OCT 14, 2020 3:07 PM PDT

That's not really how atolls form...

A study published this month in the Annual Review of Marine Science challenges the accuracy of Darwin’s theory about atolls, ring-shaped reefs or islands formed of coral. The paper was co-authored by André Droxler, formerly of professor of environmental and planetary sciences at Rice University, and Stéphan Jorry, a marine geologist and oceanographer at the French Research Institute for Exploitation of the Sea (IFREMER).

Darwin's theory about the formation of atolls was published in 1842 and theorized that atolls formed around the circumferences of volcanic islands and remained after volcanoes sank into the ocean, giving them their characteristic donut shape. At the time of this theory, geologists like Darwin believed continents were steadily rising out of the Earth and oceans were steadily sinking.

*Note: the above video shows Darwin’s theory on atoll formation, which science has now disproved.

Commenting on the contemporary popularity of this theory, despite a more accurate comprehension that has been understood since the 1930s, Droxler says, “It's so beautiful, so simple and pleasing that everybody still teaches it. Every introductory book you can find in Earth science and marine science still has Darwin's model. If they teach one thing about reefs or carbonates in marine science 101, they teach that model."

But despite that, it is not accurate. In fact, atolls are formed from sea-level changes. "Cyclic changes in sea level drive atoll formation," Droxler says. "Darwin had no concept that sea level could go up and down, because glaciation didn't become common knowledge until the 1860s." Droxler elaborates that our current atolls formed in the last 500,000 years, shaped by five huge sea-level changes (where sea level rose and fell by 120 meters or more) that occurred every 100,000 years.

These cyclical swings resulted in coral colonizing banks that were continually resubmerged during warm periods when the sea level was at its highest. Coral particularly colonized the highest parts of the eroded banks along their outer rims.

"Now you're adding carbonate on to their raised rim, and you're creating the modern atolls," Droxler said. "But this period of high sea level doesn't last for more than 10,000-12,000 years, and it goes down again. So now you are adding some reef on the rim, but then dissolving the lagoon again when sea level retreats. So, you are enhancing the morphology even more with each cycle now, growing on the margin and dissolving in the middle."

This gradual geologic process is much more complex to comprehend than Darwin’s original theory. However, theories are meant to be proved incorrect.

Photo: Pixabay

In Droxler's and Jorry's paper, they include dozens of illustrations comparing Darwin's original drawings with contemporary maps and satellite imagery. Both co-authors have spent decades in the field acquiring marine geological data from dozens of expeditions. The study draws on their combined knowledge and experience.

"For years, Stéphan had pushed me, saying we should publish this, but you know, you always kind of do something else and something else," Droxler laughed. "At the end of March, Stéphan called me and said, 'I'm stuck in France, at home, and you're stuck in the middle of the countryside in Mason County. Let's write this paper.' And so that's what we did."

Sources: Annual Review of Marine Science, Science Daily

About the Author
  • Kathryn is a curious world-traveller interested in the intersection between nature, culture, history, and people. She has worked for environmental education non-profits and is a Spanish/English interpreter.
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