Posted by: Rob Viens | July 25, 2013

Petrified Lightnin’

At long last, on the night of the 24th, the Beagle set sail on its next adventure – this time to the Rio Negro.  Amid a storm of lighting, the crew weighed anchor and turned south:

“In the evening of the 24th, after it was dark, we got under weigh & started on our cruize to the Rio Negro. The whole sky was brilliant with lightning; it was a wild looking night to go to sea, but time is too precious to lose even a bad portion of it.” (July 23/24)

Lightning strike (from National Geographic by Lynda Smith)

circle of lightning

As Darwin has noted before, lightning is a common feature on the Rio de la Plata. In fact, it even bookmarked his visit to the river – welcoming him to Uruguay when he first arrived there almost 1 year ago (see Watching a 19th Century “Plasma Screen”).  One of the striking things about lightning (no pun intended J), is that it is so ephemeral – a flash of light burned on our retina for just a few seconds.  An image we can often only record in our minds eye.  But, in fact, as Darwin discovered for himself while in Maldonado, lightning can also leave a more permanent record of its passing.  We geologists call this “fossilized lightning” a fulgurite.

While in Maldonado, Darwin finds some of his very own petrified lightning ion the beach.  He describes it in his Zoological Notebook, but let me share excerpts from the more detailed entry on fulgurites from the Voyage of the Beagle. First Dawrin describes what he found and how it came to be there:

“In a broad band of sand-hillocks which separate the Laguna del Potrero from the shores of the Plata, at the distance of a few miles from Maldonado, I found a group of those vitrified, siliceous tubes, which are formed by lightning entering loose sand. …  The sand-hillocks of Maldonado, not being protected by vegetation, are constantly changing their position. From this cause the tubes projected above the surface; and numerous fragments lying near, showed that they had formerly been buried to a greater depth. Four sets entered the sand perpendicularly: by working with my hands I traced one of them two feet deep; and some fragments which evidently had belonged to the same tube, when added to the other part, measured five feet three inches. The diameter of the whole tube was nearly equal, and therefore we must suppose that originally it extended to a much greater depth. These dimensions are however small, compared to those of the tubes from Drigg, one of which was traced to a depth of not less than thirty feet.

The internal surface is completely vitrified, glossy, and smooth. A small fragment examined under the microscope appeared, from the number of minute entangled air or perhaps steam bubbles, like an assay fused before the blowpipe. The sand is entirely, or in greater part, siliceous; but some points are of a black colour, and from their glossy surface possess a metallic lustre. The thickness of the wall of the tube varies from a thirtieth to a twentieth of an inch, and occasionally even equals a tenth. On the outside the grains of sand are rounded, and have a slightly glazed appearance: I could not distinguish any signs of crystallization. In a similar manner to that described in the Geological Transactions, the tubes are generally compressed, and have deep longitudinal furrows, so as closely to resemble a shrivelled vegetable stalk, or the bark of the elm or cork tree. Their circumference is about two inches, but in some fragments, which are cylindrical and without any furrows, it is as much as four inches. The compression from the surrounding loose sand, acting while the tube was still softened from the effects of the intense heat, has evidently caused the creases or furrows. Judging from the uncompressed fragments, the measure or bore of the lightning (if such a term may be used), must have been about one inch and a quarter.” (Voyage of the Beagle)

These fulgurites (which Darwin does a beautiful job describing accurately) are fairly simply geologic features.  They form when lightning strikes and passes through sand (dunes, beaches, etc.). During the strike, the extreme heat of the lightning (at least 1,800 °C (3,270 °F)) vitrifies the sand (i.e., turns it into glass).  Sand, if you recall, can be made of a lot of different minerals (technically the name just refers to the size of the grains). But a lot of sand you find on beaches and in deserts (especially the white or tan type that is common) is made of the mineral quartz (see The Singing Sands of Bahia). Quartz has the chemical formula SiO2 (one silica atom for every two oxygen atoms). Natural glass is also made of SiO2 (also refered to as silica or silicon dioxide). The only difference is that in glass, the atoms of silicon and oxygen are not arranged in a nice orderly crystalline structure. Imagine quartz sand as being made of tiny, nicely organized lego blocks all stacked in a particular pattern.  The heat of the lightning literally rips those blocks apart and quickly fuses them all together in a random pattern. This is why glass has no crystals in it.  The conversion from sand into a fulgurite can literally take a second or two – it all happens very fast.

Fulgurite (from National Geographic by Ken Smith)


Darwin goes on to explain another important aspect of science – experimentation.  Much of what Darwin was doing on the voyage was making observations and interpreting what they meant.  But scientists also try to better understand natural phenomenon by recreating the natural world in a laboratory.  Darwin describes an example of experiments that had been done in France to help better understand the true nature of fulgurites:

“At Paris, M. Hachette and M. Beudant succeeded in making tubes, in most respects similar to these fulgurites, by passing very strong shocks of galvanism through finely-powdered glass: when salt was added, so as to increase its fusibility, the tubes were larger in every dimension. They failed both with powdered felspar and quartz. One tube, formed with pounded glass, was very nearly an inch long, namely, ·982, and had an internal diameter of ·019 of an inch. When we hear that the strongest battery in Paris was used, and that its power on a substance of such easy fusibility as glass was to form tubes so diminutive, we must feel greatly astonished at the force of a shock of lightning, which, striking the sand in several places, has formed cylinders, in one instance of at least thirty feet long, and having an internal bore, where not compressed, of full an inch and a half; and this in a material so extraordinarily refractory as quartz!” (Voyage of the Beagle)

Another fulgurite (from NASA’s Earth Science Picture of the Day (EPOD) by Stan Celestian) [PS – EPOD last week linked to the Beagle Project when they posted a great image of Sugarloaf in Brazil.  Way cool!]


Lastly Darwin describes some of the more dramatic lightning events on the Rio de la Plata and speculates a little on their cause:

“The neighbourhood of the Rio Plata seems peculiarly subject to electric phenomena. In the year 1793, one of the most destructive thunderstorms perhaps on record happened at Buenos Ayres: thirty-seven places within the city were struck by lightning, and nineteen people killed. From facts stated in several books of travels, I am inclined to suspect that thunderstorms are very common near the mouths of great rivers. Is it not possible that the mixture of large bodies of fresh and salt water may disturb the electrical equilibrium? Even during our occasional visits to this part of South America, we heard of a ship, two churches, and a house, having been struck. Both the church and the house I saw shortly afterwards: the house belonged to Mr. Hood, the consul-general at Monte Video. Some of the effects were curious: the paper, for nearly a foot on each side of the line where the bell-wires had run, was blackened. The metal had been fused, and although the room was about fifteen feet high, the globules, dropping on the chairs and furniture, had drilled in them a chain of minute holes. A part of the wall was shattered as if by gunpowder, and the fragments had been blown off with force sufficient to dent the wall on the opposite side of the room. The frame of a looking-glass was blackened, and the gilding must have been volatilized, for a smelling-bottle, which stood on the chimney-piece, was coated with bright metallic particles, which adhered as firmly as if they had been enamelled.” (Voyage of the Beagle)

Although an interesting interpretation, the idea that river mouths are nurseries for lightning storms is one of those cases where Darwin was not really on the mark. Hey, that is the nature of science – make hypotheses based on t he available data at the time, and be willing to revise it when more data comes in. In this case, more extensive observations suggest that the mixing of fresh and salt water do not cause “electrical disequilibrium”.  Alas, no one ever said Darwin was perfect … (RJV)



  1. […] formula 1Petrified Lightnin’ At long last, on the night of the 24 th, the Beagle set sail on its next adventure – this time to […]

  2. I continue to be amazed at Chuck’s knowledge for being such a young punk…

  3. The winner of the sandcastle contest goes to… nature.

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