Usually three coral reef types are differentiated. These are defined through the morphology (size and shape) as well as the location (how close to land they form). The types are fringing reef, barrier reef and atoll.

diver at great barrier reefThis allows to distinguish between most of the reefs while some are not clearly one or the other form.

Coral reefs are complex ecosystem held together by calcium carbonate secreted by the coral and are highly important for the oceans.

The coral reefs are home to 25% of the marine life.

Many different species live in the reefs producing important nutrition or serving as food for larger animals. In addition juvenile fish grow up around reefs before they move on to more open areas.

These reefs also protect the coasts from being washed away and bring in billions of dollars to the economy of many countries through tourism.

 

 

Fringing Reef

The most common type of coral reef is the fringing reef. This type of reef grows directly from the shore out into the sea.

There might be deeper areas of water with sandy bottom forming between the beach and the edge of the coral. However, these are still shallow without creating a lagoon.

Fringing reefs are way more exposed to sediments and pollution brought into the ocean by rivers due to the location close to shore. These effects are worsened by an increasing human population and agriculture runoffs as well as removal of mangroves along the coast.

 

Barrier reef

Barrier reefs are similar to fringing reefs as they grow parallel to the coastline. However, they are separated from land by a larger body of water. The deeper water between reef and shore forms a large lagoon. The reef can grow up to shallow depth and even stick out at low tide.

Barrier reefs are less common with the largest ones in Australia and the south pacific. Only two barrier reefs can be found in the Caribbean.

 

Atoll

When a (often volcanic) islands surrounded by a fringing reef sinks entirely below sea level an atoll is formed. Atolls are usually round in shape with a large central lagoon. These are often located far away from land in the open sea.

Atolls are common in the South Pacific, Maledives and the Seychelles.

 

More types?

Some sources add patch reefs as a fourth type of coral reef. These are small, isolated patches of reef that grow up further away from shore. However, these might as well be early forms of fringing or barrier reefs and not a separate type of reef.

Other types have been described for example cays which are sandy islands formed on top of a coral reef.

 

coral reef raja ampat

 

How did the reefs develop

The three types of coral reef have first been described in Charles Darwin’s “reef theory“. He studied the reefs in the IndoPacific region.

His theory says that the reefs first grew was fringing reefs around a volcanic island. When the island started to subside into the ocean the coral kept growing building a barrier reef. At some point the island disappeared completely leaving only an atoll of coral close to the surface.

This process takes many thousand to millions of years allowing the coral enough time to grow fast enough to stay close to the surface.

The theory was confirmed later on but it has been shown that in different parts of the world reefs can form through different processes.

 

Where have you seen your favorite coral reef?

coral reef wakatobi

 

References:

What are the three main types of coral reefs? in National Ocean Service. Retrieved on 24/08/16 from http://oceanservice.noaa.gov/facts/threecorals.html
Types of Coral Reef Formation in Coral Reef Alliance. Retrieved on 24/08/16 from http://coral.org/coral-reefs-101/coral-reef-ecology/types-of-coral-reef-formations/
Types of Coral in Coral Reef Facts. Retrieved on 24/08/16 from http://www.coral-reef-info.com/types-of-coral-reefs.html