Darwin the Geologist

In an autobiographic note Charles Robert Darwin (February 12, 1809 ? 1882) remembered a childhood wish:

?It was soon after I began collecting stones, i.e., when 9 or 10, that I distinctly recollect the desire I had of being able to know something about every pebble in front of the hall door?it was my earliest and only geological aspiration at that time.?

Darwin today is mostly associated with terms like natural selection and evolution, but his first scientific achievements and publications were dealing ? even against his own preconceptions ? with geology.

As a medicine student at Edinburgh University (1825-1827) Darwin frequented various courses on natural science. Here he encountered geology in the form of the lectures by mineralogist Professor Robert Jameson. Darwin considered the teaching of Jameson as boring and despite his ambitions in collecting minerals in early years, during his remaining time at university he never again actively joined a lecture about geology.
The later time at Cambridge was more productive; he joined various privately organized geological-botanical excursions in the areas surrounding the town. In July 1831 he visited the cave of Llanymynech near his hometown of Shrewsbury and in August of the same year, after graduating from Cambridge University and pushed by his mentor and friend, botanist John S. Henslow, he accompanied Professor Adam Sedgwick (1785-1873, considered one of the founding fathers of geology in England, the Darwin Correpondence Project discusses this trip and? ?Darwin?s introduction to geology?) on a one week long geologic tour in North Wales. Twenty pages of notes made by Darwin during the tour are today conserved in the library of the Cambridge University. In his private autobiography he later remembered: ?This tour was of decided use in teaching me a little how to make out the geology of a country??

When Darwin returned to Shrewsbury, a letter from Captain Robert FitzRoy?s offered him a position as gentlemen companion on board of the ship ?Beagle?. FitzRoy was himself a gifted amateur geologist and was searching a talented naturalist with additional geological knowledge to sustain him in a personal task ? the Beagle voyage, despite improving the nautical maps of South America, could also be used to gather geological evidence for the biblical flood, a worldwide phenomena considered real by most geologists at the time. As a welcoming gift FitzRoy presented Darwin with a copy of Charles Lyell?s recently published ?Principles of Geology? (first edition 1830-1832).
Darwin will became strongly influenced by the ?slow occurring? geology as proposed by Lyell. He observed at his first stop of the Beagle on the Cape Verde Islands (January 16, 1832 to February 8,) sediments enclosed by lava flows and raised above the sea level, but with fossils similar to the shells in the sea nearby (implying no substantial change of the environment over time). He applied the principles formulated by Lyell and became convinced that the surface of earth changes over time only slowly and gradually, not as believed by many naturalists at the time by sudden catastrophic events, like the ?biblical flood?.

In a letter to his sisters Darwin confessed that he ?literally could not sleep for thinking over my [geology]?.

Fig.1. Profile of the island of St. Jago (today Santiago) as seen by Darwin in 1832. Darwin was the first to study the geology of the Cape Verde Islands (from DARWIN 1876). Darwin recognized three distinct layers of rocks, a lower series with volcanic rocks composed of volcanic breccias and magma dikes (deposited under water), a limestone with fossils and finally a cover of basaltic lava. Darwin, trained by Sedgwick, noted the contact metamorphism between the former hot molten lava and the earlier cool limestone.
It is curious to note that Darwin adopted the geological terms used by German geologists to describe the rocks observed in the field, here the strong influence of Alexander von Humboldt works, read by the young Charles, is recognizable (image in public domain).

February 20, 1835 Darwin experienced a strong earthquake that destroyed the town of Valdivia in Chile:

?I happened to be on shore, and was lying down in the wood to rest myself. It came on suddenly, and lasted two minutes, but the time appeared much longer. The rocking of the ground was very sensible.?

Darwin noted after the earthquake the raised shell beds along the coast of the Pacific and the similarities of these deposits with the layers of fossils observed on the cliffs of the islands of Cape Verde. Could it be possible that the sum of these small vertical movements could form such high mountains like the Andes? Earth had then to be very old, so that countless earthquakes would have enough time to modify the surface of the entire planet.

During the voyage (1831-1836) Darwin encountered various outcrops with magmatic and volcanic products and he became soon fascinated by these rocks. On the Galapagos Islands he carefully studied the viscosity of lava flows:

?The degree of fluidity in different lavas does not seem to correspond with any apparent corresponding amount of difference in their composition?

This is an erroneous conclusion as the viscosity of lava in fact depends of the amount of dissolved silica. However he correctly postulates that a mineralogical differentiation of magma is possible by segregation of minerals by gravity ? a fundamental point to explain the different lava types found on earth:

?Much of the difficulty which geologists have experienced, when they have compared the composition of volcanic with plutonic formations, will, I think, be removed, if we may believe, that most plutonic masses have been, to a certain extent, drained of those comparatively weighty and easily liquefied elements, which compose the trappean and basaltic series of rocks.?

At the time the mechanics and origin of volcanoes? was fiercely discussed. The eminent German geologist Leopold von Buch imagined that volcanoes form very fast like a bubble on the crust of the earth: first geologic forces push up the ground and form the mountain, then the summit collapses and the molten magma is released, causing a catastrophic eruption. Darwin did not share this vision of uprising volcanoes; in part the model proposed very fast rates of elevation and Darwin was convinced from his observations of lava flows interbedded with sediments that volcanoes form by episodic smaller eruptions. Again the geologic observations demonstrated two important facts: slow and small modifications can over vast periods of time completely reshaped the surface of earth.

In the five years that the voyage of the Beagle lasted, Darwin wrote 1.383 pages of notes about geology ? compared to a mere 368 pages of notes on plants and animals.

After returning to England (1836) Darwin presented his first scientific discourse of the geology of the Andes at the Royal Geological Society and published some preliminary results about volcanic phenomena observed in South America. His major contributions to volcanology are two later books: ?The structure and distribution of coral reefs?, published in 1842, and the ?Geological Observations on the Volcanic Islands? published in 1844. The first book covers the distribution, structure and formation of coral-riffs around the slowly drowning volcanic islands in the Pacific, the second presents the descriptions of the visited volcanic islands, like Ascension, St. Helena, the Galapagos and a short notification about the geology of South Africa and Australia.

Fig.2. Atoll formation according to Darwin, 1842. Darwin proposed that volcanic islands with fringing reefs, islands with barrier reefs and atolls (i.e. ring-shaped reefs without a volcanic island) are different stages of one process, governed by subsidence and reef growth. This famous concept is based on surface examination of reefs and comparison of islands and atolls in different stages of development (image in public domain).

In 1846 he published his ?Observations on South America?, a book that covers the continent Darwin explored and studied most.

Darwin also published some minor papers dealing with other subjects of geology encountered during the voyage (not to mention the volumes of the ?The Zoology of the Voyage of H.M.S. Beagle? dedicated to the collected fossil remains). In 1846 a description about the geology of the Falkland Islands, in 1838 about some phenomena connected with the volcanism in South America, in 1841 about the distribution of erratic blocks and sediments found in South America and in 1845 the observations about the dust that can be found, transported by the wind, on ships crossing the Atlantic Ocean.

In July 1838 he visited Glen Roy in Scotland and published his opinion on the origin of parallel terraces found along the slopes of the mountains there. Based on his observation in South America he proposed a marine origin of the terraces as ancient shores of a today vanished sea. Isolated large boulders, also found on the floors of the valleys, were according to his hypothesis the remains of debris transported by icebergs. Darwin encountered his first glaciers in January 1833 during the survey of the Beagle of Tierra del Fuego. He describes them of a beautiful ?beryl blue? and noted that the ice falling from the snouts into the sea formed icebergs and that these icebergs often incorporated and transported rocks and debris. When the ice melts the debris is released and deposited on the bottom of the flooded valleys. For Darwin the channels of Tierra del Fuego were the modern equivalent of Scotland?s valleys long time ago.

Fig.3. ?A glacier with moraines? from A.R. Wallace?s ?Island Life? (1880), image in public domain.

Two years after the publication of the ?Observations on the Parallel Roads of Glen Roy, and of Others Parts of Lochaber in Scotland, with an Attempt to Prove that They are of Marine Origin? a new theory dealing with the idea of ice ages attributed the terraces to the shores of former glacial lakes dammed up by ancient glaciers, like the examples observed in the European Alps at the time.

After these publications Darwin quickly gave up his ?geological phase? and retired from active geological research. In 1842 he visited Cwm Idwal in North Wales, one of the last geological excursion before his ill health forced him to an apparent quiet country life.

Geology played a major role in Darwin?s life and scientific work: The formation of volcanoes, the slow subsidence of coral reefs, the rising of the Andes by earthquakes, the fossil relatives to modern species in South America, these geological observations enabled Darwin to grasp two fundaments needed for his scientific theory: the deep time and the slow, but perpetual changes of earth itself.
If geology was able to such profound modifications over time, so had biology, to adapt and survive to an ever changing environment.

Bibliography:

CHIESURA, G. (2010): A Santiago sulle orme di Darwin. Darwin ? Bimestrale di Scienze No.40: 32-36
HERBERT, S. (2005): Charles Darwin, Geologist. Cornell University Press: 485
ROBERTS, M. (2001): Just before the Beagle: Charles Darwin?s geological fieldwork in Wales, summer 1831. Endeavour Vol. 25(1): 33-37
SEWARD, A.C. (2006): Darwin And Modern Science. The Echo Library, Teddington: 489
TOSATTI, G. (2008): Charles Darwin geologo. Atti Soc. Nat. Mat. Modena 139: 205-219
ZAPPETTINI, O. & MENDIA, J. (2009): The first Geological Map of Patagonia. Revista de la Asociaci?n Geol?gica Argentina 64 (1): 55 ? 59

Online Resources:

Darwin in London Project (2009): Charles Darwin: A Genius in the Heart of London. (Accessed 12.02.2011)
The Complete Work of Charles Darwin Online (2010): Geology of The Voyage of The Beagle. (Accessed 12.02.2011)

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