How coral atolls get their gorgeous ring shapes

During the famous voyage of the HMS Beagle, which circumnavigated the globe from 1831 to 1836, naturalist Charles Darwin wasn’t thinking only about evolution. He was also working with navigators to chart the coral reefs that the Beagle encountered in the South Pacific and Indian oceans. Along the way, Darwin hatched new ideas about the formation of reefs — including the shimmering island rings of coral known as atolls.

Once upon a time, Darwin proposed, there must have been a volcano rising from the seafloor. Coral grew in a ring around it, as tiny marine organisms cemented themselves into a reef circling the volcano’s flanks. And then, at some point, the volcano eroded away, sinking beneath the waves and leaving the atoll ring behind.

Now, two marine geologists say this isn’t right. And they are on a mission to get Darwin’s model taken out of textbooks. “It’s a fabulous model when you think about what little data he had,” says André Droxler, a geologist recently retired from Rice University in Houston, Texas. “But it’s not really the case.”

Instead, he says, atolls form thanks to sea levels fluctuating cyclically over hundreds of thousands of years. When seas fall, exposing a pre-existing pile of carbonate rocks, rainwater dissolves the center of those rocks and leaves behind a depression. When seas rise, corals build themselves up in a ring around that depression, forming an atoll. No volcano is needed.

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

Their work builds on decades of studies on how coral reefs are constructed. It’s important not only for revealing the basic geology of how atolls form, but also for illuminating the future for residents of the Earth’s hundreds of atoll islands as sea levels rise and threaten to drown their low-lying homes.

Drilling into the Maldives

One of Droxler and Jorry’s favorite places to study coral reefs is the Maldives, a long double chain of atolls dotting the Indian Ocean like a pearl necklace. “It’s kind of the Mecca of atolls,” says Droxler.

Which is why the scientists used the Maldives to flesh out their theory of atoll formation. Over the years, companies such as Royal Dutch Shell have drilled into some of the islands and the surrounding seafloor in search of oil and gas. There have also been a number of research voyages, including two ocean-drilling expeditions and a sonar-beam survey that revealed the topography of the seafloor around the capital island of Malé, including ancient reef terraces that were once exposed but were progressively drowned as the last ice age drew to a close. 

All these data helped Droxler and Jorry piece together a detailed picture of how the Maldives came to be. It all begins with a flat-topped bank of rocks made of carbonate minerals such as limestone. Many such banks formed in many parts of the tropical ocean between about 5 million and 2.5 million years ago, when Earth’s climate was relatively warm and sea levels didn’t change very much. In this stable environment, the skeletons of dead marine creatures drifted to the seafloor and accumulated slowly and steadily into large formations of carbonate rock.

These underwater banks served as the foundation on which atolls grew starting around half a million years ago. It’s all thanks to cyclical changes in sea level, Droxler and Jorry say.

In recent geologic time, roughly every 100,000 years the planet plunges into the deep chill of an ice age (due to the shape of Earth’s orbit around the sun). Huge sheets of ice form and spread across the continents, locking up much of the Earth’s water and causing global sea levels to plummet. Then, when Earth’s orbit causes it to once again receive more radiation from the sun, the planet warms, the ice sheets melt and sea levels rise. “You get this yo-yo of sea level changes,” says Droxler.

Over the last 500,000 years or so, those dramatic shifts in sea level — up to 135 meters (440 feet) — created many atolls, Droxler and Jorry argue. As a first step, imagine if sea levels drop and expose the top of a flat-topped bank of carbonate rocks. Rain falls from the sky, pooling on top of the carbonates. Because rainwater is usually mildly acidic, it starts to dissolve the rocks. This process is known as karstification, and it’s the same thing that happens in places like Kentucky when rainwater percolates through underground limestone and dissolves it, forming dramatic caverns such as Mammoth Cave.

Atop the exposed marine rocks, rainwater typically ponds in the center. So that’s where most of the carbonate rocks dissolve, leaving a small depression. When sea levels rise again, corals start growing mostly along the elevated ring-shaped edge surrounding the depression, where they are closest to the sea surface and have plenty of light and nutrients to grow.

Over thousands to hundreds of thousands of years, as sea levels drop and rise and drop again, a circular atoll gradually forms around the central depression, or lagoon. Darwin didn’t recognize that this process formed atolls because geologists of his era had not yet understood ice ages and the regular fluctuations in sea level, Droxler says.

The dawn of the nuclear age also provided some hints of the need to overturn Darwin’s atoll idea, Droxler adds. 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 blasting them with atomic bombs. Those expeditions discovered volcanic rock beneath the carbonate atolls — but far too deep to support Darwin’s notion of a volcano sinking and leaving behind an atoll. (Today, residents of Bikini and Eniwetok are still living with the radioactivity left behind by US nuclear tests, and the deteriorating structure of the reefs blown apart by bombs.)

Atolls may not ‘keep up’ with climate change

Darwin’s ideas about atolls might not be entirely wrong. At least some reefs in Tahiti might have formed in the way he envisioned, a team of researchers argued in a 2014 paper. “But we have also known for a long time that there are several other ways for atolls to develop,” says Anna Weiss, a paleontologist at Midwestern State University in Wichita Falls, Texas, who studies ancient reefs. “It’s a mistake to overgeneralize things in geoscience.” Several reefs in Belize, for instance, rise from atop a chunk of continental crust that tectonic forces thrust close to the ocean surface. And one study of a particular atoll in the Maldives argues that it was shaped more by waves crashing into it than by karstification.

Understanding how atolls form is important for helping residents of low-lying islands adapt to rising sea levels brought about by climate change, Weiss says. “If we operate on the assumption that all atolls are the same, we miss important nuance as to why a reef may or may not be able to ‘keep up’ with climate change,” she says. The Maldives, the flattest country in the world, faces an existential threat as rising seas lap at its islands. About half a million people live on this sprawling archipelago, where no point is higher than three meters (about 10 feet) above sea level.

In the absence of humans, atolls can grow at a rate much faster than that of sea level rise. But people have degraded natural atolls by introducing pollution and waste, altering the water table and adding concrete and asphalt that smothers the underlying coral. The Maldives face a future of flooding, water contamination and erosion that threaten its tourism and fishery industries.

For hundreds of thousands of years, these islands’ fortunes were governed by the planet’s cycles. But now human influence has grown and imperils their very existence. That’s an evolution Darwin himself couldn’t have foreseen.