2.1 Formation by increasing sea level
The continental shelves were formed either by an increase in the sea-level or the subsidence of the continental margins. 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.However, there are certain objections to this view.As some authors put it, "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."
2.2 Formation by River action
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 (fig. 3). 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 discolored 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.
Figure 3: formation of continental shelf by the action of Rivers (George et al., 2010)
2.3 Formation by faulting
There is another school of oceanographers which holds the view that because of faulting elongated troughs are formed along the coasts (fig. 4).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.
2.4 Trapping of sediments behind Natural Dams
There may be a continental shelf formed of coral reef (fig. 5). 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.A coral reef growing parallel to the coastline and separated from it by a lagoon is called a barrier reef. The lagoon may develop between the fringing reef and the land. As the reef continues to grow further and further offshore it eventually reaches the edge of the continental shelf. Barrier reefs can also originate offshore if the depth of the seabed out there is shallow enough to allow corals to grow. The most famous barrier reef is the Great Barrier Reef in Australia. It stretches over 2300 km and covers over 200'000 km2. It lies between 24 and 240 km from the main continent.
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.