Darwin's Volcano
 

Summary

• Darwin thought coral atolls grew up from sinking volcanic islands
• Agassiz thought atolls started on shallow sand banks
• It took almost 100 years to prove Darwin right

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)

Atoll arguments

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.

An 'easy' solution

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.
In 1896, a Royal Society of London expedition went to the South Pacific to drill beneath Funafuti Atoll in Tuvalu. This expedition was unsuccessful because they were only able to drill down 100 feet (30 m).

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.
The Royal Society’s final attempt in 1898 also proved to be inconclusive even though this expedition drilled down to 1114 ¼ ft (340 m). They could drill no further and still hadn’t made it through the reef.

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.

And so to Eniwetok

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:

• Shallow water organisms were dredged up from the top of guyoks (submerged seamounts that often have coral reefs on top). It was too deep for these organisms to live, and these volcanoes must have subsided faster than the reef could grow.
• Echolocation showed that atolls are steep sided rather than broad as Agassiz’ model predicted.
• Seismic soundings could determine the type of rock without the need for drilling.

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.

References

CIA. (2009, April 2). Tuvalu. Central Intelligence Agency – The world fact book. Retrieved 8 April 2009 from https://www.cia.gov/library/publications/the-world-factbook/geos/tv.html

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 https://darwin-online.org.uk/content/frameset?itemID=F1647&viewtype=text&pageseq=1

Darwin, F. ed. (1887). The life and letters of Charles Darwin, including an autobiographical chapter Volume 3. John Murray: London. The complete works of Charles Darwin online. Retrieved 8 April 2009 from https://darwin-online.org.uk/content/frameset?itemID=F1452.3&viewtype=text&pageseq=1

Dobbs, D. (2005) Reef Madness: Charles Darwin, Alexander Agassiz and the meaning of coral. Pantheon Books: New York

Ladd, H et al. (1953). Drilling on Eniwetok Atoll, Marshall Islands. AAGP bulletin. Retrieved 8 April 2009 from https://search.datapages.com/data/bulletns/1953-56/data/pg/0037/0010/2250/2257.htm

Tuvalu Online. (2008, January 2). A brief history of Tuvalu. Tuvalu Online. Retrieved 8 April 2009 from https://www.tuvaluislands.com/history.htm

Wikipedia. (2009, July 2). Eniwetok. Wikipedia. Retrieved 8 April 2009 from https://en.wikipedia.org/wiki/Enewetak

Wikipedia. (2009, March 22). Funafuti. Wikipedia. Retrieved 8 April 2009 from https://en.wikipedia.org/wiki/Funafuti