). The Indian Ocean's average depth is 12,760 feet (3,890 metres), and its deepest point, in the Sunda Deep of the Java Trench off the southern coast of Java, is 24,442 feet (7,450 metres). The Indian Ocean, with depth contours and undersea features. The Indian Ocean is bounded by Iran, Pakistan, India, and Bangladesh to the north; the Malay Peninsula, the Sunda Islands of Indonesia, and Australia to the east; Antarctica to the south; and Africa and the Arabian Peninsula to the west. In the southwest it joins the Atlantic Ocean south of the southern tip of Africa, and to the east and southeast its waters mingle with those of the Pacific. The question of defining the oceanic limits of the Indian Ocean is complicated and remains unsettled. The clearest border and the one most generally agreed upon is that with the Atlantic Ocean, which runs from Cape Agulhas, at the southern tip of Africa, due south along the 20 E meridian to the shores of Antarctica. The border with the Pacific Ocean to the southeast usually is drawn from South East Cape on the island of Tasmania south along the 147 E meridian to Antarctica. Bass Strait, between Tasmania and Australia, is considered by some to be part of the Indian Ocean and by others to be part of the Pacific. The northeastern border is the most difficult to define. The one most generally agreed upon runs northwest from Cape Londonderry in Australia across the Timor Sea, along the southern shores of the Lesser Sunda Islands and the island of Java, and then across the Sunda Strait to the shores of Sumatra. Between the island of Sumatra and the Malay Peninsula the boundary is usually drawn across the Singapore Strait. The Indian Ocean has the fewest seas of the major oceans. To the north are the inland Red Sea and Persian Gulf. The Arabian Sea is to the northwest, and the Andaman Sea to the northeast. The large gulfs of Aden and Oman are to the northwest, the Bay of Bengal is to the northeast, and the Great Australian Bight is off the southern coast of Australia. The Indian Ocean differs from the Atlantic and Pacific Oceans in several other respects. In the Northern Hemisphere it is landlocked, does not extend to Arctic waters, or have a temperate-to-cold zone. It has fewer islands and narrower continental shelves. It is the only ocean with an asymmetric and, in the north, semiannually reversing surface circulation. It has no separate source of bottom water (i.e., the Indian Ocean's bottom water originates outside its boundaries) and has two sources of highly saline water (the Persian Gulf and the Red Sea). Below the surface layers, especially in the north, the ocean's water is extremely low in oxygen. body of salt water, covering approximately one-seventh of the Earth's surface and extending between Africa in the west, Australia in the east, Asia in the north, and Antarctica in the south. The Indian Ocean constitutes the world's third largest ocean after the Pacific and Atlantic oceans and, excluding its marginal seas, has an area of about 28,360,000 square miles (73,440,000 square km). It meets the Atlantic Ocean in the southwest and the Pacific Ocean in the east and southeast. The Indian Ocean has an average depth of 12,760 feet (3,890 m); the greatest depth (24,442 feet below sea level) is in the Java Trench, south of Java. The chief marginal seas include the Red Sea, Arabian Sea, Persian Gulf, Andaman Sea, Bay of Bengal, and the Great Australian Bight. Its major islands and island groups include Madagascar, Sri Lanka, and the Mascarenes (Mauritius, Runion, and related islands). The continental shelf of the Indian Ocean consists of a narrow, flat surface that averages 75 miles (120 km) in width and broadens to about 190 miles (300 km) off Bombay and northwestern Australia. The major oceanic-ridge system forms an inverted Y of seismically active mountains, which extends southeast and south from the Arabian Sea and splits into southwestern and southeastern branches in midocean. Notable among several major aseismic ridges is the Ninetyeast Ridge, which runs north-south for 2,800 miles (4,500 km) along the 90 E meridian. The surface-water movement of the Indian Ocean is governed by the prevailing winds. Two main water-circulation systems dominate the ocean, a northern system (between latitudes 2 and 10 S) where currents are directly related to the seasonal change of the monsoon winds, and a southern system (between 10 and 20 S) where several major currents comprise a regular counterclockwise system. The Indian Ocean is rich in minerals. Petroleum and natural gas are found on the continental shelves of the Persian Gulf, the Gulf of Cambay (near Bombay), and off northwestern Australia. Sediments containing iron ore and copper have been discovered in the Red Sea, and huge accumulations of nodules containing manganese and other metals have been located on the ocean bed. Additional reading General works Oceanographic texts and atlases that include information on the Indian Ocean and its seas include Rhodes W. Fairbridge (ed.), The Encyclopedia of Oceanography (1966); James P. Kennett, Marine Geology (1982); George L. Pickard and William J. Emery, Descriptive Physical Oceanography: An Introduction, 5th enlarged ed. (1990); M. Grant Gross, Oceanography, 5th ed. (1990); Alastair Couper (ed.), The Times Atlas and Encyclopedia of the Sea (1989); and Martyn Bramwell (ed.), The Rand McNally Atlas of the Oceans (also published as Mitchell Beazley Atlas of the Oceans and The Macmillan Atlas of the Oceans, 1977, reissued 1987). Julia J. Gotthold and Donald W. Gotthold (comps.), Indian Ocean (1988), is an annotated bibliography of works on the area's history, sea power, and ocean science, on the island groups in the region, and on the Indian Ocean's strategic military importance. Physical and human geography Physical environment Indian Ocean geology and oceanography are explored in Alan E.M. Nairn and Francis G. Stehli (eds.), The Ocean Basins and Margins, vol. 6, The Indian Ocean (1982), which includes treatment of the marginal seas; United States Defense Mapping Agency Hydrographic Center, Sailing Directions (Planning Guide) for the Indian Ocean, 3rd ed. (1988); and D.J. Rochford, Salinity Maxima in the Upper 1000 m of the Indian Ocean, Australian Journal of Marine and Freshwater Research, 15:1-24 (1964). There are many useful essays in Bernt Zeitzschel and Sebastian A. Gerlach (eds.), The Biology of the Indian Ocean (1973), including a summary introduction to the area by Klaus Wyrtki, Physical Oceanography of the Indian Ocean, pp. 1836. Graphic presentations of information include United States Central Intelligence Agency, Indian Ocean Atlas (1976); Klaus Wyrtki, Edward B. Bennett, and David J. Rochford, Oceanographic Atlas of the International Indian Ocean Expedition (1971); and Stefan Hastenrath and Peter J. Lamb, Climatic Atlas of the Indian Ocean (1979 ). Economic aspects The history of commerce in the area is examined by K.N. Chaudhuri, Trade and Civilisation in the Indian Ocean: An Economic History from the Rise of Islam to 1750 (1985). Present-day sources of revenue are detailed by Ganpat Singh Roonwal, The Indian Ocean: Exploitable Mineral and Petroleum Resources (1986). Study and exploration A.L. Rice (ed.), Deep-sea Challenge: The John Murray/Mabahiss Expedition to the Indian Ocean, 193334 (1986), presents the history of the voyage and its influence on oceanographic research in the area. Other works on the area's history include Satish Chandra (ed.), The Indian Ocean: Explorations in History, Commerce, and Politics (1987); G.A. Ballard, Rulers of the Indian Ocean (1927, reprinted 1984); and Auguste Toussaint, History of the Indian Ocean (1961; originally published in French, 1961). Joseph R. Morgan Philomene A. Verlaan