Voyage of HMS Beagle
by Charles Darwin
One of the questions on Charles Darwin’s mind even before the Royal Navy’s survey ship, HMS Beagle, began its multi-year, round the world mission in 1831 was the origin of coral reefs, the only geological features formed by animals. Exactly how coral polyps by the billions formed their structures was a question of importance both for geological theorists and for the British naval strategists.
Among the instructions issued for the Beagle’s voyage was that “the circularly-formed Coral Islands in the Pacific occasionally afford excellent land-locked harbours. . . and would be well adapted to any nice astronomical observations which might require to be carried on in undisturbed tranquility. While these are quietly proceeding. . . a very interesting inquiry might be instituted respecting the formation of these coral reefs, ” including examination of the “modern and very plausible theory” that such reefs have been raised from the summits of extinct volcanoes.
Oceanic coral reefs often formed a circular island or group of islands with a central open area, or lagoon. Scottish geologist Charles Lyell, whose books Darwin studied, and to whom he would dedicate his own book about the Beagle’s voyage, believed that coral reefs grew around the summits of underwater volcanoes, with the circular atolls reflecting the opening of the volcanic crater.
Acting on the Admiralty’s orders, Beagle Captain Robert Fitzroy stopped at the Keeling (now Cocos) Islands in the Indian Ocean. Among other activities, he took soundings of the area to determine the depth at which coral formations began. These gave the depth limit for living corals as only about 20 fathoms (37 meters), but the island in question, rising more than 7200 feet (2100 meters) Darwin wrote, “forms a lofty submarine mountain, with sides steeper even than those of the most abrupt volcanic cone. . . and every single atom, from the least particle to the largest fragment of rock in this great pile, which however is small compared with very many other lagoon-islands, bears the stamp of having been subjected to organic arrangement. We feel surprise when travelers tell us of the vast dimensions of the Pyramids and other great ruins, but how utterly insignificant are the greatest of these, when compared to these mountains of stone accumulated by the agency of various minute and tender animals!”
If the sea depth had remained constant, corals could never have built their reefs on the bottom, far below their habitable limit. On the other hand, besides the fact that the Keeling Islands soundings showed they were not formed around volcanoes, how did coral reefs come into existence?
Although it pained Darwin to disagree with Lyell, he proposed a new theory: that reefs formed around the fringes of existing land, and that it was the subsequent subsidence or sinking of that land below sea level that formed the open, circular interior lagoon of atolls. (At the time, no one was aware of the additional role played by rising and lowering sea levels in the formation of coral reefs.)
Darwin would end by writing an entire book, The Structure and Distribution of Coral Reefs, to state his new theory of the place of the humble coral polyps in the formation of the Earth. Far from being offended by Darwin, Charles Lyell was delighted.
(Next Friday, Adventure classics begins a September of young adventurers with William Golding’s Lord of the Flies. And you thought your schooldays were bad. . . )