![]() It is generally believed that fringing reefs formed as a result of upward and outward growth of corals that became established on rocks near shore there is disagreement about the nature of barrier reef and atoll formation. An atoll is a reef surrounding a lagoon that has no central island, with passages through the reef to the sea. Barrier reefs are separated from the shore by a wide, deep lagoon or surround a lagoon that has a central island. Fringing reefs are coral platforms that are more or less continuous with the shore and exposed at low tide. ![]() Geologically, coral reefs are classified into three main types. However, any reef formed by a biological community is usually called a coral reef. Calcium carbonate is also deposited by coralline algae, the protozoan foraminiferans, some mollusks, echinoderms, and tube-building annelid worms. Corals are not the only, and in some cases not even the major, reef-forming organisms. Although corals are found both in temperate and tropical waters, reefs are formed only in a zone extending at most from 30°N to 30°S of the equator the reef-forming corals do not grow at depths of over 100 ft (30 m) or where the water temperature falls below 72℉ (22℃). The accumulation of skeletal material, broken and piled up by wave action, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone). ![]() Coral reefs, limestone formations produced by living organisms, found in shallow, tropical marine waters.
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