Deeper than Darwin: How were Maldivian atolls really formed?

As an archipelago formed relatively recently in geological terms, there are many factors still poorly understood about how this breathtaking ecosystem came to exist. 

The Maldives’ low-lying coral reef structures have traditionally been as hazardous for unwary travellers as they have been for unwary theorists: a lesson hard-learned by French merchant Francois Pyrard, who was shipwrecked in the Maldives in 1602. 

During his years of captivity, Pyrard learnt the Dhivehi language, later publishing its words for a global readership. Notable among them was atholhu: the coral rims encircling a shallow lagoon, often with islands at their borders, which perfectly described Goidhoo atoll, where Pyrard found himself stranded.

"Atoll" first entered the English language when Charles Darwin used it to describe the coral reef structures encountered during HMS Beagle’s infamous circumnavigation of the globe. Darwin didn’t come to the Maldives. But he sailed close to Pacific islands and studied the Cocos atoll in the southeast Indian Ocean. 

To help prevent further shipwrecks, Darwin drew detailed maps of coral reefs. A theory of atoll formation evolved based on these, suggesting the atolls had been left by volcanoes rising from the seafloor. Tiny marine organisms called polyps had fixed themselves on the volcanoes, he theorised, forming coral reefs as they excreted their limestone exoskeletons. As the volcanoes eventually sank into the sea, Darwin concluded, they left behind the structures we now know and love. 

The simplicity of Darwin’s theory fostered mass appeal. It was adopted across textbooks worldwide as the commonly-held belief for the formation of the Maldives. But modern research suggests it was plate tectonics and sea level fluctuations that birthed the atolls of Dhivehi Raajje.

At the time Darwin proposed his theory, scientists knew nothing of plate tectonics – the movement of the Earth’s surface plates across an underlying molten mantle – while major glaciers and climate shifts related to long-term sea-level change were also still unknown. 

One hundred million years ago, when the Indian plate began its migration north – and the Indian subcontinent moved away from modern-day Madagascar – it passed over the island of Rėunion, a volcanic hotspot. Here, a buoyant mantle from the Earth’s core broke through the surface as lava, solidified, and became the bed of basalt lava that now extends from the Deccan Traps in India down to Mauritius.

Over the past five million years, sea levels fluctuated, falling and rising again by 120m due to the formation of ice sheets in North America. These 100,000-year cycles periodically exposed the flat-topped bases of volcanic basalt, allowing acidic rain to erode their centres – through a process known as karstification – leaving behind central depressions. When sea levels rose again, coral polyps and other marine animals settled on the rim of these now-submerged banks, dying and adding their exoskeletons to the formation. 

As sea levels rose further, these coral-lined banks rose with it, forming the landscape today known as the Maldives: almost 1,200 islands on the world’s largest atoll chain. The main lagoons today have an average depth of 40-60 metres, with the north-central atolls forming a double-chain surrounding an ‘Inner Sea’ up to 600 metres deep. 

More than a century after Darwin tried to get to the bottom of atoll formation, it would take a different sort of exploration to explain how the Maldives got here. 

Beginning in the early 1970s, oil companies conducted exploratory drills in the Maldives to look for fossil fuel deposits. Their seismic surveys – exploring the area northeast of Malé and the inner sea southeast of Ari, Lhaviyani and (Pyrard’s) Goidhoo atoll – have since been used by marine geologists to form a new theory that dives deeper than Darwin’s. 

Interpreting the two-dimensional seismic profiles, Dr Andre Droxler and Stėphan Jorry discovered that the islands were part of a carbonate mega platform rooted in the plateau that was formed by the volcanic hotspot. Their findings were first published in full in 2021. 

“The different seismic surveys have imaged the subsea floor to be able to observe the evolution of the full reefal limestone Maldives edifice and also the volcanic plateau on top of which the edifice initially was established,” Dr Andre Droxler told Maldives Independent. 

The volcanic base’s development coincided with the period when the Maldives passed over Reunion island, and was followed by the deposit of limestone bases, which contain evidence of the developmental changes related to sea level fluctuation, his research found. 

The understanding of these historic changes came from analysis of fossilised marine organisms which contain archival environmental information. Microfossils can indicate distribution of food, the temperature of their environment, and oxygen availability. Based on the origin and extinction of planktons, the limestone carbonate platform could be dated using sediment successions, which show the carbonate platform resting on volcanic basalt from in the late early Eocene period.

“Based on this information, we are able to conclude that the volcanic plateau [of Maldives] was 57-55 million years old, the reefal limestone edifice, 2-3 km thick, was initially established 55 million years ago,” said Dr Droxler, “with its modern atolls part of the latest and most recent phase (about the last 450,000 years of its long 55 million years long evolution).”

The Maldives’ evolution remains “really complex”, but his theory suggests Darwin’s simpler solution of sunken volcanoes no longer holds water.

While Darwin’s prevailing theory might not apply to the Maldives, it might not be entirely wrong. Pacific islands research shows that his theory may still apply there and geologists accept that there are several ways for atolls to develop. 

Dr Droxler is working with educational institutes and the Maldives National University – which has published a book teaching the new theory – to ensure it becomes common sense for future generations. 

“This important knowledge needs to be included in the school curriculum in the Maldives for the new generations to understand how their own archipelago and modern atolls have formed,” he said.