Darwin’s other theory (repost)

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Diagrams from Darwin, 1842.

I originally published this on my old blog, Updates from the Paleontology Lab, on March 24, 2010. I’m republishing it here for Darwin Day.

Tim has to write an essay about a famous scientist for his science class that includes describing that person’s major contributions, and he chose Charles Darwin (that’s my boy!). To help him out, I showed him some of my Keynote presentations from my historical geology classes (I’m not teaching this semester, but I still have all my lecture slides). I came across some slides that I thought were worth reproducing here.

Darwin is obviously most famous for his work on evolution and the publication of “Origin of Species” in 1859, but that wasn’t all he did in his career. Among geologists, he’s also well known for his theories on coral reef development, which like evolution largely stemmed from his travels on the HMS Beagle. These thoughts were published in “The Structure and Distribution of Coral Reefs” in 1842 (available online here).

Darwin started by describing in detail coral reefs all over the world, concentrating on those in the Pacific and Indian Oceans that he visited while on the Beagle (see map at top). His Plate 1 included maps of some of these islands, modified from excellent Royal Navy maps:

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Compare to modern Google Earth images of the same islands:

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Darwin pointed out that there were three distinct types of reefs: fringing reefs which were right next to an island’s shore (Kosrae, below), barrier reefs which surrounded an island and were separated from it by a lagoon (Bora bora), and atolls, ring-shaped reefs surrounding a lagoon with no central island (Hao).

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(As an aside, Darwin apparently established the regular use of the term atoll in the western world, even though it had been used by some earlier European explorers. From page 2: “As the reef of a lagoon-island generally supports many separate small islands, the word ‘island,’ applied to the whole, is often the cause of confusion; hence I have invariably used in this volume the term ‘atoll,’ which is the name given to these circular groups of coral islets by their inhabitants in the Indian Ocean, and is synonymous with ‘lagoon-island.’)

With his usual meticulous attention to detail, he went through numerous examples of each type of reef, looking at how the coral were distributed laterally and vertically, what kinds of rocks were found in the area, and so on. He pulled up samples from various depths of water to show that coral couldn’t live at depth greater than about 200 feet, even though dead corals were found much deeper than that. (His descriptions include this remarkable quote on page 80: “A little further out the depth is thirty fathoms, and thence the bank slopes rapidly into the depths of the ocean. … The water was so clear outside the reef, that I could distinguish every object forming the rugged bottom.”)

Based on these observations he proposed that these three types of reef are all related to each other. Essentially atolls are formed when an island with a fringing reef subsides, but the coral growth keeps pace with the subsidence. Therefore, atolls had encountered greater subsidence than fringing reefs, and barrier reefs were intermediate. Here are Darwin’s cross-section interpretations:

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Darwin pretty much got it right. The one thing he missed (and no one realized it for more than 100 years) concerned the nature of the subsidence that formed the atolls. Darwin noticed that the atolls tended to cluster in certain areas, and believed those areas were subject to greater rates of subsidence. In fact, the atolls had encountered more subsidence not because the rate of subsidence was particularly higher there, but because they were older than the other islands.

It finally took the development of plate tectonics theory in the 1960’s to fully understand Darwin’s observations. Any given island chain (such as Hawaii) is formed from a single volcanic hot spot. As the Pacific plate moves over the hot spot, the old volcanic island is carried away and a new one forms, with a new fringing reef surrounding it. In the Hawaiian chain, Midway and Oahu are probably experiencing close to the same subsidence rates. But Midway (an atoll) has been experiencing that subsidence for about 27 million years, while Oahu (with a fringing reef) has only been subsiding for about 3 million years.

While Darwin played the key role in the development of the most important unifying theory in biological sciences (evolution), he also played a small but significant role in the development of the major unifying theory in geological sciences (plate tectonics).

Reference:

Darwin, C., 1942. The Structure and Distribution of Coral Reefs. Smith, Elder, and Co., London, 214 p.

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