Published on: 11/28/2020 12:48:53 AM

Although the Indian Ocean is much smaller than the Pacific and the Atlantic Oceans, yet it is of immense importance for us because it is located in the south of India. This is the only ocean in the world to be named after the name of a country, i.e., Indian Ocean after India. Its total area excluding marginal seas is 68,556,000 sq. km which increases to 73,425,500 sq. km when marginal seas are also included. In a way, it is just half an ocean because it does to open out northwards in the Arctic Ocean. It is bounded by South Asian the north, Indonesian islands and Australia in the East and by Africa in the west.

In the south, it extends to the Antarctic continent from where it merges with the Atlantic and the Pacific. Tropic of Cancer forms its northernmost limit. The average depth of ocean is 4,000 m which is much less than that of the other two major ocean, i.e., the Pacific Ocean and the Atlantic Ocean. Also, the regional variations in-depth are less than that of the other oceans.

Major parts of the coast are made up of hard block mountains of Gondwanaland and are compact and solid. But coasts of the East Indies are bordered by fold mountain chains.

Distinct zones on the basis of the regional characteristics

Johnson divided the Indian Ocean into three distinct zones on the basis of the regional characteristics.

  • The Western zone between the African coast and the Mid Indian Oceanic Ridge with average depth of 3,650 m having a few islands e.g. Seychelles and Madagascar.
  • The Eastern Zone is the deepest of all zones where average depth is 5,500m and some areas are over 6,000 m deep. Here the continental shelf is narrow but has steep slopes and areas deeper than 7,000 m are found along East Indies.
  • The Central Zone represents the mid-oceanic ridge where depth is less than 4,000 m. Several tiny islands are located in this zone.


Continental Shelf

Width of the continental shelf of the Indian Ocean varies widely from one area to another. The shelf is quite wide and extensive along the margins of Arabian Sea and Bay of Bengal where its width is about 650 km. It is moderately wide along the eastern coast of Africa but widens considerably around the Madagascar island which itself stands on a deep shelf projected from the African continent. It is very narrow along the coasts of Java and Sumatra (Indonesia) and west coast of Australia. It narrows down to about 160 km in these areas. It becomes further narrow along the northern coast of Australia.

Mid Ocean Ridge and Other Ridges

The central ridge or mid-oceanic ridge known as Mid-Indian Oceanic Ridge figure extends from the southern tip of Indian Peninsula in the north to Antarctica in the south almost in north-south direction and forms a continuous chain of highlands. Wherever the central ridge or its branches emerge above the sea level, islands are formed.

The main central ridge starts from the continental shelf of the southern tip of Indian Peninsula with average width of 320 km. This part of the ridge is known as Laccadive-Chagos Ridge (also known as Maldives Ridge).

The ridge further extends southward and widens near equator. It is called Chagos-St. Paul Ridge between equator and 30° S latitude where the average width becomes 320 km. The ridge further widens to 1,600 km between 30° S and 50° S latitudes and is known as Amsterdam-St Paul Plateau. The central ridge bifurcates to the south of 50° S latitude. The western branch known as Kerguelen- Gaussberg ridge extends in NW-SE direction between 48° S and 63° S and the eastern branch is known as Indian-Antarctic Ridge.

Branches of the Central Ridge

  • Socotra-Chagos Ridge also known as Carlesbreg Ridge emerges from the central ridge at 5° S latitude and extends in north-westerly direction up to Gardafuli Peninsula of N.E. Africa.
  • Seychelles-Mauritius ridge bifurcates from the main ridge around 18° S latitude near Mauritius Island and runs in roughly north-west direction in arcuate shape up to Seychelles and Amirante islands.
  • Madagascar Ridge stretches from the southern tip of Madagascar (Malagasy) to 40° S-48° S latitudes.
  • The south-western branch near 23° S latitude is known as S. W. Indian Ridge.
  • Ninety East Ridge extends from the continental shelf off the Irrawaddy river mouth and runs in almost north-south direction parallel to 90° E longitude up to 40° S where it merges with Amsterdam-St Paul Plateau.

Ocean Basins

The above-mentioned ridges have divided this ocean into several ocean basins which are 4,000-6,000 m deep. Some of the important basins are briefly described as under:

  • Oman Basin: It is found near the Gulf of Oman. This 4,000 m deep basin spreads over extensive continental shelf of Asia.
  • Arabian Basin: This circular shaped depression is situated between Laccadive-Chagos ridge in the east and Socotra-Chago ridge in the west. It extends up to 5°S latitude in the form of a south-ward narrow extension of the basin along Socotra Chago ridge where the depth is 2,000-4,000 m.
  • Somali Basin: Surrounded by Socotra-Chagos Ridge in the north-east Central Ridge in the east and by Seychelles-Mauritius Ridge in the south-west, this basin lies off the coast of Somali Republic in East Africa. It is more than 4,000 m deep and is connected with Mauritius basin in the south. It is much deeper along the African coast where its depth varies from 5,000 m to 6,000 m.
  • Mauritius Basin: It is also known as South-Eastern Madagascar Basin as it lies to the south-east of Madagascar Island. This longitudinal basin lies between Madagascar Ridge in the west and Mid Indian Rise in the east and extends from 20°S to 45°S latitudes. This basin has uniform depth of 4,000 m with two areas having depth of about 6,000 m. The deepest part measures 6,391 m depth.
  • Natal Basin: It lies to the south-west of Madagascar Island and is bounded by South Madagascar Ridge in north-east. To its west is the coast of South Africa. This basin is more than 4,000 m deep.
  • Agulhas Basin: It is an extension of Agulhas Basin in the Atlantic Ocean and is separated from the Natal Basin by Cape Rise in the Indian Ocean. Its eastern boundary is demarcated by Prince Edward Crozet Ridge. Most parts of this basin are about 6,000 m deep.
  • Atlantic-Indian-Antarctic Basin: Lile Agulhas Basin, this basin is also an extension of the Atlantic Ocean and lies to the north of Antarctic continent. It stretches up to 70°E longitude and is bordered by Prince Edward Crozet Ridge in the north. Its average depth is 3,600 m although some areas are more than 5,000 m deep.
  • Andaman Basin: It lies to the east of the Andaman Rise in the Bay of Bengal. It is small in size and is a shallow basin of 2,000 m depth only.
  • Cocos Keeling Basin: Also known as the Indo-Australian Basin or West Australian Basin, this is the largest and the most extensive basin of Indian Ocean. This more or less rectangular basin is surrounded by S.E. Indian Ridge in the South-west, Ninety Degree East Ridge in the west, Sunda Trench in the northeast. This vast basin extends from 10°N to 5°S latitudes. Its average depth varies from 3,600 m to 6,100 m but its central part is 6,459 m deep. It is comparatively deeper in areas near Sunda Trench and west of Australia between 26° and 40°S latitudes.
  • Eastern Indian-Antarctic Basin: This basin is bounded by Amsterdam-St. Paul Plateau and Indian Antarctic Ridge in the north and north-east and Antarctic continental shelf in the south. Keruguelen-Gaussberg Ridge in the west separates this basin from the Atlantic-Indian-Antarctic Basin. The major part of this basin lies between 50° and 60°S latitudes. The depth of this basin varies from 3,600 m to 4,800 m.

Deeps and Trenches

About 58.8 per cent of the total area of this ocean is covered by deep sea plains with depth ranging from 4,000 m to 6,000 m and very few trenches are found in this ocean. The major deep-sea plains are Somali Abyssal Plain, Sri Lanka Abyssal Plain, and Indian Abyssal Plain, etc. The only trenches worth mentioning are Java or Sunda Trench (7,450 m deep), Ob Trench (6,875 m deep, Mauritius Tench, Amivante Trench etc.


Islands of the Indian ocean are of varied types and they owe their origin to different geological and geographical reasons. Malagasy (Madagascar) and Sri Lanka are big islands whereas Socotra, Zanzibar and Comoro, Reunion, Seychelles, Prince Edwards, Crozet, Kerguelen, St. Paul, Rodriges, Maldives, Laccadive, Andaman-Nicobar islands have been formed by submergence of folded mountains; Laccadives and Maldives are located on submarine ridge, and other small islands are rising from plateau such as Kerguelen and the volcanic cones such as Mauritius and Re-union Islands.

Marginal Seas

This ocean does not have as many marginal seas as are possessed by other two big oceans viz. The Pacific Ocean and the Atlantic Ocean. The lack of marginal seas is due to the nature of its coasts which have been formed by old plateaus which were once parts of a huge continent called Gondwanaland. Some of the important seas are Mozambique channel, Red Sea, Persian Gulf, Andaman Sea, Arabian Sea and the Bay of Bengal.

Figure : Bottom reliefs of the Indian Ocean A-Socotra-Chagos Ridge, B-Chagos Ridge, C-Seychelles Ridge, D-Chagos-St. Paul Ridge, E-Amsterdam-St Paul Ridge, F-Indian-Antarctic Ridge, G-Kerguelen- Gassberg Ridge, H-Madagascar Ridge and I-Atlantic-Antarctic Ridge, 1. Oman basin, 2. Arabian basin, 3 Somali basin, 4. Mauritius basin, 5. Natal basin, 6, Atlantic-Indian-Antarctic Basin, 7. Andaman Basin, 8. Indian-Australia basin and 9. Antarctic basin.