Part I told the story of Charles Darwin’s early life and explained how he got interested in exploring the oceans and the life that lives within them. In this second installment, Darwin forms a theory while aboard the HMS Beagle and explores various pieces of evidence to support it.

An animated sequence showing the erosion of a ...

An animated sequence showing the erosion of a volcano, and the formation of a coral reef around it—eventually resulting in an atoll (Photo credit: Wikipedia)

While on the voyage of the Beagle, Darwin came up with a theory involving three types of coral structures. His theory was that atolls formed from evolving fringing and barrier reefs. According to NOAA (National Oceanic and Atmospheric Administration), fringing reefs are the most common coral structures and grow out towards the sea from a shore or island. Barrier reefs are comparable to fringing reefs in that they form a boundary-line around the circumference of an island or shore of an island. However, they are not attached to the land, but rather are separated from it by a lagoon-like stretch of seawater. This area of open water separating the reef from the shore can be extremely deep. Finally, atolls are the result of fringing or barrier reefs continuing to grow upwards while the land within the reef’s perimeter simultaneously sinks over long geological time, until there is no longer any land sticking out above the water’s surface (“What Are the Three Main Types of Coral Reefs?”).

During the voyage of the HMS Beagle, Darwin worked on understanding the geology of the places he visited, particularly in the southern hemisphere with Australia and South Africa as examples. He wrote three geological books: Volcanic Islands (1844), Coral Reefs (1842), and South America (1846). All three of these texts contain overlapping observations and accounts of his time on the Beagle (Wyhe, Darwin Online). In one of the volumes of his book, Coral Reefs, he included a map where he had charted the whereabouts of different coral formations as he found them scattered around the globe. His interest in volcanic islands was directly related to his inquiries about coral reefs. Darwin “believed that areas of volcanic eruption were undergoing elevation” (Herbert, 233). Based on coral fossils Darwin found in the rocks of the Andes Mountains located in Chile, he predicted that the extensive mountain chain had gradually grown up from the depths of the sea floor “by volcanic action over aeons of time” (McCalman, 188). Darwin’s theory that atolls formed from fringing and barrier reefs through time was a result of his research on volcanic islands. To explain his theory a little more in depth, one can read through Darwin’s journal records in The Voyage of the Beagle to find details similar to the following overview. His theory was that an island would start with a fringing reef bordering its perimeter. The island would begin to subside, causing the seawater to “encroach” upon the beach. This would result in a widening space between the reef and the shore of the island (Darwin, 493) . This process is the transition of a fringing reef into that of a barrier reef. Coral islets, very small islands made of coral, would be navigated by flowing channels of water though the lagoon. As the island continued to subside, ocean water would continue to pour over the coral barrier into the lagoon, making the island appear smaller and shorter due to its deeper position in the water column (Darwin, 493). Eventually, the island would entirely subside beneath the sea surface and would no longer be seen sticking out of the ocean. At this point, the barrier reef turns into an atoll.

Sources:

“What Are the Three Main Types of Coral Reefs?” Ocean Services. N.p., n.d. Web. 8 June 2015. <http://oceanservice.noaa.gov/facts/threecorals.html>.

Wyhe, John van. Ed. 2002-. The Complete Work of Charles Darwin Online (http://darwin-online.org.uk/)

Herbert, Sandra. “Toward Simplicity.” Charles Darwin, Geologist. N.p.: n.p., n.d. 233. Print.

McCalman, Iain. “Obsession.” The Reef: A Passionate History. Penguin Books, Australia: Hamish Hamilton, 2013. 187-88. Print.

Darwin, Charles. “Keeling Island:– Coral Formations.” The Voyage of the Beagle. N.p.: n.p., n.d. 472-502. Print.