During HMS Beagle’s famous voyage, which circled the world from 1831 to 1836, the naturalist Charles Darwin thought not only about evolution. He also worked with navigators to map the coral reefs the Beagle encountered in the South Pacific and Indian oceans. Along the way, Darwin hatched new ideas about reef formation – including the glittering circles of coral islands known as atolls.

Once upon a time, Darwin suggested, there must be a volcano rising from the bottom of the sea. Corals grow in circles around them, as tiny marine organisms coalesce into a reef that surrounds the flanks of the volcano. And then, at some point, the volcano erodes, sinks under the waves and leaves a ring of atolls behind.

Now, two marine geologists say this is not true. And they have a mission to get the Darwin model out of textbooks. “It’s a great model when you think about the little data he has,” said André Droxler, a geologist who recently retired from Rice University in Houston, Texas. “But that’s not really the case.”

Instead, he said, atolls were formed thanks to sea levels that fluctuated cyclically over hundreds of thousands of years. When the ocean falls, exposing piles of pre-existing carbonate rocks, rainwater dissolves the center of the rock and leaves a depression. As the seas rise, corals form a ring around the depression, forming atolls. No volcano required.

Droxler and Stéphan Jorry, a geologist at the IFREMER marine laboratory in Plouzané, France, explain their theory in the year 2021 Annual Review of Marine Science.

Their work builds on decades of study of how coral reefs are built. It is important not only to reveal the basic geology of how the atolls formed, but also to illuminate the future for the inhabitants of Earth’s hundreds of atoll islands as sea levels rise and threaten to submerge their lowland homes.

Drilling to Maldives

One of Droxler and Jorry’s favorite places to study coral reefs is the Maldives, a series of long double atolls that decorate the Indian Ocean like pearl necklaces. “It’s kind of like the Mecca of the atoll,” said Droxler.

That’s why scientists are using the Maldives to perfect their theory of atoll formation. Over the years, companies such as Royal Dutch Shell have drilled several islands and the surrounding seabed in search of oil and gas. There have also been a number of research trips, including two ocean drilling expeditions and sonar-ray surveys that revealed the topography of the seafloor around the island capital of Malé, including ancient coral terraces that were once seen but increasingly The last ice age is coming to an end.

All this data helps Droxler and Jorry gather a detailed picture of how the Maldives came about. It all starts with flat rock piles made of carbonate minerals such as limestone. Many such edges formed in many parts of the tropical oceans between about 5 million and 2.5 million years ago, when Earth’s climate was relatively warm and sea levels changed little. In this stable environment, the skeletons of dead sea creatures drift to the ocean floor and accumulate slowly and steadily into massive carbonate rock formations.

These underwater shores serve as the foundation on which the growth of atolls began about half a million years ago. It’s all thanks to cyclical changes at sea level, say Droxler and Jorry.

In recent geological times, approximately every 100,000 years the planet fell into an ice age chill (due to the shape of Earth’s orbit around the sun). Massive ice sheets formed and spread across continents, locking up most of Earth’s water and causing global sea levels to plummet. Then, when Earth’s orbit caused it to once again receive more radiation from the sun, the planet warmed, the ice sheet melted and sea levels rose. “You get this yo-yo change in sea level,” says Droxler.

Over the last 500,000 years, dramatic changes in sea level – up to 135 meters (440 feet) – created numerous atolls, say Droxler and Jorry. As a first step, imagine that the sea level drops to reveal the top of a carbonate rock cluster with a flat top. Rain fell from the sky, pooled on the carbonates. Since rainwater is usually slightly acidic, it begins to dissolve rock. This process is known as karstification, and it is the same in places like Kentucky when rainwater seeps through underground limestone and dissolves it, forming dramatic caves such as Mammoth’s Cave.

Above open sea rocks, rainwater usually pools in the middle. So that’s where most of the carbonate rock dissolves, leaving a little depression. When sea levels rose again, most corals began to grow along the high ring-shaped edges around the depression, where they were closest to sea level and had plenty of light and nutrients to grow.

Over thousands to hundreds of thousands of years, as sea levels fall and rise and fall again, circular atolls gradually form around a central depression, or lagoon. Darwin did not realize that this process formed atolls because geologists of his time did not understand the ice ages and the usual fluctuations in sea level, Droxler said.

The start of the nuclear age also provides some clues about the need to overturn the Darwin atoll idea, Droxler added. Starting in the 1940s, research teams drilled into Pacific atolls such as Bikini and Eniwetok, in the Marshall Islands, to better understand their structure before detonating them with an atomic bomb. The expedition found volcanic rock beneath the carbonate atoll – but too deep to support Darwin’s idea of ​​a volcano sinking and leaving the atoll. (Today, residents of Bikini and Eniwetok are still alive with the radioactivity left behind by the US nuclear tests, and the deteriorating structure of the coral reef that was destroyed by the bomb.)

Atolls may not ‘keep up’ with climate change

Darwin’s idea of ​​the atoll may not be completely wrong. At least some of the reefs on Tahiti may have formed in the way he envisioned, a research team said argued in a 2014 paper. “But we’ve also long known that there are other ways to develop atolls,” said Anna Weiss, a paleontologist at Midwestern State University in Wichita Falls, Texas, who studies ancient coral reefs. “It’s a mistake to over-generalize things in geoscience.” Some reefs in Belize, for example, emerge from above a chunk of the continental crust driven by tectonic forces closer to sea level. And one study of certain atolls in the Maldives argue that it is shaped more by waves crashing into it than by karstification.

Understanding how the atolls formed is important for helping lowland islanders adapt to sea level rise caused by climate change, Weiss said. “If we operate with the assumption that all atolls are the same, we lose important nuances of why reefs may or may not be able to ‘keep up’ with climate change,” he said. The Maldives, the world’s flatest country, faces an existential threat as sea levels rise on its islands. About half a million people live on this vast archipelago, where no point is higher than three meters (about 10 feet) above sea level.

In the absence of humans, atolls could grow at a much faster rate than sea level rise. But people have degraded natural atolls by incorporating pollution and waste, changing the water level and adding concrete and asphalt covering the coral underneath. The Maldives faces a future of flooding, water contamination and erosion that threatens its tourism and fishing industries.

For hundreds of thousands of years, the wealth of these islands was governed by planetary cycles. But now the human influence has grown and jeopardizes their existence. It was an evolution that Darwin could not have predicted.

Knowable Magazine is an independent journalistic effort from the Annual Review.