An atoll is a ring-shaped coral reef that surrounds a central body of water called a lagoon. Atolls are unique natural structures built by living coral ecosystems on submerged landforms. The word “atoll” originates from “atholhu,” a word from Dhivehi, the language spoken in the Maldives, reflecting the prevalence of these formations in that region.
Global Distribution
Atolls are primarily found in the warm, tropical and subtropical waters of the Pacific and Indian Oceans, where conditions are favorable for coral growth. The Pacific Ocean contains the largest concentration of atolls, with approximately 440 atolls. Specific regions in the Pacific known for numerous atolls include the Marshall Islands, the Tuamotu Archipelago in French Polynesia, and Kiribati. Kwajalein Atoll in the Marshall Islands is one of the world’s largest by lagoon size, while the Tuamotu Archipelago is the largest chain of atolls.
In the Indian Ocean, significant atoll formations are present in the Maldives, the Chagos Archipelago, and Lakshadweep. The Maldives, for instance, is a nation composed entirely of 26 natural atolls. These formations generally occur in areas with past or present volcanic activity, providing the initial foundation for coral development. While most atolls are in these two oceans, a smaller number are located in the Caribbean Sea.
Formation Conditions
Atoll formation begins with the emergence of a volcanic island from the ocean floor. As this volcanic activity ceases, corals start to grow around the island’s shores, establishing what is known as a fringing reef. Over geological time, the volcanic island gradually subsides into the ocean. As the island sinks, the coral reef continues to grow upward, maintaining its position near the sunlit ocean surface.
This continuous upward coral growth, coupled with the island’s subsidence, develops the reef structure. The fringing reef transforms into a barrier reef, separated from the sinking island by a widening lagoon. Eventually, the volcanic island completely disappears beneath the sea surface, leaving only the ring-shaped coral reef encircling a central lagoon. This process, known as Darwin’s subsidence theory, can take millions of years, with corals constantly building upon the skeletons of older generations. Coral growth requires specific conditions, including warm, clear, shallow waters and sufficient sunlight for the symbiotic algae living within coral tissues.
Ecological Features
An atoll’s structure features an outer reef, a perimeter reef rim, and a central lagoon. The reef rim, sometimes broken by channels, may include small landmasses called motu, which are formed by accumulated coral debris and sand. The central lagoon, protected by the surrounding reef, varies in depth and contains clear, calm waters. This sheltered environment contrasts with the more exposed outer reef, which faces the open ocean.
The diverse structure of atolls supports a wide array of marine life. The clear waters and abundant coral species provide habitats for numerous fish, invertebrates, and other marine organisms. The reef crest, which is the shallowest part of the reef, experiences wave action and protects the inner lagoon. These ecosystems are shaped by geological processes and biological growth, fostering biodiversity in their isolated oceanic settings.