Coral Reefs

A World of Their Own




Contents

What Are They
Ideal Environment
Different Types
Theories of Formation
For More Information



What Are They

Coral reefs are underwater wave-resistant mounds that are virtual ecosystems all by themselves. They are made up of corals, algae, mollusks, bryozoans, brachiopods, echinoderms, and sponges. Coral sands and solid limestone also play a large part in the buildup of a coral reef. Coral reefs grow upward by a rock like accumulation of calcium containing (calcareous) exoskeletons of past generations of coral animals.



(copyright 1995 Royal Tyrrell Museum)

Devonian Reef exhibit at the Royal Tyrrell Museum

Contents



Ideal Environment

In their ideal environments, coral reefs will grow from 1 to 100 centimeters per year. It has been found that reefs are very sensitive to changes in their environment. High rainfall and mainland runoff can kill reefs by increasing the amount of sediment in the water as well as decreasing the salinity of the water. Another danger to reefs is the absence of waves. In this environment, silt will accumulate and suffocate the coral. Following is a list of ideal conditions for coral growth:

Tropical waters along western coasts of ocean basins (eastern coasts of continents), and around oceanic islands are the best places to find coral reefs. They are usually only found within 30 degrees north or south of the equator.

Contents



Different Types

There are several different types of reefs. They are called fringing reefs, barrier reefs, and atolls. Fringing reefs, which are located close to the shore, are separated from land by shallow water, or no water at all. Barrier reefs are located farther offshore. Lagoons more than 10 meters deep separate barrier reefs from land. Atolls are ring-shaped reefs that enclose a circular lagoon. These are found far offshore. Atolls are also described as coral islands.



Contents



Theories of Formation

There are several theories of formation but the most correct ones are from Charles Darwin and Reginald Daly. Darwin's theory is the Subsidence Theory. According to this theory fringing reefs form along the edges of an island and then, as the island subsides, a barrier reef is created. Actual subsidence is caused usually by one of two primary reasons: sea-floor spreading or the great weight of the island itself. Continued subsidence causes the central island to become totally submerged. This leaves a circular reef, an atoll, surrounding an open body of water. This theory was proven by boring into Eniwetok Island, (an atoll in the Marshall Islands of the West Pacific). During the deep drillings, a basaltic summit of an old volcano was encountered at depths of 1200 meters.

A second theory was suggested by Daly. This theory gives another view of a beginning fringing reef. He suggested glaciation as a reason for reef formation. According to his theory, he believes that the string of events leading to the buildup of fringing reefs on a flat surface began with a small fringing reef that encountered an ice age. The cold waters prevented new growth and sea level lowered due to alot of water being used to form a glacier. The part of the reef left out of water was eroded and cut down by waves. After the end of the ice age and the melting of the glaciers, sea level rose and coral reefs began growing once more. This time they grew on flat, planed surfaces. This theory has also been proven.

A third theory suggests that the formation of reefs and atolls is due entirely to sea level rise. All of the theories describe how some reefs form and all are correct to an extent. However, none of these are entirely correct, nor do they explain how all reefs form.

Contents



For more information on coral reefs, visit Sea World. Sea World has a variety of listings of almost any topic concerning corals. Some examples of what Sea World has to offer are habitats, physical characteristics, senses, and eating habits. Sea World also has information concerning conservation of corals and coral reefs.

Contents



This document was created by Star Moss
Dekalb College, Clarkston, GA
mossmeli@gpc.edu
Last modified on November 3, 1995