The Indian Ocean is the third largest body of water in the world,
covering about 20% of the Earth's water surface. It is bounded on the
north by southern Asia; on the west by the Arabian Peninsula and
Africa; on the east by the Malay Peninsula, the Sunda Islands, and
Australia; and on the south by Antarctica. It is separated from the
Atlantic Ocean by the 20 deg east meridian south of Africa, and from
the Pacific by the 147 deg east meridian. The northernmost extent of
the Indian Ocean is approximately 30 deg north latitude in the Persian
Gulf. The ocean is nearly 10,000 km (6,200 mi) wide at the southern
tips of Africa and Australia; its area is 73,556,000 sq km (28,400,000
sq mi), including the RED SEA and the PERSIAN GULF. The ocean's
volume is estimated to be 292,131,000 km(3) (70,086,000 mi(3)). Small
islands dot the continental rims. Island nations within the ocean are
Madagascar (formerly Malagasy Republic), the world's fourth largest
island; Comoros; Seychelles; Maldives; Mauritius; and Sri Lanka.
Indonesia borders it. The ocean's importance as a transit route
between Asia and Africa has made it a scene of conflict. Because of
its size, however, no one nation had successfully dominated until the
early 1800s when Britain controlled much of the surrounding land. Its
strategic importance far outweighs the economic value of its minerals or marine life.
The African, Indian, and Antarctic crustal plates converge in the Indian Ocean. Their junctures are marked by branches of the
MID-OCEANIC RIDGE forming an inverted Y, with the stem running south from the edge of the continental shelf near Bombay, India.
The eastern, western, and southern basins thus formed are subdivided into smaller basins by ridges. The ocean's continental shelves
are narrow, averaging 200 km (125 mi) in width. An exception is found off Australia's western coast, where the shelf width exceeds
1,000 km (600 mi). The average depth of the ocean is 3,890 m (12,760 ft). Its deepest point, in the Java Trench, is estimated to be
7,450 m (24,442 ft). North of 50 deg south latitude, 86% of the main basin is covered by pelagic sediments, of which more than
one-half is globigerina ooze. The remaining 14% is layered with terrigenous sediments. Glacial outwash dominates the extreme
The climate north of the equator is affected by a MONSOON wind system. Strong northeast winds blow from October until April; from
May until October south and west winds prevail. In the ARABIAN SEA the violent monsoon brings rain to the Indian subcontinent. In
the southern hemisphere the winds generally are milder, but summer storms near Mauritius can be severe. When the monsoon
winds change, cyclones sometimes strike the shores of the Arabian Sea and the Bay of Bengal.
Among the few large rivers flowing into the Indian Ocean are the Zambezi, Shatt-al-Arab, Indus, Ganges, Brahmaputra, and
Irrawaddy. Currents are largely controlled by the monsoon. Two large circular currents, one in the northern hemisphere flowing
clockwise and one south of the equator moving counterclockwise, constitute the dominant flow pattern. During the winter monsoon,
however, currents in the north are reversed. Deepwater circulation is controlled primarily by inflows from the Atlantic Ocean, the Red
Sea, and Antarctic currents. North of 20 deg south latitude the minimum surface temperature is 22 deg C (72 deg F), exceeding 28
deg C (82 deg F) to the east. Southward of 40 deg south latitude, temperatures drop quickly. Surface water salinity ranges from 32 to
37 parts per 1,000, the highest occurring in the Arabian Sea and in a belt between southern Africa and southwestern Australia. Pack
ice and icebergs are found throughout the year south of about 65 deg south latitude. The average northern limit of icebergs is 45 deg
The warmth of the Indian Ocean keeps phytoplankton production low, except along the northern fringes and in a few scattered spots
elsewhere; life in the ocean is thus limited. Fishing is confined to subsistence levels. The ocean's most important function has been
that of trade transport. Europeans, following the ancient seafarers, had crossed its waters to reach the East and returned with silks,
rugs, tea, and spices. The Indian Ocean is also noted for its role in the shipment of petroleum from Southeast Asia to the West.
Petroleum is the most significant mineral of the area, extracted primarily on the Persian Gulf.
The earliest known civilizations, in the valleys of the Nile, Euphrates, Tigris, and Indus rivers and in Southeast Asia, have developed
near the Indian Ocean. During Egypt's 1st dynasty (c.3000 BC), sailors were sent out onto its waters, journeying to Punt, thought to
be part of present-day Somalia. Returning ships brought gold and slaves. Phoenicians of the 3d millennium BC may have entered the
area, but no settlements resulted. Marco POLO (c.1254-1324) is thought to have returned from the Far East by way of the Strait of
Malacca. Vasco da GAMA rounded the Cape of Good Hope in 1497 and sailed to India, the first European to do so. The ancient
peoples who lived along the ocean each tried unsuccessfully to control its commercial routes. Portugal attempted to achieve
preeminence for more than a century but was thwarted in the mid-1600s. The Dutch East India Company (1602-1798) sought control
of trade with the East across the Indian Ocean. France and Britain established trade companies for the area, but Britain became the
principal power. After 1815 it dominated the area.
The opening of the SUEZ CANAL in 1869 revived European interest in the East, but no nation was successful in establishing trade
dominance. Since World War II the United Kingdom has withdrawn from the area, to be only partially replaced by India, the USSR,
and the United States. The last two have tried to establish hegemony by negotiating for naval base sites. Developing countries
bordering the ocean, however, seek to have it made a "zone of peace" so that they may use its shipping lanes freely.
Bibliography: Braun, D., The Indian Ocean (1983); Chandra, S., ed., The Indian Ocean (1987); Chaudhuri, K. N., Trade and
Civilization in the Indian Ocean (1985); Cousteau, Jacques-Yves, and Diole, Philippe, Life and Death in a Coral Sea (1971); Cubitt,
Gerald, Islands of the Indian Ocean (1975); Das Gupta, A., and Pearson, M.N., India and the Indian Ocean (1987); Dowdy, W. L.,
and Trood, R., eds., The Indian Ocean (1985); Kerr, A., ed., Resources and Development in the Indian Ocean Region (1981); Nairn,
A. E., and Stehli, F. G., eds., The Ocean Basins and Margins, Vol. 6: The Indian Ocean (1982); Ostheimer, John M., ed., The
Politics of the Western Indian Ocean Islands (1975); Toussaint, Auguste, The History of the Indian Ocean, trans. by June