Indian Ocean. The Indian Ocean is 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 south latitude, in which region it opens widely into the Southern and Antarctic oceans. It gradually narrows towards the north, and is divided by the Indian peninsula into the Bay of Bengal on the east and the Arabian Sea on the west, the latter sending northward 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 sq. m.
At the dawn of history the Indian Ocean was known as the Erythrean Sea; the Phoenicians are said to have been familiar with this southern ocean at a very early date. Necho, an Egyptian monarch who flourished about 610 B.C., is reported by Herodotus to have sent some of his vessels, manned by Phoenicians, into the Erythrean Sea with orders to return by the south of Africa and the Columns of Hercules. Whether or not this voyage ever took place, it appears certain, from their reports as to the position of the sun to the north of them, that these early navigators penetrated far south (see GEOGRAPHY, Vol. V. p. 145). From a very early date there was a coasting trade between India and the Persian Gulf, but the voyage of Nearchus, one of Alexander's generals, from the Indus to the Persian Gulf, is the earliest reliable record of these coasts. Hippalus, an Egyptian navigator who flourished about the beginning of the Christian era, was the first to observe the regular alternations in the direction of the monsoons of the Indian Ocean, and to profit by them to open up a direct route across the high seas from the Red Sea to India. The shore routes were henceforth abandoned, and a fresh impulse was given to voyages into oriental waters. In the 9th century the Arabs made frequent voyages across the Indian Ocean, Soleiman of Siraf being probably the first to cross the Bay of Bengal and pass into the China Sea. 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 2300 fathoms, or slightly greater than that of the Atlantic (q.v.). The greatest depths are in the eastern part to the south of the equator, where it is estimated that there are fully 50,000 sq. m. with a depth of over 3000 fathoms. Over 13,000,000 sq. m. of this ocean's floor lie between the depths of 2000 and 3000 fathoms.
The area of land draining into the Indian Ocean is estimated at 6,813,600 sq. m., and the annual rainfall on this land is equal to 4379 cubic miles of water. The rivers flowing from the Asiatic continent are by far the most important, and they carry an immense amount of detrital matter into the Bay of Bengal and Arabian Sea, these forming extensive deposits of blue mud. Along the African coasts, in depths from 100 to 1000 fathoms, there are great deposits of glauconitic sands and muds, and on these as well as other coasts there are coral muds and sands, and blue and green muds in the shallower depths. In the deeper parts of the ocean, far from land, there are vast deposits of red clay, Radiolarian ooze, and Globigerina ooze. In the Southern Ocean, towards the Antarctic, the bed of the ocean is covered with a Diatom ooze.
The temperature of the surface waters of the Indian Ocean varies much in different parts of the ocean, and at the same place at different times of the year or states of the wind. In tropical regions the temperature usually varies from 70° to 80° F., and the yearly range is only 7° or 8° F. Off the Cape of Good Hope and off Cape Guardafui, however, the annual range of temperature may be from 20° to 30° F. For instance, sudden and great changes of temperature are often noticed off Cape Guardafui when the wind blows off shore, for in this way cold and deep water is drawn up along the African coast to take the place of the warm surface water which is driven eastward by the wind.
The temperature of the water at the bottom of the Indian Ocean is very uniform and subject to little, if any, annual variation. In the Bay of Bengal and Arabian Sea temperatures of 33°·7 F. and 34°·2 F. have been recorded at the bottom; these are not more than the fraction of a degree higher than those observed by the Challenger in 50° of south latitude. It is certain, therefore, that this deep cold water is slowly drawn into the Indian Ocean from the Antarctic to supply the place of the warm surface currents that are driven southward by the winds. 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 (see MONSOONS). Some of the most characteristic coral atolls and islands are to be found towards the central part of the Indian Ocean, such as the great Maldive group, the Chagos, Diego Garcia, and the Cocos Islands. Almost all the tropical shores are skirted by fringing and barrier reefs. Christmas Island is an upraised coral formation. St Paul's, Mauritius, Rodriguez, and others are of volcanic origin, while Madagascar, Ceylon, and Socotra are typical continental islands.