The Oceans : size, shape, and basins

The Earth’s radius is approximately 6300 km. About 71% of the Earth’s surface is covered by oceans. The average depth of the oceans is 3700 m, and they contain 97% of the Earth’s water (Fig. 1). The remaining 3% is freshwater: of this, one-third is in liquid form, and the rest is frozen in glaciers and polar icecaps. The oceans form a rather unconventional tank of water: its horizontal dimension stretches to tens of thousands of kilometres, but its vertical dimension is generally less than 4 km. If we were to make an exact replica of the oceans on an Earth of one metre diameter, the depth of the oceans on it would be less than a millimetre.

The oceans have been conveniently divided into the Indian, Atlantic, Pacific, Arctic, and Southern (Antarctic) Oceans (Fig. 2). The Indian Ocean extends between the continents of Africa (to the west) and Australia (to the east), and south Asia (to the north) and the Southern Ocean (to the south). The North Indian Ocean includes the Arabian Sea (to the west of India) and the Bay of Bengal (to the east). Countries with coastline have a fixed area earmarked exclusively for exploration and exploitation. This area is called its Exclusive Economic Zone (EEZ) (Fig. 3). Islands help increase a country’s EEZ area. The Lakshadweep are a group of 36 low-lying coral islands, 10 of which are inhabited. At their highest point they are less than a few metres high. Another group of islands, the Andaman and Nicobar Archipelago in the Bay of Bengal, comprises 554 islands, some of which are merely large rocks. If these are excluded, the total number of islands is 294, of which 36 are inhabited. The Andaman and Nicobar islands owe their existence to plate tectonic processes. These islands were formed as a result of geological processes associated with the destruction of the ocean floor.

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