INDIAN OCEAN, a body of water bounded on the W. by Africa, on the N. by Asia, on the E. by Australia and the Australasian Islands. According to modern geographers it is limited to the S. by the 40th parallel of S. latitude, in which region it opens widely into the Southern and Antarctic oceans. It grad ually narrows towards the N., and is di vided by the Indian peninsula into the Bay of Bengal on the E. and the Arabian Sea on the W., the latter sending N. two arms, the Persian Gulf and the Red Sea. Within these limits the Indian Ocean is estimated to have an area of 17,320,500 square miles.
At the dawn of history the Indian Ocean was known as the Erythrwan Sea. Necho, an Egyptian monarch who flour ished about 610 B. c., is reported by He rodotus to have sent some of his vessels, manned by Phcenicians, into the Ery thran Sea with orders to return by the S. of Africa and the Pillars of Hercules. From a very early date there was a coasting trade between India and the Persian Gulf, but the voyage of Near chus, one of Alexander's generals, from the Indus to the Persian Gulf, is the earliest reliable record of these coasts. In 1486 the Portuguese rounded the Cape of Good Hope, and in 1498 Vasco da Gama reached the coasts of India by the same route. In 1521 the one remaining. ship of Magellan's squadron crossed the Southern Indian Ocean in completing the first circumnavigation of the world.
The mean depth of the Indian Ocean is estimated at about 2,300 fathoms, or slightly greater than that of the Atlan tic.
The area of land draining into the In dian Ocean is estimated at 6,813,600 square miles, and the annual rainfall on this land is equal to 4,379 cubic miles of water. The rivers flowing from the Asi
atic continent are by far the most im portant, and they carry an immense amount of detrital matter into the Bay of Bengal and Arabian Sea, these form ing extensive deposits of blue mud.
The temperature of the surface waters of the Indian Ocean varies inuch in dif ferent parts of the ocean, and at the same place at different times of the year or states of the wind. In tropical re gions the temperature usually varies from 70° to 80' F., and the yearly range is only 7° or 8° F.
The temperature of the water at the bottom of the Indian Ocean is very uni form and subject to little, if any, annual variation. In the Bay of Bengal and Arabian Sea temperatures of 33.7° F. 34.2° F. have been recorded at the bot tom. The currents of the Indian Ocean are less constant than in the other great oceans, and are largely controlled by the direction and strength of the monsoons. Some of the most characteristic coral atolls and islands are to be found toward the central part of the Indian Ocean, such as the great Maldive group, the Chagos, Diego Garcia, and the Cocos Is lands. Christmas Island is an upraised coral formation. St. Paul's, Mauritius, Rodriguez, and others are of volcanic or igin, while Madagascar, Ceylon, and So cotra are typical continental islands.