Coral Reefs
Coral reefs are build by the actions of colonial stone corals belonging to the subclass zooantharia. The initial colony is formed on relatively shallow water since the corals are dependent on the symbiotic relationship with the photosynthesizing zooxanthellae. There are basically three types of coral reefs: fringing reefs, barrier reefs and atolls. Each of the three types of reefs will be explained in the following text.

 

Fringing reefs:
The fringing reefs usually border the shores of tropical coast and islands, where they develop along rocky shorelines. The fringing reefs exhibit some degree of zonation, that depends primarily on exposure to wave action and water depth. The

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fringing reefs can be divided into two zones: the reef crest(exposed at low tides, may be subjected to strong wave action) and the reef flat(extends from the front of the reef to the shore, exposed at low tides, subjected to freshwater inputs and sediment inputs from the land).  
 

Barrier reefs:
The formation of barrier reefs are thought to be the result of the sinking of land masses , which would separate the typical fringing reef from the shoreline. These reefs parallel the shoreline, but are usually found at greater distances from land(as far as a 100 km).

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Several zones exists in the typical barrier reef: the back reef slope(positioned between the reef and the shore in a relatively shallow lagoon, this part of the reef is subjected to low mechanical stress due to a protected environment),  the reef flat(mentioned above), and the fore reef slope(extends outward from the reef, subjected to wave action and sediment deposition). 

Atolls:
This type of coral reef is formed when small volcanic islands surrounded by a fringing reef disappears below the ocean surface. An atoll may rest on very deep layers of dead coral, since the corals have to keep growing upwards in order to keep their position in the photic zone of the water column(the foundation of the atoll keeps sinking). On the Enewetak Atoll positioned in the Pacific Ocean it was found that the atoll rested on 1400 mīs of shallow water coral reef deposits!  

 

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