UNIT 2: ADVANCED PHYSICAL GEOGRAPHY: COASTAL ENVIRONMENTS
CORAL REEFS
Nature of corals:
Corals are limestone rocks made up of skeletons of tiny marine organisms called polyps.
Coral polyps cannot grow out of water and they have therefore been formed below the
low tide level. Coral polyps are formed and survive under the following conditions;
⮚ The sea temperature must be about 210C
⮚ Sunlight must be present
⮚ Clear salt water down to a depth of about 55 metres.
Extensive coral formation develops between 30 North and 30 South of the Equator
where warm currents flow near to the coast. They do not develop on the Western coast
in these latitudes because of the cool currents which flows along these coasts. Coral
masses are called reefs and there are three types:
1. Fringing reef
2. Barrier reef
3. Atolls.
1. Fringing reef
A fringing reef consists of a platform of canal which is connected to and which is built
out from a coast. The surface of the platform is usually flat or slightly concave and its
outer edge drops away steeply to the surrounding seafloor. A shallow lagoon usually
occurs between the coast and the outer edge of the cliff. In other words a fringing
reef is a narrow coral platform separated from the coast by a lagoon which may
disappear at low tides (diagram below).
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, UNIT 2: ADVANCED PHYSICAL GEOGRAPHY: COASTAL ENVIRONMENTS
2. Barrier reef:
A barrier reef is similar to a fringing reef except that it is situated several miles off
the coast and it is separated from the coast by a deep water lagoon. The corals of the
barrier reef are often joined to the coast although the lagoon may be too deep for
corals to grow on its bed. Some barrier reefs lie off the coast of continents, for
example the Great Barrier Reef along the East coast of Australia. Others occur around
islands forming a continuous reef except for openings on the leeward side.. In some
cases fringing reefs develop on the inner side of lagoons which lie between a barrier
reef and the coast of the islands that it encircles. (Diagram below)
3. Atolls
An atoll is a circular coral reef which encloses a lagoon. Atolls are particularly common
in the pacific and Indian ocean (diagram below).
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