The continental margins which have been flooded by the oceans are called the continental shelf. It is geologically part of the continent. This part of the sea floor has a very gentle slope. “The continental shelf may be defined as a shelf like zone extending from the shore beneath the ocean surface to a point at which a marked increase in slope occurs.”

It extends to a depth of 100 fathoms. Actually on the seaward margin of the continental shelf the depth of the sea is not uniform. According to Shepard, the average depth of water on the margin of the shelf is 72 fathoms, even though in certain areas it is as much as 250 fathoms.

The width of continental shelves varies in different areas. In certain areas the shelf is practically nonexistent, and in other areas its width may exceed 1300 km. The broadest continental shelf is found in the North and West Pacific Oceans from Alaska to Australia.

Another area where the broadest shelf occurs is in the Arctic Ocean off the northern coasts of Siberia and North America. However, the average width of the continental shelf is about 70 km, and the average depth at which the continental break occurs is about 135 meters.

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As an exception, the continental break occurs at a depth of 350 meters around the Antarctica. The continental break is the point where an increase in the rate of slope occurs. The steeper slope beyond this break is called the continental slope.

The average slope of the continental shelves varies from 0.2% to 1°. But depending on the nature of the coastline and the variation in the form of the adjoining continent the slope of the shelf may also vary.

In those areas, for example, where the mountains or plateaus flank the sea coast, the slope of the continental shelf is relatively steeper. As per calculations of Kossina, the average slope of the continental shelves is about 2 fathoms per mile, even though the shelves have much steeper slopes towards the sea.

But it is true that the slope of the shelf is never gradual, because in certain areas there are many submarine terraces, hillocks and various types of depressions on the continental shelf.

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On certain continental shelves there are submarine canyons and deep trenches as well. Glaciated coastal regions are characterized by great variations in the bottom relief of the shelf.

Thus, the topography of the continental shelf is not as smooth as we might expect. This is so because of the effects of constant wave abrasion, and the settling of sediments brought from the adjacent land.

During the Pleistocene the sea level was lowered, as much water in the seas was deposited on land as the glacial ice. “The continental shelves in that period must have been exposed to the atmosphere, and were grazed by animals. The teeth of these Pleistocene animals are occasionally dredged up by fishermen.”

The width of the continental shelf is marked by great variations. Where the plains over the continents extend to the sea coast, the shelf zone is usually very wide. On the contrary, in mountainous coastal regions, the width narrows down.

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Thus, it is clear that the width of the shelves is largely controlled by the topography of the coastal region. The average width of the continental shelves is 48 km. But actually the width may vary from 0 to 1280 km.

For example, there is a great difference in the width of the shelves on the eastern and western coasts of the United States of America. In the coastal regions of the Atlantic Ocean and the Hudson Bay, the width of the shelf is up to 480 km, while along the Pacific coast, because of the mountain chains reaching up to the coast, the width of the shelf is negligible.

Near the coast of Ireland the width of the shelf is 80 km or even more than that. It may be stated that in the broader continental shelves the slope is very gentle. On the west coast of Ireland, for example, the slope of the shelf is less than 1°.

However, the slope of the shelf is nowhere more than 2 or 3°. Whereas the width of the shelf along the west coast of South America seldom exceeds 16 km, the width of the continental shelves in the East Indies is many hundred kilometers.

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The continental shelves occupy 7.6% of the total area of the oceans and seas. In the Atlantic Ocean the continental shelf occupies 13.3% of its total area, but in the Pacific and Indian Oceans the percentage is 5.7 and 4.2 respectively.

According to Wegener, the total area of the continental shelves is 30.6 km2, whereas according to Kegel, their total area is only 29.5 km2.

The continental shelf area is one of the most useful parts of the ocean to man. The water on the continental shelf is shallow so that light penetrates to the sea floor.

Because of the availability of light and nutrients, plants and animals grow and thrive in abundance in these regions. Naturally, therefore, the most abundant supplies of fish can be found in the continental shelf areas.

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Origin of Continental shelf:

There is divergence of opinion regarding the origin of the continental shelf. However, it is undoubtedly true that different processes have been responsible for the formation of the shelf in different parts of the world. A brief discussion of various hypotheses regarding the origin of shelf is presented here.

1. Some of the oceanographers are of the opinion that the continental platforms extend up to the seaward margin of the continental shelf. According to them, in the distant past the sea water extended up to the upper part of the continental slopes.

Later on, the sea-level was raised and the sea water inundated the rim of the continents. In support of this hypothesis, the evidence of such river valleys are produced which extend up to the continental slope beneath the sea water on the shelf.

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It is, therefore, obvious that, according to this hypothesis, the continental shelves were formed either by an increase in the sea-level or the subsidence of the continental margins. However, there are certain objections to this view.

As P. Lake has put is, “since the shelf is almost continuous around the shores of the Atlantic, the movement must have affected half the globe, and it must have been remarkably uniform in amount.

Unless there has been an increase in the quantity of water in the ocean, it is difficult to understand how a change of level so widespread and regular could be brought about.”

2. A shelf may be formed by the action of the rivers. Rivers erode the land and the eroded material is brought down by the rivers as their load, which is ultimately laid down beneath the sea water. Remember that the deposition occurs near the continental margin.

Since this type of deposition is a continuous process, and, in due course of time, under the pressure of the deposited material the continental shelf is depressed. Gradually a huge amount of sediments is deposited giving rise to a new land adjoining the sea shore.

Such a type of shelf is called the constructive continental shelf. This type of a shelf is relatively broader in extent. Most of the large rivers play dominant role in this regard. For example, the Amazon River deposits such a huge amount of sediments in the ocean that the sea water is discoloured up to a distance of about 480 kilometers.

All the big rivers deposit a part of their load near the shore. Thus, the eroded material from the continental platform accumulates as a submarine terrace upon the continental rim. However, the terrace thus created gradually grows seawards by addition of more and more of the eroded material.

There are some such types of continental shelves which are the creation of the erosive action of the waves and currents. If there is stability in the levels of the sea as well as the land, the margin of the land will undergo continuous erosion so that a submarine platform will come into existence backed by a cliff.

Again on this platform the waves and the rivers will continue to deposit sediments. After sometime when the accumulated debris becomes compact and hard, the continental shelf comes into existence. This type of shelf is generally narrow. Such continental shelves are found around Iceland and Faroe Islands.

3. Shepard, F.P. holds quite a different view regarding the origin of continental shelf. In his opinion, a continental shelf is created by the combined action of erosion and deposition. This type of shelf is relatively broader.

Some of the continental shelves in East Asia were formed in this way. In the distant past these shelves were surrounded by island areas which, after being eroded, got submerged under the sea water. These are called the depositional shelves. Hard rocks are found along their seaward margins.

Thus, the enclosed basins offer ideal conditions for the deposition of sediments, and the shelf created in this manner would be one of deposition, and it will be flanked by a rim of solid rocks.

4. In the high latitude regions, glaciation has played an important role in the formation of continental shelf. Such continental shelves are seen on coasts which were glaciated in the remote past.

These shelves are invariably deeper towards their seaward margins. The main cause of the greater width of the continental shelves of the glaciated coastal regions is the glaciation of the continental margins.

5. There is another school of oceanographers which holds the view that because of faulting elongated troughs are formed along the coasts. Typical examples are found in the coastal regions of the Red Sea and Queensland. Where the continental margins collapse due to faulting, a continental shelf is created.

6. Delta growth is also an important factor in the formation of the continental shelf. However, this is found in certain localities only. Example may be cited of the Mississippi delta where there is a huge accumulation of debris. Moreover, this delta, as it grows at a fast rate, helps in the formation of a shelf, though on a local scale.

7. Even the convectional currents are supposed to be one of the factors that help in the formation of continental shelf. Wherever because of the unequal distribution of sial and sima near sea coasts the convectional currents arise, there is subsidence of the land along the coast.

The direction of movement is from sima beneath the oceans, towards the sial, the base of the continents. Eroded material derived from the land is deposited in the subsided submarine depression just for compensation. It results in the gradual uplift of the continents and subsidence of the continental shelf.

8. According to Daly, the origin and growth of the continental shelves in certain areas of the oceans is attributed to the falling of the sea-level by about 38 fathoms during the Pleistocene period.

Due to the lowering of the sea-level, the submerged parts of the coastal regions became dry land areas. The processes of glacial erosion and deposition were operative on these newly emerged land areas.

This resulted in the creation of various types of land forms. With the subsequent rising of the sea-level at the end of the Pleistocene period, these land areas adjoining the sea coasts were again submerged under the sea – water.

This type of shelf is found along the Norwegian coast. The Norwegian shelf is characterized by the presence of some of the glaciated topographical features. The continental shelf of this nature is usually very wide.

9. There may be a continental shelf formed of coral reef. The major characteristic of this type of shelf is the very shallow water at the edge of the shelf. On the other hand, the offshore slope is generally very steep. The depth at the edge of this type of shelf is about 20 meters.

However, all these views regarding the origin of continental shelf are open to criticism. But it is undoubtedly true that continental shelves in different areas were formed by different processes. Thus, it is unlikely that any one explanation accounts for all the continental shelves of the world.