In this article we will discuss about Coral Reefs:- 1. Definition of Coral Reef 2. Distribution and Conditions of Coral Reef Formation 3. Components 4. Types 5. Formation.
Definition of Coral Reef:
Vaughan (1917) has defined coral reef as “a ridge or mound of lime stone, the upper surface of which is near the surface of the sea and which is formed of calcium carbonate by the actions of organisms, chiefly corals.”
The coral reefs are, in fact, produced by corals belonging to Anthozoa, particularly by stony corals, the Madreporaria. Hence, these are supposed to be the principal builders of coral reefs though there are certain other contributors also. A coral reef, in fact, is a ridge of lime stone whose upper surface is just below the sea surface and it is exposed at low tides.
Distribution and Conditions of Coral Reef Formation:
Corals have built a thick stratum of the earth’s crust, they have coral reefs in the Caribbean seas and in the Indo-Pacific region from east coast of Africa to the north-eastern coast of Australia which is known as the Great Barrier Reef. However, Fiji islands of Pacific Ocean and those situated in Bahama islands region are the best known coral islands of the world.
Bermuda is a coral island where houses are built of coral blocks. Around India, coral reefs are found off Port Okha and Dwarka in the gulf of Kutch and also off Rameshwaram in the gulf of Mannar between India and Sri Lanka. The coral reefs are also located at Andaman and Nicobar islands and at Lakshadweep Islands.
The coral reefs grow best at a depth of about 30 metres or less and normally in warm water up to about 20°C. Light and amount of sediment also limit the reef forming corals. They also fail to grow in dark shaded areas and they completely die in total darkness. Below 50 metres no reef building corals are found though some solitary corals exist up to 8000 metres.
Components of Coral Reefs:
In addition to stony corals, other components in the formation of coral reefs are Millepora, Tubipora, Heliopora, Alcyonaria, Gorgonians, Foraminifera, Coralline algae and branching algae, etc. The coral reefs are also inhabited by a number of sponges, anaemones, sea-urchins, starfishes, crabs, snails, bivalves, etc.
Types of Coral Reefs:
The various types of coral reefs are grouped into three major types:
1. Fringing Reefs:
The fringing reefs also referred to as the shore reefs are built from the sea bottom and extend from the shore up to 1/4 miles having no navigable channel between the shore and reef. This zone of the sea is called edge or front. However, sometimes reef beds are broken to result into irregular channels called lagoon.
Such reefs are composed largely of coral sand having living and dead corals building reefs, mud and other animals. Fringing reef is very common in East Indies.
2. Barrier Reef:
The barrier reefs are like fringing reefs but they are situated in the sea nearly 1 km to 15 km away from the shore. Therefore, navigable channel called lagoon separates these reefs from the shore. The lagoon may be 20 to 40 fathoms deep, hence, it becomes navigable. The Great Barrier Reef on the north-east coast of Australia is 1,200 miles long, about 20-70 miles wide and situated nearly 90 miles away from the shore.
3. Atoll Reef:
The atoll reef, also referred to as coral island or lagoon island, is a circular or horse-shoe-shaped reef enclosing a lagoon of water which may be small or large up to 50 miles across.
Atoll reef may be broken to form channels; some suitable for navigation and other may not be suitable for it. An interesting atoll reef example is Aldabra in the Indian Ocean, about 260 miles northeast of the Malagasy Republic and 400 miles from the coast of Africa.
Formation of Coral Reefs:
The coral reefs have great vertical thickness though reef-building corals live only up to depth of 50 metres, and those of past geological ages also lived in shallow littoral waters. How the great thickness of coral reefs has been made is explained by several theories of which two may be considered.
1. Darwin’s Subsidence Theory:
This theory states that coral reefs were first formed as fringing reefs on sloping shores, they became barrier reefs when the shores sank with water channel between them and the land. If the land is an island which sinks completely, then an atoll is formed. Thus, sinking or subsidence has caused the thickness of the reefs.
2. Daly’s Glacial-Control Theory:
This theory states that during the last glacial period the formation of ice caps lowered the ocean level by 60 to 70 metres below the present surface. Waves cut the shores to make flat platforms suitable for growth of corals.
As the ice caps melted and temperature rose, corals began to grow on these platforms and rose upwards with rising ocean level, and all types of reefs were formed on the pre-existing platforms. There is evidence that coral reefs are growing today on submerged land and the foundations of reefs are now at a much greater depth than they were when corals first began to grow.
Observation of living corals shows that their rate of growth is from 5 mm to 20 cm per year, thus, a 50-metre deep reef could be formed in less than 8,000 years, and all the known reefs could have been built in under 30,000 years.
Some borings made in coral reefs showed that the reef rested on level platforms, but some other borings showed that reefs had no underlying platforms but had only sand and shell below them. It appears from these facts that some reefs were laid down on pre-existing platforms, but many reefs were formed according to Darwin’s subsidence theory.