How is an atoll formed?

Charles Darwin provided one of the first explanations for how atolls form.

An atoll is one of the beautiful but mysterious land formations that occur in remote locations in warm tropical waters such as the Pacific Ocean. Formed over 30 million years ago, an atoll is home to up to 300 species of coral and a host of other marine life. Seen from above, it looks like a flat ring of sandy land with a lagoon in the center. An atoll should not be confused with an island, as their geographical formation is significantly different.

Coral reefs are a key element in the formation of atolls.

Charles Darwin provided an explanation for the formation of an atoll in 1842 that is still widely accepted today. Based on his rough observations of South Pacific atolls, Darwin concluded that these unique land creations were the result of two factors that occurred simultaneously – the growth of a coral reef and the gradual sinking of an oceanic island.

The atoll formation process takes up to 30 million years. It starts with a new tropical oceanic island, which exists due to collisions of tectonic plates or movement of plates over oceanic hot spots of volcanic activity. Most atolls, especially those located in the Pacific Ocean, are products of previous volcanic islands. These newly formed volcanic islands will have no sign of any form of life, either on land or in the water. This is because tropical waters are relatively warm and lack sufficient nutrients to support marine life.

The central lagoons of atolls are formed by coral growth and volcanic activity.

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One hundred thousand years later, a coral reef surrounds the volcanic island’s shores. Coral reefs are built by hermatypic corals, organisms that thrive in warm surface waters. These organisms multiply and reproduce corals along the coast of the island. New corals grow on top of older or dead ones, and the process continues as the volcanic island begins to sink. This is the second step in the formation of the atoll.

Waves cause erosion.

Over the next 25 million years, fringing coral reefs transform into barrier reefs. Barrier reefs are formed when the island sinks almost to sea level while corals continue to grow. These geographical movements in opposite directions result in the development of lagoons between the sinking island and the growing coral reef.

An atoll is born five million years after that, when the island sinks completely below sea level and the coral reef continues its growth towards and above the surface of the water. The lagoon that was produced in the previous stage now dominates the center of the atoll. Eventually, due to wind and wave erosion, the corals break up into pieces and become sandy, creating a land surface called an islet.

The atoll formation process begins with a new tropical oceanic island, which exists due to the movements of tectonic plates.

The atolls are known for their beautiful coral reefs and colorful marine life. Many are attractive sights, particularly for diving enthusiasts and snorkelers. Famous atolls around the world include those within the Tuamotu Archipelago, the Coral Sea Islands and the Caroline Islands in the Pacific Ocean, as well as the Chagos Archipelago and the Maldives in the Indian Ocean.

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