Atolls

You may have amazed by seeing through photographs or really, an isolated ring shaped island or coral reef in the ocean. Stop wondering now. Here is what they are and how they are formed.

A ring shaped coral reef including a coral rim that encircles a lagoon completely or partially is known as an atoll. Water body surrounded by the atoll is called a lagoon. They can be formed in tropical and former tropical seas. Sometimes atoll and lagoon protect a central island.

People in the past began to wonder how these atolls formed. They came up with many theories about the atoll formation. Among them, most famous, widely used and quoted theory was presented by Charles Darwin. As Darwin presented this, it was known as Darwin’s theory of atoll formation.

Charles Darwin was one of the first scientists who explained the geology and the theory of the origin of coral reefs. According to this theory there are three types of coral reefs. They are as follows;

1. Fringing reefs.
2. Barrier reefs.
3. Atolls.

According to this theory, the process of atoll formation starts with a volcano building up to the surface from the seafloor. First the volcano erupts and piles lava on the seafloor. This continues until this volcano becomes a volcanic island. When it becomes inactive there develops a fringing reef around the volcanic island. The coral type which makes this reef is called hermatypic or hard corals. They create a hard limestone exoskeleton. This fringing reef can be directly attached to the shore or separated from the shore by a shallow channel or lagoon.

After thousands of years the centre of the volcanic mountain starts to subside. With this, the top of the mountain becomes flat by the strong powerful waves and makes a Guyot. Then after, the previous fringing reef becomes a larger barrier reef. Barrier reefs occur further offshore separated by a deep channel or lagoon.

With this subsidence, it changes the reef lot. While the reef facing oceanward remains healthy, corals facing lagoon side start to decay slowly. Limestones decay and change the colour of the lagoon from deep ocean blue to bright colour.

At the final stage, because of the subsidence the volcanic island sinks below the sea. Decaying part of the reef break and erode the corals into tiny grains of sand. This sand and other sediments pile up on the reef and form ring-shaped island or islets. So, the barrier reef develops into an atoll enclosing an open lagoon. This lagoon forms over the volcanic carter.

Most famous atolls of the world are atoll islands of Maldives, Lighthouse reef with the famous great blue hole, Bikini atoll, Aitutaki atoll, Tikehan atoll and many more. Today these atolls have become most famous tourist destinations and diving and snorkeling sites, because of its biodiversity and captivating seascapes.