DEFINITION OF REEF

Framework-supported carbonate mounds built by carbonate

secreting organisms, or in some usages any shallow

ridge of rock lying near the surface of the water. Reefs contain

a plethora of organisms that together build a wave-resistant

structure to just below the low-tide level in the ocean

waters and provide shelter for fish and other organisms. The

spaces between the framework are typically filled by skeletal

debris, which together with the framework become cemented

together to form a wave-resistant feature that shelters the

shelf from high-energy waves. Reef organisms (presently consisting

mainly of zooxanthellae) can only survive in the photic

zone, so reef growth is restricted to the upper 328 feet (100

m) of the seawater.

Reefs are built by a wide variety of organisms, today

including red algae, mollusks, sponges, and cnidarians

(including corals). The colonial Scleractinia corals are

presently the principal reef builders, producing a calcareous

external skeleton characterized by radial partitions known as

septa. Inside the skeleton are soft-bodied animals called

polyps, containing symbiotic algae that are essential for the

life cycle of the coral, and the building of the reef structure.

The polyps contain calcium bicarbonate that is broken down

into calcium carbonate, carbon dioxide, and water. The calcium

carbonate is secreted to the reef building its structure,

whereas the algae photosynthesize the carbon dioxide producing

food for the polyps.

There are several different types of reefs, classified by

their morphology and relationship to nearby landmasses.

Fringing reefs grow along and fringe the coast of a landmass,

and are often discontinuous. They typically have a steep

outer slope, an algal ridge crest, and a flat, sand-filled channel

between the reef and the main shoreline. Barrier reefs

form at greater distances from the shore than fringing reefs

and are generally broader and more continuous than fringing

reefs. They are among the largest biological structures on the

planet—for instance, the Great Barrier Reef of Australia is

1,430 miles (2,300 km) long. A wide deep lagoon typically

separates barrier reefs from the mainland. Atolls or atoll reefs

form rings around emergent or submerged volcanic islands,

growing progressively upward as the central volcanic island

subsides below sea level.

Reefs are rich in organic material and have high primary

porosity, so they are a promising target for many hydrocarbon

exploration programs. Reefs are well represented in the

geological record, with examples including the Permian reefs

of west Texas, the Triassic of the European Alps, the Devonian

of western Canada, Europe, and Australia, and the Precambrian

of Canada and South Africa. Organisms that

produced the reefs have changed dramatically with time, but

surprisingly, the gross structure of the reefs has remained

broadly similar.

See also ATOLL; CARBONATE; CORALS; PASSIVE MARGIN.

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