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 (the Indian Sub-continent); 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 the Southern Ocean. It is separated from the Atlantic Ocean by the 20° east meridian south of Africa, and from the Pacific by the 147° east meridian. The northernmost extent of the Indian Ocean is approximately 30° 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 km² (28,400,000 sq mi), including the Red Sea and the Persian Gulf. The ocean's volume is estimated to be 292,131,000 km³ (70,086,000 mi³). 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.
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° 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° south latitude the minimum surface temperature is 22°C (72°F), exceeding 28°C (82°F) to the east. Southward of 40° 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° south latitude. The average northern limit of icebergs is 45° south latitude.
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. 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.
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. 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.
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.
The Indian Ocean is far calmer and thus open to trade earlier than the Atlantic or Pacfic. The powerful monsoons also meant ships could easily sail them west early in the season, then wait a few months a return eastwards. This allowed Indonesian peoples crossed the Indian Ocean to settle in Madagascar.
In 1497 Vasco da Gama rounded the Cape of Good Hope and sailed to India, the first European to do so. The European ships, aremd with heavy cannon, quicky came to dominate the trade. Portugal at first attempted to achieve pre-eminence setting up forts an the important straights and ports. But the small nation was unable to support such a vast project and they were replaced in the mid-1600s by other European powers. 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. Eventually Britain became the principal power and by 1815 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.
Southern Ocean: A spring 2000 decision by the International Hydrographic Organization delimited a fifth world ocean, stripping the southern portions of the Indian Ocean. The new ocean extends from the coast of Antarctica north to 60° south latitude which coincides with the Antarctic Treaty Limit. The Indian Ocean remains the third-largest of the world's five oceans.
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
Ports and harbors: Calcutta (India), Chennai (Madras; India), Colombo (Sri Lanka), Durban (South Africa), Jakarta (Indonesia), Karachi (Pakistan), Fremantle (Australia), Mumbai (Bombay; India), Richards Bay (South Africa)
Much of this text is based on public domain text by US Naval Oceanographer at: http://oceanographer.navy.mil/indian.html
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