An atoll is a ring of coral encircling a shallow lagoon. The coral rises from great depths to just below high-tide level. The surface of the reef may extend above sea level as a series of low, flat islands containing wave-tossed and eroded coral debris. In the Pacific Ocean, Polynesians and Micronesians settled many of these islands. Because corals require sunlight, only the upper portion of the coral is living, to depths of 150 m in the Indian and Pacific oceans and 50 m in the Caribbean Sea. In addition, almost all corals need warm water and do best when the temperature is between 23 and 25 degrees centigrade, which keeps them from growing in cold-water currents and cold water at depth.

That reefs extend to great depths provided a puzzle to early geologists and naturalists. For example, modern drilling in Enewetok Atoll passed through 1,400 m of coral before coming to volcanic rock. Many theories were proposed to explain these observations, but it was Charles Darwin, in 1837, who presented the explanation that coral reefs and volcanic islands were connected. He recognized that there are three types of islands in the sea: volcanic islands, corals reefs, and a combination of the two; he proposed that they represent stages in a single sequence of development. After a volcanic island is formed and activity ceases, corals begin to grow in the shallow water on the edge of the volcano, forming a fringing reef. As the oceanic plates move and slowly subside, carrying the volcanic edifice downward with them, the reef builds upward and outward continually, to maintain its position within the zone of light. When subsidence is too rapid, the reef is brought into sunless depths where it dies, never forming an atoll unless uplift brings the mass close to the surface and new coral builds upon it.

As subsidence continues, a barrier reef forms adjacent to the partially submerged volcano and is separated from it by a lagoon. Eventually the volcano subsides completely, leaving

an atoll continually growing upward as sinking continues. Reefs are among the most diverse environments on earth and have complex assemblages of plants and animals that are intensely competitive with one another. The atoll surfaces, on the other hand, are not diverse but are usually home to unique species, as are many oceanic islands.

—Sidney Horenstein

See also: Coral Reefs; Lagoons; Oceans Bibliography

Hamblin, W. Kenneth and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Nunn, P. D. 1994. Oceanic Islands. Oxford: Blackwell; Press, Frank and Raymond Siever. 2000. Understanding Earth, 3rd ed. New

York: W. H. Freeman and Company; Spalding, Mark. 2001. World Atlas of Coral Reefs. Berkeley: University of California Press

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