Atoll Facts and Information

What is an “atoll”? For reef scientists, atolls are broadly defined as a large, ring-shaped coral reefs that surround a central lagoon.

Coming up with a more specific and unambiguous definition of such reef formations is not all that easy however.

Charles Darwin, one of the first scientists to formally study and classify coral reefs, focused on the geological factors related to reef formation as well as the general structure of the reef system.

Over time, other scientists have defined atolls in somewhat different ways, but all emphasize several common factors that separate atolls from barrier and fringing reef systems.

Perhaps the best modern formal articulation defines atolls as, “more or less continuous reefs surrounding a distinctly deeper lagoon with or without lagoon reefs. . . which rise from a sea bottom which is too deep for the growth of coral reefs”. This definition is perfectly illustrated by the reef system pictured above, left.

A circular ring of coral encloses an open central lagoon

Global Distribution of Atolls

Most of the world’s coral atolls occur in mid-ocean

The Pacific Ocean is home to far more atolls than any of the other ocean basins, but the Indian Ocean also contains numerous examples this reef type. Only a relative few such reef formations occur in the tropical western Atlantic, or “Greater Caribbean” region however.

Indo-Pacific Region

In the Pacific Ocean, there are more than 80 atolls in French Polynesia alone, most of which occur in the Tuamotu Island group.

Such formations are also extensively distributed through the Caroline and Marshall Islands, and frequently occur in the Federated States of Micronesia and the Cook Islands as well.

The northernmost of all Pacific atolls is Kure in the northwestern Hawaiian Islands, while the southernmost is Ducie Island in the Pitcairn Islands group.

The Indian Ocean contains numerous examples of this reef type, particularly in the Maldive and Chagos island groups. Such reefs are also found in the Seychelles and scattered through parts of the southwest Indian Ocean, while in the eastern Indian Ocean they can be found in the Cocos Island group.

Atoll Structure (Geomorphology)

Atolls are relatively rare in the Caribbean, with counts ranging from 10 -27 depending upon the scientist doing the classification.

The three best examples (Glover’s, Lighthouse, and Turneffe) can be found just off the the Belize barrier reef.

Reefs such as Hogsty Reef and the Cay Sal Bank in the Bahamas, and Roncador Bank off the east coast of Nicaragua, have also sometimes been described as atolls.

However, these reefs clearly differ in origin and geomorphology from superficially similar reef structures of the Indo-Pacific region, and most geologists do not consider them to be true atolls.

The typical structure of atolls derives from the most common method of the formation of such reefs; specifically the gradual subsidence of tropical oceanic islands (see right).

This hypothesis was first developed and described by Charles Darwin and its basic tenets are now widely accepted. An overview of the general process of coral reef formation as first described by Darwin is presented elsewhere on this website.

The large-scale structure (geomorphology) of atolls has historically been mapped by in situ field studies, but more recently high-resolution satellite imagery and remote sensing algorithms have been used to map reef morphology and habitat distribution.

The general topography and geomorphology of these reef formations are characterized by three distinctive features, respectively called the:

Outer Reef (Fore Reef)
Perimeter Rim (Reef Rim)
Central Lagoon

Outer Reef

The outer reef (fore reef) is generally continuous. The shallower portions are subject to ocean swells that break on all sides of the reef.

The lower (deeper) portions of outer reefs are typically very steep, rising abruptly from ocean depths of thousands of meters. In contrast, the upper portions of the outer reefs often take the form of well developed spur-and-groove systems that run at right angles to the seaward margin of the main reef.

Perimeter Rim

The perimeter rim (also often simply called the reef rim) is highly variable in width, and generally contains two main structural elements: reef flats, and islands.

Numerous or large passages through the perimeter rim are effective at ensuring that the lagoon is flushed by daily tidal cycles, whereas atolls with almost continuous perimeter rims depend instead upon the far less effective process mechanism of wave-generated water exchange. Where tidal exchange is strong, very large and well-developed patch reefs often occur.

While waves break on all sides of those portions of the reef that reach sea level, the largest waves strike the windward margin. In these high-energy environments, the seaward margin of the reef rims generally display a prominent algal ridge dominated by calcareous red algae.

Reef Flats are shallow structures and often much of the flat is exposed during the lowest tides. On large atolls, reef flats occur in sections that often range from about 1,000 to 2,000 m in width.

Where islands are not present, a broad reef flat often comprises the bulk of the area behind the algal ridge, and extend all the way to the lagoon. In some cases, reef flats display a zonation pattern in which the algal ridge merges into a back reef zone formed by wave-scattered coral debris.

Reef islands often develop on reef rims.Islands are often more abundant on the higher energy, windward portions of perimeter rims and are characteristically low-lying land formations.

On many Indo-Pacific atolls, only about one-third of the island surface is more than 2 m above mean sea level. Such islands usually consist of a distinct seaward ridge and a lesser lagoon-ward ridge, with an intervening depression (swale) in the middle.

However, there is considerable natural variation to this basic pattern, along with (in many cases) human modification.

Waves are the main factor leading to the building of reef islands. Much of the energy of incoming ocean swells is diminished by impact with the reef rim, but some of the wave energy crosses the reef flat and reaches island shores. This in turn results in the accumulation of calcareous sediment and coral rubble, and ultimately to the building of low-lying reef islands.

The Lagoon

Lagoons are prominent features of most atolls. The lagoon is sheltered in comparison to the outer reef and reef rim, and may contain scattered patch reefs. In some cases these may be relatively sparsely distributed; in other cases extensive patch reef mosaics can occupy large portions of the lagoon.

These sheltered, watery expanses vary considerably in depth, ranging in most cases from an average depth of just a few meters to over 70 meters in depth. Lagoons are dynamic features, gradually being filled with coralline sediments derived from the reef rim, along with sediments produced within the lagoon itself.

Island Life

Typically, the communities that occupy the islands rimming the central lagoons of coral atolls are composed largely or entirely of widely distributed, opportunistic plant and animal species.

Endemism is generally very low. The most likely reasons for this are that (1) these reef formations are typically located far from large land masses that might serve as sources of recruitment (and subsequent speciation), and (2) because these islands are generally only a few thousand years old at most.

Aldabra (part of the Seychelles in the Indian Ocean) is an exception to this generalization. Here, an endemic giant tortoise (Aldabrachelys gigantea) can be found.

On unpopulated islands where human visitors are rare, huge colonies of seabirds are common. Most often such colonies include frigate birds, boobies, and noddy and/or sooty terns.

What Are Coral Reefs? | Where Are Coral Reefs Located? | How Are Coral Reefs Formed? | Types of Coral Reefs | Coral Reef Zones

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