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Indian Ocean

the five Oceans

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 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≥ (70,086,000 mi≥). 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.

Table of contents
2 Economy
3 History


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 Mumbai, 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 southern latitudes.


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 south latitude.


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 3rd millennium BC may have entered the area, but no settlements resulted. The Greeks and Romans knew something of the ocean; the unknown author of the Periplus of the Erythraean Sea describes ports and trade goods along the coasts of Africa and India from around the 2nd century AD.

Indonesian peoples crossed the Indian Ocean to settle in Madagascar. Marco Polo (c. 1254-1324) is thought to have returned from the Far East by way of the Strait of Malacca. Chinese expeditions of exploration reached East Africa in the 15th century, but Arab traders dominated shipping in the Indian Ocean before 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 pre-eminence 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 Guicharnaud (1966).

Background: A spring 2000 decision by the International Hydrographic Organization delimited a fifth world ocean from the southern portions of the Atlantic Ocean, Indian Ocean, and Pacific Ocean. The new ocean extends from the coast of Antarctica north to 60 degrees south latitude which coincides with the Antarctic Treaty Limit. The Indian Ocean remains the third-largest of the world's five oceans (after the Pacific Ocean and Atlantic Ocean, but larger than the Southern Ocean and Arctic Ocean).

Location: body of water between Africa, the Southern Ocean, Asia, and Australia

Geographic coordinates: 20 00 S, 80 00 E

Map references: World


Area - comparative: about 5.5 times the size of the US

Coastline: 66,526 km

Climate: northeast monsoon (December to April), southwest monsoon (June to October); tropical cyclones occur during May/June and October/November in the northern Indian Ocean and January/February in the southern Indian Ocean

Terrain: surface dominated by counterclockwise gyre (broad, circular system of currents) in the southern Indian Ocean; unique reversal of surface currents in the northern Indian Ocean; low atmospheric pressure over southwest Asia from hot, rising, summer air results in the southwest monsoon and southwest-to-northeast winds and currents, while high pressure over northern Asia from cold, falling, winter air results in the northeast monsoon and northeast-to-southwest winds and currents; ocean floor is dominated by the Mid-Indian Ocean Ridge and subdivided by the Southeast Indian Ocean Ridge, Southwest Indian Ocean Ridge, and Ninetyeast Ridge

Elevation extremes:

Natural resources: oil and gas fields, fish, shrimp, sand and gravel aggregates, placer deposits, polymetallic nodules

Environment - current issues: endangered marine species include the dugong, sealss, turtles, and whales; oil pollution in the Arabian Sea, Persian Gulf, and Red Sea

Geography - note: major chokepoints include Bab el Mandeb, Strait of Hormuz, Strait of Malacca, southern access to the Suez Canal, and the Lombok Strait

Economy - overview: The Indian Ocean provides major sea routes connecting the Middle East, Africa, and East Asia with Europe and the Americas. It carries a particularly heavy traffic of petroleum and petroleum products from the oilfields of the Persian Gulf and Indonesia. Its fish are of great and growing importance to the bordering countries for domestic consumption and export. Fishing fleets from Russia, Japan, South Korea, and Taiwan also exploit the Indian Ocean, mainly for shrimp and tuna. Large reserves of hydrocarbons are being tapped in the offshore areas of Saudi Arabia, Iran, India, and western Australia. An estimated 40% of the world's offshore oil production comes from the Indian Ocean. Beach sands rich in heavy minerals and offshore placer deposits are actively exploited by bordering countries, particularly India, South Africa, Indonesia, Sri Lanka, and Thailand.

Ports and harbors: Calcutta (India), Chennai (Madras; India), Colombo (Sri Lanka), Durban (South Africa), Jakarta (Indonesia), Fremantle (Australia), Mumbai (Bombay; India), Richards Bay (South Africa)

Disputes - international: some maritime disputes (see littoral states)

Much of this text is based on public domain text by US Naval Oceanographer at:

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