The depression can belong to the emergent island or part of the sea (i.e., a Lagon) or, more rarely, of a closed enclosure filled with water fresh, brackish, or strongly saltworks.
More modern definitions of atoll are those of McNeil (1954, p. 396) as “… annular reef locking up a lagoon in which there are no headlands other than reefs and small islands made up of refuse of reef” and Fairbridge (1950, p. 341) “… in an exclusively morphological direction,… a reef in the annular shape of ribbon locking up a lagoon in the center. ”
ThreatsMany flat and low atolls are threatened by the Tsunami S, and recently by an increase in the sea level so fast that the corals, in addition in bad health could not be able to grow rather quickly to save the atolls of the immersion. They appear among the first Écoréfugié S. The corals contribute in addition to the Cycle of carbon because they consist of Calcaire which is rich in durably fixed Carbone. The Pollution is also a threat for the part interns relatively closed atolls inhabited or exploited for their richnesses.
Some atollsThe largest atoll of the world, in term of surface, is the atoll of Christmas in the Kiribati: it is also oldest.
Largest by its lagoon is Kwajalein (the Marshall Islands).
Clipperton is a French possession off Mexico and is the only atoll of the Northern hemisphere.
- description of the atolls and their formation process
- Vidéos on Channel IRD - Tikehau: an atoll observatory of the coral ecosystems
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