Indian Ocean

The Indian Ocean shares the broad ecological and oceanographic features of the other major oceans of the world, but it possesses many unique and interesting characteristics. In general terms, the Indian Ocean’s size means that both its shores and its depths are so varied that it is difficult to describe it as a single geographical unit. Its westernmost tides arrive at the shores of a continent that bears little resemblance to the shores of its easternmost limits. Most of the truly unique characteristics of the Indian Ocean, however, lie beneath its surface in the form of extensive mountain ridges and, in one case, at least, a very deep rift that represents an unparalleled underwater world of its own.

Geological Origins

The geological origins of the Indian Ocean make it unique among the world’s major oceans. Compared with the Pacific and Atlantic, the Indian Ocean is considerably smaller (covering an area of about 73 million square kilometers, in contrast to the Atlantic’s nearly 84 million and the Pacific’s nearly 166 million square kilometers). It is also of more recent geological origin than the Atlantic, Pacific, or Arctic Oceans. Geologists specializing in the evolutionary history of Earth and plate tectonics estimate that about 150 million years ago, a giant southern continent called Gondwanaland began to break apart. The movement of segments both westward (to what became the African continent) and northeastward (to what became India, a central section of which became known as Gondwana) took at least 100 million years. The collision of the Indian subcontinent with the Eurasian landmass about 50 million years ago brought about the violent upheaval of the Himalayan Mountains that continues in the twenty-first century. One of the effects of this phenomenon was to define new shorelines of the “youngest” of the world’s major oceans.

The final product of these major geological upheavals was an oceanic body extending over the area between Australia in the east and Africa to the west. Its northernmost point corresponds to the Tropic of Cancer, where the Indian subcontinent joins the Eurasian landmass. From India’s western coast to the southeastern tip of Arabia, the waters of the Indian Ocean form the Arabian Sea. On the opposite side of India, the Bay of Bengal and the Andaman Sea extend eastward to the coasts of Southeast Asia (Myanmar, Thailand, the Malay Peninsula, and the Indonesian Archipelago). If one includes the two smaller subsidiary seas, the Persian Gulf and the Red Sea, the Indian Ocean extends even farther north, to 30 degrees north latitude. To the south, the Indian Ocean technically goes as far as Antarctica. Two features of the Indian Ocean’s floor define the point at which it separates the Atlantic and Pacific Oceans: the Atlantic Indian Basin and the South Indian Basin.

Associated Seas and Major Rivers

Although the formation of the Indian Ocean created several important seas and gulfs as distinct subsections of its total surface, there are fewer such bodies here than in the other oceans of the world. One should contrast general geographical denominations such as the Bay of Bengal (comprising most of the area east of India and touching the coasts of Southeast Asia) or the Arabian Sea (separating western India from the coasts of the Arabian Peninsula) with the geographical uniqueness of the Red Sea and the Persian Gulf. The latter two are, in fact, clearly separated from the main body of the Indian Ocean by narrow straits—the Mandab Straits and the Straits of Hormuz, respectively. The Red Sea and the Persian Gulf have ecologies that are very different from that of the main body of the Indian Ocean. This is not the case for the two other semi-contained gulfs off the Arabian coast, the Gulf of Oman and the Gulf of Aden. The area known as the Great Australian Bight is simply the slightly curved central southern coast of Australia and is, therefore, even less circumscribed than the Bay of Bengal west of India. The Andaman Sea lies north of the Indonesian Archipelago. It is enclosed geographically from the Bay of Bengal by a line of islands (actually an extension of the Indonesian islands) called the Nicobar and Andaman Islands.

Several major rivers pour large amounts of freshwater into the Indian Ocean. Such rivers are probably much older than the Indian Ocean itself, even though their pattern of flow was different in earlier geological ages. The Zambezi in East Africa, the Indus in northwest India, and the Ganges in northeast India probably flowed from their respective continental freshwater sources toward what eventually became the Indian Ocean’s coastline. Each of these has had a notable effect on the configuration of the coast where it empties into the ocean over the long period since the formation of the ocean. Freshwater currents have, for example, cut actual canyons into the continental shelf area adjacent to the coast. In the case of the Ganges, an immense zone of sediment has built up, affecting both marine life and local currents in its delta area in the Bay of Bengal.

Ocean Depths and Submarine Geological Features

The average depth of the Indian Ocean is in the range of 3,636 to 3,940 meters. Several extremely deep but limited areas, notably the Java Trench to the south of Indonesia, are nearly twice as deep. The continental shelf along the coasts of the Indian Ocean is generally narrower than that of the other oceans, averaging 122 kilometers before deeper waters begin. The area west of the Indian coast, off the major city of Mumbai, is an exception. There, the continental shelf extends almost 325 kilometers into the ocean.

Several underwater mountain ranges crisscross the floor of the Indian Ocean. Although notable ridges exist, its underwater topography is nowhere near as complex or spectacular as the eastern half of the Pacific, where extensive archipelagos with many small islands and some very large island formations (Japan, the Philippines, New Guinea, and New Zealand, for example) predominate. The most concentrated area of subsurface mountains in the Indian Ocean is centered near 30 degrees longitude, about halfway between the west coast of India and the Gulf of Aden, south of Arabia. Several small but historically important islands mark high points along the mountain ridges between 30 and 60 degrees longitude. Mauritius and the Seychelles are examples of these. The huge island of Madagascar (588,000 square kilometers) is the most prominent surface example of the complex north-to-south submarine mountain systems located east of the African coast. The first of these is the Mauritius Ridge, marked on the surface at its southernmost point by Mascarene Island (due east from Madagascar) and by the Seychelles Islands to the north. The next range, the Carlsberg Ridge, is longer than the Mauritius Ridge, but none of its peaks emerge to form islands.

Finally, a long ridge extends due south off the southwest coast of India. This range is marked at the surface by the Laccadive and Maldive Islands. Again, like Mascarene Island and Seychelles near Madagascar, the Maldives are dwarfed by the single major island just to the southeast of the tip of India, Sri Lanka (formerly Ceylon). Sri Lanka, however, is the tailing part of the Indian subcontinental landmass rather than the tip of a submarine mountain range.

Beginning with the 90 degree east longitude line and moving toward the eastern shores of the Indian Ocean, the ocean floor's topography is quite different from that of the western half. First, the name of one subsurface range, the Ninety East Ridge, suggests a regular pattern extending from north to south. The Ninety East Ridge, discovered only as recently as the 1960s, has become the longest and straightest underwater mountain range in the world. Unlike the other ridges of the eastern basin of the Indian Ocean, most notably those south of the Indonesian coast, the Ninety East Ridge appears to be seismically inactive.

Between 90 degrees east longitude and the western shores of Australia, one finds the third-deepest point in the Indian Ocean, the Wharton Basin, measuring nearly 6,364 meters deep. Farther south, off the southwestern tip of Australia, is the Diamantina Deep. However, neither of these deep points is associated with the rapid fall-off from mountain ridges to deep valleys characteristic of ocean trenches. Trenches are characteristic of the eastern rim of the Pacific Ocean, but only one such phenomenon occurs in the Indian Ocean. The Java Trench off the southern coast of Indonesia is more than 6,060 meters deep. Pioneer scientists examining flora and fauna in this area of what were then still undiscovered ocean trenches hypothesized that Indonesia was very close to the dividing line between tectonic plates. They observed that plants and animals to the east of Java appear to be biologically isolated in their evolution from species farther west.

The long, curved pattern of the ranges that constitute the Indonesian Archipelago actually extends far beyond the northern tip of Sumatra. Its peaks can be found in the chain of islands known as the Nicobar and Andaman Islands. The presence of these island chains west of the coasts of Thailand and Myanmar helps define the Andaman Sea area east of the main body of the Bay of Bengal.

Tides and Currents

The immense size of the Indian Ocean means that tidal phenomena are variable in type and the volume of tidal movement registered. The most common tides are semidiurnal (occurring twice daily). These are characteristic of the subequatorial eastern shores of Africa and, farther north and much farther east, in the Bay of Bengal. Australia’s southwest coast, which is roughly opposite the subequatorial eastern coast of Africa, has an entirely different tidal pattern. Australia’s Indian Ocean shores experience diurnal (once per day) tides that are extremely light compared to other coasts.

The Indian Ocean is the only ocean in the world with asymmetric reversing surface currents. Asymmetric conditions apply when currents in the northern half of the ocean are moving in a different direction from those of the southern half. The complexity of Indian Ocean currents close to the surface goes well beyond the relatively simple question of north-south asymmetry. One finds, for example, that wind conditions contribute to creating gyres, circular or spiral movements that break the broad pattern of the surface current into localized segments.

It is particularly important to note that broad patterns of currents in the Indian Ocean reverse according to the season. Currents, like so many other factors determining the overall ecology of the Indian Ocean, are largely affected by the major monsoonal wind and weather conditions that are characteristic of this region of the world.

Probably the most famous current in the Indian Ocean is the so-called Somali Current, which moves, in particular seasons of the year, in a relatively rapid clockwise direction from the northeast coast of the Horn of Africa. In summer, this current goes as far as the coast of India. At its farthest point, moving east in the summer months, it meets the southwesterly monsoon current off the Indian subcontinent. During the winter, the direction of the Somali Current reverses, creating near-ideal seasonal sailing conditions for centuries for ships sailing between the Arabian peninsular zone, particularly the Persian Gulf and the Horn of Africa.

Climatic Zones and Wind Patterns

Because of the great north-to-south distance covered by the Indian Ocean and its associated seas, there are several distinct climatic zones according to geographical location. The most famous and most important for sustaining seasonal agriculture in the entire Indian Ocean area is the so-called monsoon zone, which runs from 10 degrees north of the equator to about 10 degrees south. The region between 10 and 30 degrees south of the equator is the zone of what has traditionally been called the trade winds. The predictability of steadily blowing southeast winds in this wide region (as distinct from the area of the Somali Current) made maritime communication, and therefore trade, between the opposite shores of the Indian Ocean possible.

Near the global band of the Tropic of Capricorn (running through the island of Madagascar on the western shores and Australia on the east) lies the subtropical to temperate zone, between 30 and 45 degrees latitude. Some 20 degrees south of the Tropic of Capricorn, climatic conditions begin to show the temperate cooling influence of the extreme southern extent of Indian Ocean waters leading to the last climate zone. From 45 degrees latitude southward to the Antarctic ice cap, the beginnings of sharply cold Antarctic waters mark the end of the gradual transition separating some of the world’s hottest climates in the Indian Ocean proper from the extreme cold of the southern zone of the globe. Here, three of the world’s four oceans almost literally fuse in the Antarctic ice cap.

Monsoon Wind Conditions

Generally stated, monsoon conditions are semiannual reversing wind patterns. Extensive areas of high pressure “empty” their air in the direction of equally vast low-pressure zones. When this happens, winds moving across water carry the moisture they pick up, typically precipitated as rain before reaching their low-pressure destinations. In the case of Indian Ocean monsoons, the widespread heating of the landmass in the Northern Hemisphere during summer creates low atmospheric pressure conditions over Asia. This low-pressure zone becomes an attractive force for air masses pressed downward by high-pressure conditions over Australia. The resultant winds that move northwesterly across Southeast Asia and the Indian subcontinent bring a much-needed monsoon season of heavy rains that lasts until the particular atmospheric conditions cease to apply. Generally, the monsoon rainy season is predictable, but the arrival of torrents of rain to quench the dry agricultural fields of South Asia and Southeast Asia is not guaranteed. When the typical monsoon wind pattern develops, but insufficient moisture is collected to bring rains, areas that depend on monsoon waters can face serious drought for at least one year, as there is no chance of humid air movements over the landmass once the directional wind pattern created by Asia’s summer heating ends.

Natural Resources

The Indian Ocean contains many key minerals extracted to supplement the local economies of several countries along its shores. Manganese is found in several areas off South Africa and Australia. Other minerals include tin and chromite. Mineral wealth in the water is, however, overshadowed by the vast petroleum reserves concentrated in one of its seas: the Persian Gulf. These are estimated to be the largest oil reserves in the world. Other locations where petroleum wealth is important are on the island of Sumatra and its offshore waters. A similar intermediate potential for petroleum production is found in the Red Sea and off the shores of western India.

The fishing industries that depend on the Indian Ocean are varied. Many depend on upwellings, movements of water from lower depths that carry phytoplankton—a basic food source for many fish species—close to the surface. The most common commercialized fisheries seek large schools of sardines, mackerel, and anchovies. Shrimp is the principal single species fished in many areas of the Indian Ocean.

Significance

The Indian Ocean and its associated seas represent one of the world's most diversified ecological marine environments. This applies both to its coastlines and to the world beneath its surface. The African landmass along the ocean’s western shores reveals various natural characteristics that hardly resemble what one finds along its eastern shores. Dry coastal regions characterize both the southern and northern reaches of the African landmass, with some of the driest deserts in the world extending from the Somalian Horn of Africa into the Red Sea and around the southern shores of Arabia into the Persian Gulf. Although these conditions continue along the northwestern coastline to the Gujarati coast of India, the Indian Ocean ecosystem is distinct once one passes the Indian subcontinent. From the Bay of Bengal southward along the eastern shores of the Indian Ocean, the environment becomes increasingly tropical, passing through some of the most extensive tropical rainforests in the world, especially in the Indonesian Archipelago. Indeed, parts of the west coast of Australia, generally considered a desert environment, exhibit a tropical ecology that contrasts notably with the dry climates of the African coastline of the Indian Ocean. With this physical and ecological diversification comes an enormous variation in plant and animal life along the ocean’s shorelines and in its relatively shallow continental shelf waters. Studying the diversity of the Indian Ocean’s biological ecology and submarine geology is like observing multiple different worlds on one segment of the planet. Further, political and economic control and strategic planning for the diversity of people and nations that border the Indian Ocean have become increasingly important in the twenty-first century as climate change also continues to alter the Indian Ocean landscape.

Principal Terms

Gondwanaland: an ancient supercontinent that geologists theorize broke into at least two large segments; one segment became India and pushed northward to collide with the Eurasian landmass, while the other, Africa, moved westward

gyres: circular patterns in the movement of surface currents that create nearly self-contained local subsections within the larger pattern of a typical ocean current

Java Trench: one of the deepest areas of the Indian Ocean, located off the southern coast of Java in Indonesia; it is a form of geological canyon created by the upward thrust of mountain ridges from the ocean floor

monsoon: a seasonal movement of winds into and out of the Indian Ocean region caused by variations of atmospheric pressure over the Indian Ocean and the interior land mass of Asia

Somali Current: a seasonally reversing current that moves between the eastern coasts of Africa and the Arabian Peninsula

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