This occurs when a fringing reef grows upwards so that it can stay near the surface of the water. Coral reefs like to grow just below the waterline so that they have the best access to the sunlight. It is best if a reef grows at the same rate as the rise of the water. Barrier reefs also grow where the land is sinking faster than the water. The number of barrier reefs has increased dramatically as the greenhouse effect has warmed up our atmosphere causing the water levels to rise.
Atolls are reefs surrounding a lagoon.
They are created when an island surrounded by barrier reefs sinks below the ocean surface, leaving a circular reef, called an atoll. Atolls are also created when water levels rise to cover an island and the surrounding reef grows to keep up with the surface of the water.