• A coral reef is a ridge in the sea with its upper surface nearer to the surface of water.
  • This ridge is formed by calcium carbonate secreted coral organisms of cnidarians.
  • These reefs are formed in warm, clear and shallow sea waters.
  • Hence these reefs are confined to tropical regions only.
  • These reef building organisms require 22 to 28°C and shallow depths up to 30 meters.
  • There are 3 types of coral reefs.

1. Fringing reef  2. Barrier reef  3. Atolls reef


1. Fringing reef:
  • It lies close to the shore.
  • The reef may extend to a distance of 200metres from the shore into the sea. Its active growing region is called reef front.
  • It is facing the sea.
  • The narrow shallow water channel between shore and reef is called reef flat.
  • This reef is largely composed of calcium carbonate, sand, dead, and living coral colonies and other organisms. 
2. Barrier reef:
  • This type of coral reef is located some distance from the shore.
  • The reef front is facing the sea.
  • The reef and shore are separated by a deep stretch of water called lagoon. This lagoon is suitable for navigation.
  • The best example for barrier reef is "Great barrier reef of north eastern Australia.
  • It is 15 to 250 km away from shore. It extends up to 2000 km in length.
3. Atoll reef:
  • It is also called coral island.
  • It is a ring like or horse-shoe shaped reef that encircles a lagoon.
  • The lagoon is very deep and is suitable for navigation.
  • The ring like reef may be broken by a number of channels.
  • This is called atoll.
  • The biggest Atoll in the world is seen in the form of Suva diva of Maldives.
Theories explaining the formation of coral reefs:
Many theories were put forward to explain the formation of coral reefs.
1. Darwin - Dana subsidence theory:
  1. Reefs start as fringing reefs on a shore slop.
  2. This fringing reef by the subsidence of the reef flat into lagoon becomes barrier reef.
  3. A fringing reef formed around an island becomes an atoll if the island is fully submerged.'
  4. This theory is supported by Dana and Davis.
2. Submerged bank theory:
  1. Because of the sedimentation, shallow slopes are formed, on which reefs are developed.
  2. By the growth of the outer edges of the reef and dissolution of inner regions of the reefs barrier reefs and atolls are formed.
3. Daly Glacial-epoch theory:
  1. In the last glacial period water was drawn from ocean to form ice caps .
  2. Because of which the ocean level was lowered by 60 to 70 meters.
  3. Rat slopes are formed, which are suitable for the development of reefs.
  4. When there is a rise in temperature and with the rising of ocean level by melting of ice the reef had grown further.
  5. Thus barrier reefs and Atolls are formed.
  6. None of the theories gave satisfactory account of coral reef formation.
The following are the pictures of coral reefs.The first one is 1.The Great Barrier Reef and the remaining are 2.Fringing Reef 3. Atolls Reef

The Great Barrier Reef

The following image shows coral reefs around the word in different zoogeographical regions. In the below shown corals reef picture the red color dotted area indicates the coral reefs locations on the globe.
coral reefs around the world

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