There is no clear or well-defined line separating oceans from continents.
In fact, continents do not end abruptly at shoreline.
They slope seaward from the coast to a point where the slope becomes very steep.
The shallow submerged extension of continent is called the continental shelf.
The depth of this shallow sea water over the continental shelf ranges between 120 to 370 metres.
The width of the continental shelf varies greatly ranging between a few kilometres to more than 100 kilometres.
This variation can be seen even in the context of Indian peninsula.
The continental shelf off the eastern coast of India is much wider than that of the western coast. Similar variations are seen all over the world.
Off the coast of West Europe, it extends to 320 kilometres from the Cape of Land’s End.
Off the coast of Florida the shelf is 240 kilometres wide.
They are much narrower or absent in some continents, particularly where Fold Mountains run parallel or close to the coast as along the eastern Pacific Ocean.
Most of the continental shelves represent land which has been inundated by a rise in sea level.
Many regard their formation due to the erosional work of waves or due to the extension of land by the deposition of river borne material on the off-shore terraces.
Off the coast regions which were once covered by ice sheets, they may have developed due to glacial deposits.
The continental shelves are of great importance to man.
The shallow water over the shelf enables sunlight to penetrate through the water to the bottom and encourages growth of microscopic plants and animals called planktons.
These planktons are the food for fishes.
Continental shelves are the source of fishes, mineral including sand and gravel.
A large quantity of the world’s petroleum and natural gas is obtained from these shelves.
The Bombay High and the recent discovery of petroleum in the Godavari basin are examples of on shore drilling on the continental shelf.
Coral reefs and lipoclastic materials are also common on continental shelves.
One of the features of the continental shelf is the presence of submarine canyons which extend to the continental slope.
These canyons are ‘steepsided valleys’ cut into the floor of the seas.
They are very similar to the gorges found on the continents. Godavari Canyon in front of the Godavari river mouth is 502 metres deep.
One of the reasons for the formation of submarine canyon is underwater landslide.
The sediments collected on the continental shelves get dislodged by a storm or a earthquake.
The force of these moving sediments erode the slopes as they come down and as a result submarine canyons are carved out.
The continental shelf is generally considered to be territorial water extent of the nations to which it adjoins.