That whole biological evolution thing that Charles Darwin figured out quite often makes people ignore the fact he was a first-rate geologist as well. While collecting meticulous samples of the flora and fauna he encountered on his voyages with the H.M.S. Beagle, he also thoroughly investigated the geology of the places they visited. And he discovered things no other person had discovered before; understood processes no one before him had figured out. If he hadn't become famous as the man who co-discovered evolution, he still would have been noted as one of the fathers of geology. And the Galapagos Islands would have remained central to his scientific discoveries.
The Galapagos, a fairly young and still quite active group of volcanic islands, are an ideal playground for a keen young geologist. The hot, dry climate, combined with lava flows that erupted quite recently in geologic terms, presents a sparsely-vegetated and quite lightly eroded rock record. For a lover of the good science of rock-breaking such as Darwin, it was enthralling. He scrambled over several of the islands, taking notes and rock samples and puzzling over the origins of what he saw. Less than ten years after the voyage of the Beagle concluded, he published his findings on the Galapagos and other islands in Geological Observations on the Volcanic Islands, visited during the Voyage of the H.M.S. Beagle.
Now, keep in mind that Charles Lyell, the father of modern geology, had published the last volume of his Principles of Geology barely a decade before. Western scientists were only just beginning to grapple with the great age of the earth. When Darwin began his travels, James Hutton and other Plutonists had only just convinced most of the scientific world that basalt was deposited by volcanoes, not laid down by seawater. And the science of volcanology hadn't really been born yet when Darwin first laid eyes upon the Galapagos. Yet he saw things no one had eyes to see.
He looked at the tuff cones dotted along the rocky shores, and saw a marvelous type of basaltic rock that wouldn't even be named until ten years after his visit: palagonite. But he didn't just observe this fascinating rock. He saw how it must have evolved from the basalt itself, and recognized that the tuff cones must have erupted under water. He made conclusions about how these unique volcanoes erupted and eroded that have been confirmed by much later study. And he understood the basics of palagonite long before petrologists studied the stuff in thin sections and saw the delicate variations he saw.
We're going to investigate both palagonite and tuff rings here very shortly. I can't think of a better place to begin our explorations than with Darwin's own observations:
CHATHAM ISLAND. CRATERS COMPOSED OF A SINGULAR KIND OF TUFF.
Towards the eastern end of this island there occur two craters composed of two kinds of tuff; one kind being friable, like slightly consolidated ashes; and the other compact, and of a different nature from anything which I have met with described. This latter substance, where it is best characterised, is of a yellowish-brown colour, translucent, and with a lustre somewhat resembling resin; it is brittle, with an angular, rough, and very irregular fracture, sometimes, however, being slightly granular, and even obscurely crystalline: it can readily be scratched with a knife, yet some points are hard enough just to mark common glass; it fuses with ease into a blackish-green glass. The mass contains numerous broken crystals of olivine and augite, and small particles of black and brown scoriae; it is often traversed by thin seams of calcareous matter. It generally affects a nodular or concretionary structure. In a hand specimen, this substance would certainly be mistaken for a pale and peculiar variety of pitchstone; but when seen in mass its stratification, and the numerous layers of fragments of basalt, both angular and rounded, at once render its subaqueous origin evident. An examination of a series of specimens shows that this resin-like substance results from a chemical change on small particles of pale and dark-coloured scoriaceous rocks; and this change could be distinctly traced in different stages round the edges of even the same particle. The position near the coast of all the craters composed of this kind of tuff or peperino, and their breached condition, renders it probable that they were all formed when standing immersed in the sea; considering this circumstance, together with the remarkable absence of large beds of ashes in the whole archipelago, I think it highly probable that much the greater part of the tuff has originated from the trituration of fragments of the grey, basaltic lavas in the mouths of craters standing in the sea. It may be asked whether the heated water within these craters has produced this singular change in the small scoriaceous particles and given to them their translucent, resin-like fracture. Or has the associated lime played any part in this change? I ask these questions from having found at St. Jago, in the Cape de Verde Islands, that where a great stream of molten lava has flowed over a calcareous bottom into the sea, the outermost film, which in other parts resembles pitchstone, is changed, apparently by its contact with the carbonate of lime, into a resin-like substance, precisely like the best characterised specimens of the tuff from this archipelago. (The concretions containing lime, which I have described at Ascension, as formed in a bed of ashes, present some degree of resemblance to this substance, but they have not a resinous fracture. At St. Helena, also, I found veins of a somewhat similar, compact, but non- resinous substance, occurring in a bed of pumiceous ashes, apparently free from calcareous matter: in neither of these cases could heat have acted.)
To return to the two craters: one of them stands at the distance of a league from the coast, the intervening tract consisting of a calcareous tuff, apparently of submarine origin. This crater consists of a circle of hills some of which stand quite detached, but all have a very regular, qua- qua versal dip, at an inclination of between thirty and forty degrees. The lower beds, to the thickness of several hundred feet, consist of the resin- like stone, with embedded fragments of lava. The upper beds, which are between thirty and forty feet in thickness, are composed of a thinly stratified, fine-grained, harsh, friable, brown-coloured tuff, or peperino. (Those geologists who restrict the term of "tuff" to ashes of a white colour, resulting from the attrition of feldspathic lavas, would call these brown-coloured strata "peperino.") A central mass without any stratification, which must formerly have occupied the hollow of the crater, but is now attached only to a few of the circumferential hills, consists of a tuff, intermediate in character between that with a resin-like, and that with an earthy fracture. This mass contains white calcareous matter in small patches. The second crater (520 feet in height) must have existed until the eruption of a recent, great stream of lava, as a separate islet; a fine section, worn by the sea, shows a grand funnel-shaped mass of basalt, surrounded by steep, sloping flanks of tuff, having in parts an earthy, and in others a semi-resinous fracture. The tuff is traversed by several broad, vertical dikes, with smooth and parallel sides, which I did not doubt were formed of basalt, until I actually broke off fragments. These dikes, however, consist of tuff like that of the surrounding strata, but more compact, and with a smoother fracture; hence we must conclude, that fissures were formed and filled up with the finer mud or tuff from the crater, before its interior was occupied, as it now is, by a solidified pool of basalt. Other fissures have been subsequently formed, parallel to these singular dikes, and are merely filled with loose rubbish. The change from ordinary scoriaceous particles to the substance with a semi-resinous fracture, could be clearly followed in portions of the compact tuff of these dikes.
CRATERS OF TUFF.
About a mile southward of Banks' Cove, there is a fine elliptic crater, about five hundred feet in depth, and three-quarters of a mile in diameter. Its bottom is occupied by a lake of brine, out of which some little crateriform hills of tuff rise. The lower beds are formed of compact tuff, appearing like a subaqueous deposit; whilst the upper beds, round the entire circumference, consist of a harsh, friable tuff, of little specific gravity, but often containing fragments of rock in layers. This upper tuff contains numerous pisolitic balls, about the size of small bullets, which differ from the surrounding matter, only in being slightly harder and finer grained. The beds dip away very regularly on all sides, at angles varying, as I found by measurement, from twenty-five to thirty degrees. The external surface of the crater slopes at a nearly similar inclination, and is formed by slightly convex ribs, like those on the shell of a pecten or scallop, which become broader as they extend from the mouth of the crater to its base. These ribs are generally from eight to twenty feet in breadth, but sometimes they are as much as forty feet broad; and they resemble old, plastered, much flattened vaults, with the plaster scaling off in plates: they are separated from each other by gullies, deepened by alluvial action. At their upper and narrow ends, near the mouth of the crater, these ribs often consist of real hollow passages, like, but rather smaller than, those often formed by the cooling of the crust of a lava-stream, whilst the inner parts have flowed onward;—of which structure I saw many examples at Chatham Island. There can be no doubt but that these hollow ribs or vaults have been formed in a similar manner, namely, by the setting or hardening of a superficial crust on streams of mud, which have flowed down from the upper part of the crater. In another part of this same crater, I saw open concave gutters between one and two feet wide, which appear to have been formed by the hardening of the lower surface of a mud stream, instead of, as in the former case, of the upper surface. From these facts I think it is certain that the tuff must have flowed as mud. (This conclusion is of some interest, because M. Dufrenoy "Mem. pour servir" tome 4 page 274, has argued from strata of tuff, apparently of similar composition with that here described, being inclined at angles between 18 degrees and 20 degrees, that Monte Nuevo and some other craters of Southern Italy have been formed by upheaval. From the facts given above, of the vaulted character of the separate rills, and from the tuff not extending in horizontal sheets round these crateriform hills, no one will suppose that the strata have here been produced by elevation; and yet we see that their inclination is above 20 degrees, and often as much as 30 degrees. The consolidated strata also, of the internal talus, as will be immediately seen, dips at an angle of above 30 degrees.) This mud may have been formed either within the crater, or from ashes deposited on its upper parts, and afterwards washed down by torrents of rain. The former method, in most of the cases, appears the more probable one; at James Island, however, some beds of the friable kind of tuff extend so continuously over an uneven surface, that probably they were formed by the falling of showers of ashes.
Within this same crater, strata of coarse tuff, chiefly composed of fragments of lava, abut, like a consolidated talus, against the inside walls. They rise to a height of between one hundred and one hundred and fifty feet above the surface of the internal brine-lake; they dip inwards, and are inclined at an angle varying from thirty to thirty-six degrees. They appear to have been formed beneath water, probably at a period when the sea occupied the hollow of the crater. I was surprised to observe that beds having this great inclination did not, as far as they could be followed, thicken towards their lower extremities.
You can find all of Darwin's books on geology for free on Kindle:
Geological Observations on South America
And you can also pick up a copy of Charles Lyell's ground-breaking work: