1. Hard corals build by secreting calcium carbonate skeletons.
2. Boring organisms such as sponges, worms, and bivalves; along with grazers such as parrotfish and sea urchins break down the coral skeletons. Borers and grazers usually attack dead coral. The resulting sediment settles into spaces in the reef.
3. Coralline algae, encrusting bryozoans, and minerals cement the dead organic matter, stabilizing the reef structure.
1. At one time it was mistakenly thought that coral grew at the bottom of deep tropical seas and succeeding generations grew on top of the dead calcium carbonate skeletons. This idea was dispelled by dredging operations that indicated that reef corals were able to grow only in shallow water.
2. Naturalist Charles Darwin's theory of coral formation is widely accepted. This theory recognizes three types of reefs: the fringing reef, the barrier reef, and the atoll.
a. The first type is a fringing reef. Fringing reefs border shorelines of continents and islands in tropical seas. Fringing reefs are commonly found in the South Pacific Hawaiian Islands, and parts of the Caribbean.
b. The next type is the barrier reef, which occurs farther offshore. Barrier reefs form when land masses sink, and fringing reefs become separated from shorelines by wide channels. Land masses sink as a result of erosion and shifting crustal plates of the earth. (Crustal plates lift or sink the seafloor and adjacent land masses.) Barrier reefs are common in the Caribbean and Indo-Pacific. The Great Barrier Reef off northern Australia in the Indo-Pacific is the largest barrier reef in the world. This reef stretches more than 1,240 miles (2,000 km).
c. If the land mass is a small island, it may eventually disappear below the ocean
surface, and the reef becomes an atoll. Atolls are reefs that surround a central lagoon.
The result is several low coral islands around a lagoon. Atolls commonly occur in the
Indo- Pacific. The largest atoll, named Kwajalein, surrounds a lagoon over 60 miles (97
km) long.
There are three types of coral reefs.
3. Existing coral reefs have been formed since the last of three glacial periods in the Pleistocene epoch, 10,000 years ago. Seawater trapped as ice in enormous glaciers caused sea level to fall. Consequently, all previously formed coral reefs probably died from exposure. When the glaciers melted, sea level rose to its current position and present-day reefs began to develop.
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