They can start out as underwater volcanoes, take up to 30 million years to form – and are home to about a million people.

What is an atoll?

An atoll is a ring-shaped island (often made up of several small islands) with Coral A rim encircles the lake. Atolls are located in warm tropical or subtropical parts of oceans and seas, because this is where reef-building corals can develop.

The exact number of atolls around the world varies depending on the source, but a Census It was published in 2023 and yielded 494 oceanic atolls and 104 continental atoll systems. Of these, 344 are located in the Pacific Ocean (including Japan), 77 in Indonesian waters, 68 in the China Sea, 54 in the Indian Ocean, and 29 in the Australian Coral Sea and the Pacific and Indian Ocean waters surrounding the continent.

The word “atoll” is derived from the Maldivian word “athulhu”, which can be understood as “a circular group of atolls”.

How are coral islands formed?

It was Charles Darwin Who first proposed a theory about the formation of atolls in 1842 – this theory is called the subsidence model. According to her, coral islands develop from Underwater volcanoescalled seamounts. First, the volcano erupts and lava accumulates on the sea floor. The elevation grows higher and higher, eventually emerging above the surface of the water and becoming an island.

In the next stage, the corals build a reef around the island – a fringing reef forms surrounding the island just below the surface. The strip of water between the reef and the island is the lagoon.

Over time, the volcanic island erodes and sinks, dissipating into the sea in a process called subsidence. The top of the reef continues to grow upward at roughly the same rate as the island sinks, so it stays close to the surface even as the land beneath it disappears. Eventually, the volcanic summit disappears, leaving only the coral ring, now a barrier reef.

The atoll is now fully formed – this whole process can take between 1 million and 30 million years.

However, this model has been challenged and an alternative theory was first proposed in the 1930s Previous karst model.

In this model, a limestone island built from an ancient, long-dead coral reef is exposed above the sea during a period of low sea levels. Rainwater slowly dissolves the limestone through a process called karstation, forming a hollow basin in the center of the island. When sea level rises, coral colonizes the raised outer edge of this basin, submerging the center to form the lagoon.

What is the largest atoll?

Greater Chagos Bank – largest total area

The Greater Chagos Bank is located in the Indian Ocean about 500 kilometers south of the Maldives, and extends over a massive area of ​​12,642 square kilometres. Most of it is located underwater. Only a few small islands appear on the surface, and their combined land area is only 5.6 square kilometers. Administered by the United Kingdom as part of the British Indian Ocean Territory, it is also a designated nature reserve.

Kwajalein Atoll – the largest lagoon

Located in the Marshall Islands in the Pacific Ocean, Kwajalein is made up of about 97 islands and islets surrounding a lagoon that is often cited as the largest in the world with an area of ​​about 2,174 square kilometers – even though it has a land area of ​​only 16.4 square kilometers.

Kiritimati Atoll – the largest land area

Kiritimati, also known as Christmas Island, has the largest area of ​​dry land of any atoll on the planet at 321 square kilometres. This atoll is part of Kiribati, an island nation in the Pacific Ocean.

Huvadhu Atoll – Most of the individual islands

This atoll is located in the south of the Maldives, in the Indian Ocean, and includes a large number of islands: a total of 255 islands, distributed over a total area of ​​3,152 square kilometers.

How many people live on atolls?

About one million people inhabit atolls worldwide, and approximately half of this population resides in major island nations and territories, such as the Maldives, Kiribati, Marshall Islands and Tuvalu.

What wildlife lives on atolls?

The atolls’ wildlife is sparse due to their small land areas, but they still support a rich diversity of life, from thriving marine ecosystems in their shallow lagoons to seabird breeding grounds on their sandy islands.

According to A He studies Published in the magazine Nature ecology and evolutionThere are 37 Classify Of the seabirds that breed on atolls – the numbers of each range from a few dozen to more than 3 million individuals on each atoll. The study estimates that more than 31 million seabirds nest on atolls, about a quarter of all tropical seabirds in the world.

Atolls are crucial to the survival of species such as White sea ternBlack gestures and less Frigate birdas more than half of the world’s population nests there.

Are atolls at risk due to climate change?

Atolls are particularly vulnerable to Climate change Because most of their land is located only 1-2 meters above sea level. This makes it highly vulnerable to even modest increases in sea level, as a sea level rise of just half a meter could dramatically reduce the area of ​​habitable land.

The land itself is not sinking (in most cases) – sea levels are rising around it, and coral’s ability to grow upward to keep up is also under threat from ocean warming, acidification and bleaching.

Not only is the island at risk of sinking, but the low elevation makes the atolls vulnerable to storm surges, saltwater intrusion into drinking water supplies, and coastal erosion.

Some studies have found that some atolls actually grow in land, as waves redistribute sediment onto the island. For example, 2020 He studies Published in the magazine Geophysical Research Letters It has been documented that J Island, part of the Marshall Islands, grew by merging at least two islands between 1943 and 2006.

Top image: An atoll near Ternate Island, Indonesia. Credit: Velvetfish/Getty Images