Coral islands are (usually) low-lying islands formed by hermatypic, or reef-building, corals, chiefly scleractinian corals and hydrocorallians. Reef-building corals occur in a broad band stretching around the globe from 25 degrees north of the equator to 25 degrees south of the equator and require an average water temperature of about 68–77°F (20–25°C). They do not grow below 165 ft (50 m) in depth. They also have specific needs for water salinity, clarity, calmness, and sunlight. Sunlight aids in formation of the living corals' exoskeleton, and so aids in reef-building. Corals anchor on something—seamounts, submarine slopes of islands, or debris such as abandoned army vehicles and bedsprings—and therefore are generally found at the edges of continents or existing islands. If the surface of a reef emerges into the air—through, for example, a slight drop in sea level—the creatures dry up and die. The exposed, dead surface of the reef then serves as a platform for the accumulation of sediment, which may in turn become sufficient to support plant and animal life. Thus, offshore islands in tropical and semitropical zones around the world often have a core of emerged, dead coral reef. For example, a reef that emerged in about 3450 B.C. provided the base on which all of the islands in the Maupihaa Atoll, in the Society Islands, are founded. Indeed, study of the rate of uplift of emerged coral reefs has helped scientists determine local sea levels in past eras.
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