Atolls

Atolls

Atolls are ring-shaped reef formations with a central lagoon. At this stage in darwinian development, the island young fringing and barrier reefs surrounded has now been completely submerged and a ring of coral 1-20 miles in diameter is left. Atolls are often uninhabited desert islands, relatively untouched and unexplored by humans. However, some atolls grow beyond short reef formations, forming substantial landmass. These islands and islets have become the home of several island nations including the Republic of the Marshall Islands in the Pacific Ocean and the Maldives in the Indian Ocean.

Formation

Atolls are the third and final stage of Darwin's subsidence theory. After corals have colonized a newly developing island forming a fringing reef, the volcano gradually sinks as it cools and condenses leaving the ring of corals behind. This middle stage where the top of the island still emerges from the ocean but a lagoon has formed in the separation of the reef and island is known as a barrier reef. The corals continue to build upwards to stay in the sunlit range. Eventually the island will submerge completely leaving one large lagoon and a ring of reef formations. This is an atoll.

Morphology and Marine Species

There are two sides to an atoll. The outer edge facing the ocean slopes steeply to the ocean floor. Here, calcareous algaes flourish. The seaward margin faces into the center of the circle and drops much more gently. Here, you may find coral spurs or sediment-filled canyons.

Location

Atolls are much more prevalent in the Indo-Pacific than in the Caribbean because of the relative abundance of submarine volcanoes. The few that are exhibited in the Caribbean are surrounded by fire coral instead of algal ridges.