The Indian Ocean has been known by its present name since at least 1515 when the

Latin form Oceanus Orientalis Indicus ("Indian Eastern Ocean") is attested, named
for India, which projects into it. It was earlier known as the Eastern Ocean, a
term that was still in use during the mid-18th century (see map), as opposed to the
Western Ocean (Atlantic) before the Pacific was surmised.[7]

Conversely, Chinese explorers in the Indian Ocean during the 15th century called it
the Western Oceans.[8] The ocean has also been known as the Hind Mahasagar and
Indic Ocean in various languages.[citation needed]

In Ancient Greek geography, the Indian Ocean region known to the Greeks was called
the Erythraean Sea.[9]

Geography

The ocean-floor of the Indian Ocean is divided by spreading ridges and crisscrossed
by aseismic structures

A composite satellite image centred on the Indian Ocean

Extent and data
The borders of the Indian Ocean, as delineated by the International Hydrographic
Organization in 1953 included the Southern Ocean but not the marginal seas along
the northern rim, but in 2000 the IHO delimited the Southern Ocean separately,
which removed waters south of 60°S from the Indian Ocean but included the northern
marginal seas. [10][11] Meridionally, the Indian Ocean is delimited from the
Atlantic Ocean by the 20° east meridian, running south from Cape Agulhas, and from
the Pacific Ocean by the meridian of 146°49'E, running south from the southernmost
point of Tasmania. The northernmost extent of the Indian Ocean (including marginal
seas) is approximately 30° north in the Persian Gulf.[11]

The Indian Ocean covers 70,560,000 km2 (27,240,000 sq mi), including the Red Sea
and the Persian Gulf but excluding the Southern Ocean, or 19.5% of the world's
oceans; its volume is 264,000,000 km3 (63,000,000 cu mi) or 19.8% of the world's
oceans' volume; it has an average depth of 3,741 m (12,274 ft) and a maximum depth
of 7,906 m (25,938 ft).[5]

All of the Indian Ocean is in the Eastern Hemisphere and the centre of the Eastern
Hemisphere, the 90th meridian east, passes through the Ninety East Ridge.

Coasts and shelves

In contrast to the Atlantic and Pacific, the Indian Ocean is enclosed by major
landmasses and an archipelago on three sides and does not stretch from pole to
pole, and can be likened to an embayed ocean. It is centered on the Indian
Peninsula. Although this subcontinent has played a significant role in its history,
the Indian Ocean has foremostly been a cosmopolitan stage, interlinking diverse
regions by innovations, trade, and religion since early in human history.[12]

The active margins of the Indian Ocean have an average depth (land to shelf
break[clarification needed]) of 19 ± 0.61 km (11.81 ± 0.38 mi) with a maximum depth
of 175 km (109 mi). The passive margins have an average depth of 47.6 ± 0.8 km
(29.58 ± 0.50 mi).[13] The average width of the slopes of the continental shelves
are 50.4–52.4 km (31.3–32.6 mi) for active and passive margins respectively, with a
maximum depth of 205.3–255.2 km (127.6–158.6 mi).[14]

In correspondance of the Shelf break, also known as Hinge zone, the Bouguer gravity
ranges from 0 to 30 mGals that is unusual for a continantal region of around 16 km
thick sediments. It has been hypothesized that the "Hinge zone may represent the
relict of continental and proto-oceanic crustal boundary formed during the rifting
of India from Antarctica."[15]
Australia, Indonesia, and India are the three countries with the longest shorelines
and exclusive economic zones. The continental shelf makes up 15% of the Indian
Ocean. More than two billion people live in countries bordering the Indian Ocean,
compared to 1.7 billion for the Atlantic and 2.7 billion for the Pacific (some
countries border more than one ocean).[2]

Rivers
The Indian Ocean drainage basin covers 21,100,000 km2 (8,100,000 sq mi), virtually
identical to that of the Pacific Ocean and half that of the Atlantic basin, or 30%
of its ocean surface (compared to 15% for the Pacific). The Indian Ocean drainage
basin is divided into roughly 800 individual basins, half that of the Pacific, of
which 50% are located in Asia, 30% in Africa, and 20% in Australasia. The rivers of
the Indian Ocean are shorter on average (740 km (460 mi)) than those of the other
major oceans. The largest rivers are (order 5) the Zambezi, Ganges-Brahmaputra,
Indus, Jubba, and Murray rivers and (order 4) the Shatt al-Arab, Wadi Ad Dawasir (a
dried-out river system on the Arabian Peninsula) and Limpopo rivers.[16] After the
breakup of East Gondwana and the formation of Himalayas, the Ganges-Brahmaputra
rivers flowed into the world's largest Bengal delta.[15]

Marginal seas
Marginal seas, gulfs, bays and straits of the Indian Ocean include:[11]

Along the east coast of Africa, the Mozambique Channel separates Madagascar from
mainland Africa, while the Sea of Zanj is located north of Madagascar.

On the northern coast of the Arabian Sea, Gulf of Aden is connected to the Red Sea
by the strait of Bab-el-Mandeb. In the Gulf of Aden, the Gulf of Tadjoura is
located in Djibouti and the Guardafui Channel separates Socotra island from the
Horn of Africa. The northern end of the Red Sea terminates in the Gulf of Aqaba and
Gulf of Suez. The Indian Ocean is artificially connected to the Mediterranean Sea
without ship lock through the Suez Canal, which is accessible via the Red Sea. The
Arabian Sea is connected to the Persian Gulf by the Gulf of Oman and the Strait of
Hormuz. In the Persian Gulf, the Gulf of Bahrain separates Qatar from the Arabic
Peninsula.

Along the west coast of India, the Gulf of Kutch and Gulf of Khambat are located in
Gujarat in the northern end while the Laccadive Sea separates the Maldives from the
southern tip of India. The Bay of Bengal is off the east coast of India. The Gulf
of Mannar and the Palk Strait separates Sri Lanka from India, while the Adam's
Bridge separates the two. The Andaman Sea is located between the Bay of Bengal and
the Andaman Islands.

In Indonesia, the so-called Indonesian Seaway is composed of the Malacca, Sunda and
Torres Straits. The Gulf of Carpentaria of located on the Australian north coast
while the Great Australian Bight constitutes a large part of its southern coast.
[17][18][19]

Arabian Sea - 3.862 million km2

Bay of Bengal - 2.172 million km2
Andaman Sea - 797,700 km2
Laccadive Sea - 786,000 km2
Mozambique Channel - 700,000 km2
Timor Sea - 610,000 km2
Red Sea - 438,000 km2
Gulf of Aden - 410,000 km2
Persian Gulf - 251,000 km2
Flores Sea - 240,000 km2
Molucca Sea - 200,000 km2
Oman Sea - 181,000 km2
Great Australian Bight - 45,926 km2
Gulf of Aqaba - 239 km2
Gulf of Khambhat
Gulf of Kutch
Gulf of Suez
Climate

During summer, warm continental masses draw moist air from the Indian Ocean hence
producing heavy rainfall. The process is reversed during winter, resulting in dry
conditions.
Several features make the Indian Ocean unique. It constitutes the core of the
large-scale Tropical Warm Pool which, when interacting with the atmosphere, affects
the climate both regionally and globally. Asia blocks heat export and prevents the
ventilation of the Indian Ocean thermocline. That continent also drives the Indian
Ocean monsoon, the strongest on Earth, which causes large-scale seasonal variations
in ocean currents, including the reversal of the Somali Current and Indian Monsoon
Current. Because of the Indian Ocean Walker circulation there are no continuous
equatorial easterlies. Upwelling occurs near the Horn of Africa and the Arabian
Peninsula in the Northern Hemisphere and north of the trade winds in the Southern
Hemisphere. The Indonesian Throughflow is a unique Equatorial connection to the
Pacific.[20]

The climate north of the equator is affected by a monsoon climate. Strong north-
east winds blow from October until April; from May until October south and west
winds prevail. In the Arabian Sea, the violent Monsoon brings rain to the Indian
subcontinent. In the southern hemisphere, the winds are generally milder, but
summer storms near Mauritius can be severe. When the monsoon winds change, cyclones
sometimes strike the shores of the Arabian Sea and the Bay of Bengal.[21] Some 80%
of the total annual rainfall in India occurs during summer and the region is so
dependent on this rainfall that many civilisations perished when the Monsoon failed
in the past. The huge variability in the Indian Summer Monsoon has also occurred
pre-historically, with a strong, wet phase 33,500–32,500 BP; a weak, dry phase
26,000–23,500 BC; and a very weak phase 17,000–15,000 BP, corresponding to a series
of dramatic global events: Bølling-Allerød, Heinrich, and Younger Dryas.[22]

Air pollution in South Asia spread over the Bay of Bengal and beyond.
The Indian Ocean is the warmest ocean in the world.[23] Long-term ocean temperature
records show a rapid, continuous warming in the Indian Ocean, at about 1.2 °C (34.2
°F) (compared to 0.7 °C (33.3 °F) for the warm pool region) during 1901–2012.[24]
Research indicates that human induced greenhouse warming, and changes in the
frequency and magnitude of El Niño (or the Indian Ocean Dipole), events are a
trigger to this strong warming in the Indian Ocean.[24]

South of the Equator (20-5°S), the Indian Ocean is gaining heat from June to
October, during the austral winter, while it is losing heat from November to March,
during the austral summer.[25]

In 1999, the Indian Ocean Experiment showed that fossil fuel and biomass burning in
South and Southeast Asia caused air pollution (also known as the Asian brown cloud)
that reach as far as the Intertropical Convergence Zone at 60°S. This pollution has
implications on both a local and global scale.[26]

Oceanography
40% of the sediment of the Indian Ocean is found in the Indus and Ganges fans. The
oceanic basins adjacent to the continental slopes mostly contain terrigenous
sediments. The ocean south of the polar front (roughly 50° south latitude) is high
in biologic productivity and dominated by non-stratified sediment composed mostly
of siliceous oozes. Near the three major mid-ocean ridges the ocean floor is
relatively young and therefore bare of sediment, except for the Southwest Indian
Ridge due to its ultra-slow spreading rate.[27]

The ocean's currents are mainly controlled by the monsoon. Two large gyres, one in
the northern hemisphere flowing clockwise and one south of the equator moving
anticlockwise (including the Agulhas Current and Agulhas Return Current),
constitute the dominant flow pattern. During the winter monsoon (November–
February), however, circulation is reversed north of 30°S and winds are weakened
during winter and the transitional periods between the monsoons.[28]

The Indian Ocean contains the largest submarine fans of the world, the Bengal Fan
and Indus Fan, and the largest areas of slope terraces and rift valleys. [29]

The inflow of deep water into the Indian Ocean is 11 Sv, most of which comes from
the Circumpolar Deep Water (CDW). The CDW enters the Indian Ocean through the
Crozet and Madagascar basins and crosses the Southwest Indian Ridge at 30°S. In the
Mascarene Basin the CDW becomes a deep western boundary current before it is met by
a re-circulated branch of itself, the North Indian Deep Water. This mixed water
partly flows north into the Somali Basin whilst most of it flows clockwise in the
Mascarene Basin where an oscillating flow is produced by Rossby waves.[30]

Water circulation in the Indian Ocean is dominated by the Subtropical Anticyclonic

Gyre, the eastern extension of which is blocked by the Southeast Indian Ridge and
the 90°E Ridge. Madagascar and the Southwest Indian Ridge separate three cells
south of Madagascar and off South Africa. North Atlantic Deep Water reaches into
the Indian Ocean south of Africa at a depth of 2,000–3,000 m (6,600–9,800 ft) and
flows north along the eastern continental slope of Africa. Deeper than NADW,
Antarctic Bottom Water flows from Enderby Basin to Agulhas Basin across deep
channels (<4,000 m (13,000 ft)) in the Southwest Indian Ridge, from where it
continues into the Mozambique Channel and Prince Edward Fracture Zone.[31]

North of 20° south latitude the minimum surface temperature is 22 °C (72 °F),
exceeding 28 °C (82 °F) to the east. Southward of 40° south latitude, temperatures
drop quickly.[21]

The Bay of Bengal contributes more than half (2,950 km3 (710 cu mi)) of the runoff
water to the Indian Ocean. Mainly in summer, this runoff flows into the Arabian Sea
but also south across the Equator where it mixes with fresher seawater from the
Indonesian Throughflow. This mixed freshwater joins the South Equatorial Current in
the southern tropical Indian Ocean.[32] Sea surface salinity is highest (more than
36 PSU) in the Arabian Sea because evaporation exceeds precipitation there. In the
Southeast Arabian Sea salinity drops to less than 34 PSU. It is the lowest (c. 33
PSU) in the Bay of Bengal because of river runoff and precipitation. The Indonesian
Throughflow and precipitation results in lower salinity (34 PSU) along the Sumatran
west coast. Monsoonal variation results in eastward transportation of saltier water
from the Arabian Sea to the Bay of Bengal from June to September and in westerly
transport by the East India Coastal Current to the Arabian Sea from January to
April.[33]

An Indian Ocean garbage patch was discovered in 2010 covering at least 5 million
square kilometres (1.9 million square miles). Riding the southern Indian Ocean
Gyre, this vortex of plastic garbage constantly circulates the ocean from Australia
to Africa, down the Mozambique Channel, and back to Australia in a period of six
years, except for debris that gets indefinitely stuck in the centre of the gyre.
[34] The garbage patch in the Indian Ocean will, according to a 2012 study,
decrease in size after several decades to vanish completely over centuries. Over
several millennia, however, the global system of garbage patches will accumulate in
the North Pacific.[35]
There are two amphidromes of opposite rotation in the Indian Ocean, probably caused
by Rossby wave propagation.[36]

Icebergs drift as far north as 55° south latitude, similar to the Pacific but less
than in the Atlantic where icebergs reach up to 45°S. The volume of iceberg loss in
the Indian Ocean between 2004 and 2012 was 24 Gt.[37]

Since the 1960s, anthropogenic warming of the global ocean combined with
contributions of freshwater from retreating land ice causes a global rise in sea
level. Sea level increases in the Indian Ocean too, except in the south tropical
Indian Ocean where it decreases, a pattern most likely caused by rising levels of
greenhouse gases.[38]

Marine life

A dolphin off Western Australia and a swarm of surgeonfish near Maldives Islands
represents the well-known, exotic fauna of the warmer parts of the Indian Ocean.
King Penguins on a beach in the Crozet Archipelago near Antarctica attract fewer
tourists.
Among the tropical oceans, the western Indian Ocean hosts one of the largest
concentrations of phytoplankton blooms in summer, due to the strong monsoon winds.
The monsoonal wind forcing leads to a strong coastal and open ocean upwelling,
which introduces nutrients into the upper zones where sufficient light is available
for photosynthesis and phytoplankton production. These phytoplankton blooms support
the marine ecosystem, as the base of the marine food web, and eventually the larger
fish species. The Indian Ocean accounts for the second-largest share of the most
economically valuable tuna catch.[39] Its fish are of great and growing importance
to the bordering countries for domestic consumption and export. Fishing fleets from
Russia, Japan, South Korea, and Taiwan also exploit the Indian Ocean, mainly for
shrimp and tuna.[3]

Research indicates that increasing ocean temperatures are taking a toll on the
marine ecosystem. A study on the phytoplankton changes in the Indian Ocean
indicates a decline of up to 20% in the marine plankton in the Indian Ocean, during
the past six decades. The tuna catch rates have also declined 50–90% during the
past half-century, mostly due to increased industrial fisheries, with the ocean
warming adding further stress to the fish species.[40]

Endangered and vulnerable marine mammals and turtles:[41]

Name Distribution Trend

Endangered
Australian sea lion
(Neophoca cinerea) Southwest Australia Decreasing
Blue whale
(Balaenoptera musculs) Global Increasing
Sei whale
(Balaenoptera borealis) Global Increasing
Irrawaddy dolphin
(Orcaella brevirostris) Southeast Asia Decreasing
Indian Ocean humpback dolphin
(Sousa plumbea) Western Indian Ocean Decreasing
Green sea turtle
(Chelonia mydas) Global Decreasing
Vulnerable
Dugong
(Dugong dugon) Equatorial Indian Ocean and Pacific Decreasing
Sperm whale
(Physeter macrocephalus) Global Unknown
Fin whale
(Balaenoptera physalus) Global Increasing
Australian snubfin dolphin
(Orcaella heinsohni) Northern Australia, New Guinea Decreasing
Indo-Pacific humpback dolphin
(Sousa chinensis) Southeast Asia Decreasing
Indo-Pacific finless porpoise
(Neophocaena phocaenoides) Northern Indian Ocean, Southeast Asia Decreasing
Australian humpback dolphin
(Sousa sahulensis) Northern Australia, New Guinea Decreasing
Leatherback
(Dermochelys coriacea) Global Decreasing
Olive ridley sea turtle
(Lepidochelys olivacea) Global Decreasing
Loggerhead sea turtle
(Caretta caretta) Global Decreasing
80% of the Indian Ocean is open ocean and includes nine large marine ecosystems:
the Agulhas Current, Somali Coastal Current, Red Sea, Arabian Sea, Bay of Bengal,
Gulf of Thailand, West Central Australian Shelf, Northwest Australian Shelf, and
Southwest Australian Shelf. Coral reefs cover c. 200,000 km2 (77,000 sq mi). The
coasts of the Indian Ocean includes beaches and intertidal zones covering 3,000 km2
(1,200 sq mi) and 246 larger estuaries. Upwelling areas are small but important.
The hypersaline salterns in India covers between 5,000–10,000 km2 (1,900–3,900 sq
mi) and species adapted for this environment, such as Artemia salina and Dunaliella
salina, are important to bird life.[42]

Left: Mangroves (here in East Nusa Tenggara, Indonesia) are the only tropical to
subtropical forests adapted for a coastal environment. From their origin on the
coasts of the Indo-Malaysian region, they have reached a global distribution.
Right: The coelacanth (here a model from Oxford), thought extinct for million
years, was rediscovered in the 20th century. The Indian Ocean species is blue
whereas the Indonesian species is brown.
Coral reefs, sea grass beds, and mangrove forests are the most productive
ecosystems of the Indian Ocean — coastal areas produce 20 tones per square
kilometre of fish. These areas, however, are also being urbanised with populations
often exceeding several thousand people per square kilometre and fishing techniques
become more effective and often destructive beyond sustainable levels while the
increase in sea surface temperature spreads coral bleaching.[43]

Mangroves covers 80,984 km2 (31,268 sq mi) in the Indian Ocean region, or almost
half of the world's mangrove habitat, of which 42,500 km2 (16,400 sq mi) is located
in Indonesia, or 50% of mangroves in the Indian Ocean. Mangroves originated in the
Indian Ocean region and have adapted to a wide range of its habitats but it is also
where it suffers its biggest loss of habitat.[44]

In 2016 six new animal species were identified at hydrothermal vents in the
Southwest Indian Ridge: a "Hoff" crab, a "giant peltospirid" snail, a whelk-like
snail, a limpet, a scaleworm and a polychaete worm.[45]

The West Indian Ocean coelacanth was discovered in the Indian Ocean off South
Africa in the 1930s and in the late 1990s another species, the Indonesian
coelacanth, was discovered off Sulawesi Island, Indonesia. Most extant coelacanths
have been found in the Comoros. Although both species represent an order of lobe-
finned fishes known from the Early Devonian (410 mya) and though extinct 66 mya,
they are morphologically distinct from their Devonian ancestors. Over millions of
years, coelacanths evolved to inhabit different environments — lungs adapted for
shallow, brackish waters evolved into gills adapted for deep marine waters.[46]

Biodiversity
Of Earth's 36 biodiversity hotspot nine (or 25%) are located on the margins of the
Indian Ocean.

Madagascar's Elephant bird, Mauritius's Dodo bird and ostrich (from left to right)
Madagascar and the islands of the western Indian Ocean (Comoros, Réunion,
Mauritius, Rodrigues, the Seychelles, and Socotra), includes 13,000 (11,600
endemic) species of plants; 313 (183) birds; reptiles 381 (367); 164 (97)
freshwater fishes; 250 (249) amphibians; and 200 (192) mammals.[47]
The origin of this diversity is debated; the break-up of Gondwana can explain
vicariance older than 100 mya, but the diversity on the younger, smaller islands
must have required a Cenozoic dispersal from the rims of the Indian Ocean to the
islands. A "reverse colonisation", from islands to continents, apparently occurred
more recently; the chameleons, for example, first diversified on Madagascar and
then colonised Africa. Several species on the islands of the Indian Ocean are
textbook cases of evolutionary processes; the dung beetles, day geckos, and lemurs
are all examples of adaptive radiation.[citation needed] Many bones (250 bones per
square metre) of recently extinct vertebrates have been found in the Mare aux
Songes swamp in Mauritius, including bones of the Dodo bird (Raphus cucullatus) and
Cylindraspis giant tortoise. An analysis of these remains suggests a process of
aridification began in the southwest Indian Ocean began around 4,000 years ago.[48]

Maputaland-Pondoland-Albany (MPA); 8,100 (1,900 endemic) species of plants; 541 (0)

birds; 205 (36) reptiles; 73 (20) freshwater fishes; 73 (11) amphibians; and 197
(3) mammals.[47]
Mammalian megafauna once widespread in the MPA was driven to near extinction in the
early 20th century. Some species have been successfully recovered since then — the
population of white rhinoceros (Ceratotherium simum simum) increased from less than
20 individuals in 1895 to more than 17,000 as of 2013. Other species are still
dependent of fenced areas and management programs, including black rhinoceros
(Diceros bicornis minor), African wild dog (Lycaon pictus), cheetah (Acynonix
jubatus), elephant (Loxodonta africana), and lion (Panthera leo).[49]

Coastal forests of eastern Africa; 4,000 (1,750 endemic) species of plants; 636
(12) birds; 250 (54) reptiles; 219 (32) freshwater fishes; 95 (10) amphibians; and
236 (7) mammals.[47]
This biodiversity hotspot (and namesake ecoregion and "Endemic Bird Area") is a
patchwork of small forested areas, often with a unique assemblage of species within
each, located within 200 km (120 mi) from the coast and covering a total area of c.
6,200 km2 (2,400 sq mi). It also encompasses coastal islands, including Zanzibar
and Pemba, and Mafia.[50]

Horn of Africa; 5,000 (2,750 endemic) species of plants; 704 (25) birds; 284 (93)
reptiles; 100 (10) freshwater fishes; 30 (6) amphibians; and 189 (18) mammals.[47]

Coral reefs of the Maldives

This area, one of the only two hotspots that are entirely arid, includes the
Ethiopian Highlands, the East African Rift valley, the Socotra islands, as well as
some small islands in the Red Sea and areas on the southern Arabic Peninsula.
Endemic and threatened mammals include the dibatag (Ammodorcas clarkei) and Speke's
gazelle (Gazella spekei); the Somali wild ass (Equus africanus somaliensis) and
hamadryas baboon (Papio hamadryas). It also contains many reptiles.[51] In Somalia,
the centre of the 1,500,000 km2 (580,000 sq mi) hotspot, the landscape is dominated
by Acacia-Commiphora deciduous bushland, but also includes the Yeheb nut
(Cordeauxia edulus) and species discovered more recently such as the Somali
cyclamen (Cyclamen somalense), the only cyclamen outside the Mediterranean.
Warsangli linnet (Carduelis johannis) is an endemic bird found only in northern
Somalia. An unstable political regime has resulted in overgrazing which has
produced one of the most degraded hotspots where only c. 5 % of the original
habitat remains.[52]

The Western Ghats–Sri Lanka; 5,916 (3,049 endemic) species of plants; 457 (35)
birds; 265 (176) reptiles; 191 (139) freshwater fishes; 204 (156) amphibians; and
143 (27) mammals.[47]
Encompassing the west coast of India and Sri Lanka, until c. 10,000 years ago a
landbridge connected Sri Lanka to the Indian Subcontinent, hence this region shares
a common community of species.[53]

Indo-Burma; 13.500 (7,000 endemic) species of plants; 1,277 (73) birds; 518 (204)
reptiles; 1,262 (553) freshwater fishes; 328 (193) amphibians; and 401 (100)
mammals.[47]

Aldabra giant tortoise from the islands of the Aldabra Atoll in the Seychelles
Indo-Burma encompasses a series of mountain ranges, five of Asia's largest river
systems, and a wide range of habitats. The region has a long and complex geological
history, and long periods rising sea levels and glaciations have isolated
ecosystems and thus promoted a high degree of endemism and speciation. The region
includes two centres of endemism: the Annamite Mountains and the northern highlands
on the China-Vietnam border.[54] Several distinct floristic regions, the Indian,
Malesian, Sino-Himalayan, and Indochinese regions, meet in a unique way in Indo-
Burma and the hotspot contains an estimated 15,000–25,000 species of vascular
plants, many of them endemic.[55]

Sundaland; 25,000 (15,000 endemic) species of plants; 771 (146) birds; 449 (244)
reptiles; 950 (350) freshwater fishes; 258 (210) amphibians; and 397 (219) mammals.
[47]
Sundaland encompasses 17,000 islands of which Borneo and Sumatra are the largest.
Endangered mammals include the Bornean and Sumatran orangutans, the proboscis
monkey, and the Javan and Sumatran rhinoceroses.[56]

Wallacea; 10,000 (1,500 endemic) species of plants; 650 (265) birds; 222 (99)
reptiles; 250 (50) freshwater fishes; 49 (33) amphibians; and 244 (144) mammals.
[47]
Southwest Australia; 5,571 (2,948 endemic) species of plants; 285 (10) birds; 177
(27) reptiles; 20 (10) freshwater fishes; 32 (22) amphibians; and 55 (13) mammals.
[47]
Stretching from Shark Bay to Israelite Bay and isolated by the arid Nullarbor
Plain, the southwestern corner of Australia is a floristic region with a stable
climate in which one of the world's largest floral biodiversity and an 80% endemism
has evolved. From June to September it is an explosion of colours and the
Wildflower Festival in Perth in September attracts more than half a million
visitors.[57]

Geology

Left: The oldest ocean floor of the Indian Ocean formed c. 150 Ma when the Indian
Subcontinent and Madagascar broke-up from Africa. Right: The India–Asia collision
c. 40 Ma completed the closure of the Tethys Ocean (grey areas north of India).
Geologically, the Indian Ocean is the ocean floor that opened up south of India.
Main category: Landforms of the Indian Ocean
As the youngest of the major oceans,[58] the Indian Ocean has active spreading
ridges that are part of the worldwide system of mid-ocean ridges. In the Indian
Ocean these spreading ridges meet at the Rodrigues Triple Point with the Central
Indian Ridge, including the Carlsberg Ridge, separating the African Plate from the
Indian Plate; the Southwest Indian Ridge separating the African Plate from the
Antarctic Plate; and the Southeast Indian Ridge separating the Australian Plate
from the Antarctic Plate. The Central Indian Ridge is intercepted by the Owen
Fracture Zone.[59] Since the late 1990s, however, it has become clear that this
traditional definition of the Indo-Australian Plate cannot be correct; it consists
of three plates — the Indian Plate, the Capricorn Plate, and Australian Plate —
separated by diffuse boundary zones.[60] Since 20 Ma the African Plate is being
divided by the East African Rift System into the Nubian and Somalia plates.[61]

There are only two trenches in the Indian Ocean: the 6,000 km (3,700 mi)-long Java
Trench between Java and the Sunda Trench and the 900 km (560 mi)-long Makran Trench
south of Iran and Pakistan.[59]

A series of ridges and seamount chains produced by hotspots pass over the Indian
Ocean. The Réunion hotspot (active 70–40 million years ago) connects Réunion and
the Mascarene Plateau to the Chagos-Laccadive Ridge and the Deccan Traps in north-
western India; the Kerguelen hotspot (100–35 million years ago) connects the
Kerguelen Islands and Kerguelen Plateau to the Ninety East Ridge and the Rajmahal
Traps in north-eastern India; the Marion hotspot (100–70 million years ago)
possibly connects Prince Edward Islands to the Eighty Five East Ridge.[62] These
hotspot tracks have been broken by the still active spreading ridges mentioned
above.[59]

There are fewer seamounts in the Indian Ocean than in the Atlantic and Pacific.
These are typically deeper than 3,000 m (9,800 ft) and located north of 55°S and
west of 80°E. Most originated at spreading ridges but some are now located in
basins far away from these ridges. The ridges of the Indian Ocean form ranges of
seamounts, sometimes very long, including the Carlsberg Ridge, Madagascar Ridge,
Central Indian Ridge, Southwest Indian Ridge, Chagos-Laccadive Ridge, 85°E Ridge,
90°E Ridge, Southeast Indian Ridge, Broken Ridge, and East Indiaman Ridge. The
Agulhas Plateau and Mascarene Plateau are the two major shallow areas.[31]

The opening of the Indian Ocean began c. 156 Ma when Africa separated from East
Gondwana. The Indian Subcontinent began to separate from Australia-Antarctica 135–
125 Ma and as the Tethys Ocean north of India began to close 118–84 Ma the Indian
Ocean opened behind it.[59]

History
The Indian Ocean, together with the Mediterranean, has connected people since
ancient times, whereas the Atlantic and Pacific have had the roles of barriers or
mare incognitum. The written history of the Indian Ocean, however, has been
Eurocentric and largely dependent on the availability of written sources from the
colonial era. This history is often divided into an ancient period followed by an
Islamic period; the subsequent periods are often subdivided into Portuguese, Dutch,
and British periods.[63]

A concept of an "Indian Ocean World" (IOW), similar to that of the "Atlantic

World", exists but emerged much more recently and is not well established. The IOW
is, nevertheless, sometimes referred to as the "first global economy" and was based
on the monsoon which linked Asia, China, India, and Mesopotamia. It developed
independently from the European global trade in the Mediterranean and Atlantic and
remained largely independent from them until European 19th-century colonial
dominance.[64]

The diverse history of the Indian Ocean is a unique mix of cultures, ethnic groups,
natural resources, and shipping routes. It grew in importance beginning in the
1960s and 1970s and, after the Cold War, it has undergone periods of political
instability, most recently with the emergence of India and China as regional
powers.[65]

First settlements

According to the Coastal hypothesis, modern humans spread from Africa along the
northern rim of the Indian Ocean.
Pleistocene fossils of Homo erectus and other pre-H. sapiens hominid fossils,
similar to H. heidelbergensis in Europe, have been found in India. According to the
Toba catastrophe theory, a supereruption c. 74000 years ago at Lake Toba, Sumatra,
covered India with volcanic ashes and wiped out one or more lineages of such
archaic humans in India and Southeast Asia.[66]

The Out of Africa theory states that Homo sapiens spread from Africa into mainland
Eurasia. The more recent Southern Dispersal or Coastal hypothesis instead advocates
that modern humans spread along the coasts of the Arabic Peninsula and southern
Asia. This hypothesis is supported by mtDNA research which reveals a rapid
dispersal event during the Late Pleistocene (11,000 years ago). This coastal
dispersal, however, began in East Africa 75,000 years ago and occurred
intermittently from estuary to estuary along the northern perimeter of the Indian
Ocean at a rate of 0.7–4.0 km (0.43–2.49 mi) per year. It eventually resulted in
modern humans migrating from Sunda over Wallacea to Sahul (Southeast Asia to
Australia).[67] Since then, waves of migration have resettled people and, clearly,
the Indian Ocean littoral had been inhabited long before the first civilisations
emerged. 5000–6000 years ago six distinct cultural centres had evolved around the
Indian Ocean: East Africa, the Middle East, the Indian Subcontinent, South East
Asia, the Malay World, and Australia; each interlinked to its neighbours.[68]

Food globalisation began on the Indian Ocean littoral c. 4.000 years ago. Five
African crops — sorghum, pearl millet, finger millet, cowpea, and hyacinth bean —
somehow found their way to Gujarat in India during the Late Harappan (2000–1700
BCE). Gujarati merchants evolved into the first explorers of the Indian Ocean as
they traded African goods such as ivory, tortoise shells, and slaves. Broomcorn
millet found its way from Central Asia to Africa, together with chicken and zebu
cattle, although the exact timing is disputed. Around 2000 BCE black pepper and
sesame, both native to Asia, appear in Egypt, albeit in small quantities. Around
the same time the black rat and the house mouse emigrate from Asia to Egypt. Banana
reached Africa around 3000 years ago.[69]

At least eleven prehistoric tsunamis have struck the Indian Ocean coast of
Indonesia between 7400 and 2900 years ago. Analysing sand beds in caves in the Aceh
region, scientists concluded that the intervals between these tsunamis have varied
from series of minor tsunamis over a century to dormant periods of more than 2000
years preceding megathrusts in the Sunda Trench. Although the risk for future
tsunamis is high, a major megathrust such as the one in 2004 is likely to be
followed by a long dormant period.[70]

A group of scientists have argued that two large-scale impact events have occurred
in the Indian Ocean: the Burckle Crater in the southern Indian Ocean in 2800 BCE
and the Kanmare and Tabban craters in the Gulf of Carpentaria in northern Australia
in 536 CE. Evidences for these impacts, the team argue, are micro-ejecta and
Chevron dunes in southern Madagascar and in the Australian gulf. Geological
evidences suggest the tsunamis caused by these impacts reached 205 m (673 ft) above
sea level and 45 km (28 mi) inland. The impact events must have disrupted human
settlements and perhaps even contributed to major climate changes.[71]

Antiquity
The history of the Indian Ocean is marked by maritime trade; cultural and
commercial exchange probably date back at least seven thousand years.[72] Human
culture spread early on the shores of the Indian Ocean and was always linked to the
cultures of the Mediterranean and the Persian Gulf. Before c. 2000 BCE, however,
cultures on its shores were only loosely tied to each other; bronze, for example,
was developed in Mesopotamia c. 3000 BCE but remained uncommon in Egypt before 1800
BCE.[73] During this period, independent, short-distance oversea communications
along its littoral margins evolved into an all-embracing network. The début of this
network was not the achievement of a centralised or advanced civilisation but of
local and regional exchange in the Persian Gulf, the Red Sea, and the Arabian Sea.
Sherds of Ubaid (2500–500 BCE) pottery have been found in the western Gulf at
Dilmun, present-day Bahrain; traces of exchange between this trading centre and
Mesopotamia. The Sumerians traded grain, pottery, and bitumen (used for reed boats)
for copper, stone, timber, tin, dates, onions, and pearls.[74] Coast-bound vessels
transported goods between the Indus Valley Civilisation (2600–1900 BCE) in the
Indian subcontinent (modern-day Pakistan and Northwest India) and the Persian Gulf
and Egypt.[72]

The Austronesian maritime trade network was the first trade routes in the Indian
Ocean.
The Red Sea, one of the main trade routes in Antiquity, was explored by Egyptians
and Phoenicians during the last two millennia BCE. In the 6th century, BCE Greek
explorer Scylax of Caryanda made a journey to India, working for the Persian king
Darius, and his now-lost account put the Indian Ocean on the maps of Greek
geographers. The Greeks began to explore the Indian Ocean following the conquests
of Alexander the Great, who ordered a circumnavigation of the Arabian Peninsula in
323 BCE. During the two centuries that followed the reports of the explorers of
Ptolemaic Egypt resulted in the best maps of the region until the Portuguese era
many centuries later. The main interest in the region for the Ptolemies was not
commercial but military; they explored Africa to hunt for war elephants.[75]

The Rub' al Khali desert isolates the southern parts of the Arabic Peninsula and
the Indian Ocean from the Arabic world. This encouraged the development of maritime
trade in the region linking the Red Sea and the Persian Gulf to East Africa and
India. The monsoon (from mawsim, the Arabic word for season), however, was used by
sailors long before being "discovered" by Hippalus in the 1st century. Indian wood
have been found in Sumerian cities, there is evidence of Akkad coastal trade in the
region, and contacts between India and the Red Sea dates back to 2300 B.C. The
archipelagoes of the central Indian Ocean, the Laccadive and Maldive islands, were
probably populated during the 2nd century B.C. from the Indian mainland. They
appear in written history in the account of merchant Sulaiman al-Tajir in the 9th
century but the treacherous reefs of the islands were most likely cursed by the
sailors of Aden long before the islands were even settled.[76]

Greco-Roman trade with ancient India according to the Periplus Maris Erythraei 1st
century CE
Periplus of the Erythraean Sea, an Alexandrian guide to the world beyond the Red
Sea — including Africa and India — from the first century CE, not only gives
insights into trade in the region but also shows that Roman and Greek sailors had
already gained knowledge about the monsoon winds.[72] The contemporaneous
settlement of Madagascar by Austronesian sailors shows that the littoral margins of
the Indian Ocean were being both well-populated and regularly traversed at least by
this time. Albeit the monsoon must have been common knowledge in the Indian Ocean
for centuries.[72]

The Indian Ocean's relatively calmer waters opened the areas bordering it to trade
earlier than the Atlantic or Pacific oceans. The powerful monsoons also meant ships
could easily sail west early in the season, then wait a few months and return
eastwards. This allowed ancient Indonesian peoples to cross the Indian Ocean to
settle in Madagascar around 1 CE.[77]

In the 2nd or 1st century BCE, Eudoxus of Cyzicus was the first Greek to cross the
Indian Ocean. The probably fictitious sailor Hippalus is said to have learnt the
direct route from Arabia to India around this time.[78] During the 1st and 2nd
centuries AD intensive trade relations developed between Roman Egypt and the Tamil
kingdoms of the Cheras, Cholas and Pandyas in Southern India. Like the Indonesian
people above, the western sailors used the monsoon to cross the ocean. The unknown
author of the Periplus of the Erythraean Sea describes this route, as well as the
commodities that were traded along various commercial ports on the coasts of the
Horn of Africa and India circa 1 CE. Among these trading settlements were Mosylon
and Opone on the Red Sea littoral.[9]

Age of Discovery

The economically important Silk Road was blocked from Europe by the Ottoman Empire
in c. 1453 with the fall of the Byzantine Empire. This spurred exploration, and a
new sea route around Africa was found, triggering the Age of Discovery.

Preferred sailing routes across the Indian Ocean

Unlike the Pacific Ocean where the civilization of the Polynesians reached most of
the far-flung islands and atolls and populated them, almost all the islands,
archipelagos and atolls of the Indian Ocean were uninhabited until colonial times.
Although there were numerous ancient civilizations in the coastal states of Asia
and parts of Africa, the Maldives were the only island group in the Central Indian
Ocean region where an ancient civilization flourished.[79] Maldivians, on their
annual trade trip, took their oceangoing trade ships to Sri Lanka rather than
mainland India, which is much closer, because their ships were dependent of the
Indian Monsoon Current.[80]

Arabic missionaries and merchants began to spread Islam along the western shores of
the Indian Ocean from the 8th century, if not earlier. A Swahili stone mosque
dating to the 8th–15th centuries has been found in Shanga, Kenya. Trade across the
Indian Ocean gradually introduced Arabic script and rice as a staple in Eastern
Africa.[81] Muslim merchants traded an estimated 1000 African slaves annually
between 800 and 1700, a number that grew to c. 4000 during the 18th century, and
3700 during the period 1800–1870. Slave trade also occurred in the eastern Indian
Ocean before the Dutch settled there around 1600 but the volume of this trade is
unknown.[82]

From 1405 to 1433 admiral Zheng He said to have led large fleets of the Ming
Dynasty on several treasure voyages through the Indian Ocean, ultimately reaching
the coastal countries of East Africa.[83]

For most of the 16th century, the Portuguese dominated the Indian Ocean trade.
The Portuguese navigator Vasco da Gama rounded the Cape of Good Hope during his
first voyage in 1497 and became the first European to sail to India. The Swahili
people he encountered along the African east coast lived in a series of cities and
had established trade routes to India and to China. Among them, the Portuguese
kidnapped most of their pilots in coastal raids and onboard ships. A few of the
pilots, however, were gifts by local Swahili rulers, including the sailor from
Gujarat, a gift by a Malindi ruler in Kenya, who helped the Portuguese to reach
India. In expeditions after 1500, the Portuguese attacked and colonised cities
along the African coast.[84] European slave trade in the Indian Ocean began when
Portugal established Estado da Índia in the early 16th century. From then until the
1830s, c. 200 slaves were exported from Mozambique annually and similar figures has
been estimated for slaves brought from Asia to the Philippines during the Iberian
Union (1580–1640).[82]

The Ottoman Empire began its expansion into the Indian Ocean in 1517 with the
conquest of Egypt under Sultan Selim I. Although the Ottomans shared the same
religion as the trading communities in the Indian Ocean the region was unexplored
by them. Maps that included the Indian Ocean had been produced by Muslim
geographers centuries before the Ottoman conquests; Muslim scholars, such as Ibn
Battuta in the 14th Century, had visited most parts of the known world;
contemporarily with Vasco da Gama, Arab navigator Ahmad ibn Mājid had compiled a
guide to navigation in the Indian Ocean; the Ottomans, nevertheless, began their
own parallel era of discovery which rivalled the European expansion.[85]

The establishment of the Dutch East India Company in the early 17th century lead to
a quick increase in the volume of the slave trade in the region; there were perhaps
up to 500,000 slaves in various Dutch colonies during the 17th and 18th centuries
in the Indian Ocean. For example, some 4000 African slaves were used to build the
Colombo fortress in Dutch Ceylon. Bali and neighbouring islands supplied regional
networks with c. 100,000–150,000 slaves 1620–1830. Indian and Chinese slave traders
supplied Dutch Indonesia with perhaps 250,000 slaves during the 17th and 18th
centuries.[82]

The East India Company (EIC) was established during the same period and in 1622 one
of its ships carried slaves from the Coromandel Coast to Dutch East Indies. The EIC
mostly traded in African slaves but also some Asian slaves purchased from Indian,
Indonesian and Chinese slave traders. The French established colonies on the
islands of Réunion and Mauritius in 1721; by 1735 some 7,200 slaves populated the
Mascarene Islands, a number which had reached 133,000 in 1807. The British captured
the islands in 1810, however, and because the British had prohibited the slave
trade in 1807 a system of clandestine slave trade developed to bring slaves to
French planters on the islands; in all 336,000–388,000 slaves were exported to the
Mascarene Islands from 1670 until 1848.[82]

In all, European traders exported 567,900–733,200 slaves within the Indian Ocean
between 1500 and 1850 and almost that same amount were exported from the Indian
Ocean to the Americas during the same period. Slave trade in the Indian Ocean was,
nevertheless, very limited compared to c. 12,000,000 slaves exported across the
Atlantic.[82]

Modern era

Malé's population has increased from 20,000 people in 1987 to more than 220,000
people in 2020.
Scientifically, the Indian Ocean remained poorly explored before the International
Indian Ocean Expedition in the early 1960s. However, the Challenger expedition
1872–1876 only reported from south of the polar front. The Valdivia expedition
1898–1899 made deep samples in the Indian Ocean. In the 1930s, the John Murray
Expedition mainly studied shallow-water habitats. The Swedish Deep Sea Expedition
1947–1948 also sampled the Indian Ocean on its global tour and the Danish Galathea
sampled deep-water fauna from Sri Lanka to South Africa on its second expedition
1950–1952. The Soviet research vessel Vityaz also did research in the Indian Ocean.
[1]

The Suez Canal opened in 1869 when the Industrial Revolution dramatically changed
global shipping – the sailing ship declined in importance as did the importance of
European trade in favour of trade in East Asia and Australia.[86] The construction
of the canal introduced many non-indigenous species into the Mediterranean. For
example, the goldband goatfish (Upeneus moluccensis) has replaced the red mullet
(Mullus barbatus); since the 1980s huge swarms of scyphozoan jellyfish (Rhopilema
nomadica) have affected tourism and fisheries along the Levantian coast and clogged
power and desalination plants. Plans announced in 2014 to build a new, much larger
Suez Canal parallel to the 19th-century canal will most likely boost the economy in
the region but also cause ecological damage in a much wider area.[87]

An unnamed Chagossian on Diego Garcia in 1971 shortly before the British expelled
the islanders when the island became a U.S. military base. The man spoke a French-
based creole language and his ancestors were most likely brought to the uninhabited
island as slaves in the 19th century.
Throughout the colonial era, islands such as Mauritius were important shipping
nodes for the Dutch, French, and British. Mauritius, an inhabited island, became
populated by slaves from Africa and indenture labour from India. The end of World
War II marked the end of the colonial era. The British left Mauritius in 1974 and
with 70% of the population of Indian descent, Mauritius became a close ally of
India. In the 1980s, during the Cold War, the South African regime acted to
destabilise several island nations in the Indian Ocean, including the Seychelles,
Comoros, and Madagascar. India intervened in Mauritius to prevent a coup d'état,
backed up by the United States who feared the Soviet Union could gain access to
Port Louis and threaten the U.S. base on Diego Garcia.[88] Iranrud is an unrealised
plan by Iran and the Soviet Union to build a canal between the Caspian Sea and the
Persian Gulf.

Testimonies from the colonial era are stories of African slaves, Indian indentured
labourers, and white settlers. But, while there was a clear racial line between
free men and slaves in the Atlantic World, this delineation is less distinct in the
Indian Ocean — there were Indian slaves and settlers as well as black indentured
labourers. There were also a string of prison camps across the Indian Ocean, from
Robben Island in South Africa to Cellular Jail in the Andamans, in which prisoners,
exiles, POWs, forced labourers, merchants, and people of different faiths were
forcefully united. On the islands of the Indian Ocean, therefore, a trend of
creolisation emerged.[89]

On 26 December 2004 fourteen countries around the Indian Ocean were hit by a wave
of tsunamis caused by the 2004 Indian Ocean earthquake. The waves radiated across
the ocean at speeds exceeding 500 km/h (310 mph), reached up to 20 m (66 ft) in
height, and resulted in an estimated 236,000 deaths.[90]

In the late 2000s, the ocean evolved into a hub of pirate activity. By 2013,
attacks off the Horn region's coast had steadily declined due to active private
security and international navy patrols, especially by the Indian Navy.[91]

Malaysian Airlines Flight 370, a Boeing 777 airliner with 239 persons on board,
disappeared on 8 March 2014 and is alleged to have crashed into the southern Indian
Ocean about 2,500 km (1,600 mi) from the coast of southwest Western Australia.
Despite an extensive search, the whereabouts of the remains of the aircraft is
unknown.[92]

The Sentinelese people of North Sentinel Island, which lies near South Andaman
Island in the Bay of Bengal, have been called by experts the most isolated people
in the world.[93]

The sovereignty of the Chagos Archipelago in the Indian Ocean is disputed between
the United Kingdom and Mauritius.[94] In February 2019, the International Court of
Justice in The Hague issued an advisory opinion stating that the UK must transfer
the Chagos Archipelago to Mauritius.[95]

Trade
Main article: Indian Ocean trade
Major ocean trade routes in the world includes the northern Indian Ocean.
The sea lanes in the Indian Ocean are considered among the most strategically
important in the world with more than 80 percent of the world's seaborne trade in
oil transits through the Indian Ocean and its vital chokepoints, with 40 percent
passing through the Strait of Hormuz, 35 percent through the Strait of Malacca and
8 percent through the Bab el-Mandab Strait.[96]

The Indian Ocean provides major sea routes connecting the Middle East, Africa, and
East Asia with Europe and the Americas. It carries a particularly heavy traffic of
petroleum and petroleum products from the oil fields of the Persian Gulf and
Indonesia. Large reserves of hydrocarbons are being tapped in the offshore areas of
Saudi Arabia, Iran, India, and Western Australia. An estimated 40% of the world's
offshore oil production comes from the Indian Ocean.[3] Beach sands rich in heavy
minerals, and offshore placer deposits are actively exploited by bordering
countries, particularly India, Pakistan, South Africa, Indonesia, Sri Lanka, and
Thailand.

Mombasa Port on Kenya's Indian Ocean coast

In particular, the maritime part of the Silk Road leads through the Indian Ocean on
which a large part of the global container trade is carried out. The Silk Road runs
with its connections from the Chinese coast and its large container ports to the
south via Hanoi to Jakarta, Singapore and Kuala Lumpur through the Strait of
Malacca via the Sri Lankan Colombo opposite the southern tip of India via Malé, the
capital of the Maldives, to the East African Mombasa, from there to Djibouti, then
through the Red Sea over the Suez Canal into the Mediterranean, there via Haifa,
Istanbul and Athens to the Upper Adriatic to the northern Italian junction of
Trieste with its international free port and its rail connections to Central and
Eastern Europe.[97][98][99][100]

The Silk Road has become internationally important again on the one hand through
European integration, the end of the Cold War and free world trade and on the other
hand through Chinese initiatives. Chinese companies have made investments in
several Indian Ocean ports, including Gwadar, Hambantota, Colombo and Sonadia. This
has sparked a debate about the strategic implications of these investments.[101]
There are also Chinese investments and related efforts to intensify trade in East
Africa and in European ports such as Piraeus and Trieste.[102][103][104]

See also