Different reefs

We generally speak of “reefs”, but this term is general. There are instead a number of coral formations with different origins, shapes and relations to the mainland.
Fringing reefs
Fringing reefs look like coral belts parallel to the coast; they get bigger as they are farther out to the sea and are linked to the coast by an internal flat reef. Its actively growing part is in the area of the barrier that looks onto the open sea because the environmental conditions (light, oxygen, food) there are more favourable to the growth of corals; on the contrary, the shallowness of the internal areas increase the temperature, salinity and the sediments that reduce the growth of the corals.
Surf barriers are typically to be found in most coastal reefs in the Read Sea, Eastern Africa and the Caribbean.
Barriers reefs
Barriers reefs develop parallel to the coast, but, unlike fringing reefs, are not linked to it.
They can also be in the open sea and grow in all directions, with different shapes. They start to grow as small barriers; then, the seabed starts to sink so the corals may grow vertically. When completely developed, this type of barrier may extend into a grid of reefs and islets, separated by lagoons and canals formed by the currents, waves and winds.
Barriers like these can be found in Australia, for instance the Great Reef, in Papua-New Guinea, off the New Caledonian coast, in the Fiji islands and off the coasts of Belize and Bahamas.
Atolls
The term ‘atoll’ means a coral formation encircling a round lagoon; the term actually comes from the Maldivian “atholu” which means “islands arranged in a ring”.
These madreporic structures generally occur in deep oceanic waters at the level of ancient submerged volcanic islands. Most atolls are in the Pacific, but there are some in the Indian Ocean as well. Some countries consist only of atolls, for instance the Maldives.

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