Topic summary
Barrier reefs
Extracted from the Wikipedia article Coral reef.
Barrier reef
Barrier reefs are separated from a mainland or island shore by a deep channel or lagoon. They resemble the later stages of a fringing reef with its lagoon, but differ from the latter mainly in size and origin. Their lagoons can be several kilometres wide and 30 to 70 metres deep. Above all, the offshore outer reef edge formed in open water rather than next to a shoreline. Like an atoll, it is thought that these reefs are formed either as the seabed lowered or the sea level rose. Formation takes considerably longer than for a fringing reef; thus, barrier reefs are much rarer.