atoll


atoll
/at"awl, -ol, -ohl, euh tawl", euh tol", euh tohl"/, n.
a ring-shaped coral reef or a string of closely spaced small coral islands, enclosing or nearly enclosing a shallow lagoon.
[1615-25; earlier atollon < F: a word used in early descriptions of the Maldive Island atolls; said to be Divehi (Indo-Aryan language of the Maldives) atolu]

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Coral reef enclosing a lagoon.

Atolls consist of ribbons of reef that may not be circular but that are closed shapes, sometimes miles across, around a lagoon that may be 160 ft (50 m) deep or more. Most of the reef itself is usually below the water surface; around the rim along the top are usually low, flat islands or more continuous strips of low, flat land.

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 coral reef enclosing a lagoon. Atolls consist of ribbons of reef that may not always be circular but whose broad configuration is a closed shape up to dozens of kilometres across, enclosing a lagoon that may be approximately 50 m (160 feet) deep or more.

      A brief treatment of atolls follows. For full treatment, see ocean: Coral reefs, coral islands, and atolls (ocean).

      Most of the reef itself is a submarine feature, rising from the abyssal floors of the ocean to just beneath high-tide level. Around the rim along the top of the reef there are usually low, flat islands or more continuous strips of low, flat land. Some of these islands have been settled by oceanic peoples like the Maldivians, Polynesians, and Micronesians for many centuries.

 The origin of atolls has always fascinated sailors and naturalists, who early appreciated that, although reef-building organisms inhabit only the shallowest depths of the sea (about 100 m [330 feet]), the reefs rose from much deeper. The modern explanation of atolls incorporates the theory of Charles Darwin (Darwin, Charles), who suggested that atolls represented the final stage of a continuing upgrowth of reef around a sinking extinct volcanic island that had long since disappeared from view.

      Reefs tend to grow outward from a fringing-reef stage toward the better conditions of open water and also grow upward if the foundations beneath are sinking. After thousands of years the actively growing reef structure becomes separated from the volcanic shoreline by an intervening stretch of lagoon water. This is the barrier-reef (barrier reef) stage. The volcanic island eventually subsides from view, leaving a reef whose uppermost part is like a saucer whose rim reaches up to sea level and whose deeper central area is a lagoon.

      Different kinds of reefs and volcanic islands are found together in the tropical oceans, related to each other in such a way that they can be interpreted as representing the progressive stages postulated by the subsidence theory. Stronger direct evidence for subsidence has come from geologic drilling of atolls (first at Enewetak atoll in 1952), which revealed the presence of volcanic rock about 1,400 m (4,600 feet) below the modern reef top. Changes in sea level complicate the subsidence model. These have been relatively frequent during the last 2,000,000 years or more and mostly result from cycles of glaciation. See also coral reef.

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Universalium. 2010.

Look at other dictionaries:

  • atoll — atoll …   Dictionnaire des rimes

  • atoll — [ atɔl ] n. m. • attôle 1773; atolon 1611; mot des îles Maldives , par l angl. ♦ Récif corallien annulaire des mers chaudes, enfermant un lagon communiquant avec la haute mer. Des atolls. ● atoll nom masculin (maldive atolu, du cinghalais ätul, à …   Encyclopédie Universelle

  • Atoll — Sn Koralleninsel per. Wortschatz exot. (19. Jh.) Entlehnung. Entlehnt aus ne. atoll, das seinerseits aus einheimischen Bezeichnungen wie atollon und atoll für die Malediven übernommen ist. Im Englischen ist das Wort seit dem 16. Jh. als Exotismus …   Etymologisches Wörterbuch der deutschen sprache

  • Atoll — Жанры прогрессив рок Годы 1972 1976 1989 2003 …   Википедия

  • Atoll (BD) — Atoll (revue) Pour les articles homonymes, voir Atoll (homonymie). Atoll est une revue de Petit format de l éditeur Jeunesse et Vacances qui a eu 121 numéros de mars 1967 à novembre 1981. Ce fascicule est l un des plus gros succès de l éditeur… …   Wikipédia en Français

  • Atoll K — ((it) Atollo K) est un film franco italien réalisé par Léo Joannon, sorti en 1951, avec dans les rôles principaux le duo comique Laurel et Hardy dont c est le dernier film tourné en commun. Sommaire 1 Synopsis 2 Fiche technique …   Wikipédia en Français

  • Atoll 66 — 20e album de la série Natacha Scénario Guy d Artet Dessin François Walthéry Bruno Di Sano Couleurs Cerise Éditeur Marsu Productions …   Wikipédia en Français

  • Atoll — das; s, e <über gleichbed. engl. atoll aus Malayalam aḍal »verbindend«> ringförmige Koralleninsel …   Das große Fremdwörterbuch

  • Atoll — Atoll: Die im Deutschen seit dem 19./20. Jh. übliche Bezeichnung für eine ringförmige Koralleninsel stammt vermutlich aus der südwestindischen Drawidasprache Malayalam, wo aḍal »verbindend« bedeutet. Ins Dt. gelangte das Wort durch Vermittlung… …   Das Herkunftswörterbuch

  • Atoll — A*toll , n. [The native name in the Indian Ocean.] A coral island or islands, consisting of a belt of coral reef, partly submerged, surrounding a central lagoon or depression; a lagoon island. [1913 Webster] …   The Collaborative International Dictionary of English

  • atoll — 1620s, atollon, from Malayam atolu reef, probably from adal closing, uniting. Popularized in present form by Darwin s writings …   Etymology dictionary


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