The Indian Ocean

Size and Shape

Although the Indian Oceans is much smaller in size 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 is 73, 442, 700 sq km. In a way, it is just half an ocean because it does not open out northwards in the Arctic Ocean. It is bounded by South Asia in 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.


Continental shelf in this ocean is much less as compared to Atlantic Ocean. Its average depth is 4,000 metres and its 60% part is 4,000 to 6,000 metres deep sea plain. The door of the Indi an Ocean has fewer irregularities in comparison to the other two oceans. Linear deeps are almost absent. The only exception is the Sunda Trench, which lies south of the island of Java and runs parallel to it.


There are a number of broad submarine ridges on the floor of the Indian Ocean. The most important submarine ridge runs from Kanyakumari continuously southward to Antarctica. It is situated in the middle and divides the ocean into two basins on either side. It is called the Lakshdweep–Chagos Ridge in the north, the St. Paul Ridge in the middle and the Amsterdam St. Paul Plateau in the south, where it widens out considerably.

The central ridge is bifurcated into many small ridges which reach the coast of Africa and India. Two minor and parallel ridges run north-westward. These are known as the Socotra-Chagos Ridge and the Seychelles Ridge.

Another ridge, known as the South Madagascar Ridge, runs southward from the Madagascar island. It widens in the south, where it is called the Prince Edward Crozet Ridge. In the Bay of Bengal, another ridge called the Andaman Nicobar extends from the mouth of the Ayeyarwadi (Irrawaddy) to the Nicobar Islands. Beyond this towards the south, a very important ridge runs along the 90°E longitude and is known as Ninety East Ridge. The Carlsberg Ridge has been discovered by recent surveys and it divides the Arabian Sea into two parts.


The central and other ridges divide the Indian Ocean into many basins. Chief among are the Central or Mid Indian Basin, Arabian Basin, South Indian Basin, Mascarene Basin, West Australian, and South Australian Basins.


Most of the islands near the coast of the Indian Ocean are detached parts of the continental blocks. e Andaman and Nicobar, Sri Lanka, Madagascar and Zanzibar are important examples of this group. The Lakshdweep and Maldive Islands are coral islands. The Mauritius and Reunion islands to the east of Madagascar are of volcanic origin. The eastern section of the Indian Ocean is almost devoid of islands.

Marginal Seas

There are only two marginal seas in the form of Red Sea and Persian Gulf. The Arabian Sea and Bay of Bengal are also treated as seas but they are just northern extensions of the Indian Ocean.


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