What Is an Atoll and How Does It Form?

An atoll is an oceanic coral island or a set of various small islands known as islets which are characterized by their ringed shape. The circular or annular shape encloses an interior lagoon which has access to the sea, generally of shallow depth.

Most of the atolls are located in the Pacific Ocean, with some in the Indian Ocean and even fewer in the Atlantic. Some atolls like Bora Bora have a central island, which is actually an ancient volcano. Although the term atoll comes from an Indo-Aryan language (the ‘Dhivehi’) spoken in the Maldives, the word was popularized by Charles Darwin.

The father of evolutionary theory, Darwin used his observations to develop a theory on the formation of coral reefs and atolls. He included this theory in his landmark scientific work The Origin of Species. This theory still stands, but it has been refined and expounded upon since the 19th century when it was first proposed. We look at how an atoll is formed in the section below.

Discover more about the types of coral species which may appear on an atoll with our related article.

Atolls originate from volcanic islands located in the oceans. After extended periods of time of volcanic activity, these islands emerge above the ocean surface. The particular conditions between seawater and the bedrock of volcanoes provide a favorable environment for the growth of reef-forming corals.

Over the years, coral reefs gradually accumulate at the submerged base of the island. As the volcanic island moves away from the region of hot spot activity, it sinks due to rising sea levels or experiences a progressive decline due to the effects of erosion. The reef forms an outer ring that encloses an inner lagoon. This lagoon gradually covers the structure of the submerged volcano.

It is important to highlight that the process of formation of an atoll is a phenomenon that can extend over approximately 30 million years. As the reef continues to grow, sediment accumulates in its structures, leading to the development of low islands at the top of the reefs. These islands, covered in sand and vegetation, are an integral part of the atoll.

To give a better idea of atoll formation, we condense the generally accepted steps of how an atoll is formed below:

1. Original formation: the original volcanic island is formed from the Earth's crust as magma is cooled by the ocean. This has occurred millions of years ago. Discover in greater detail how volcanoes are formed with our related article.
   
   
2. Coral formation: coral reefs form on the coral island. Coral are colonies of cnidarians which only exist in their polyp stage. Hard coral polyps develop skeletons from calcium carbonate which grow into massive colonies over time until they become a coral reef. Discover the difference between hard and soft coral.
   
   
3. Sinking: the original volcanic island and the coral which builds upon it sinks for various reasons, with the tectonic movement which created the original structure being one of the most significant.
   
   
4. Lagoon formation: although the volcanic island submerges further into the water, the coral reefs continue to grow up towards the sunlight above the water's surface. It is this growth which forms the islets and ring-shaped island. Within the middle of this ring, the lagoon forms.

Once the atoll has developed over millions of years, it will have reached its annular shape. The lagoon is in the middle and a circular strip of land surrounds it.

In most cases, very little of the original volcanic island material is present since this has sunk beneath the surface millions of years ago. Most of the islands closer to the surface are made from the calcified coral skeletons which have formed the reefs over millions of years.

Although the structure of the atolls is made from the calcified coral skeletons, millions of years has allowed sediment and sand to grow on top of them. This sediment and sand is made from coral, shells and other organic matter which has accumulated. Organic matter such as plants and trees can grow on top of it.

Below we provide some examples of atolls from around the world with photos.

Bikini Atoll is one of the atolls that are part of the Marshall Islands and is world famous for the nuclear tests that have been carried out there. These have been orchestrated by the United States which tested atomic and hydrogen bombs there between 1946 and 1958. The result is a devastated site for its former local inhabitants. Currently it has radiation higher than that of Fukushima.

Bora Bora Atoll is located in French Polynesia and is one of the Leeward Islands of the Society Islands. A distinctive characteristic is that it has a central mountainous area (extinct volcano seen in the photo below) surrounded by a barrier of reefs and islets with white sandy beaches called motus. The island is surrounded by crystal clear waters that are home to a variety of marine life, including rays and sharks. In addition, Bora Bora has lush tropical vegetation inside.

Discover how much of the Earth is covered by water in our related article.

Located in the Indian Ocean, Aldabra Atoll is part of the Outer Islands and is the second largest atoll in the world after Kiritimati. It has a great wealth of flora and fauna and among its most important animals stands out the Aldabra giant tortoise (Geochelone gigantea) endemic to the island.

Kiritimati Atoll, also known as Christmas Island, is the largest atoll in the world. It is located in the Pacific Ocean, towards the north of the Line Islands archipelago. Kiritimati has been inhabited by Polynesians for centuries. The arrival of European and American explorers left their mark on the island's history, as during World War II it was an important military site for the United States. An aerial photo of the island can be seen below.

The Rowley Shoals constitute a trio of coral reefs (Mermaid, Clerke and Imperieuse) with atoll characteristics, located in the southern region of the Timor Sea. It is approximately 260 km west of Broome, Australia. These atolls have a remote and pristine location, offering a natural sanctuary for a diverse range of marine species.

The Australian Government has taken significant steps to protect and conserve the Rowley Shoals, recognizing the importance of these coral reefs. In a conservation initiative, Clerke Reef and Imperieuse Reef have been designated as marine parks, while Mermaid is a nature reserve.

Now that you know what an atoll is and how it is formed, you may also be interested in some related articles which explain how oceanic landmarks are formed. This includes what is a mid-ocean ridge and what is coastal erosion??

If you want to read similar articles to What Is an Atoll and How Does It Form?, we recommend you visit our Ecosystems category.