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Atolls – how these remote, circular coral reefs could form puzzled many 19th century scientists. Coral reefs are typically associated with land because reef-building coral only grow in shallow water. If this is the case then how can atolls form in the middle of the Pacific far from any apparent land?
In 1837, Charles Darwin proposed a simple theory to explain atoll formation – atolls begin as a fringing reef growing around a volcanic island. If this island sinks beneath the ocean, but the reef continues to grow upwards an atoll forms. (See "4 kinds of reef" for more detail)
This theory was pretty well accepted until the late 1870’s when geologist Alexander Agassiz and others proposed a different idea – that atolls grew up from shallow sand banks on the bottom of the ocean. The reef that grows on this bank eventually grows so large that coral the middle of the reef dies, and is dissolved away creating the distinctive shape of an atoll.
Both of these theories were simple. The way to resolve this dispute was also simple. However, it took almost 100 years to determine the right theory.
Darwin wrote to Agassiz in 1881 suggesting that the answer could be gained by boring 500- 600 ft (150-180 m) beneath an atoll. If there was volcanic rock below the atoll Darwin would be right, but if sand was found then Agassiz would be right.
One year later, a second Royal Society expedition drilled down to 698 ft (210 m). This was deeper than Darwin suggested, but results were inconclusive because they were still drilling through the reef. Darwin had under estimated how thick atolls could be.
After 3 expeditions, the Royal Society had to abandon their attempts with inconclusive results. The technology of the time was simply unable to drill down deep enough to resolve the dispute, and it wouldn’t be for another 50 years.
Eniwetok Atoll lies in the Marshall Islands in the middle of the Pacific Ocean. This atoll was used for nuclear weapon testing from 1948 -1958 by the Atomic Energy Commission.
In the summer of 1951, this group drilled to a depth of 4154 ft (1260 m), and at the depth of almost a mile the drill struck volcanic rock – Darwin was right.
Also at around this time other evidence came to light supporting Darwin:
After 100 years, Darwin’s theory of atoll building proved to be the correct one. Darwin just had to wait for technology caught up with his ideas.
See "4 kinds of reefs" for more on building an atoll.
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Darwin, C. (1842). The structure and distribution of coral reefs. [Reprint by University of California Press: Berkeley 1962]
Darwin, C. (1837). On certain areas of elevation and subsidence in the Pacific and Indian oceans, as deduced from the study of coral formations. Proceedings of the Geological Society of London 2: 552-554. The complete works of Charles Darwin online. Retrieved 8 April 2009 from http://darwin-online.org.uk/content/frameset?itemID=F1647&viewtype=text&pageseq=1
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