Indian Ocean

The Indian Ocean is a vast, dynamic marine domain combining unique geological, climatic, ecological, historical and strategic dimensions. Below is a more exhaustive compendium of key facts, drawn largely from the Wikipedia-derived material you provided and supplemented with further detail.

Geography and Extent

The Indian Ocean is the third-largest of the world’s five oceans, covering about 70,560,000 km² (or about 27,240,000 sq mi), which is roughly 20 % of Earth’s ocean surface area . Its total volume is estimated at 264,000,000 km³, or about 19.8 % of the world’s oceanic volume .
It is bounded:

• to the north by Asia, including the Indian subcontinent and the Arabian Peninsula,
• to the west by Africa,
• to the east by Australia and parts of Southeast Asia,
• to the south by either the Southern Ocean or Antarctica depending on the definition adopted .

In terms of meridional (north–south) and longitudinal boundaries:

• The northernmost extent (including marginal seas) is approximately 30° N, at the Persian Gulf .
• The Indian Ocean is delimited from the Atlantic by the 20° E meridian (running south from Cape Agulhas, South Africa) and from the Pacific by the 146° 49′ E meridian (south from Tasmania) .
• All of the Indian Ocean lies in the Eastern Hemisphere; the 90° E meridian (the centre of the Eastern Hemisphere) passes through the Ninety East Ridge .

Coasts, Shelves and Marginal Seas

• Unlike the Pacific and Atlantic, the Indian Ocean is more “embayed”—surrounded by landmasses or archipelagos on three sides. Its continental margins are comparatively constrained.
• Its continental shelves tend to be narrow. On active margins, the average shelf width is around 19 km (max ~175 km). On passive margins, average increases to ~47.6 km. The slopes from shelf break to foot of slope average ~50–52 km horizontally .
• The continental shelf comprises about 15 % of the Indian Ocean’s area .

Marginal seas, gulfs, and connected water bodies include:

• Arabian Sea (~3,862,000 km²)
• Bay of Bengal (~2,172,000 km²)
• Andaman Sea (~797,700 km²)
• Laccadive (Lakshadweep) Sea (~786,000 km²)
• Red Sea (~438,000 km²), Gulf of Aden, Persian Gulf, Gulf of Oman, Gulf of Khambhat, Gulf of Kutch, Gulf of Mannar, Palk Strait, Adam’s Bridge, Mozambique Channel, and others

Some of the largest marginal seas by area are the Arabian Sea and Bay of Bengal .
The Laccadive Sea (also called Lakshadweep Sea) lies to the west of southern India and near Maldives, with a maximum depth ~4,131 m and supports rich marine biodiversity, e.g. 3,600 species in the Gulf of Mannar region .
Islands within the Indian Ocean include Madagascar, Sri Lanka, Maldives, Seychelles, Mauritius, Comoros, Réunion, and the Andaman & Nicobar Islands, among others .

Bathymetry, Geology and Ocean Floor

• The average depth of the Indian Ocean is approximately 3,741 m (12,274 ft) .
• Its maximum depth is about 7,290 m (23,920 ft) .
• Geologically it is considered the youngest ocean, featuring active spreading ridges and a dynamic plate tectonic environment .
• Key mid-ocean ridge systems include: the Carlsberg Ridge, Central Indian Ridge, Southwest Indian Ridge, Southeast Indian Ridge. These ridges help partition the ocean floor into distinct basins .
• The old notion of a single “Indo-Australian Plate” has been revised: there are now thought to be three plates — Indian, Capricorn, and Australian — separated by diffuse boundaries .
• Only two major trenches exist: the Java Trench (between Java and the Sunda Trench) and the Makran Trench (south of Iran–Pakistan) .
• Numerous hotspot tracks traverse the Indian Ocean floor:  • The Réunion hotspot links Réunion and the Mascarene Plateau to the Chagos-Laccadive Ridge and the Deccan Traps in India.  • The Kerguelen hotspot connects the Kerguelen Islands and Plateau with the Ninety East Ridge and Rajmahal Traps.  • The Marion hotspot is also implicated in links between the Prince Edward Islands and ridge structures.These hotspot tracks are often intersected or broken by active spreading ridges .
• The Indian Ocean contains some of the world’s largest submarine fans, especially the Bengal Fan and Indus Fan, which receive tremendous sediment deposition from rivers .
• Sediment types vary: terrigenous (land-derived) sediments dominate near continental slopes, whereas pelagic (fine, open ocean) sediments prevail away from margins. South of ~50°S, siliceous oozes dominate .

A noteworthy geophysical feature is the Indian Ocean Geoid Low (IOGL): a gravity anomaly forming a depression in Earth’s geoid (i.e. the gravitational equipotential surface) of ~3 million km², located just south of the Indian Peninsula. Discovered in 1948, the IOGL remains poorly understood, though recent seismic and geophysical modelling has sought to explain it .

Climate, Circulation and Oceanographic Phenomena

Because of its tropical and subtropical location, the Indian Ocean is the warmest ocean on average, and its sea surface temperatures (SSTs) are generally above 22 °C north of ~20° S, with the eastern parts exceeding 28 °C. South of ~40° S, temperatures fall sharply .

Monsoon Winds & Seasonal Reversal

One of the Indian Ocean’s defining features is the monsoon climate that drives seasonal reversal in wind and current patterns:

• Summer (May–October): The southwest monsoon draws moist air from the ocean toward the land, causing heavy rainfall over India and environs.
• Winter (October–April): The northeast monsoon brings drier air from the land back toward the sea.

These winds drive seasonal reversal of major currents, especially north of 30° S. For example, the Somali Current reverses direction between summer and winter .
Other circulation features include:

• Two large gyres: one north of the equator flowing clockwise, and another south of the equator flowing anticlockwise, including the Agulhas Current and its Agulhas Return Current.
• The Indian Ocean Walker circulation, which is a longitudinal atmospheric-ocean coupling system that modifies equatorial wind patterns and helps suppress steady equatorial easterlies in this region .
• Upwelling zones in the northern Indian Ocean near the Horn of Africa and Arabian Peninsula (driven by monsoon winds) and in seasonal locations south of the trade winds .
• The Indonesian Throughflow offers a unique oceanic connection between the Indian and Pacific Oceans via the maritime passages of Indonesia .

Temperature, Salinity, and Freshwater Input

• Salinity varies significantly: high salinity (>36 PSU) in the Arabian Sea (due to high evaporation, low precipitation) and lower salinity (~33 PSU) in the Bay of Bengal (due to heavy river discharge and rainfall) .
• In seasonal patterns, saltier water is transported east from the Arabian Sea to the Bay of Bengal (June–September) and then westward via coastal currents (January–April) .
• The Bay of Bengal delivers a large volume of freshwater runoff into the Indian Ocean—over 2,950 km³ annually—some of which flows across the equator and influences southern tropical currents .
• Over the 20th and early 21st centuries, the Indian Ocean has experienced rapid warming (≈1.2 °C from 1901 to 2012) and accelerated projected warming of 1.7–3.8 °C per century in many model scenarios .The northeastern Indian Ocean, especially the Arabian Sea, is experiencing the strongest warming, whereas warming is relatively milder off Sumatra and Java .
• Because of such warming trends, marine heatwaves are projected to increase dramatically in frequency (e.g. from ~20 days/year historically to 220–250 days/year by end of century) .
• The Indian Ocean Experiment (INDOEX, 1999) revealed that anthropogenic aerosols (from biomass burning and fossil fuel emissions in South/Southeast Asia) reduced solar radiation reaching the ocean surface by ~10 %, altered cloud dynamics, and influenced regional climate via the “Asian Brown Cloud” effect .

Ecology, Marine Life and Biodiversity

The Indian Ocean hosts diverse ecosystems and is of high ecological significance.

Ecosystems

• Coral reefs, mangroves, and seagrass beds are among the most productive and biodiverse coastal ecosystems. Coral coverage is around 200,000 km². Coastal zones and estuaries (3,000 km² and 246 major estuaries) support rich biodiversity .
• Mangroves are extensive in the Indian Ocean realm, covering ~80,984 km² (~half of world mangrove area contributes) with Indonesia alone contributing ~42,500 km². Mangrove systems originated in the Indian Ocean region but also face severe habitat loss .
• Coastal environments often yield up to 20 tonnes of fish per km² in productive zones, although pressures from urbanisation and destructive fishing are rising .

Fisheries and Marine Resources

• The Indian Ocean provides the second-largest share globally of the economically valuable tuna catch . Coastal and oceanic fisheries (shrimp, tuna, etc.) are heavily exploited by local and foreign fleets (Russia, Japan, South Korea, Taiwan) .
• Over recent decades, phytoplankton biomass in many parts has declined by up to 20 %, partly due to warming and stratification, reducing primary productivity .
• Tuna catch rates have dropped by 50–90 % in some regions over decades, largely due to overfishing coupled with thermal stress on species .

Faunal Diversity & Conservation

Many endangered and vulnerable marine mammals, turtles, and species inhabit the Indian Ocean. Some examples:

Also notable: coelacanths—“living fossil” fishes—are found in the Indian Ocean (especially near the Comoros). Two species exist (one in Indian Ocean, one in Indonesian waters), and they illustrate deep evolutionary lineages .
On land, biodiversity hotspots on Indian Ocean margins are significant:

• Madagascar and nearby islands host ~13,000 plant species (≈11,600 endemic), many birds, reptiles, amphibians, mammals, lots of endemism.
• Other hotspots include the Western Ghats–Sri Lanka, Indo-Burma, Sundaland, Wallacea, Southwest Australia, Horn of Africa, Coastal forests of east Africa, Maputaland–Pondoland–Albany, etc. Many of these lie on or near the Indian Ocean frontier and contribute to high global biodiversity in the region .

Overall, about 25 % of Earth’s biodiversity hotspots (9 out of 36) lie on or near Indian Ocean coasts .

Rivers, Sediment Flux and Land–Ocean Connectivity

• The Indian Ocean drainage basin spans ~21,100,000 km², a size comparable to the Pacific’s catchment, and about half that of the Atlantic basin. This basin is divided into ~800 sub-basins: ~50 % in Asia, ~30 % in Africa, ~20 % in Australasia .
• Rivers flowing into the Indian Ocean tend to be short on average (~740 km) relative to those feeding other oceans .
• Major rivers (order 5) include Ganges–Brahmaputra, Indus, Zambezi, Jubba, and Murray. (Order 4: Shatt al-Arab, Wadi Ad Dawasir, Limpopo) .
• After the breakup of Gondwana and uplift of the Himalayas, the Ganges–Brahmaputra system drains into one of the world’s largest deltas (the Bengal Delta / Sundarbans) before reaching the sea .
• Sediment delivery is concentrated: ~40 % of Indian Ocean sediment is deposited in the Indus and Ganges fans .

Human History, Trade, Culture, Geopolitics

Etymology & Early Names

• The name “Indian Ocean” dates at least to 1515, from the Latin Oceanus Orientalis Indicus, with reference to India projecting into it. Earlier, it was known as the “Eastern Ocean” (in contrast to the Western / Atlantic) even up through the mid-18th century. “Afro-Asian Ocean” has occasionally been used in modern times .
• In Hindi, it is हिंद महासागर (Hind Mahāsāgar).
• Chinese explorers (e.g. Zheng He) called it the “Western Oceans” (西洋) in the 15th century. In ancient Greek geography, parts of the Indian Ocean were referred to as the Erythraean Sea .

Human Migrations & Early Trade

• The coastal hypothesis suggests modern humans migrated “Out of Africa” along the northern rim of the Indian Ocean, with DNA evidence supporting coastal dispersal ~75,000 years ago .
• By ~5,000–6,000 years ago, six distinct cultural centres had evolved along the Indian Ocean rim (East Africa, Middle East, Indian Subcontinent, Southeast Asia, Malay world, Australia) connected by maritime exchange .
• Crops and domesticated species moved along these routes: African crops (sorghum, millet, cowpea) reached Gujarat, and Asian species such as black pepper, poultry, bananas moved westward ~2,000–3,000 years ago .
• The Periplus of the Erythraean Sea (1st century CE) is an Alexandrian maritime guide describing trade between Roman Egypt, East Africa, Arabia, and India using monsoon wind routes . Greek/Roman trade expanded due to monsoon-based navigation.
• The Austronesian maritime network, starting about the same period, enabled migrations across the Indian Ocean, such as the settlement of Madagascar by Austronesians from Southeast Asia. Their outward crossings utilised monsoon patterns .

Age of Discovery & Colonial Era

• With the Ottoman capture of Constantinople (1453), the traditional overland routes to Asia from Europe became difficult. Sea routes were sought, and in 1497–98 Vasco da Gama rounded the Cape of Good Hope and reached India, initiating European dominance of Indian Ocean trade .
• The Portuguese established fortified ports on African and Indian coasts and controlled key chokepoints. Later, the Dutch East India Company, French, and British became dominant players in maritime trade and colonisation.
• The Suez Canal (opened 1869) dramatically transformed trade, bringing Europe–Asia shipping through the Mediterranean and reducing dependence on circumnavigating Africa . The canal also facilitated the biological invasion of species into the Mediterranean via the Red Sea (Lessepsian migration) .
• The islands of the Indian Ocean (e.g. Mauritius, Réunion, Chagos Archipelago) became important colonial outposts, often linked with plantation economies and slave/indentured labour systems.
• The British Indian Ocean Territory (BIOT) includes the Chagos Archipelago, which remains contested between the UK and Mauritius. Diego Garcia, within BIOT, hosts a UK/US military base. The native Chagossians were forcibly removed in the 1960s–70s .

Modern Era, Security & Disasters

• In 2004 a massive undersea earthquake off Sumatra triggered a tsunami that struck Indian Ocean coasts, killing over 230,000 people across multiple countries .
• Piracy surged in the early 2000s, particularly off the Horn of Africa and the Somali coast, prompting coordinated international naval patrols. By 2013, pirate activity had declined significantly due to persistent naval presence and deterrence efforts .
• In March 2014, Malaysia Airlines Flight MH370 disappeared and is believed to have crashed in the southern Indian Ocean, though its wreckage has never been conclusively located .
• The strategic importance of the Indian Ocean remains paramount: over 80 % of global seaborne oil trade transits through it, with 40 % passing through the Strait of Hormuz, 35 % through the Strait of Malacca, and 8 % via Bab el Mandab .
• In recent years, major powers (India, China, the United States, Australia) have increased naval deployments, port investments (e.g. Gwadar, Hambantota, Colombo), and strategic partnerships in the region, making the Indian Ocean a theatre of power competition.

India’s Defence Minister has explicitly highlighted the importance of maintaining a strong naval presence in the Indian Ocean as 95 % of India’s trade (by volume) passes through it .

Challenges, Threats & Future Outlook

• Climate change and ocean warming threaten coral reefs via bleaching, alter species distributions, and increase frequency/severity of cyclones and marine heatwaves.
• Overfishing, illegal/unregulated/unreported (IUU) fishing, and fisheries collapse risk food security for coastal nations.
• Marine pollution (plastics, oil spills, nutrient runoff) is a serious concern: the Indian Ocean hosts a garbage patch (≥ 5 million km²), circulating debris across the southern Indian gyre .
• Sea level rise threatens low-lying island nations (Maldives, Seychelles, etc.) and coastal populations.
• Territorial disputes over islands, Exclusive Economic Zones (EEZs), and maritime boundaries remain unresolved in some areas (e.g. Chagos Archipelago).
• Geopolitical rivalry over influence—especially between India and China—for port infrastructure, naval bases, trade corridors, and resource access is heating up further.
• The Indian Ocean Geoid Low (IOGL) poses an ongoing scientific puzzle about Earth’s internal structure and geodynamics.

Indian Ocean is a richly complex and deeply consequential region—from its geological youth and dynamic circulation to its role as a crucible of biodiversity, human history, trade, and strategic power. Its future will be shaped by how coastal nations manage climate change, resource sustainability, and geopolitical competition in the decades ahead.