What are the stages of atoll formation?

Three classic stages, each still visible somewhere in the tropical Pacific today.

For a classroom-style walk-through of each step, see describe the formation of an atoll reef.

Why does the volcano sink?

Because the ocean plate it sits on cools, contracts and sags as it moves away from the hot spot or ridge where it formed.

New oceanic crust and the volcanoes built on it are hot and buoyant. As the plate drifts away from its heat source over millions of years it cools, becomes denser and settles deeper into the mantle. The enormous weight of the volcano itself also bends the crust downward. Erosion by rain and waves wears the exposed peak down from above at the same time. Subsidence rates are slow, generally fractions of a millimetre per year, which is well within the rate at which a healthy reef can build upward (typically several millimetres a year).

How did Darwin work it out?

On the voyage of HMS Beagle (1831–1836) Darwin saw fringing reefs, barrier reefs and atolls and realised they could be stages of one process.

Darwin visited Tahiti and the Cocos (Keeling) Islands and reasoned that reef corals grow only in shallow, sunlit water, yet atolls rise from very deep ocean. The only way shallow-water reefs could sit on top of deep foundations was if the foundations had sunk while the reef kept pace. He published the idea in The Structure and Distribution of Coral Reefs in 1842. Rival explanations followed, including Reginald Daly’s glacial-control theory, which emphasised sea-level changes during the ice ages. Read more about the man in our Charles Darwin profile.

What proved Darwin right?

Deep drilling. Funafuti (Tuvalu) was drilled in the 1890s without reaching bedrock, but Enewetak in the Marshall Islands finally hit basalt in 1952.

In 1951–52, while Enewetak was being used as a US nuclear test site, the US Geological Survey drilled two deep holes through the atoll. Both passed through more than a kilometre of shallow-water reef limestone, with fossils showing it had all formed near the surface, before reaching volcanic basalt at roughly 1,270–1,400 m. That is exactly what Darwin’s theory predicts: a reef that grew upward for tens of millions of years on a sinking base. See the Marshall Islands atolls.

Did sea-level change play a part?

Yes. Subsidence explains the foundation; ice-age sea-level swings shaped the details.

During glacial periods sea level fell by up to about 120 m, exposing atolls as limestone islands that were dissolved by rain into bowl shapes with raised rims. When the sea rose again, corals recolonised the rims first, which helps explain why many atoll lagoons are so neat and basin-like. Modern geologists combine Darwin’s subsidence with these sea-level cycles. Today, rising seas and bleaching raise the question of whether atoll reefs can keep up again.

What did the Enewetak boreholes actually find?

The 1951–52 drilling on Enewetak in the Marshall Islands remains the classic test of Darwin’s idea, and the details are worth knowing. Harry Ladd and colleagues of the US Geological Survey sank two holes on opposite sides of the atoll. Hole E-1, on the south-east side, brought up fresh basalt cuttings at a depth of 4,154 feet, about 1,266 m, and then a solid core of olivine basalt a little deeper. Hole F-1, on the north-west side, struck hard basement rock at 4,610 feet, about 1,405 m, though no sample was recovered.

Above the basalt, both holes passed through more than a kilometre of limestone. The uppermost several hundred feet were young, soft reef limestone from the last few hundred thousand years. Below that was a thick sequence of older limestone, with minor dolomite, reaching back to the Eocene. Throughout the column, fossils showed that the limestone had formed in shallow water, at depths reached by sunlight. That was the critical point: shallow-water reef rock more than a kilometre below sea level could only be explained if the base had sunk.

The drillers summed up the structure as a thick cap of limestone sitting on the summit of a volcano that rises about two miles from the deep ocean floor. Below about 1,400 feet, the two holes differed markedly in rock type, fossils and the distribution of hard layers and cavities, a reminder that an atoll is not a uniform cake but a complex pile of reef, sediment and old land surfaces.

Why did earlier attempts fail?

Darwin’s theory was not tested quickly. In the 1890s the Royal Society of London sent three expeditions to Funafuti, now in Tuvalu, specifically to drill through an atoll and look for shallow-water organisms at depth. The first expedition in 1896 was led by William Johnson Sollas of Oxford University. The second, in 1897, was led by Edgeworth David of the University of Sydney, and the third in 1898 by Alfred Edmund Finckh.

The Funafuti bore reached about 340 m. It found reef material throughout, which was consistent with Darwin’s prediction, but it never struck volcanic rock. Critics could argue that the reef might simply have grown outward over its own debris, or on a platform that had always been at that depth. Without reaching the foundation, the result could not settle the debate.

Drilling technology improved during the first half of the 20th century, largely because of the oil industry. After the Second World War, the United States government began studying Pacific atolls in detail ahead of and alongside its nuclear weapons tests in the Marshall Islands. Drill holes were sunk on Bikini Atoll in 1947 but did not reach basement. Enewetak was the first to go deep enough, more than a century after Darwin had published his theory.

Where do atolls stop forming?

Atolls can only grow where reef-building corals and coralline algae can add limestone faster than the foundation sinks and waves erode it. Reef growth slows in cooler water. At some latitude, upward growth and subsidence balance, and beyond it reefs fall behind and drown. In 1982 Richard Grigg named this boundary the Darwin Point, describing it as a threshold for atoll formation.

The Hawaiian-Emperor chain shows the Darwin Point in action. The Hawaiian hot spot builds a volcano, the Pacific plate carries it north-west, and it sinks as it moves away. In the warm waters near the main Hawaiian Islands, fringing reefs form. Farther along the chain, Midway and then Kure atolls show the classic atoll stage. Kure, at about 28°25′N, is the northernmost coral atoll in the world, and at roughly 30 million years old it is also the oldest island in the Hawaiian chain.

Beyond Kure, the reefs could not keep pace. The older volcanoes of the chain have sunk below the reach of sunlight and continued north-west as flat-topped seamounts. As the Pacific plate drifts, Kure itself is being carried slowly toward the same fate. In time, its reef will drown and it will join the line of submerged mountains to the north.

What are guyots, and how are they related to atolls?

Guyots are flat-topped seamounts whose summits lie more than 200 m below the surface. They were first recognised during the Second World War by Harry Hess, a geologist who commanded a US Navy transport ship and kept its echo sounder running across the Pacific. He named them after the 19th-century geographer Arnold Guyot and suggested that their flat tops had been planed off by waves when they were at sea level.

Later work confirmed that many guyots are drowned islands and atolls. Dredging and drilling on their summits recovered shallow-water reef limestones, some from the Cretaceous period, now lying more than a kilometre below the surface. These are the remains of ancient atolls that drowned when reef growth could no longer keep up with subsidence, or when environmental changes such as warming, nutrient changes or sea-level shifts stalled reef growth.

Guyots are common in the western Pacific, where the oldest sea floor has had the longest time to subside. Together with living atolls, they make a continuous story: a volcano builds an island, the island sinks, corals keep a reef at the surface for as long as they can, and eventually the whole structure drowns and becomes a flat-topped seamount on the deep sea floor.

How did ice ages reshape atoll tops?

Darwin’s subsidence model explains why atolls sit on drowned volcanoes, but it does not explain every detail of their shape. In the 20th century, geologists such as J. E. Hoffmeister and others developed what is called the antecedent karst model. It argues that the present-day form of many atolls was strongly influenced by repeated exposure during ice-age low sea levels.

When sea level fell by more than 100 m during glacial periods, the limestone tops of atolls were exposed to rain. Slightly acidic rainwater dissolved the limestone, especially in the centre, where water collected. The edges, which were harder and better cemented, dissolved more slowly. The result was a saucer-shaped platform with a raised rim. When sea level rose again, corals recolonised the rim first, and the central depression became the lagoon.

Seismic surveys and drill cores from many atolls support this model. Under the modern reef, there are buried old land surfaces, soils and cave systems from earlier low stands. Most geologists now see the two ideas as complementary. Subsidence over millions of years builds the thick limestone cap. Glacial cycles over the last few million years shape the top into the familiar ring and lagoon.

How do the islands on an atoll form?

The small islands perched on an atoll’s reef, called motu in Polynesia, are much younger than the reef itself. They are piles of sand and gravel made from broken coral, shells, coralline algae and the skeletons of tiny single-celled organisms called foraminifera. Waves and storms sweep this sediment onto the reef flat, where it accumulates into low islands, usually less than 5 m above sea level.

In 2005, Paul Kench and colleagues published a new model of reef-island evolution based on drilling and radiocarbon dating in the Maldives. They found that many islands formed relatively quickly in the middle of the Holocene, as rising seas flooded reef platforms and generated sediment, and that they have remained broadly stable in size since. Similar studies across the Pacific have shown that reef islands are dynamic: their shape and position shift as storms, currents and sediment supply change.

That dynamism has surprised many people. In 2018, Kench and colleagues analysed aerial photographs and satellite images of all 101 islands in Tuvalu over four decades, a period when local sea level rose at about twice the global average. They found a net increase in land area of 2.9 per cent, with about three-quarters of islands growing and the rest shrinking. The authors argued that atoll islands are not simply drowning but changing shape, and that adaptation plans should take this variety into account.

Do all atolls sit on volcanoes?

Most Pacific atolls do, but there are important variations. The atolls of the Maldives and the Chagos Archipelago in the Indian Ocean rest on a long volcanic ridge formed by a hot spot, but they sit on a thick carbonate platform rather than on separate volcanic cones. Over tens of millions of years, reefs grew up on the sinking ridge, forming the chain of atolls seen today.

In the Caribbean, a handful of atolls, including Lighthouse Reef, Glover’s Reef and Turneffe off Belize, sit on blocks of continental crust tilted by faulting, not on volcanoes. They look like Pacific atolls but have a different foundation, showing that any shallow platform that sinks slowly in warm water can carry an atoll. The Atlantic has no large groups of atolls, but there are several atolls in the western Caribbean belonging to Colombia’s San Andrés and Providencia.

Some atolls have been lifted rather than lowered. When nearby volcanoes grow or plates bend, an atoll can be raised above sea level and its old lagoon left high and dry. Aldabra in the Seychelles, Niue in the South Pacific and Henderson Island in the Pitcairn group are examples. Lifou in the Loyalty Islands of New Caledonia is often given as the largest raised coral atoll by total area. Kiritimati, in Kiribati, is often listed as the atoll with the largest land area.

What does the future hold for atolls?

Atolls are one of the clearest examples of how sensitive the ocean’s living architecture is to change. For millions of years they have survived by keeping pace with subsidence and sea-level change. Today they face rising seas, warming water, ocean acidification, coral bleaching and human pressure, all at once.

The outcome depends on how healthy the reef remains. A thriving reef can produce the sediment that builds and maintains islands and can grow upward with moderate sea-level rise. A reef weakened by repeated bleaching and acidification may produce less carbonate and grow more slowly, making it harder to keep pace. Studies such as the Tuvalu analysis show that islands can be resilient, but they also show that change is uneven and difficult to predict.

For the people who live on atolls, in nations such as the Marshall Islands, Kiribati, Tuvalu and the Maldives, these are not abstract questions. Their homes are built on structures that took tens of millions of years to form. Understanding how atolls grow, from Darwin’s insight to modern drilling and satellite mapping, is essential to understanding whether, and how, those islands will persist.