Indian Ocean

Background

The Indian Ocean is the third largest of the world's five oceans (after the Pacific Ocean and Atlantic Ocean, but larger than the Southern Ocean and Arctic Ocean). Four critically important access waterways are the Suez Canal (Egypt), Bab el Mandeb (Djibouti-Yemen), Strait of Hormuz (Iran-Oman), and Strait of Malacca (Indonesia-Malaysia). The decision by the International Hydrographic Organization in the spring of 2000 to delimit a fifth ocean, the Southern Ocean, removed the portion of the Indian Ocean south of 60 degrees south latitude.

Location

body of water between Africa, the Southern Ocean, Asia, and Australia

Geographic coordinates

20 00 S, 80 00 E

Map references

Political Map of the World

Area

total: 70.56 million sq km

note: includes Andaman Sea, Arabian Sea, Bay of Bengal, Great Australian Bight, Gulf of Aden, Gulf of Oman, Mozambique Channel, Persian Gulf, Red Sea, Savu Sea, Strait of Malacca, Timor Sea, and other tributary water bodies

Area - comparative

almost 7 times the size of the US

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

Ocean volume

ocean volume: 264 million cu km

percent of World Ocean total volume: 19.8%

Major ocean currents

the counterclockwise Indian Ocean Gyre comprised of the southward flowing warm Agulhas and East Madagascar Currents in the west, the eastward flowing South Indian Current in the south, the northward flowing cold West Australian Current in the east, and the westward flowing South Equatorial Current in the north; a distinctive annual reversal of surface currents occurs 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 clockwise currents, while high pressure over northern Asia from cold, falling, winter air results in the northeast monsoon and northeast-to-southwest winds and counterclockwise currents

Bathymetry

continental shelf:

The continental shelf (see Figure 1), a rather flat area of the sea floor adjacent to the coast that gradually slopes down from the shore to water depths of about 200 m (660 ft). Dimensions can vary: they may be narrow or nearly nonexistent in some places or extend for hundreds of miles in others. The waters along the continental shelf are usually productive in both plant and animal life, both from sunlight and nutrients from ocean upwelling and terrestrial runoff. The following are examples of features found on the continental shelf of the Indian Ocean (see Figure 2).

Exmouth Plateau
Indus Canyon
The Swatch of No Ground/Ganges Canyon (Bay of Bengal)
Sunda Shelf

The continental slope (see Figure 1) is where the ocean bottom drops off more rapidly until it meets the deep-sea floor (abyssal plain) at about 3,200 m (10,500 ft) water depth. The deep waters of the continental slope are characterized by cold temperatures, low light conditions, and very high pressures. Sunlight does not penetrate to these depths, having been absorbed or reflected in the water above. The continental slope can be indented by submarine canyons, often associated with the outflow of major rivers. Another feature of the continental slope are alluvial fans or cones of sediments carried downstream to the ocean by major rivers and deposited down the slope. The following are examples of features found on the continental slope of the Indian Ocean (see Figure 2).

Bengal Fan
Indus Fan

The abyssal plains (see Figure 1), at depths of over 3,000 m (10,000 ft) and covering 70% of the ocean floor, are the largest habitat on earth. Sunlight does not penetrate to the sea floor, making these deep, dark ecosystems less productive than those along the continental shelf. Despite their name, these “plains” are not uniformly flat; they are interrupted by features like hills, valleys, and seamounts. The following are examples of features found on the abyssal plains of the Indian Ocean (see Figure 2).

Arabian Basin
Crozet Basin
Madagascar Basin
Mid-Indian Basin
Mozambique Basin
Wharton Basin

The mid-ocean ridge (see Figure 1), rising up from the abyssal plain, is an underwater mountain range, over 64,000 km (40,000 mi) long, rising to an average depth of 2,400 m (8,000 ft). Mid-ocean ridges form at divergent plate boundaries where two tectonic plates are moving apart and new crust is created by magma pushing up from the mantle. Tracing their way around the global ocean, this system of underwater volcanoes forms the longest mountain range on Earth. Fracture Zones are linear transform faults that develop perpendicular to the line of the mid-ocean ridge which can offset the ridge line and divide it into segments. The following are examples of mid-ocean ridges found on the floor of the Indian Ocean (see Figure 2).

Central Indian Ridge
Davie Ridge
Southeast Indian Ridge
Southwest Indian Ridge

Seamounts (see Figure 1) are submarine mountains at least 1,000 m (3,300 ft) high formed from individual volcanoes on the ocean floor. They are distinct from the plate-boundary volcanic system of the mid-ocean ridges, because seamounts tend to be circular or conical. A circular collapse caldera is often centered at the summit, evidence of a magma chamber within the volcano. Flat topped seamounts are known as guyots. Long chains of seamounts are often fed by "hot spots" in the deep mantle. These hot spots are associated with stationary plumes of molten rock rising from deep within the Earth's mantle. These hot spot plumes melt through the overlying tectonic plate as it moves and supplies magma to the active volcanic island at the end of the chain of volcanic islands and seamounts. The following are examples of seamounts found on the floor of the Indian Ocean (see Figure 2).

Andaman-Nicobar Ridge
Chagos-Laccadive Ridge
Kerguelen Plateau
Madagascar Plateau
Mascarene Plateau
Mozambique Plateau
Ninetyeast Ridge

Ocean trenches (see Figure 1) are the deepest parts of the ocean floor and are created by the process of subduction. Trenches form along convergent boundaries where tectonic plates are moving toward each other, and one plate sinks (is subducted) under another. The location where the sinking of a plate occurs is called a subduction zone. Subduction can occur when oceanic crust collides with and sinks under (subducts) continental crust resulting in volcanic, seismic, and mountain-building processes. Subduction can also occur in the convergence of two oceanic plates where one will sink under the other and in the process create a deep ocean trench. Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form a volcanic island. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The following are examples of ocean trenches found on the floor of the Indian Ocean (see Figure 2).

Java/Sunda Trench; note - deepest point in the Indian Ocean

Atolls (see Figure 1) are the remains of dormant volcanic islands. In warm tropical oceans, coral colonies establish themselves on the margins of the island. Then, over time, the high elevation of the island collapses and erodes away to sea level leaving behind an outline of the island in the form of the fringing coral reef. The resulting low island is typified by the coral reef surrounding a low elevation of sand and coral above sea level with an interior shallow lagoon. Often times the remaining dry land is broken into a ring of islets. Some lagoons can be hundreds of square kilometers. It may take as long as 300,000 years for an atoll formation to occur. Guyots are submerged atoll structures, which explains why they are flat topped seamounts. The following are examples of atolls found in the Indian Ocean (see Figure 2).

Bassas da India
Chagos Archipelago/Diego Garcia
Europa Island
Juan de Nova Island
Lakshadweep Islands
Maldive Islands
Seychelles

Elevation

highest point: sea level

lowest point: Java Trench -7,192 m unnamed deep

mean depth: -3,741 m

ocean zones: Composed of water and in a fluid state, the oceans are delimited differently than the solid continents. Oceans are divided into three zones based on depth and light level. Although some sea creatures depend on light to live, others can do without it. Sunlight entering the water may travel about 1,000 m into the oceans under the right conditions, but there is rarely any significant light beyond 200 m.

The upper 200 m (656 ft) of oceans is called the euphotic, or "sunlight," zone. This zone contains the vast majority of commercial fisheries and is home to many protected marine mammals and sea turtles. Only a small amount of light penetrates beyond this depth.

The zone between 200 m (656 ft) and 1,000 m (3,280 ft) is usually referred to as the "twilight" zone, but is officially the dysphotic zone. In this zone, the intensity of light rapidly dissipates as depth increases. Such a minuscule amount of light penetrates beyond a depth of 200 m that photosynthesis is no longer possible.

The aphotic, or "midnight," zone exists in depths below 1,000 m (3,280 ft). Sunlight does not penetrate to these depths and the zone is bathed in darkness.

Natural resources

oil and gas fields, fish, shrimp, sand and gravel aggregates, placer deposits, polymetallic nodules

Natural hazards

occasional icebergs pose navigational hazard in southern reaches

Geography - note

major chokepoints include Bab el Mandeb, Strait of Hormuz, Strait of Malacca, southern access to the Suez Canal, and the Lombok Strait

Environment - current issues

marine pollution caused by ocean dumping, waste disposal, and oil spills; deep sea mining; oil pollution in Arabian Sea, Persian Gulf, and Red Sea; coral reefs threatened due to climate change, direct human pressures, and inadequate governance, awareness, and political will; loss of biodiversity; endangered marine species include the dugong, seals, turtles, and whales

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

Marine fisheries

the Indian Ocean fisheries are the third most important in the world accounting for 15.5%, or 12,220,000 mt of the global catch in 2020; tuna, small pelagic fish, and shrimp are important species in these regions; the Food and Agriculture Organization delineated two fishing regions in the Indian Ocean:

Eastern Indian Ocean region (Region 57) is the most important region and the fifth largest producing region in the world with 8.4%, or 6,590,000 mt, of the global catch in 2020; the region encompasses the waters north of 55º South latitude and east of 80º East longitude including the Bay of Bengal and Andaman Sea with the major producers including India (2,362,481 mt), Indonesia (1,940,558 mt), Burma (1,114,777 mt), Bangladesh (877,837 mt), and Sri Lanka (373,369 mt); the principal catches include shad, Skipjack tuna, mackerel, shrimp, and sardinellas

Country name

etymology: named for the country of India, which makes up much of its northern border

Ports and terminals

major seaport(s): Chennai (Madras, India); Colombo (Sri Lanka); Durban (South Africa); Jakarta (Indonesia); Kolkata (Calcutta, India); Melbourne (Australia); Mumbai (Bombay, India); Richards Bay (South Africa)

Military - note

according to the International Maritime Bureau, areas of high risk for piracy and armed robbery against ships in territorial waters of littoral states and offshore waters include the Gulf of Aden, along the east coast of Africa, the Bay of Bengal, and the Strait of Malacca; in addition, the Maritime Administration of the US Department of Transportation advises that regional conflict, military activity, and political tensions pose threats to commercial vessels in the Persian Gulf, Strait of Hormuz, Gulf of Oman, Arabian Sea, Gulf of Aden, Bab al Mandeb Strait, Red Sea, and Somali Basin (2023)

Disputes - international

some maritime disputes (see littoral states)