Coral reef

The coral reefs are a form of life among oldest and primitive, going back to at least 500 million years. Largest of these formations is the Grande barrier of coral, off the Australian coasts. It extends over some 2000 km, the second is the coral barrier of New Caledonia (France).

A coral Récif results from the construction of a durable mineral substrate (calcium carbonate) by symbiotic living beings commonly called corals, makes of it the meeting of a Algue, the Zooxanthelle and of a Cnidaire, the polyp. There exist very many coral species which form complex marine ecosystems and among richest in biodiversity, generally at a shallow depth. The coral solid masses, in particular in tropical area get ecological niches with many animals which find there food, refuge, protection and shelter. Very many fish species of it are dependant.

However, contrary to the general belief, the corals are not pledged with the hot seas; one finds some for example off Norway.

The coral reefs differ from the Côtes of accumulation in what they are of biological origin. These reefs are also part of the oceanic Puits of carbon, threatened by degradation of the reefs. approximately 50% of the coral reefs were in bad health at the end of the 20th century.

Formation of the reefs

The coral reefs tropical follow a cycle now well-known.

  • an opening of the terrestrial coat lets escape a flood from magma. A volcanic island then is formed;
  • the coral settles in fringing reef in broad this island;
  • as the island starts to be inserted under the effect of the Subsidence, the coral colonizes all the circumference of the island (except for a channel of current fort), forming a reef barrier ;
  • the island completes to be inserted in the ocean whereas the coral continues to grow in the form of atoll .

The flood of magma is not, contrary to what occurs at the time of the volcanic eruptions that one usually knows, a specific phenomenon. On the contrary, we assist here with a continuous phenomenon of matter ejection in fusion (one knows today with precision neither the cause, nor the mechanisms of these ejections). These “chimneys” remain relatively fixed whereas the Tectonic plate that they bore continues to be driven, involving the creation of a volcanic string of islands; one sees very well since the sky or the space which these islands are placed on the same trajectory.

One can very simply visualize this phenomenon, by lighting a lighter and by moving the hand horizontally before behind above the flame: it is exactly what occurs for creation from these volcanic islands; it is not the increase of magma (the flame) which moves but the Earth's crust (the hand).

Types of reefs

One distinguishes three types of coral reefs, corresponding to three chronological stages of the evolution of the volcanic island of substrate:

  • the fringing reefs . In the shape of “comet tail”, they are the first to be formed slightly in broad island. Indeed, one notes one (more or less) narrow channel between the littoral and the reef, channel due to:

    • the excessive Turbidity of immediately littoral water;
    • dessalure of coastal water by mixture with fresh water of the rivers.

  • the reefs barrier . With time, the colony forms a crown around the island. This crown forms a kind of “split” (where the rock would be replaced by the coral) with a trough of water. In the center of this split the remainder of island is. The unit crowns récifale-vestige of island is called Lagon. If the fringing reefs do not continue their growth indefinitely, but transform themselves into crown, it is because the tropical coral can live only thanks to one abundant source of light, that it does not find any more in lower part of 25 meters under water. Consequently, only the littoral zones of the emerged grounds are likely to accommodate coral.

  • the Atoll S . The volcanic island from now on is completely immersed. Just like the lagoons, the atolls are surrounded by a crown récifale with a channel. But contrary to them, they do not have any more a central island; the only outcrops are of coral nature. There exist two types of atoll:

    • “normal” atolls, as described above;
    • atolls of faro, the “atolls of atolls”. The faro are circular atolls which form themselves a crown called large atoll. One especially sees them in the Indian Ocean (in particular in the archipelago of the Maldives).

Geographical distribution

Although certain coral species exist out of cool water, in the areas moderated, or in-depth, the vast majority of the reefs is distributed in the zone of light of the first 20 meters of tropical water , in particular in the West of the Oceania (Indonesia, Australia, New Caledonia…), with broad of the Mexico (Riviera Maya) and with the the Antilles.

More

  • Reefbase.org

Simple: Coral reef Zh-min-nan: San-O·- chiau

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