What is a Coral Reef?

A quarter of marine species make their homes in coral reefs.

A coral reef is a ridge or a mound of limestone whose upper surface is near the surface of the sea. Coral reefs are made up of calcium carbonate. The last glacial period marked the formation of coral reefs when melting ice caused the sea levels to rise and flood the continental plates. Known coral reefs are found in the deep sea far from the continental shelves and around oceanic islands as atolls, most of which are volcanic in derivation. The few exceptions have tectonic origins where plate movements have lifted the ocean floor on the surface.

Formation of Coral Reefs

Several theories try to explain the formation of coral reefs. So far, only two have been approved as they explain the great vertical thickness of coral reefs. These two include:

Darwin's Subsidence Theory

According to this theory, coral reefs were initially fringing reefs on slanting shores, after which they became barrier reefs when the shores sank, with a water channel between them and the land. If the land is an island that sinks entirely, it results in the formation of an atoll, which means the sinking causes the thickness of the reefs.

Daly's Glacial-control Theory

This theory states that during the last glacial period, the formation of ice caps lowered the ocean levels by 60 to 70 meters below the current ocean levels. Waves then cut the shores to make flat platforms suitable for the growth of coral reefs. Over time as the ice caps melted and the levels of temperatures rose, coral began to grow on those platforms and rose upwards managing to raise the ocean levels. After that, all types of reefs were formed on the existing platforms.

Types of Coral Reefs

Coral reefs can be sorted into the following three groups:

Fringing Reefs

They are also referred to as shore reefs. The fringing reefs can be built from the sea bottom and later extend from the shore having no navigable channel between the shore and the reef. Most of these reefs are composed solely of coral sand with both living and dead coral building reefs and mud. Most fringing reefs have been located in East Indies.

Barrier Reef

Barrier reefs are almost the same as fringing reefs except a navigable channel, also called a lagoon, separates the reef from the shore.

Atoll Reef

We mostly refer to atoll reefs as a coral island or lagoon island. The reef is circular or horseshoe-shaped and encloses a lagoon of water. In some occasions, the reefs can be broken to form navigational channels, some of which are suitable and others are not.


Coral reefs are an important factor in the ecosystem, in tourism, fisheries, and coastline protection. Coral reefs offer coastline protection to nearby islands by absorbing wave energy. The global economic value of coral reefs ranges from US $29.8 billion to US $375 billion per year. About 25% of all marine species live in shallow coral reefs, although they occupy less than 0.1% of the world's ocean surface. An annual average of six million tons of fish being taken. As such, overfishing is a major concern, along with fish die-off due to disease.

However, coral reefs are under threat around the world. Due to rising ocean temperatures and pH changes, coral reefs are dying off, and are in danger of extinction. About 10% of coral reefs are now dead, and about 60% are considered at risk. By 2050, all of the world's coral reefs are estimated to be in danger. Protection and restoration processes are now of the utmost importance.


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