The Structure and Distribution of Coral Reefs - Theory of Coral Atoll Formation

Theory of Coral Atoll Formation

When the Beagle set out in 1831, the formation of coral atolls was a scientific puzzle. Advance notice of her sailing, given in the Athenaeum of 24 December, described investigation of this topic as "the most interesting part of the Beagle's survey" with the prospect of "many points for investigation of a scientific nature beyond the mere occupation of the surveyor. In 1824 and 1825, French naturalists Quoy and Gaimard had observed that the coral organisms lived at relatively shallow depths, but the islands appeared in deep oceans. In books that were taken on the Beagle as references, Henry De la Beche, Frederick William Beechey and Charles Lyell had published the opinion that the coral had grown on underwater mountains or volcanoes, with atolls taking the shape of underlying volcanic craters. The Admiralty instructions for the voyage stated:

The circularly-formed Coral Islands in the Pacific occasionally afford excellent land-locked harbours, with a sufficient entrance, and would be well adapted to any nice astronomical observations which might require to be carried on in undisturbed tranquillity. While these are quietly proceeding, and the chronometers rating, a very interesting inquiry might be instituted respecting the formation of these coral reefs..... A modern and very plausible theory has been put forward, that these wonderful formations, instead of ascending from the bottom of the sea, have been raised from the summits of extinct volcanoes...

As a student at the University of Edinburgh in 1827, Darwin learnt about marine invertebrates while assisting the investigations of the anatomist Robert Edmond Grant, and during his last year at the University of Cambridge in 1831, he had studied geology under Adam Sedgwick. So when he was unexpectedly offered a place on the Beagle expedition, as a gentleman naturalist he was well suited to FitzRoy's aim of having a companion able to examine geology on land while the ship's complement carried out its hydrographic survey. FitzRoy gave Darwin the first volume of Lyell's Principles of Geology before they left. On their first stop ashore at St Jago island in January 1832, Darwin saw geological formations which he explained using Lyell's uniformitarian concept that forces still in operation made land slowly rise or fall over immense periods of time, and thought that he could write his own book on geology. Lyell's first volume included a brief outline of the idea that atolls were based on volcanic craters, and the second volume, which was sent to Darwin during the voyage, gave more detail. Darwin received it in November 1832.

While the Beagle surveyed the coasts of South America from February 1832 to September 1835, Darwin made several trips inland and found extensive evidence that the continent was gradually rising. After witnessing an erupting volcano from the ship, he experienced an earthquake on 20 February 1835. In the following months he speculated that as the land was uplifted, large areas of the ocean bed subsided. It struck him that this could explain the formation of atolls.

Darwin's theory followed from his understanding that coral polyps thrive in the clean seas of the tropics where the water is agitated, but can only live within a limited depth of water, starting just below low tide. Where the level of the underlying land stays the same, the corals grow around the coast to form what he called fringing reefs, and can eventually grow out from the shore to become a barrier reef. Where the land is rising, fringing reefs can grow around the coast, but coral raised above sea level dies and becomes white limestone. If the land subsides slowly, the fringing reefs keep pace by growing upwards on a base of dead coral, and form a barrier reef enclosing a lagoon between the reef and the land. A barrier reef can encircle an island, and once the island sinks below sea level a roughly circular atoll of growing coral continues to keep up with the sea level, forming a central lagoon. Should the land subside too quickly or sea level rise too fast, the coral dies as it is below its habitable depth.