Depending on where they are located and how they are formed, shallow- water tropical reefs can be classified into one of three major groups: fringing reefs, barrier reefs, and atoll reefs. The Figure above illustrates the structure of each reef type. Fringing reefs, which form along a coastline, are the most common type. These develop at the margin of a landmass where conditions are suitable for coral growth. They are normally located only in shallow waters and border the coast very closely with only a narrow stretch of water separating the reef from the shore. Because sediment washes from the land out to the sea, most fringing reefs have very little coral growing on the shore side. However, the ocean side, which is not exposed to as much sediment, is home to large populations of live coral. Fringing reefs are common in the Caribbean and around the Hawaiian Islands.
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| Three types of coral reefs are fringing reefs, barrier reefs, and atoll reefs. |
Like fringing reefs, barrier reefs run parallel to the shoreline, but they are located further out in the ocean. A barrier reef is separated from the shoreline by a lagoon, a deep, open body of water with a sandy bottom. Lagoons are home to many forms of life. The shallow sections contain large underwater fields of grass. The root systems of these plants help to trap sand, further adding to the base of the lagoon. The barrier reefs are so named because they form a barrier between the lagoon and the ocean. The largest reef in the world, the Great Barrier Reef, is located off the eastern coast of Australia. The Great Barrier Reef is more than 500,000 years old.
The third classification, the atoll reef, is made of circular coral structures. These formations grow on top of volcanoes that lie below the ocean surface. Like barrier reefs, atolls surround central lagoons. These coral reefs are commonly found in the Indo-Pacific regions with the largest atoll being Kwajalein, which surrounds a 60-mile (97-km) wide lagoon.