Coral reefs are marine ridges or mounds, which have formed as a result of the deposition of calcium carbonate by living organisms, predominantly corals, but also a rich diversity of other organisms such as coralline algae and shellfish. 

Coral Reef Formation :-

• Coral reefs begin to form when free-swimming coral larvae attach to submerged rocks or other hard surfaces along the edges of islands or continents.

Condition for Coral Growth :

• There are certain special conditions which are necessary prerequisites for the growth and development of corals. That is why the reef building corals and their associates are not uniformly deposited throughout the tropical warm ocean waters.
• The most important of these conditions are the following:

The surface temperature:-

• For the growth and development of corals the surface temperature of the ocean must be above 20°C. Corals cannot live if the temperature of the sea water falls below this.

Depth:-

• The water must be shallow, with a depth not exceeding 25-30 fathoms. With increasing depth, the amount of calcium as well as the temperature of water goes on decreasing, so that the coral polypsand other sea creatures which live on calcium carbonate do not thrive. That is why the reef building corals live in shallow waters and on the surface of the seas.

Salinity:-

• The water must be normally saline, and, therefore, along the ocean margins where the water becomes fresh by the inflow of rivers, reef building corals do not grow and develop.
• Therefore the average salinity ranging from 27% to 40% is ideal for the proper growth of corals and other reef forming creatures.

Sediment:-

• Corals need clear water that is free from abundant sediments. That is why coral reefs cannot develop where rivers enter the sea or where wave-erosion causes muddy coastal water.

• There must be adequate food supply to nourish the abundant life of the coral reef. The most favourable condition for this purpose is the presence of continuously flowing ocean currents which provide to the stationary reef building organisms the much needed food supply For example, extensive coral reefs are found on the east coasts of Australia, Central America, and Africa, which are washed by warm ocean currents flowing along them. On the contrary, corals are found only in scattered patches on the west coasts of these continents.
• The most essential prerequisite for the formation of coral reefs is the presence of sub-marine platforms which must lie near the sea shore, or should be attached to some islands. The depth of water on such platforms should not exceed 50 fathoms.
• Corals cannot live for long out of water, and are therefore, rarely found above the low-tide level. On the other hand, their growth is retarded at depths much exceeding 25 or 30 fathoms.
• Corals need clear oxygenated water with sufficient supplies of microscopic life as food.

Types of reefs:-

• As the corals grow and expand, reefs take on one of three major characteristic structures – fringing, barrier or atoll. 
• Fringing reefs:  
  1. They are the most common, project seaward directly from the shore, forming borders along the shoreline and surrounding islands. 
  2. They are located very close to land, and often form a shallow lagoon between the beach and the main body of the reef.
  3. A fringing reef runs as a narrow belt [1-2 km wide]. This type of reef grows from the deep sea bottom with the seaward side sloping steeply into the deep sea.
  4. Coral polyps do not extend outwards because of sudden and large increase in depth. The fringing reef is by far the most common of the three major types of coral reefs, with numerous examples in all major regions of coral reef development
• Barrier reefs: 
  1. Grow at border shorelines, but at a greater distance. They are separated from their adjacent land mass by a lagoon of open, often deep water. 
  2. It is largest in size of the three reefs.
  3. Barrier reefs are far less common than fringing reefs or atolls, although examples can be found in the tropical Atlantic as well as the Pacific.
• Atoll:
  1. If a fringing reef forms around a volcanic island that subsides completely below sea level while the coral continues to grow upward, an atoll forms.
  2. Atolls are usually circular or oval, with a central lagoon. 
  3. An atoll is a roughly circular (annular) oceanic reef system surrounding a large (and often deep) central lagoon.
  4. The lagoon has a depth 80-150 metres and may be joined with sea water through a number of channels cutting across the reef.
  5. Atolls are located at great distances from deep see platforms, where the submarine features may help in formation of atolls, such as a submerged island or a volcanic cone which may reach a level suitable for coral growth.