An atoll is an island formation characterized by a ring-shaped coral reef that encircles a body of water called a lagoon. They often feature small, low-lying islands, called motus, along the reef rim. Atolls are geological formations, resulting from interactions between marine life and geological processes.
What Defines an Atoll
Atolls are distinguished by several physical components. The primary feature is the coral reef, which forms a circular or oval ribbon extending up to dozens of kilometers across. This reef is built by hermatypic corals, which secrete calcium carbonate skeletons that accumulate over thousands of years. The reef structure includes a reef rim, the main part that breaks the surface, and a reef flat, a level coral platform that may be exposed during low tides.
The central body of water enclosed by the reef is the lagoon, often much shallower than the surrounding ocean, with depths that can reach approximately 50 meters (160 feet). Lagoon waters are generally calm and clear, connecting to the open ocean through channels or passes that cut through the reef rim, allowing for tidal water exchange. Within the lagoon, smaller patch reefs can sometimes be found.
Small, low-lying islands, or motus, frequently form on the reef rim. These islets are composed of unconsolidated reef-derived detritus, such as sand, gravel, and coral fragments. Motus typically rise only a few feet above sea level and can change shape regularly due to currents and wave action. While some motus are barren, others can support vegetation, contributing to the atoll’s distinct appearance.
The Formation of Atolls
Atoll formation is best explained by Charles Darwin’s subsidence theory, first proposed in 1842. This theory begins with a volcanic island emerging from the ocean. Coral larvae attach to its shores, initiating the growth of a fringing reef directly connected to the island’s perimeter.
Over time, the volcanic activity ceases, and the island gradually subsides beneath the ocean’s surface. As the island subsides, the coral reef continues to grow upward and outward, maintaining its position in the sunlit shallow waters necessary for coral survival. This growth leads to the formation of a barrier reef, which is separated from the shrinking island by a widening and deepening lagoon.
Eventually, the volcanic island completely submerges, leaving only the ring of coral reef encircling the central lagoon. This final stage is the atoll, where the lagoon occupies the space once filled by the volcanic peak. The atoll’s existence relies on the coral’s ability to grow upward, keeping pace with the island’s subsidence and any changes in sea level.
Atolls Across the Globe
Atolls are predominantly found in the warm, tropical, and subtropical waters of the Pacific and Indian Oceans. These regions provide suitable conditions for coral growth, including warm temperatures and clear, shallow waters. Approximately 440 atolls exist worldwide, with the vast majority located in the Pacific.
Notable concentrations are found in the Maldives in the Indian Ocean, which is entirely composed of atolls, and in French Polynesia, Micronesia, and the Marshall Islands in the Pacific Ocean. In contrast, atolls are less common in the Atlantic Ocean, with one example being the Rocas Atoll off the coast of Brazil. The scarcity of atolls in the Atlantic is often attributed to a lack of volcanic activity in areas conducive to reef formation.