Marine, colonial, polypoid coelenterates which live in a secreted skeleton of their own.
Calcareous skeleton is called the coral (secreted by epidermal cells).
Belong to Class Hydrozoa and Anthozoa.
Kinds of Coral Reefs:
Vaughan (1917): Coral Reef is a ridge or mound of limestone, formed from calcium carbonate produced by marine organisms (corals), the upper surface of which is near the sea surface.
Confined to about 28ยบ on either side of equator.
Fringing Reefs
Coral reefs lying close to shore of some volcanic island or part of some continent.
May extend out to a km from the shore, most active zone of coral growth faces the sea (front).
Shallow water channel (50-100m broad) lies between reef edge and shore.
Most common type of coral reef.
Barrier Reefs
Like fringing reefs, but they are located some distance away from the shore and parallel to it.
Stretch of water separating the barrier reef from land is a deeper and wider lagoon.
e.g. Great barrier reef
Atoll
Coral Island or Lagoon Island formed when island surrounded by fringing reef sinks.
Ring like horse shoe shaped reef that surrounds a lagoon.
May be complete or broken by a number of channels.
e.g. Bikini Atoll in Pacific Ocean
Theories of Formation:
Subsidence Theory by Darwin
Fringing reef was first formed on the sloping shores of an island.
Subsidence of sea floor in regions of the reef, followed by upward and outward growth of coral.
Fringing reef becomes Barrier reef.
Gradual sinking leads to vanishing of island (Barrier reef becomes Atoll).
Glacial control Theory by Daly
Lowering of Ocean level by withdrawal of water for glacial formation.
Exposed several flat platforms cut out by action of waves.
When glaciers melted and temperatures became favourable, coral grew on these platforms, building higher as the ocean level rose
Fringing reefs grew on narrow wave cut platforms, barrier reefs on broad wave cut platforms.