EXPLANATION

Darwin’s theory of the formation of coral reefs and atolls was published in 1842, six years after his legendary voyage aboard the British survey ship HMS Beagle.

During the fiveyear voyage, Darwin assembled information from existing, surprisingly accurate, nautical charts into a single map showing global distribution of coral reefs. He classified all the known reefs on Earth at that time into three categories: fringing reefs, barrier reefs, and atolls.

Darwin observed that both fringing and barrier reefs tended to surround volcanic islands; he reasoned that reefs initially formed on the fringe of volcanic islands. When the volcano became extinct and slowly subsided below sea level becoming a separated barrier reef and once the volcanic edifice was entirely buried it eventually became an atoll.

What Darwin didn’t know at that time was that sea level could fluctuate in 100 m amplitudes.

We now know that Earth’s climate has changed quite dramatically. More than 3 million years ago, climate and sea level remained surprisingly constant over much longer periods than in recent times.
Lagoons of existing atolls during these periods would have likely filled up, resulting in flat topped coral banks.

Earth’s climate drastically deteriorated at the onset of major glaciations in the northern hemisphere 2.5 million years ago, and ice caps systematically and cyclically expanded, inducing high amplitude sea level variations and drops down to 135 m below the presentday level.

The coral flattopped banks remained largely exposed during a 2millionyearlong interval of stepwise increase in ice volume. During this exposure, relatively acidic rain partially dissolved the coral limestone and created karstified banktop morphologies, characterized by central depressions where dissolution was maximum and surrounded by raised rims where chemical erosion was minimal.

Modern atolls have developed over the last 450 thousand years, a period uniquely characterized by five sealevel cycles of highamplitude of around 130 m.

The atolls formed periodically during the short, warm interglacial intervals when the karstified exposed flattopped banks were reflooded. At times of sea level rise, coral reestablished themselves preferentially on their raised rims. However, the reflooding intervals did not last long enough for the sediment to fill their lagoons and so created flattopped.


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