Coral Reefs: Definition, Types, Formation and Economic Importance

Table of Contents

What are Coral Reefs?

Coral colonies grow continuously in size by budding of polyps and often form extensive masses, known as coral reefs.
According to T. Wayland Vaughan (1917), A coral reef is a ridge or mound of limestone, the upper surface of which is near the surface of sea and which is formed chiefly of CaCO₃ secreted by coral polyps.
Principal builders of coral reefs are stony corals (Madreporaria), but other important contributors are the hydrocorallines and alcyonarians.
Coralline algae and Foraminiferan Protozoa also take part in the formation of coral reefs.
Reef building corals require warm shallow waters (normally above 20^oC).
They are therefore limited to the Indo-Pacific, the Central-Western Pacific, and the Caribbean regions north of Bermuda.
About 50 species of corals contribute in the formation of reefs along the Florida keys and in the West-Indies.

Coral reefs lying close to the shores of some volcanic island or part of some continent are termed fringing reefs.
A fringing reef may extend out to a distance of a quarter mile from the shore with the most active zone of the coral growth facing the sea.
This seaward zone is commonly called the edge or front.
A shallow water channel, 50 to 100 meters broad, lies between the reef-edge and shore.
At low tide, water of channel recedes at quickly exposing a flat bottom surface, called reef flat.
It is largely composed of coral sand, mud, dead and living coral colonies and other animals.

Barrier reefs are like fringing reefs but they are located some distance away from the shore.
The stretch of water, separating the barrier reef from land, may be half a mile to 10 miles or more in width. It is called a lagoon.
It is 10 to 50 fathoms deep and suitable for navigation.
Most notable example of barrier reef is the Great Barrier Reef along the North-eastern coast of Australia.
It is about 2,000km long and up to 150 km from shore.

An atoll is also termed a coral island or lagoon island.
It is a ring-like or horse-shoe-shaped reef that encircles a lagoon but not an island.
The lagoon varies from a few to about 90km across.
It may be complete or broken by a number of channels, of which only a few are navigable.
Outer side of the reef slopes off rather steeply into the depth of ocean.
The atoll of Bikini, famous for atomic and hydrogen bomb tests, lies in the Pacific Ocean.

Many theories have been advanced to explain coral-reef formation, but none are entirely satisfactory.
Two theories seem to be of some convincing importance:-

According to this theory, as put forth by Darwin (1831), fringing reef was first formed on the sloping shore of an island.
Subsidence of sea-floor then commenced in the regions of reef followed by upward and outward growth of coral.
Thus, the fringing reef became the barrier reef.
By gradual sinking the island ultimately vanished and the barrier reef became a coral atoll with a central lagoon.
In time it acquired a growth of vegetation.

Another theory, as propounded by Daly, accounts for the lowering of the ocean level by the withdrawal of water for glacial formation.
This resulted in the exposing of several flat platforms cut out by the action of waves.
When the glaciers melted and the temperature became favorable, corals began to grow on these platforms, building higher as the ocean level rose.
Most reefs grew at the rate of 10-200 mm each year.
Most of the exciting reefs could have formed with in a period of 15,000 to 30,000 years.

Corals of the remote geological past formed reef structures that were highly favourable sites for the accumulation of petroleum deposits.
Thus, coral reefs are of much importance to oil industry.
Large quantities of corals are shipped every year for the curio trade.
The coral reefs serve as habitats for many plants and animals like sponges, molluscs, echinoderms, fishes, etc.
Some coral reefs are used as habitations by man as well.
Some corals are highly priced for their decorative value.
Corallum rubrum is considered to be a precious stone in India and China and treated as auspicious.
The red coral and organ pipe coral are used in some indigenous system of medicine in S. India.
Chunks of coral skeleton belonging to species Porites are used as building material.
Coral skeletons serve as raw material for the preparation of lime, mortar and cement because of their calcium carbonate and magnesium carbonate content.
Coral skeletons are also helpful in making ridges that may act as natural barriers against sea erosion and cyclonic storms.
Coral reefs serve as good nursery grounds for commercially important fishes.
Reef fish varieties are more colorful than others.