Geography of Atolls

Caitlin Dempsey


Atolls are ring-shaped coral reefs, islands, or a series of islets that encircle a lagoon either partially or completely. The shape of atolls varies with some being elliptical or circular in shape and others having an angular shape. Atolls also vary in size.

How atolls form

Atolls require warm ocean waters, which are essential for coral growth, and near areas of volcanic activity for their formation.

1. Volcanic Island Formation

The base geographic feature upon which atolls form is volcanic islands. These islands emerge from the ocean floor due to volcanic activity, where magma from the Earth’s mantle reaches the surface and solidifies, creating an island above sea level.

2. Coral Reef Development

Surrounding the newly formed volcanic island, coral reefs start to develop, forming a fringing reef. Corals are marine invertebrates that thrive in warm, shallow waters. Photosynthetic algae, called zooxanthellae, live in the tissues of coral polyps and provide the corals with food through photosynthesis.

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Coral reefs live in compact colonies of many identical individual polyps which secrete calcium carbonate. This accumulates to form the hard, limestone skeleton of the coral reef. Coral reefs thrive in warm, shallow waters where there is plenty of sunlight available for the photosynthetic algae, called zooxanthellae, which live in the tissues of coral polyps and provide the corals with food through photosynthesis.

Step by step diagram showing how an atoll forms from a subsiding island with a volcano.
An illustration showing how atolls are formed. Image: USGS, public domain.

3. Island Subsidence

Over time, the volcanic island begins subside. This subsidence can occur due to several reasons: the cooling and contraction of the Earth’s crust beneath the island, the weight of the volcanic island pressing down on the crust, and natural erosive processes. As the island subsides, the coral reef along the edges of the island continues to grow upward toward the light, maintaining its position near the ocean surface.

4. Atoll Formation

Eventually, the central island disappears completely beneath the ocean’s surface, leaving behind a ring of growing coral that marks the island’s former location. This ring of coral forms a barrier reef enclosing a central lagoon. The result is an atoll, a circular or horseshoe-shaped coral reef that encircles a lagoon partially or fully. The coral reef and the lagoon together create a unique ecosystem that supports a wide variety of marine life.

5. Island Development on the Reef

Over time, coral debris and sand can accumulate on the reef, creating small islands or islets along the atoll’s perimeter. These islands can be colonized by plants and animals, further contributing to the biodiversity of the atoll ecosystem.

Where are atolls located?

Atolls are mostly found in the Pacific and Indian Oceans, with some of the most famous examples including the Maldives, the Marshall Islands, and the Tuamotu Archipelago in French Polynesia.

Different types of atoll shapes

The shape of an atoll is influenced by several factors, including the original shape of the volcanic island around which it formed, the pattern of coral growth, ocean currents, wind patterns, and the geological activity beneath the sea floor.

Most atolls are either ellipsoidal (or ring shaped), or angular in shape.

Circular atolls

Circular atolls are perhaps the most iconic and easily recognizable shape. These atolls form around a roughly circular volcanic island, resulting in a nearly complete ring of coral reef surrounding a central lagoon. The uniform growth of the coral reef around the perimeter of the island contributes to this symmetrical shape.

Oval atolls

Oval-shaped atolls are similar to circular atolls but with a more elongated form.

Angular or irregular atolls

Irregular atolls do not fit neatly into the other shape categories and can have complex, asymmetric forms. These atolls may have multiple lagoons, uneven reef thickness, and varied island sizes along the reef. Irregular shapes can result from a combination of factors, including the uneven subsidence of the volcanic island, variations in coral growth rates, and past geological events that altered the reef’s development.

Crescent atolls

Crescent-shaped atolls are characterized by a curved, arc-like formation of the coral reef, often with a more open lagoon. These atolls may form when one side of the volcanic island subsides faster than the other, or when prevailing winds and currents favor coral growth on one side of the island over the other.

Examples of atolls

Rocas atoll

Rocas Atoll (Atol das Rocas) is located in the Atlantic Ocean, about 160 miles (260 kilometers) northeast of the Brazilian state of Rio Grande do Norte. The atoll has the classic atoll shape.

Rocas Atoll is the only atoll in the South Atlantic Ocean. The atoll serves as a critical nesting site for the threatened green sea turtle (Chelonia mydas) and a breeding ground for various seabirds. The atoll is designated as a Brazilian federal biological reserve and is also a UNESCO World Heritage Site.

Aldabra Atoll

Aldabra Atoll is an example of an oval shaped atoll, including a coral rim that encircles a lagoon. Considered one of the largest raised coral reefs in the world, Aldabra Atoll is located in the Indian Ocean about 375 miles (600 kilometers) east of mainland Africa. The atoll forms part of the Seychelles archipelago found northeast of Madagascar.

Aldabra Atoll’s remote location has resulted in the evolution of quite a few endemic species and supports a wide diversity of other species. One of the notable endemic species is the Aldabra giant tortoise, known for being one of the largest land tortoises in the world.

Another endemic species, the Aldabra rail, is known for having undergone iterative evolution. This means that the flightless bird originally went extinct about 136,000 years ago after the atoll submerged. Birds similar to the ancestors of the Aldabra rail re-colonized the island. With the same environmental conditions of a lack of predators or the need to migrate, the rail’s development mirrored the evolutionary history of its extinct predecessor by becoming a flightless bird on the same remote atoll.

Aldabra has been recognized as an Important Bird Area, providing a haven for various bird species, including frigatebirds and the Aldabra rail, the latter being the only flightless bird in the Indian Ocean region.

Manihiki atoll

Part of the Cook Island chain, Manihiki Island is a 6.5 mile long atoll located in the Pacific Ocean. Tauhunu and Tukao are two small


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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.