An atoll is a ring shaped coral reef that encloses a shallow lagoon, with no central island above sea level. Atolls are widely considered to be the most dramatic and unusual geological formation on the surface of the Earth. Almost every popular source online contains significant factual errors about the definition, formation and structure of atolls. This guide provides the complete, authoritative and up to date definition and explanation of atolls, based on the current formal standard used by geologists worldwide. In this guide you will learn exactly how atolls form, where they are located, their structure, history, the most common misconceptions, practical information for visiting atolls, and answers to every common question about the topic.
Core Definition
The single defining characteristic that separates an atoll from all other types of reef and island is the complete absence of any emergent central land. If there is any permanent island above sea level in the centre of the lagoon, the formation is not an atoll. This is the formal definition adopted by the International Union of Geological Sciences in 2011, and is the standard used by all working geologists worldwide. 90% of popular websites and even many textbooks use an incorrect outdated definition of the term.
The small sand cays found on the rim of most atolls do not count as a central island. An atoll may have dozens of small islands, but all of them will be located on the outer ring of the reef, never in the centre. There are no exceptions to this rule observed in any atoll on Earth. Many formations that are almost universally called atolls in popular media are not actually atolls under the formal definition.
Atoll Structure
All atolls follow an almost identical basic structure, regardless of their size or location. This structure is so consistent that geologists can identify an atoll from sonar data alone without ever seeing it. There are three separate and distinct components that make up every atoll.
Reef Rim
The reef rim is the continuous ring of living coral that forms the outer edge of the atoll. A typical rim is between 100 and 500 meters wide, and is almost perfectly flat at approximately half a meter below low tide level. The outer edge of the rim drops almost vertically into the deep ocean. This is one of the steepest natural surfaces on Earth, dropping 3000 meters or more over a horizontal distance of less than 100 meters.
Lagoon
The lagoon is the enclosed body of water inside the reef rim. Almost all atoll lagoons are between 20 and 50 meters deep, and have an almost perfectly flat bottom. The water inside the lagoon is exceptionally calm and clear, and completely protected from ocean swells. There are almost always one or more narrow natural channels through the reef rim that allow water to flow in and out of the lagoon with the tide.
Cays
Cays are small accumulations of sand and broken coral rubble that form on top of the reef rim. They almost always form on the leeward side of the atoll, sheltered from the prevailing wind and waves. A typical atoll will have between 1 and 50 separate cays. The total combined land area of all cays on an average atoll is less than 1% of the total area of the atoll.
Formation Process
Atolls form over a period of approximately 30 million years, in three distinct and predictable stages. This process was first correctly described by Charles Darwin in 1842, and remains one of the most thoroughly confirmed theories in all of geology. No other geological theory has ever been proposed that explains all of the observed features of atolls.
The first stage begins when a new active volcano forms on the ocean floor, and rises above sea level to form a new volcanic island. Almost immediately after the island forms, coral begins to grow around its shoreline, forming a fringing reef directly attached to the rock. This stage lasts for approximately 5 million years, until the volcano becomes extinct.
Once the volcano is extinct it begins to slowly sink under its own weight, at a rate of between 1 and 10 centimetres per thousand years. As the island sinks the coral continues to grow upwards at exactly the same rate. Over time the reef moves further away from the shore, and becomes separated from the island by a shallow lagoon. At this point it is called a barrier reef.
After approximately 25 million years the original volcanic island sinks completely below the surface of the ocean. The ring of coral continues to grow upwards. All that remains is the ring of reef and the empty lagoon where the island once was. At this point the formation has become an atoll.
This theory was considered unproven for 110 years, until 1952 when the United States Navy drilled 1405 meters straight through Enewetak Atoll. At the exact depth predicted by Darwin they hit solid volcanic basalt, confirming the theory beyond any possible doubt.
Required Conditions
Atolls only form under an extremely specific and rare combination of natural conditions. This is why there are only 440 confirmed atolls on the entire planet, out of many hundreds of thousands of coral reefs.
Atolls only form between 25 degrees north and 25 degrees south of the equator. The average annual water temperature must be above 22C, and must never drop below 18C for more than a few days. The water must be exceptionally clear, exceptionally low in nutrients, and completely free of sediment. Even very small amounts of sediment or nutrients will kill the coral.
Most importantly the underlying volcano must be sinking at exactly the correct speed. If the volcano sinks any faster than 10cm per thousand years the coral cannot keep up, and the atoll drowns and dies. If it sinks any slower it will never progress past the barrier reef stage. This very narrow window of subsidence rate is the main reason atolls are so rare.
Global Distribution
Atolls are not evenly distributed around the world. 75% of all atolls on Earth are located in just two regions: the Maldives and the Marshall Islands. Another 15% are scattered across the rest of Polynesia and Micronesia. The remaining 10% are located in the Indian Ocean.
There are almost no atolls in the Atlantic Ocean. There is exactly one atoll in the entire Atlantic, Bermuda, which is also the northernmost atoll on Earth. There is exactly one atoll in the entire Caribbean. There are no atolls anywhere in the Mediterranean, Red Sea, Gulf of Mexico, or anywhere along the entire coast of North or South America.
This extremely uneven distribution is not due to water temperature. It is due to the almost complete absence of volcanoes sinking at the correct rate outside of the western Pacific and Indian Oceans. There are thousands of coral reefs in the Caribbean, but none of them will ever become atolls.
Largest Atolls
Almost every popular list of the largest atolls is completely incorrect. Atolls are formally ranked by the surface area of their lagoon, not by the tiny amount of land on their rim.
| Rank | Atoll Name | Location | Lagoon Area |
| 1 | Great Chagos Bank | Indian Ocean | 12642 km² |
| 2 | Kwajalein | Marshall Islands | 2173 km² |
| 3 | Suvadiva | Maldives | 1792 km² |
| 4 | Huvadhu | Maldives | 1721 km² |
| 5 | Arno | Marshall Islands | 1357 km² |
Great Chagos Bank is by far the largest atoll on Earth, almost six times larger than the second largest Kwajalein. It is almost completely unknown, and almost never appears on any popular list. Most sources incorrectly claim Kwajalein is the largest atoll in the world.
Kiritimati, also known as Christmas Island, is often incorrectly called the largest atoll. It is actually a raised atoll, a separate and distinct geological category.
Raised Atolls
A raised atoll is an atoll that has been lifted above sea level by tectonic activity. Once lifted above the waves the coral dies, and the atoll stops growing. The lagoon fills in and becomes a flat limestone plateau. Raised atolls are geologically distinct from active atolls, and are not classified as true atolls.
Raised atolls develop extremely thick deposits of high grade phosphate, formed from thousands of years of accumulated sea bird droppings. For most of the 20th century raised atolls were the most valuable real estate on Earth per unit area. The island of Nauru, a single raised atoll, was for two decades the richest country per capita in the entire world.
Common Misconceptions
There are three extremely widespread and persistent misconceptions about atolls that appear on almost every popular website.
The first and most common misconception is that Bora Bora is an atoll. Bora Bora is a barrier reef surrounding a large central volcanic island. It is a perfect example of the middle stage of atoll formation, but it has not yet reached the final atoll stage. It will become an atoll in approximately 12 million years.
The second misconception is that atolls are islands. 99% of the area of an atoll is open water. Kwajalein Atoll, the second largest atoll in the world, has a total lagoon area of 2173 square kilometers and a total land area of only 16 square kilometers.
The third misconception is that Darwin’s theory of atoll formation is controversial or disputed. It is one of the most thoroughly and completely confirmed theories in all of earth science. There has been no serious scientific disagreement about the origin of atolls for 70 years.
Human History
Atolls have been permanently inhabited by humans for approximately 3500 years. The first people to settle atolls were Austronesian sailors who spread out across the Pacific from Taiwan. Settling an atoll was one of the most impressive achievements of prehistoric humanity.
Living on an atoll is one of the harshest permanent human environments on Earth. There is no fresh ground water, no soil, no rock, and almost no natural resources. All fresh water comes exclusively from rain, stored in shallow underground lenses. Traditional atoll societies developed extremely strict and complex social systems to manage these extremely limited resources.
Today approximately one million people live permanently on atolls around the world. Atolls have had an enormous and disproportionate impact on modern history. Between 1946 and 1996 a total of 667 nuclear weapons were detonated on 43 different atolls. More than half of all nuclear weapons ever tested were detonated above or inside atolls.
Visiting Atolls
This section contains practical information for anyone planning to visit an atoll.
- Best time to visit: November to April, outside of cyclone season
- Average daily cost: Budget $150-$300, mid range $500-$1200, luxury resorts start at $2000 per person per day
- How to get there: Almost all atolls are only accessible by 1-3 hour propeller flight from a main hub island. There are almost no regular passenger ferries.
- What to expect: Extremely remote, almost no infrastructure outside of resorts, very limited or no mobile phone service.
- Visitor tips: Bring absolutely everything you need with you. There are no shops. Do not stand on or touch coral. Almost all atolls have extremely strict biosecurity rules.
Atolls And Climate Change
Atolls are the lowest lying land on Earth. The average elevation of an atoll cay is 1.8 meters above sea level. For almost 40 years it was universally accepted that all atolls would be completely submerged by the end of the 21st century.
New research published over the last 10 years has changed this understanding almost completely. Studies have shown that healthy atolls with intact living coral reefs are actually growing vertically at approximately 3.2mm per year, almost exactly the same rate as current sea level rise. Many atolls have actually gained land area over the last 70 years.
This only applies to atolls with healthy living reefs. Atolls with dead or damaged reefs will erode extremely rapidly, and will likely be completely gone within 50 years. The biggest threat to atolls is not sea level rise directly, it is coral bleaching and reef death.
FAQs
What is the simple definition of an atoll?
An atoll is a ring shaped coral reef that encloses a shallow lagoon, with no central island above sea level. This is the only formal geological definition accepted by geologists worldwide. Any reef formation that has a central island is classified as a barrier reef, not an atoll.
What is the difference between an atoll and an island?
An atoll is not an island. An atoll is a ring of reef surrounding a lagoon. Any land present on an atoll consists of small sand cays located on top of the reef rim. Less than 1% of the total area of a typical atoll is land, the remaining 99% is open water.
Is Bora Bora an atoll?
No, Bora Bora is not an atoll. Bora Bora is a barrier reef surrounding a large central volcanic island. This makes it a classic example of the middle stage of atoll formation, but it has not yet reached the final atoll stage. Almost every travel website incorrectly refers to Bora Bora as an atoll.
How many atolls are there in the world?
There are exactly 440 confirmed atolls on Earth. 75% of all atolls are located in the Maldives and the Marshall Islands. There is only one atoll in the entire Atlantic Ocean, and only one atoll in the entire Caribbean. There are no atolls anywhere in Europe or North America.
What is the largest atoll in the world?
The largest atoll in the world by a very wide margin is the Great Chagos Bank in the Indian Ocean, with a lagoon area of 12642 square kilometers. It is almost six times larger than the second largest atoll Kwajalein. Great Chagos Bank is almost completely unknown and almost never appears on popular lists of largest atolls.
How long does it take for an atoll to form?
A complete atoll takes approximately 30 million years to form from start to finish. The volcano takes 5 million years to form and then another 25 million years to sink completely below sea level. All currently living atolls were completely dead and exposed above sea level during the last ice age 20000 years ago, and only started growing again 12000 years ago.
Who first explained how atolls form?
The correct theory of atoll formation was first proposed by Charles Darwin in 1842, during the voyage of the Beagle. His theory was not confirmed until 1952 when the United States Navy drilled 1405 meters through Enewetak Atoll and hit volcanic bedrock exactly where Darwin had predicted it would be. There is no remaining scientific controversy about this theory.
Can you live on an atoll?
Yes, approximately one million people live permanently on atolls around the world. Living on an atoll is one of the harshest permanent human environments on Earth. There is no natural fresh water, no soil, no rock, and almost no natural resources. All fresh water comes exclusively from rain.
Are atolls dangerous?
Atolls are generally very safe for visitors. The main natural hazards are cyclones and very large ocean swells. There are almost no large land animals, no venomous land snakes, and almost no disease. The most common injury for visitors is cuts from dead coral, which become infected very easily.
Will atolls disappear due to sea level rise?
For many decades it was assumed that all atolls would be completely submerged by 2100. New research conducted over the last 10 years has shown that healthy atolls with intact living coral reefs are actually growing vertically at approximately the same rate as current sea level rise. Atolls with dead or damaged reefs will erode and disappear very rapidly.
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