The following points highlight the three main types of coral reefs. The types are: 1. Fringing Reef 2. Barrier Reef 3. Atoll.
Type # 1. Fringing Reef:
Coral reefs developed along the continental margins or along the islands are called fringing reefs (fig. 30.1). The seaward slope is steep and vertical while the landward slope is gentle. The upper surface is uneven and corrugated.
Though fringing reefs are usually attached to the coastal land but sometimes there is gap between them and land and thus lagoon is formed between the fringing reef and the land. Such lagoon is called boat channel. Coral reefs are generally long but narrow in width.
The continuity of coral reefs is broken wherever rivers drain into the seas and oceans.
Coral reefs are basically of two types e.g.:
(i) Coral reefs facing open ocean, and
(ii) Coral reefs protected by a barrier.
Such fringing reefs are found along Sakau island, southern Florida, Mehetia island (of Society Group of Islands) etc.
Type # 2. Barrier Reef:
The largest coral reefs off the coastal platforms but parallel to them are called barrier reefs (fig. 30.2). Barrier reefs are the largest, most extensive, highest and widest reefs of all types of coral reefs. The average slope is about 45° but some barrier reefs are characterized by 15°-25° slope. There is extensive but shallow lagoon between the coastal land and barrier reef.
Barrier reefs are seldom found as continuous chains rather they are broken at many places and thus the lagoons have contact with the open seas and oceans through tidal inlets. Sometimes, tidal inlets are so wide that ships enter the lagoons through them. Sometimes, the base of barrier reefs exceeds the required depth for the development of coral polyps i.e., 300 feet (91 m).
Thus, the existence of barrier reefs at such greater depth (beyond the permissible depth of 60-77m) poses the problem of their formation. It may be argued that barrier reefs might have been formed at suitable depth but at much later date there might have been subsidence.
Great Barrier Reef, located parallel to the east coast of Australia, is the largest of all the barrier reefs of the world. This reef is located between 9°S to 22°S latitudes and stretches for a length of 1200 miles (1920 km) and thus covers about two-third of the coastal length of Queensland province of Australia. The northern and the southern parts of this reef are 80 miles (128 km) and 7 miles (11km) away from the coast the reef from the Coast Ranges between 20 to 30 miles (32 to 48 km).
The depth of lagoon between the coast and the reefs is 40 fathoms (240 feet) whereas the width ranges between 7 to 80 miles (11 km to 128 km). The reef is broken at places and hence there are frequent openings in the form of tidal inlets which enable the lagoon to maintain contacts with the open ocean.
Type # 3. Atoll:
A ring of narrow growing corals of horseshoe shape and crowned with palm trees is called atoll (fig. 30.3). It is generally found around an island or in elliptical form on a submarine platform. There is a lagoon in the middle of coral ring. The depth of lagoon ranges between 40 to 70 fathoms (240 to 420 feet).
Atolls are divided into 3 types e.g.:
(i) True atoll characterized by circular reef enclosing a shallow lagoon but without island,
(ii) Island atoll having an island in the central part of the lagoon enclosed by circular reef, and
(iii) Coral island or atoll island does not have island in the beginning but later on island is formed due to erosion and deposition by marine waves.
Atolls are found in Antilles Sea, Red Sea, China Sea, Australian Sea, Indonesian Sea. Funfutti Atoll of Ellice Island is a famous atoll. The enclosed lagoon is 12.8 km wide and 19.2 km long.
Shallow lagoon reefs are minor reef features which are annular in shape and are found in epicontinental seas like Indonesian Sea, South China Sea etc. The lagoon is a small pool. Faros are chains of small atolls having shallow small lagoons. Coral banks are isolated shapeless reefs. Coral pinnacles are small ridges which rise within the lagoons.