According to the nature and mode of occurrence, Darwin divided reefs into three main forms:

(i) The fringing reef,

(ii) The barrier reef, and

(iii) The atolls.

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The fringing reef:

Fringing reefs are formed on the submarine platforms attached to the shores of the continents. However, there is no problem concerning the origin of such reefs. They simply grow towards the sea from the land. Between the fringing reef and the sea coast there is usually a shallow lagoon.

The surface of a fringing reef is rough and uneven above the level of low water. Its growth is more outwards than upwards with the result that its outer rim reaches the sea surface much earlier, and when it so happens a shallow lagoon is brought into existence between the reef and the shore.

The outer rim of this type of reef seldom goes beyond a depth of 30 fathoms. Its outer edge slopes downwards into the sea. The lagoon is filled with water even at low tide. The growth of the fringing reef is continuous and uniform unless broken by large rivers coming from the main lands.

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The fringing reefs grow at a fast rate in those areas where there is a continuous supply of food for the coral forming insects by the sea waves with oxygenated water. In such areas the width of the fringing reefs goes on increasing towards the seas.

On the contrary, on such coasts where the inflowing rivers form deltas on their mouths, the sea water is usually turbid and full of sediments, and the sea water is freshened by the river water. This inhibits the growth of a fringing reef.

Remember that in the shallow and protected lagoons the growth of lime secreting corals is rather limited and at a very slow rate. The main reason appears to be the dearth of sufficient food for the coral polyps in these lagoons where the supply of food is not replenished by sea waves.

On the sea side of the reef some detritus is broken off and forms a fore-reef talus. The finer detritus is swept into the sheltered region behind the reef. More delicate corals grow in the lee of the seaward surf, adding more detritus sediment.

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On the rough and uneven surface of the fringing reefs, sea waves deposit the broken pieces of corals in the form of a boulder zone, which is known as the reef flat. Most reef flats are dotted with large blocks of coral rock.

On the reef flat, islands of coralline sand and gravel may build up to such heights as to support a fresh ground water lens and vegetation. The tropical and subtropical western margins of the oceans, which are generally warmer than the eastern sides, are the main areas of vegetation of reef forming corals. The reefs can grow at a rate of 1-2 cm. per year.

Another interesting thing about the fringing reefs is that many of them have grown on the inner side of relatively shallow lagoons enclosed by barrier reefs.

Barrier reefs:

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A barrier reef lies at some distance from the shore and is separated from it by a lagoon usually too deep to permit coral growth. The lagoon may vary in width from a narrow channel to many kilometers. The Great Barrier Reef off the coast of northeast Australia extends for about 1600 km with only minor breaks in it.

The barrier reefs invariably rise from deep water. The lagoons associated with barrier reefs are usually wide and deep. This type of reef is the greatest and most extensive of all the reefs. The outer slope of a barrier reef is about 45°.

It is noteworthy that the Great Barrier Reef is the largest in the world. However, there are many smaller exam­ples which often encircle an island.

They are broken only by narrow and often dangerous channels through which shipping can pass. These narrow gaps occur at intervals in barrier reefs. Through these openings, excess water from breaking waves is returned from the lagoon to the open sea.

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The barrier reefs are composed of numerous reefs of different sizes spread over a submarine platform. In fact the Great Barrier Reefs comprise a large number of individual reefs some of which have islands enclosed within them.

Throughout the length, the distance of these reefs from the mainland of Australia varies from 12 km in the south to 128 km in the extreme north. The latitudinal extent of the Great Barrier Reefs is from lat. 9°S to lat. 22°S.

The average depth of the shallow and broad lagoon between these reefs and the sea coast of Queen-land is 40 fathoms. The maxi­mum width of this reef is 16 km, but at certain places its width reaches 144 km. However, this huge wall of corals is hardly visible except at low tides. As stated earlier, the continuity of this reef is broken at many places.

Atoll:

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When a coral reef resembles a ring or a horseshoe and encloses within it a lagoon, it is called an atoll. Sometimes there is an island within this lagoon. There are certain openings in this circular reef through which the enclosed lagoon is connected with the open ocean.

Since the atolls and barrier reefs seem to rise from considerable depths, certainly from far below the level at which coral can grow, various theories have been propounded to explain their formation. However, the problem of their formation still remains unsolved.

The various hypotheses and ideas put forward may be placed into four categories: The Darwin – Dana hypothesis of subsidence; secondly, the non subsidence hypo-theses put forward by Murray, Semper, Aga-ssiz etc.; thirdly, the glacial control hypothesis advanced by Daly; and finally Davis application of the physiography of the shores around which the reefs are built.