Added on Feb. 7. In this video, you can see the air shock sweeping rapidly down the flank of the volcano in the first few seconds, and you can hear the shock at about 33 seconds.
|Set from the "You Only Live Twice"|
The shock wave that produced the boom you heard in the video did not have a visible presence, only audible. However, visible shock waves at volcanoes were first described at Mount Vesuvius by F.A. Perret who called them "flashing arcs." In his article in the American Journal of Science (volumes 183-184, Article XXXII, pp. 329-333, 1912). From that article: "The frequency of the explosions varied from approximately one every three or four seconds to at least three per second. Although powerful, they were verysharp and sudden in their nature, and at the instant of each--but before it could be sensed by the eye or ear--a thin, luminous arc flashed upward and outward from the crater and disappeared in space. Then came the sound of the explosion and the projection of gas and detritus above the lip of the crater. The motion of translation of the arcs, while very rapid in comparison with that of the detritus, was not above the limits of easy observation and there could be no doubt as to the reality of the phenomenon, which was repeated some hundreds of times. ...of the actual phenomenon, the beauty of which lies in the delicate luminosity, the elegance and perfection of form, and the grace and vivacty of the arcs amid the contrasting color and relatively sluggish movement of their surroundings."
|F.A. Perret at Campi Flagrei, Italy, possibly 1906-1907|
photo is from
Ah, wouldn't it be nice if some of our peer reviewed journals today allowed us to regain some of this style of writing!
Perret travelled to volcanoes around the world, but it was not until four years later that he saw them again, this time at Vesuvius. After proposing that the arcs were related to sound waves, he concluded "...the flashing arcs may be considered one of the most beautiful of all volcanic phenomena."
This posting got me to a wonderful blog by David Bressen on the history of geology, and I'll refer you there "for the rest of the story". The discussion of Perret that accompanies the picture above is on the January 18, 2011, blog.
Other volcanoes that have shocks are Ngauruhoe (New Zealand), Sakurajima (Japan), Matua (Siberia), and, just last summer, Eyjafjallajokull.