Competitive Exams: Oceanography Reefs

Coral Reefs

They are significant sub marine features. They are formed due to accumulation and compaction of skeletons of limesecreting organisms known as ‘Coral Polyps’ Coral Polyps thrive in the Tropical Oceans confined between 250 N to 25° S Coral Reefs are formed due to formation of a shell calcereous at suitable depth.

Preconditions for formation of Coral Polyps

  1. corals are formed mainly in Tropical Oceans because they require high mean annual temperature ranging between 20° C to 21° C or 68° F to 70° F their survival

  2. corals do not live in deeper waters i.e.. Not more than 200 − 250 feet or 60 − 77 m. Below sea level, because of two factors

    1. due to lack of sufficient sunlight

    2. due to lack of oxygen

  3. there should be clean sediment free water

  4. fresh water is also required

  5. a very high oceanic salinity is also injurious for growth because such water contains little amount of CaCo3 Salinity should be 27 − 30%

  6. the corals grow in open seas and oceans but die in lagoons and small enclosed seas because of lack of supply of food.

Types of Coral Reefs

  1. Fringing Reef

  2. Barrier Reef

  3. Atoll

Fringing Reef

  1. in between land and continental shelf and it is closed to the shore

  2. the upward slope is steep and vertical while the landward slope is gentle

  3. though fringing reefs are attached to the coastal land but sometimes there is gap between them and in this gap water-boat channel

  4. Fringing coral reefs are generally long but narrow in width. E. g. Sakan island, Southern Florida, Mehetia Island (of society group of island)

  5. it develops along the continental margin.

Barrier Reefs

  1. the largest coral reef off the coastal platforms but parallel to coast are barrier reef

  2. the average slope is about 45° jiome barriers are characterized by 1525° slope

  3. it is separated from the coast by a much wider and deeper channel or lagoon:

  4. the reef is partially submerged

  5. in this the lagoon, sometimes its depth goes upto 240 − 340feet

  6. examples are Great Barrier Reef off the coast of Queensland, it stretches upto 1920 km. Or 1200 miles which covers ⅔ of the coastal length of Queensland province of Australia.


  1. Atolls are similar to barrier reefs except that they are circular in shape

  2. enclosed by a shallow lagoons without any land in the centre

  3. generally found along an island

  4. the depth of lagoons ranges between 240 − 420 feet or 40 − 70 fathoms

  5. Atolls are of 3 types:

    1. True Atoll: Characterized by circular reefs enclosing a shallow lagoon

    2. Shapeless reefs.

    3. Coral Pinnacles small ridges which rise within the lagoon.

Theories Regarding Origin of Coral reefs

  1. Subsidence theory of Darwin.

  2. Standstill theory by Murray.

  3. Glacial Control theory by Daly.