Indian Ocean ranks as the third largest ocean in the world. The Indian Ocean is less than half the size of the Pacific Ocean and is about one-fifth smaller than the Atlantic Ocean.
The Indian Ocean is bordered by Australia and Indonesia on the east and Africa on the west. Asia lies to the north and Antarctica to the south. India and Sri Lanka divide the northern Indian Ocean into the Arabian Sea and the Bay of Bengal. The Red Sea and the Persian Gulf are also considered part of the Indian Ocean.
A wide variety of animals live throughout the Indian Ocean, especially in the Red Sea, the Persian Gulf, and along coastal waters. Most ocean animals depend—directly or indirectly—on microscopic organisms called phytoplankton for survival. These organisms are especially abundant near northern India and in the Persian Gulf. Ocean animals eat the phytoplankton or animals that feed on them.
Pollution is increasing in the Indian Ocean from metal and chemical products, sewage, petroleum, and food-processing waste. In addition, the Persian Gulf suffered severe oil pollution as a result of the Persian Gulf War of 1991. Iraq dumped well over 200 million gallons (760 million liters) of Kuwaiti crude oil into the gulf. The United Nations Regional Seas Programme and other international organizations are studying ways to combat pollution in and around the Indian Ocean.
Boundaries and size.
The Atlantic and Indian oceans meet off the southern tip of Africa. Australia and the Sunda Islands of Indonesia divide the Indian and Pacific oceans. The southern boundary of the Indian Ocean is 60° south latitude, where the Southern Ocean begins.
The Indian Ocean covers about 26,600,000 square miles (69,000,000 square kilometers). Its average depth is 12,600 feet (3,840 meters). The Indian Ocean drops to its deepest known point in the Java Trench, which plunges 23,812 feet (7,258 meters). The Indian Ocean’s greatest width spans about 6,200 miles (10,000 kilometers) between Africa and Australia. The longest north-south distance in the Indian Ocean measures about 5,500 miles (9,000 kilometers) and extends from Pakistan to the Southern Ocean.
Ocean floor.
The part of the ocean floor that borders the continents is called the continental shelf. The continental shelf stretches for up to 124 miles (200 kilometers) into the Indian Ocean. Around Africa, Asia, and Australia, it slopes gently to an average depth of 460 feet (140 meters).
Beyond the continental shelf, the ocean floor becomes rugged, consisting of mountain ranges, broad plateaus, and deep basins. A mountain range called the Mid-Indian Ridge runs through the center of the Indian Ocean. The ridge begins in the Gulf of Aden and separates into two parts at about 25° south latitude. Plateaus in the Indian Ocean may rise more than 10,000 feet (3,050 meters) above the ocean floor. Basins may drop more than 16,400 feet (5,000 meters) below the water’s surface.
According to the theory of plate tectonics, scientists believe the Indian Ocean began to form about 200 million years ago. At that time, a single, huge land mass called Pangaea began to break up into the world’s continents. The continents gradually drifted toward their present location. India broke away from Antarctica and Australia as early as 130 million years ago and moved north at a rate of a few inches per year. About 45 million years ago, India collided with Asia. India’s northward movement created scars and ridges on the present Indian Ocean floor. See Plate tectonics.
Temperatures.
Surface water temperatures in the Indian Ocean vary with the seasons and distance from the equator. But the ocean’s tropical waters do not reach the same temperature extremes found in the Atlantic and Pacific oceans.
Part of the Indian Ocean lies in the Northern Hemisphere and part in the Southern. The occurrence of the seasons is reversed in the two hemispheres. In January, surface temperatures in most of the Northern Hemisphere part of the ocean range between 70 and 80 °F (21 and 27 °C). They vary from 80 to 85 °F (27 to 29 °C) in most of the ocean’s Southern Hemisphere. July surface temperatures in parts of the Northern Hemisphere may reach 90 °F (32 °C). Southern Hemisphere temperatures in July may fall below 30 °F (–1 °C) near the Southern Ocean.
Currents and tides.
The movement of the wind determines the direction of currents in the Indian Ocean. Monsoon winds, or seasonal winds, cause currents north of the equator to move northeast beginning in July and southwest beginning in January. In the Southern Hemisphere, large-scale wind patterns, such as the trade winds, generate a large gyre circulation (circular current) similar to gyre currents found in the South Atlantic and the South Pacific oceans (see Trade wind). Water flows into the Indian Ocean from the Pacific Ocean around Indonesia and Australia. Water flows out into the Atlantic Ocean around the southern tip of Africa.
The tides of the Indian Ocean vary greatly, but not so much as those in the Atlantic or Pacific oceans. In Collier Bay, near Derby, Australia, the high and low tides may differ by as much as 36 feet (11 meters). The difference may be little more than an inch near Geraldton and near Bunbury, Australia.
Industry.
The natural resources found in the Indian Ocean are used by industrialized countries worldwide. Petroleum is produced by drilling into the ocean floor near India. Huge oil reserves also lie under the Persian Gulf. The transportation of petroleum accounts for a high percentage of ship traffic in the Indian Ocean. Tin, titanium, and phosphorite deposits lie beneath various coastal waters. In the early 2000’s, piracy directed at shipping vessels in the Gulf of Aden increased significantly. Most of the pirates were based in Somalia.
Major Indian Ocean ports include Kolkata and Mumbai in India, Dar es Salaam in Tanzania, Durban in South Africa, and Perth in Australia. The Suez Canal connects the Red Sea to the Mediterranean Sea, providing a shipping link from the Indian Ocean to the ports of southern Europe and North Africa.
The Indian Ocean provides about 11.2 million tons (10.2 million metric tons) of fish per year, about 8.6 percent of the world’s fish catch. The ocean’s tropical climate hinders commercial fishing because fish spoil in the heat unless quickly processed or refrigerated.
Exploration.
The Indian Ocean has been an important trade and travel route throughout history. Its early navigators were the ancient Arabs, Chinese, and Indians. The Greek historian Herodotus wrote about expeditions to the ocean around 600 B.C. In A.D. 1498, the Portuguese explorer Vasco da Gama sailed across the ocean. After the Suez Canal opened in 1869, the Indian Ocean became the most direct route between Europe and the Far East.
