Atoll

An atoll is a distinctive ring-shaped island or archipelago formed around a central lagoon and composed primarily of coral reefs. Found almost exclusively in tropical and subtropical waters where reef-building corals thrive, atolls are among the most iconic geological and ecological features of the world’s oceans. With approximately 440 recognised examples globally, most atolls are located in the Pacific Ocean, particularly in regions such as the Tuamotu Islands, Kiribati, the Marshall Islands, and other island groups across French Polynesia. Additional clusters occur in the Indian Ocean, including the Maldives, Lakshadweep, and the Chagos Archipelago, while only a few are found in the Atlantic.

Characteristics and Formation

Atolls consist of a coral rim or ring that encloses a shallow lagoon. The rim may be continuous or broken into separate islets or cays composed of coral debris. Although the lagoon is central to the structure, it does not represent the crater of a submerged volcano, despite the long-standing assumption of a volcanic origin for many islands.
Two major scientific models provide explanations for how atolls develop, each supported by substantial research.

The Subsidence Model

Charles Darwin’s subsidence model is the most traditional explanation for atoll formation. According to this theory, an atoll begins as a volcanic island that rises from the ocean floor. A fringing reef develops around this island. Over geological time, the volcanic island becomes extinct, erodes, and gradually sinks. As it subsides, the coral continues growing upward at a pace that matches changes in relative sea level.
Eventually, the volcanic island descends entirely beneath the sea surface, leaving only the ring-shaped coral reef structure. The lagoon forms where the central island once stood, although it is not the original volcanic crater but rather a space created by subsidence and reef growth. The survival of an atoll relies on corals keeping pace with sea-level changes through continued biological growth.

The Antecedent Karst Model

More recent research has proposed the antecedent karst model, offering a substantially different mechanism. In this model, an island—whether volcanic or non-volcanic—subsides below sea level and becomes capped by a flat-topped coral reef mound. When sea level subsequently falls, this flat surface is exposed as land.
Rainfall and weathering then dissolve the exposed limestone, but at varying rates: dissolution is slowest along the edges and faster in the centre. This produces a saucer-shaped island with a raised rim. When sea level rises again, the rim becomes the solid foundation for renewed coral growth, while the interior fills with seawater to create the lagoon.
Studies of modern atolls indicate that many of them rest upon buried, flat-topped Neogene reefs rather than ancient volcanic remnants, supporting this model’s validity and showing that atoll morphology may be independent of any initial volcanic structure.

Etymology and Terminology

The word atoll originates from the Maldivian term atholhu, from the Dhivehi language spoken in the Maldives. Its first recorded use in English dates to 1625. Charles Darwin adopted the word in his work The Structure and Distribution of Coral Reefs, acknowledging its indigenous origin. Modern definitions describe an atoll as any annular or ring-shaped coral reef that encloses a lagoon without prominent land features apart from cays or islets made of reef material.

Global Distribution and Extremes

Atolls are geographically restricted to warm tropical and subtropical oceans where coral reefs can flourish. Their distribution spans several major oceanic regions:

• Pacific Ocean: Home to the highest concentration of atolls, including well-known clusters in the Marshall Islands, Kiribati, Tuvalu, Tokelau, the Coral Sea Islands, and many island chains of French Polynesia.
• Indian Ocean: Notable for large atoll systems such as those of the Maldives, Lakshadweep, and the Chagos Archipelago. The Seychelles also contain several raised and submerged atolls.
• Atlantic Ocean: Contains only a limited number, including eight Caribbean atolls belonging to Colombia.

Certain atolls mark global extremes of latitude and size:

• Northernmost atoll: Kure Atoll (28°25′N), in the Northwestern Hawaiian Islands.
• Southernmost atolls: Elizabeth Reef (29°57′S) and Middleton Reef (29°27′S) in the Tasman Sea.
• Closest to the Equator: Aranuka, in Kiribati.
• Pseudo-atoll: Bermuda, which resembles an atoll in shape but differs fundamentally in geological origin.

Atoll islands are generally low-lying with elevations rarely exceeding a few metres above sea level, rendering them particularly vulnerable to climate change and rising ocean levels. Although the land area of atolls is typically small, lagoon areas can be extensive. Lifou and Rennell Island are among the largest raised coral atolls by area, while Kiritimati is commonly recognised as the largest by total land area. Aldabra and Huvadhu Atoll also rank among the world’s most expansive atolls.

Notable Examples

Atolls appear in diverse geological contexts and support a range of ecosystems. Pacific atolls such as Penrhyn, Nukuoro, and the many atolls of the Maldives illustrate the typical circular reef structures. Some atolls, like Bikini Atoll, have been sites of significant human activity, including nuclear testing, while others lie within marine protected areas known for exceptional biodiversity.

Scientific Understanding and Modern Research

Darwin’s original framework for atoll formation influenced scientific thinking for more than a century, shaping concepts of reef development and island evolution. Modern geological studies, however, have revealed that atoll formation may be more varied and complex than initially proposed. Research on Pleistocene sea-level fluctuations, coral growth rates, and the dissolution of limestone terrains has shown that many atolls are influenced by both tectonic subsidence and karst processes.
Recent findings demonstrate that numerous modern atolls overlie ancient submerged reef platforms rather than volcanic foundations, suggesting that atolls in many regions did not form during subsidence but rather after former islands were fully submerged and buried. These insights highlight the role of changing sea levels and long-term reef growth patterns in shaping modern atoll systems.