Bottom Reliefs of the Indian Ocean

• The Indian Ocean is smaller than the Pacific and Atlantic Ocean in areal extent and is bounded by, on all of its sides, Asia in the north, Africa in the west, Asia in the east, Australia in the south-east and Antarctica in the south.
• The ocean has contact with the Pacific and the Atlantic oceans in the south near Antarctica.
• The aver­age depth of the ocean is 4000m.
• Major parts of the coastal lands of the Indian Ocean formed by the block mountains of Gondwanaland are compact and solid.
• The coasts of the East Indes are bordered by fold mountain chains.
• The marginal seas are less in number than the Pacific and the Atlantic oceans.
• Significant marginal seas are Mozambique Channel, Red Sea, Per­sian Gulf, Andman Sea, Arabian Sea, Bay of Bengal etc.
• Malgasy (Madagascar) and Sri Lanka are the big islands whereas Suqutra, Zanzibar, Comoro, Reunion, Secychelles, Prince Edwards, Crozet, Kerguelen, St. Paul, Rodriges, Maldive, Laccadive, Andman-Nicobar, Christmas etc. belong to the category of small and tiny islands.
• Indian subcontinent in the north divides the Indian Ocean into Arabian Sea and Bay of Bengal.
• The ocean widens in the south

The Indian Ocean is divided into  3 zones on the basis of regional characteristics:

(1) The Western Zone between African coast and the mid-Indian Oceanic Ridge has large number of islands and the average depth is 3650 m (2000 fathoms).

(2) The Eastern Zone is deepest of all the zones with average depth of 550 m (3000 fathoms). The continental shelves are narrow but have steep slopes.

(3) The Central Zone represents the mid-oceanic ridge where many tiny islands are located.

Continental Shelf:

• There is wide range of variation in the continen­tal shelves of the Indian Ocean.
• Quite extensive shelves are found along the margins of Arabian Sea and Bay of Bengal.
• Similarly, extensive shelves are observed along the eastern coast of Africa and around Madagascar which is itself located on the continental shelves.
• On an average, the continental shelves are very wide (640 km) in the west whereas these are narrow (160 km) along the coast of Java and Sumatra.
• These become further narrow along the northern coast of Antarctica.

Mid-Oceanic Ridge:

• The central ridge or mid-oceanic ridge known as Mid-Indian Oceanic Ridge 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 Peninsisula with average width of 320 km.
• This part of the ridge is known as Laccadive-Chagos Rigde (also known as Maldive 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:

(1) Socotra-Chagos Ridge also known as Carlesbreg Ridge emerges from the central ridge at 5°S latitude and extends in north­westerly direction upto Gardafuli Peninsula of N. E. Africa,

(2) 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 upto Seychelles and Amirante islands.

(3) Madagascar Ridge stretches from the southern tip of Madagascar (Malagasy) to 40°S latitude. Its further southward extension is known as Prince Edward – Crozet Ridge between 40°S -48°S latitudes.

(4) The south-western branch near 23°S latitude is known as S.W. Indian Ridge.

(5) Ninety East Ridge extends from the continental shelf off the Irrawadi river mouth and runs in almost north-south direction parallel to 90°E longitude upto 40°S where it merges with Amsterdam- St Paul Plateau.

Ocean Basins:

The mid-Indian Oceanic Ridge divides the In­dian Ocean into two major basins-the eastern and the western basins. These basins are further divided into sub-baisns by the branches of the central ridge .

(1) Oman basin faces the Gulf of Oman and is spread over the extensive continental shelf with aver­age depth of 3,658 m.

(2) Arabian basin is located in almost circular shape between Laccadive-Chagos ridge and Socotra – Chagos Ridge with the depth of 3,600m – 5,486m.

(3) Somali basin is bordered by Socotra – Chagos ridge in the north-west. Central Ridge in the east, Seychelles – Mauritius Ridge in the south-west and African coast in the west. The average depth is 3,600m.

(4) Mauritius basin is located between S.W. Indian Ridge and South Madagascar Ridge and ex­tends from 20°S to 40°S latitude. The depth varies between 3,600m and 5,486 m. The deepest part measures 6,391 m depth.

(5) Mascarene basin of oval shape extends between Madagascar and Seychelles – Mauri­tius Ridge.

(6) Agulhas-Natal basin is an elongated basin which is bordered by Madagascar ridge in the north and north-east, Prince Edward Crozet Ridge in the east and the S.E. African coast in the west and north-west, Average depth is 3,600m.

(7) Atlantic- Indian – Antarctic basin is in fact the eastward continu­ation of Atlantic – Antarctic Basin. It stretches upto 70°E longitude and is bordered by Prince Edward Crozet Ridge in the north, Antarctica in the south and Kerguelen Gassberg Ridge in the north-east. Average depth is 3,600m.

(8) Eastern Indian-Antarctic basin is located between Amsterdam – St. Paul Plateau and Indian-Antarctic Ridge in the north and north-east and Antarctica in the south. The depth varies from 3,600m to 4,800m. Kerguelen – Gassberg Ridge separates the basin from the Atlantic – Indian-Antarctic Basin.

(9) West Australian basin is the most extensive basin and forms rectangular shape surrounded by S.E. Indian Ridge in the south – west, Ninety East Ridge in the west, continental shelves of Java-Sumatra in the north­east and the continental shelf of west Australia, Aver­age depth varies from 3,600m to 6,100m but the central part of the basin is 6,459 m deep.

(10) Mid-Indian basin is bordered by the central ridge in the west and the south-west, by Ninety East Ridge in the east and by the Bengal plateau in the north. The average depth of outer part ranges from 3,600m to 6,800m while the depth of the central part of the basin ranges between 4,800m and 6,100m.

Deeps and Trenches:

• There are very few deeps and trenches in the Indian Ocean. About 60 per cent of the Ocean consists of deep sea plains with depth ranging from 3,600m to 5,487m.
• Important deep sea plains are Somali Abyssal plain.
• Ceylon (Sri Lanka) Abyssal plain, Indian Abyssal Plain, (4,380m) etc.
• Significant trenches are Java or Sunda Trench (7,450m deep), Ob Trench (6,875m deep), Mauritius Trench, Amirante Trench etc.