What is a Coral Reef?

WHAT IS A CORAL REEF?

Coral reefs have traditionally been divided into three basic types: fringing reefs, barrier reefs, and atolls.

These correspond with stages of development, with fringing reefs representing the earliest stage and barrier reefs and atolls representing the mature stage. Fringing reefs are relatively young, generally narrow platforms that extend a short distance from shore and do not contain a substantial lagoon. As the reef grows outward, or upward if the seafloor sinks or the sea level rises, the innermost corals cannot keep pace and a lagoon may develop. The reef has now become a barrier reef. Atolls start from fringing reefs surrounding volcanic islands. Erosion and subsidence of the surrounding seafloor cause the island to shrink and sink. The seaward margin of the fringing reef keeps pace with the sea surface, leaving a lagoon along the shore. An atoll results when the island has completely disappeared beneath the surface. Other factors that affect reef development such as changing sea levels, uplift, subsidence, temperature, and freshwater intrusion shape what is a coral reef into a wide variety of forms. In sheltered seas, steep-sided flat-topped platform reefs may occur far offshore. At this stage of development the seaward slope is steep, often reaching depths of 200 m or more within 1.5 km of shore.

Coral reefs are unique among biological environments in that their inhabitants create and maintain major geological features of the Earth itself. They take several different forms related to age and situation. Fringing reefs are geologically young structures encrusting rocky shorelines. Barrier reefs are older, more massive structures separated from the coast by a lagoon of open water usually with depths between three and 50 metres.

Atolls are isolated oceanic reefs of great age rising steeply from abyssal depths. They take the shape of circular or irregular rings of reef enclosing an open lagoon with the only land, if any, being small low coral isles on the reef itself. Atolls are the culmination of a process which begins as a fringing reef around the shore of geologically young volcanic islands such as Hawaii.

Another major reef type, platform reefs, occur as extensive areas of reef forming a thin veneer on the shallow margins of continents submerged by the rise in sea level accompanying the end of the most recent glacial period about 9000 years ago. The lagoons of barrier reefs and atolls are commonly dotted with still another type of smaller reef called patch reefs. These range in size from only a few to tens of metres across and from low mounds to towering pinnacles forty metres or more in height.

I hope you know now what is a coral reef. Other related pages: