Types of coral reefs
By Marjorie Henry, Contributor

AS I promised in the last lesson, I will begin a series of lessons based on geomorphic systems with specific reference to coral reefs. My close association with the Caribbean examinations informs me that candidates seldom do questions based on coral reefs and, when they are attempted, most times they are poorly done. In view of this, I have chosen to do this topic with you. The specific objectives for this topic have been already stated. The information I will share with you on this topic comes from the recommended texts also given in the last lesson.

The definition given for coral reefs is:

'An offshore ridge, mainly of calcium carbonate, formed by the secretions of small marine animals.' (Oxford Dictionary of Geography)

'Coral reefs are often called the 'rainforests of the sea' because of their biological productivity and species diversity. They provide food, shelter and nurseries for many species of crustaceans and fish, but are very sensitive to environmental change.' (The Contemporary Caribbean by Robert B. Potter et al)

The first specific objective stated in the syllabus is:

'... name, locate and describe the types of coral reefs found within the Caribbean.'

There are three types of coral reefs found in the Caribbean Sea.

FRINGING REEF

The first is the fringing reef. The fringing reef, formed by corals and other calcareous organisms growing on the bed of the shallow seas round a continent or island, lies near the shore. It consists of a low platform, separated from the landmass by a narrow lagoon. The surface of the platform is usually flat or slightly concave and its outer edge drops away steeply to the surrounding sea floor.

The corals grow outwards from the shore to a depth of about 5.5km, and upwards to low water level, so that the edge of the platform lies at approximately the 5.5km line. As the outer corals grow most freely, they may reach the surface first, leaving a shallow channel or lagoon between the platform and the land.

There are fringing reefs off the coast of many Caribbean islands, including Jamaica, Barbados, Tobago and Antigua.

BARRIER REEF

The second is the barrier reef. It lies at some distance from the shore. Its outer side rises from deep water where no reef-building corals live, and the lagoon is wide and deep. The coral of a barrier reef is often joined to the coast although the lagoon may be too deep for coral to grow on its bed. The second largest barrier reef of the world is found off the coast of Belize. There are also large barrier reefs off the coast of Andros in the Bahamas, and Anegada in the British Virgin Islands.

ATOLL REEF

The third coral reef is the atoll. The New Caribbean Geography informs us that this is circular, ring-like or horse-shoe in shape comprising several islands and enclosing a lagoon. The reefs are built around the rim of a sunken volcanic cone or guyots. Guyots are flat-topped, circular mountains rising from the sea floor. The atoll comprises an (i) outer or seaward reef (ii) an inner or lagoon section. Atolls are widespread in the Bahamas Islands and off the Belizean coast, for example, Turneffe Islands.

PATCH REEFS

Another source, Caribbean Natural Landscape, informs us that patch reefs are also found in the Caribbean. These are isolated boulders of coral that form in shallow water in irregular patches on the sea level.

The topic continues next week. Please read on it.