Geoscientist Online

Sally Gibson* traces Darwin’s geological footsteps in the Galapagos and reveals the seminal contribution to igneous petrology he made there…

Geoscientist 19.2 February

“He realised that the confinement of low-density trachyte to the upper slopes and higher-density basalt to the lower slopes of the same volcano meant that different types of magma could form in different parts of a chamber undergoing crystal settling.”

This month we celebrate the 200^th birthday of Charles Darwin (1809-1892). So widely recognised is he today for his work on organic evolution, culminating in his Origin of Species (1859), that Darwin’s contribution to geological science has, until recently, been almost completely overshadowed¹. However, over 75% of Darwin’s notes from the Beagle voyage consist of geological observations. These, together with transcriptions of Darwin’s correspondence² are available at www.darwin-online.org.uk and www.darwinproject.ac.uk respectively, and offer an insight in to Darwin’s thoughts and inspirations – see also: Darwin’s geological formation.

These facts, taken with what we found at Isla Santiago (formerly James Island) on two recent expeditions show that Darwin was not only an efficient collector and documenter of the geology he encountered, but a pioneering theorist in igneous petrology.

Discovery in the Galapagos

In 1835, towards the end of the epic voyage, HMS Beagle’s captain charted the coastlines of the Galapagos Islands. Darwin had already described, in a letter to his Cambridge friend and mentor J S Henslow, how eager he was to reach the islands and witness an active volcano. There were no eruptions at the time of Darwin’s visit but his geological observations there assumed a prominent place in Volcanic Islands.

Between 16 September and 20 October 1835 Darwin visited four islands in the Galapagos (Fig. 1): San Cristobal (Chatham), Floreana (Charles), Isabela (Albemarle) and last, Isla Santiago (James). Some of his most significant geological observations were made on Santiago, where he spent the most time (10 days). Darwin was left on the west coast of Santiago with a small party of men, while the Beagle returned to San Cristobal to collect water and continued her survey of coastlines in the north of the archipelago. He was accompanied by his servant Syms Covington, the ship’s surgeon Bynoe and his assistant Fuller – both of whom were collecting geological, ornithological, zoological and botanical specimens for FitzRoy. The landing point at Buccaneer Cove (known to Darwin as Fresh-water Bay; Fig. 2) and nearby freshwater springs had been recommended by the English Governor, Mr Lawson, whom Darwin had met on Floreana. Darwin and his men set up camp at Buccaneer Cove, where he collected most (10) of his geological samples. These included feldspar-rich lavas and ‘ejected fragments’ of angular and granular rocks.

In the first few days, Darwin made detailed geological observations of a promontory at the north end of the Cove and correctly identified the ‘wreck of a large crater’ (Fig. 3). He also described a “200’ thick” lens-shaped mass that had filled up the former basin. Darwin noted that the lower part (2ft) of the outcrop was highly vesicular and contained numerous crystals of glassy feldspar, whereas the rock higher up was more compact and contained less feldspar. He had noticed this relationship between abundance of feldspar and degree of vesicularity elsewhere in Buccaneer Cove. At the north end of the beach he found ‘ejected fragments’ in thin lava flows (Fig. 4). These were highly oxidised but rich in feldspar and Darwin initially believed that they were granites. Following Scrope, Darwin contemplated (in his notes) the possibility that the feldspars may have been derived from the ‘granitic’ fragments rather than crystallised from the liquid lava itself. He did, however, express his concern over the lack of vesicles in the fragments and how the concentration of gas bubbles might be related to the feldspar crystals in the lava.

While on Santiago Darwin and his party made two separate c. eight-mile treks into the interior and higher parts of the island, each lasting two days. Together with a guide from a group of Spanish-speaking hunters, they followed the paths of giant tortoises (Geochelone elephantopus darwinii), which led to springs in the island’s interior. Their route, along a south-east traverse from Buccaneer Cove, passed several ‘ancient craters’ and through a habitat of unique flora that is confined to the west of the island. Darwin and his men collected tens of plant, bird, reptile and insect specimens and two rock samples. Precise details of the collection sites were not, however, recorded in Darwin’s notebooks. Indeed, he simply described the rock specimens as “Base (black grey) with scarcely any crystals” and “Compact lava with many small glassy Feldspar Basis finely Crystalline.--latter more central parts”. Although his notes were brief, Darwin had made a crucial observation that lavas erupted from the same volcano can give rise to different types of volcanic rocks.

Return of the Beagle

After leaving the Galapagos, the Beagle set sail for Tahiti and then returned to England via Australia and the Cape of Good Hope. Darwin arrived back in England on 2 October 1836, having been able during the latter part of the voyage Darwin to consolidate his Galapagos findings to observe similar volcanic rock types and landforms at brief stopovers in St Helena, Ascension and the Azores.

On returning to Shrewsbury Darwin was keen to share his findings and wrote almost immediately to Henslow (6 October 1836) emphasising his enthusiasm for geology over other aspects of natural history and asking for advice on how to deal with his specimens.

“My chief puzzle is about the geological specimens, who will have the charity to help me in describing their mineralogical nature?— “

A few weeks later Darwin travelled to London where he met several prominent naturalists, including Lyell and Owen. Although helpful, neither showed any great interest in his geological specimens. Darwin returned to Cambridge at the end of 1836 taking many of his rocks, minerals and fossils with him. At the Cambridge natural history museum, however, space was not available for geological material and the specimens remained in Darwin’s possession.

Correspondence and scraps of paper inserted in to Darwin’s notebooks indicate that once back in England, he was influenced by W H Miller, an eminent Cambridge mineralogist, and also by literature published during and after the Beagle voyage, (e.g. by von Buch¹². In the Galapagos, he had relied on the work of d’Aubisson de Voisins⁷ and used the term “trachyte” to describe cellular (vesicular) rocks with abundant feldspar. Darwin revised this usage to describe any feldspar-rich volcanic rock containing hornblende and iron oxide.

Darwin’s revised nomenclature enabled him to develop his ideas on the spatial variation of rock types around an individual volcano. This became the key to his theory of density-controlled crystal settling. He realised that the confinement of low-density trachyte to the upper slopes and higher-density basalt to the lower slopes of the same volcano meant that different types of magma could form in different parts of a chamber undergoing crystal settling.

Darwin’s ideas on the diversity of magmas stood in stark contrast to those proposed by Wernerian scientists like von Humboldt, who thought that the composition of igneous rocks varied with time. It was, however, consistent with the uniformitarian ideas of scientists such as Scrope, who believed that both trachyte and basalt were derived from a granitic source. Scrope wrote:

“It would not indeed be difficult to conceive the production of ordinary trachyte.. from granitic origin; for the process of intumescence...may easily be supposed to change the feldspar crystals from compact to glassy; to dissolve the whole or the greater part of the quartz in the aqueous vehicle; forcing it to assume the crystalline form on consolidation.”

Darwin had been heavily influenced by Scrope’s ideas but unlike him, thought that it was the density contrast between the crystals and surrounding body of magma which caused variations in composition. His ideas stemmed from experiments on lead-silver alloys in which the lead sank once it had formed granules, leaving a residue of molten metal rich in silver¹³. Darwin wrote6:

“Lavas are chiefly composed of three varieties of feldspar, varying in specific gravity from 2.4 to 2.74; of hornblende and augite, varying from 3.0 to 3.4; or olivine, varying from 3.3 to 3.4; and lastly, of oxides of iron, with specific gravities from 4.8 to 5.2. Hence crystals of feldspar, enveloped in a mass of liquefied, but not highly vesicular lava, would tend to rise to the upper parts; and crystals or granules of the other minerals, thus enveloped, would tend to sink.”

He also speculated that progressive cooling of plutons would create a crystal framework that would “eventually prevent the heavier ones from sinking and the lighter ones from rising”.

Darwin on Isla Santiago

In 1897, five years after Darwin’s death, his geological specimens from the Galapagos were incorporated by the Woodwardian Museum (now the Sedgwick Museum of Earth Sciences) at Cambridge University (Fig. 5), but by then were treated as historical material rather than being integrated with the general collections. Darwin’s samples of ‘basalt’ and ‘trachyte’ from the Galapagos were examined by Alfred Harker¹⁴ and subsequently in more detail by Richardson¹⁵. Both made full use of thin sections and confirmed much of Darwin’s petrological nomenclature. However, with the exception of work by science historians¹⁶, there have been very few recent attempts to study Darwin’s collection of rocks from the Galapagos. This may be because his samples are small (typically <5cm) and covered in lichen - offering only limited scope for a modern-day petrological study. Yet despite their size, the specimens are highly varied and confirm that Darwin had a keen collector’s eye.

Partly supported by a fieldwork grant from the Society, I participated in two international scientific expeditions (2007, 2008) to the uninhabited Isla Santiago, the fourth largest island (585 km²) lying at the centre of the archipelago and consisting of a large alkaline shield volcano to the west and predominantly tholeiitic monogenetic cones to the east. Only one pioneering geological expedition17 and one detailed field investigation¹⁸ had previously been attempted there, and one of our main goals was to locate Darwin’s route and the places from which he drew inspiration.

The expedition to the north-west of Isla Santiago (2007) visited the sites at Buccaneer Cove described in Darwin’s notebook and Volcanic Islands. His field notes are not always easy to follow and it was only once we had visited and collected samples from most of Darwin’s localities that we became aware of systematic discrepancies between these two references.

For example, we sampled what we believe to be Darwin’s ‘200 ft’ lenticular mass (a solidified lava lake) on the promontory (Fig. 3). The lava is highly vesicular and contains numerous plagioclase crystals (it is a trachyandesite). In the field, Darwin described this as trachyte; but in Volcanic Islands (1944) he reclassified it as basalt. Also, some of our observations differed from Darwin’s. The precipitous cliffs that form the promontory are ~200ft high but the lava lake itself is closer to 50ft thick. Since this is the only lens-shaped body on the promontory and occurs in the centre of what was once a volcanic crater just as Darwin says, we concluded that Darwin probably later confused the lava’s thickness with his estimate of cliff height.

We also visited Darwin’s xenolith (ejected fragment) locality at the north end of Buccaneer Cove. This is one of the few places where he drew a sketch, and it can be recognised today (Fig. 4) despite the lack of any scale. (It is interesting that the sketch was reversed in Volcanic Islands.) Darwin’s xenolith samples are all heavily oxidised (reddened), making the constituent phases difficult to identify in hand specimen. This may account for his incorrect field identification of these as granites16. On his return to England he realised they contained no quartz6 (thin sections of his samples show that they are in fact gabbros). We found similar reddened xenoliths in thin lavas at Buccaneer Cove but also located abundant fresh gabbro xenoliths in a small bay north of the promontory. It seems that Darwin did not go there, which is unfortunate because here the lavas display a clear relationship between their crystal content (olivine-, pyroxene- or plagioclase-rich) and the type of xenolith that they contain. The xenoliths appear to represent a crystal pile, dredged by different eruptions. Disaggregation of this poorly annealed crystal mush gave rise to individual crystals and small aggregates.

Nevertheless, once Darwin had realised that the xenoliths contained no quartz he was able to account for their origin correctly⁶:

“It is interesting thus to trace the steps by which a compact granular rock becomes converted Into a vesicular, pseudo-porphyritic lava, and finally into red scoriae. The structure and composition of the embedded fragments show that they are parts either of a mass of primary rock which has undergone considerable change from volcanic action, or more probably of the crust of a body of cooled and crystallised lava, which has afterwards been broken up and re-liquefied; the crust being less acted on by the renewed heat and movement.”

During our fieldwork we also successfully located a small trachyte dome near the summit. The presence of this rock type on Isla Santiago had previously been questioned¹⁷ and the suggestion made that Darwin’s sample may have been incorrectly labelled. Nevertheless, McBirney & Williams’s fieldwork had focused on the lower slopes, where subsequent investigations have shown only basaltic flows. The trachyte that we collected from the summit - and also that in Darwin’s collection - contain alkali feldspar, quartz, olivine, pyroxene and apatite together with highly distinctive blue-green (riebeckite) and green-brown amphiboles. It seems likely therefore that Darwin’s sample was indeed from Isla Santiago, and was probably collected from the same location, since we found no trachyte elsewhere on the island. As far as we know this is the first confirmation of Darwin’s diagnosis, and evidence that he reached the summit (his ‘Central parts’).

A ridge leading to the summit of Santiago is formed from coarse-grained non-vesicular feldspar-rich rock. It is much more evolved than the coarse-grained material (gabbro) that occurs as xenoliths on the lower slopes of the volcano at Buccaneer Cove. Given Darwin’s interest in feldspar-rich rocks it is slightly surprising that he does not mention this outcrop. However he reported the presence of a thick mist on the summit of the island, and this may have obscured the ridge. This and the nearby trachyte appear to represent the most extreme products of crystal fractionation that were erupted from a shallow magma chamber beneath the west of Isla Santiago.

Epiphany

It seems largely due to good fortune that, while in the Galapagos, Darwin spent most time on Isla Santiago (James Island) with its diverse range of volcanic rock types, all close to supplies of fresh water. Our fieldwork has shown that despite the small number of specimens in Darwin’s collection, they are in fact representative of almost all the different igneous rock types there. Buccaneer Cove, and a small bay to the north, are the only locations on the island where abundant coarse-grained pyroxene-, olivine- and plagioclase feldspar-bearing xenoliths occur. These samples were important to his ideas on magma chambers.

Furthermore, had it not been for the presence of hunters following the trails of tortoises on Isla Santiago, Darwin would not have been able to explore the volcano’s densely vegetated slopes. He would then certainly not have found the small trachyte outcrop near the summit – his specimen of which proved vital to one of Darwin the geologist’s most significant contributions to our present-day understanding of magmatic processes - his theory of density-controlled crystal settling.

Acknowledgements

Fieldwork in the Galapagos was initiated by Dr Sandra Herbert of the University of Maryland. I am extremely grateful to her and Dr David Norman, Director of the Sedgwick Museum, for inviting me to participate in the Darwin Project. Fieldtrips to Santiago have been made possible and truly memorable experiences thanks to them, co-geologist Dennis Geist and the rest of the team of scientists, natural historians and field assistants (Greg Estes, Thalia Grant, Andrew Miles, Andy Thurman, Melina Fowler, Maria De la Torre). Fieldwork was undertaken with permission of Galapagos National Park authorities and in collaboration with the Charles Darwin Research Station. We are extremely grateful to them for their co-operation. Fieldwork was generously funded by The Geological Society of London, the Mineralogical Society of Great Britain and Ireland, the University of Cambridge and the National Science Foundation. Photographs of the Galapagos courtesy, Andy Thurman.

References

1. Herbert, S 2005: Charles Darwin, Geologist Cornell University Press, Ithaca & London
2. Burkhardt, F et al. (eds) 1985: The Correspondence of Charles Darwin 16+ vols Cambridge: Cambridge University Press
3. Worsley, Peter 2008: Rocks of Ages Geoscientist 18.11 pp 20-2
4. Darwin, C 1842: The Structure and Distribution of Coral Reefs Being the First Part of the Geology of the Voyage of the Beagle, under the Command of Capt FitzRoy, RN during the Years 1832 to 1836 London: Smith, Elder and Co
5. Darwin, C 1846: Geological Observations on South America Being the Third Part of the Geology of the Voyage of the Beagle Under the Command of Capt FitzRoy, RN during the Years 1832 to 1836 London: Smith, Elder and Co
6. Darwin, C 1844: Geological Observations on the Volcanic Islands, Visited during the Voyage of HMS Beagle, Together with Some Brief Notices on the Geology of Australia and the Cape of Good Hope Being the Second Part of the Geology of the Voyage of the Beagle, under the Command of Capt FitzRoy RN during the Years 1832 to 1836 London: Smith, Elder and Co
7. d'Aubuisson de Voisins, J F, 1819: Traité de Géognosie Strasbourg, F G Levrault
8. Daubeny, C 1826: A description of active and extinct volcanos; with remarks on their origin, their chemical phaenomena, and the character of their products, as determined by the condition of the earth during the period of their formation. Being the substance of some lectures delivered before the University of Oxford, with much additional matter. London: W Phillips
9. von Humboldt, A , 1790: Mineralogische Beobachtungen über einige Basalte am Rhein Brunswick
10. Lyell, Charles 1830-33: Principles of geology, being an attempt to explain the former changes of the earth's surface, by reference to causes now in operation. Vols 1-3. London: John Murray
11. Scrope, G J P 1825: Considerations on volcanos, the probable causes of their phenomena, the laws which determine their march, the disposition of their products, and their connexion with the present state and past history of the globe; leading to the establishment of a new theory of the earth London: W Phillips
12. von Buch, L , 1836: Über Erhebungskrater und Vulkane Annalen der Physik 37, 169-190
13. Pattinson, H L, 1838: On a new process for the extraction of silver from lead Report of the eighth meeting of the British Association for the Advancement of Science, 7, 50-55
14. Harker, A C, c.1907: Catalogue of the “Beagle” Collection of Rocks, made by Charles Darwin during the voyage of H M S “Beagle”, 1832–6
15. Richardson, Constance 1933: Petrology of the Galapagos Islands in Chubb, Lawrence John, & Richardson, Constance: Galapagos Islands: Regional Geology Cocos Islands: Regional Geology Easter Island: Regional Geology Petrology, p45-67. Honolulu: Bernice P Bishop Museum
16. Pearson, P, 1996: Charles Darwin on the origin and diversity of igneous rocks Earth Sciences History, 15, 49-67
17. McBirney, A R, Williams, H, 1969: Geology and petrology of the Galapagos Islands Geol. Soc. Am. Mem. 118
18. Baitis, H W, 1976: Geology and petrography of Pinzon and Santiago Islands: Galapagos Archipelago. Oregon