"From about 5 million years ago to 2.5 million years ago, we had a rather warm climate that did not change very much, and sea level remained relatively constant. "The Earth's climate has changed quite dramatically in the last 5 million years," he said. But to fully appreciate how changing sea level created atolls, it helps to start much earlier, Droxler said. In each cycle, sea level rose and fell by 120 meters or more. Today's atolls formed in the past 500,000 years, Droxler said, driven by five wild swings in sea level that occurred every 100,000 years. A more accurate description of atoll formation has been around since the 1930s, but it is considerably more complicated and much of the evidence to support it is more recent, coming in the past 40 years from dozens of scientific and oil industry drilling expeditions as well as from the compiled record of Earth's climate and sea level history. "Darwin had no concept that sea level could go up and down, because glaciation didn't become common knowledge until the 1860s."ĭroxler said the simplicity of Darwin's classification system and theory could play a role in its continued appeal. "Cyclic changes in sea level drive atoll formation," Droxler said. But beauty aside, Darwin could not have accurately predicted how atolls form because he lacked the key piece of information, Droxler said. When the volcano died and slowly sank back into the ocean, the reef remained, first becoming a separated barrier reef and eventually, after the volcano sank entirely, an atoll.ĭroxler said he's awed by Darwin's ability to synthesize all that was known about reefs in his day and come up with such a simple, comprehensive and compelling theoretical model. Darwin noticed that both fringing and barrier reefs tended to surround volcanic islands, and he reasoned reefs initially formed on the fringe of volcanic islands. The third category was atolls, ring reefs that enclosed a central lagoon but no land.Īt the time, geologists believed continents were steadily rising out of the Earth and oceans were steadily sinking. Those detached from land and separated by a lagoon were barrier reefs. Those attached to land, he called fringing reefs. It's unbelievably accurate."īut unlike navy maps that simply marked reefs as hazards to navigation, Darwin systematically classified each into one of three categories. "It's amazing, when you compare satellite images of reefs today versus his map. "He published a beautiful map that compiled all the known reefs on Earth," Droxler said. The Beagle, like every Royal Navy vessel, carried charts with the marked location of every known reef, and Darwin put these to use in his 1842 paper. "So they needed to know exactly where they were located." "They come out of the abyssal plain of the ocean to almost no depth," Droxler said. Some were topped with low-lying islands but many were jagged rings of coral-topped rock that sat just below the water's surface, ready to rip the bottom out of unwary passing ships. Atolls were particularly interesting and dangerous. "They spent a lot of time mapping reefs because they were such hazards to shipping," Droxler said of the Royal Navy. The ship's primary mission was surveying coastlines and hazards to navigation, and the ship's orders included collecting detailed observations of the tides and ocean depths around a coral atoll. A geologist by training, Darwin was keenly interested in the rocks and landforms he encountered in his five years aboard the Beagle. Published this month in the Annual Review of Marine Science, the paper was co-authored by Droxler and longtime collaborator Stéphan Jorry, a marine geologist and oceanographer at the French Research Institute for Exploitation of the Sea (IFREMER).ĭarwin's theory about the formation of atolls was published in 1842, six years after his legendary voyage aboard the British survey ship HMS Beagle. If they teach one thing about reefs or carbonates in marine science 101, they teach that model."ĭroxler, a professor of Earth, environmental and planetary sciences at Rice for 33 years, is hoping to set the record straight with a 37-page, tour de force paper about the origins of atolls. "Every introductory book you can find in Earth science and marine science still has Darwin's model. "It's so beautiful, so simple and pleasing that everybody still teaches it," said Droxler, who recently retired from Rice University.
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