Types of Coral Reefs

Most reef scientists generally recognize three basic types of coral reefs: atolls, barrier reefs, and fringing reefs.

The differences between these three main reef types are pronounced in terms of large-scale structure. Nonetheless, there is often a good deal of similarity between them within a given biogeographic region in terms of dominant species of coral reef fishes, reef-building corals, and other forms of marine life, as well as their ecological interactions within the coral reef biome.

Atolls

Atolls are roughly circular (or occasionally horseshoe-shaped) oceanic reef complexes surrounding a large, deep central lagoon.

Indo-Pacific coral atolls. © Fotolia

They are most common in the Indo-Pacific region where over 300 atolls are found, but rare in the Greater Caribbean which houses only about 10-15. The four best developed Caribbean atolls are found off southern Mexico and the coast of Belize.

Atolls can exceed 100 miles in diameter and contain lagoons several thousand square miles in extent. The best developed parts of reefs surrounding atolls are on the windward side, where wave energy is greatest.

Barrier Reefs

Barrier reefs are reef systems that parallel the shore and are separated from it by a wide lagoon that contain at least some deep portions.

A barrier reef surrounds Bora Bora. Courtesy NASA

Examples of large barrier reefs can be found in both the Indo-Pacific and Greater Caribbean, with the Great Barrier Reef of Australia being the prime example.

The very largest barrier reefs develop on the edges of continental shelves (e.g., Great Barrier Reef; Belize Barrier Reef). These massive reef complexes are sometimes referred to as "shelf barrier reefs" to differentiate them from the much smaller barrier reefs surrounding islands (see photo; left).

The back reef zones and lagoons of shelf barrier reefs are often very extensive, in some cases lying over 100 miles from the mainland in some areas. In contrast, the barrier reef surrounding the narrow lagoon of Bora Bora (above) actually transitions into a fringing reef in a few places.

Fringing Reefs

The third major coral reef type are fringing reefs.

A fringing reef rings an Indian Ocean island. Courtesy NASA

These are reef systems growing fairly close to or directly from shore, with an entirely shallow lagoon or no lagoon at all.

Fringing reefs are by far the most common reef type in the Greater Caribbean region and Red Sea. Fringing reefs also surround many South Pacific and Indian Ocean islands.

Because they are situated relatively close to island or mainland shores, fringing reefs are generally the most susceptible to coastal development, agriculture, pollution, and other human activities that result in sedimentation and freshwater runoff.

Development Of Reef Type Classification

The basic coral reef classification scheme described above was first proposed by Charles Darwin, and is still widely used today.

The evolution of the three main types of coral reefs, as first proposed by Charles Darwin

Darwin spent most of his coral reef explorations in the Indo-Pacific region, and viewed the three types of coral reefs he described as simply different stages in the geological 'evolution" of Pacific oceanic islands.

Darwin theorized that fringing reefs began to grow near the shorelines of new islands as ecological conditions became ideal for hard coral growth. Then, as the island began to gradually subside into the sea, the coral was able to keep pace in terms of growth and remained in place at the sea surface, but farther from shore; it was now a barrier reef.

Eventually, the island disappeared below the sea surface, leaving only the ring of coral encircling the central lagoon; an atoll had formed (see diagram, right).

Darwin's general "reef evolution" theory was finally verified for Indo-Pacific reefs in the early 1950s after analyses of the results of deep core drilling at Bikini and Eniwetok Atolls.

However, it has also now become apparent that each of these three types of coral reef can, in some instances, also be formed by different processes as well.

A Note On Patch Reefs

Numerous patch reefs. © Fotolia

The term "patch reef" is commonly used to refer to comparatively small, isolated outcrops of coral surrounded by sand and/or seagrass (see photo, left). These have sometimes been described as a fourth "coral reef type", but such comparisons are clearly not appropriate.

Patch reefs are microscale reef features common to fringing reefs, atolls and barrier reef systems throughout the world. They are not remotely comparable to, and should not be confused with, the three macroscale types of coral reef systems first described by Darwin and still used in that context today.