22 February 2012

“Magnificent column of smoke” (Accretionary Wedge #43 Entry)

Posted by Jessica Ball

My choice of favorite geologic illustration, for Accretionary Wedge # 43, comes from a book that geobloggers (and others) have written about in the past: Sir William Hamilton’s Campi Phlegraei, Observations on the Volcanoes of the Two Sicilies. I won’t repeat all the background about Hamilton, who was a British natural historian who observed¬†eruptions of Mount Vesuvius in the 1760s and 1770s. Campi Phlegraei, a three-part work, contained wonderful descriptions of the volcanoes and eruptions of Naples and Sicily, including the 1779 eruption of Mount Vesuvius (discussed and illustrated in a supplement to the first two volumes). It also discussed the debate between the Neptunists and the Plutonists, and shows that Hamilton favored Plutonist theory, which stated that heat was a central driving force of geologic processes, and that volcanoes and volcanic areas were a surface expression of the Earth’s great internal engine:

If the circuit I have described, can be fairly proved to owe it’s very existence to Volcanick explosions, at various, and in some parts at very remote periods, and be not meerly a country torn to pieces by subterraneous fires, as has been hitehrto the generally received opinion, I flatter myself, I shall have open’d a new field for observation on this curious subject.

But since this Accretionary Wedge is about the illustrations, I’d really better comment on Hamilton’s Illustrator, Pietro Fabri (or Peter Fabris, as Hamilton calls him):

…I employed Mr. Peter Fabris, a most ingenious and able artist, a native of Great Britain, to take Drawings of every interesting spot, descibed in my letters, in which each stratum is represented in its proper colours; The exteriour, and interiour forms of Mount Vesuvius, the Solfaterra, and of every other ancient Volcano in the neighbourhood of Naples, are represented faithfully in these Drawings, as are likewise the different specimens of Volcanick matter, such as lava’s, Tufa’s, pumice stones, ashes, sulphurs, salts &c., of which the whole country I have described, is evidently comprised.

I haven’t been able to discover much more about Fabri, but it’s clear that he was a brilliant illustrator, and managed to capture some really beautiful (and useful!) moments of ephemeral geologic phenomena. Which brings us to the illustration I like the most:

"View of the eruption of Mount Vesuvius Monday morning August the 9th 1779 and of the magnificent column of smoke, which attended that eruption; taken from an original Drawing done from nature at Pausilipo by Mr. Peter Fabris."

This is my favorite geologic illustration for a number of reasons. The first is simply aesthetic; it’s a lovely piece of work, and I particularly enjoy the observers in the foreground, for whom this seems to be an occurrence that merits a bit of sightseeing but not panic. But there are geologic aspects that I find intriguing as well. Fabri has captured not only the Plinian (or sub-Plinian) eruption column, but the ballistics falling out of the cloud (some of which must have been quite large, judging from the scale at which they’re depicted). In addition, he’s managed to show a vital process in the growth and maintenance of a volcanic plume – the entrainment and heating of outside air. This is occurring where Fabri shows the column curling in on itself, and it shows that this column was probably in its initial stages of formation, when there was still enough hot material (ash or hot gas jets) in the plume to trap and heat outside air. This heated air provides lift to the column, and allows it to grow to the impressive heights that we see in Plinian eruptions.

Entrainment is part of what ‘feeds’ an eruption column, so seeing that Fabri and Hamilton took note of it even this early on in the development of volcanology is pretty exciting. I will note that some of his ballistic trajectories are a bit suspect – material falling out of an eruption column should pretty much just ‘rain’ out, and probably wouldn’t be arcing like the upper ballistics in this illustration. But the lower ones, which are presumably being expelled directly from the vent and not rising in the column, are doing precisely what they should. It’s also unlikely that the upper bombs are going to be the same size as the lower ones; material that rises in an eruption plume has to be light enough for the upward thrust of gases and hot air to push it that high in the first place, so bigger bombs will fall out earlier. (Want to see what paths ballistics follow depending on things like velocity, ejection angle and bomb size? Try the bomb trajectory simulation at Stromboli Online.)

Finally, I always liked to think about the woman standing in the foreground. Who was she? What was she saying about the eruption? She doesn’t look particularly upset at the sight of a volcanic plume, but why is that? Has she seen so many that she’s jaded by them? (This eruptive period lasted for a decade, after all!) Is she describing something to her friends, or pointing out a feature of the plume? Or is she just a creation of the artist, there for scale and color and perhaps to balance the center of the image? Since it’s hard to find much information about the artist, we’ll probably never know anything about his subjects – or if they were even real. But it’s always fun to imagine things.