Whispering, singing or shouting of erupting volcanoes: the music of gigantic volcanic bubbles

The magic and the strength of erupting volcanoes have always been perceived by people, who have developed many tales for coping with their danger. The dimensions of what can be observed close to a volcano varies from a few meters oscillations at the top of a quasi-stagnant vertical magma column to violent disruptions of a gas jet containing ashes and being propelled at several kilometers in the atmosphere (Fig 1). However despite the obvious differences seen at the surface during various types of eruptions, most of the volcanic activity at the surface is driven by the gas. The bubble size can vary by several orders of magnitude (Fig. 1 and 2). It has a dimension of a few millimeters in diameter in quiet lava flows (Fig 1a) or in violent gas jet prior to its disruption at the surface (Fig 1d) but it can also reach radius of several meters with lengths varying between a few meters as in Strombolian activity (Fig 1b) to a few hundreds of meters as in fire fountains (Fig 1c).

Fig 1: Photos of the four main types of volcanic eruptions : a) lava flow at Piton de la Fournaise on december 27 2006 (courtesy of A. Dupont); b) a Strombolian explosion at Etna (Italy) on July 25, 2001 (Courtesy of Tom Pfeiffer / www.VolcanoDiscovery.com); c) a fire fountain, reaching six hundreds of meter high on september 26 1989, at Etna (courtesy of A. Bertagnini); d) an eruptive column, whose the darkest lowermost part had reached a height of several kilometers, at Lopevi (Vanuatu) on June 8 2003 (courtesy of P. Leloup and Air Vanuatu).

Fig 2: A sketch of the uppermost portion of the volcanic conduit for each of the four volcanic regimes presented in figure 1: a) small bubbles in suspension in magma as an equivalent to lava flows; b) a metric bubble as long as wide at the origin of Strombolian explosions; c) a long inner gas core expelling the annular ring of magma for interpreting fire fountain; d) droplets of magma in suspension within a powerful gas jet producing eruptive columns.

The sound produced by an erupting volcano is also very striking and can appear as loud as the sound of an approaching train (100-120 dB) (Fig. 1b). However the most energetic part of the sound is not detectable by human hearing (above twenty cycles per second) due to its very low frequency content (one to ten cycles per second) even if the pressure wave, when strong, can sometimes be felt on the body. But a volcano can whisper as well as shout depending on its mood, which can be grossly related to the viscosity of the magma. As a rule of a thumb, a violent and energetic eruption is asssociated with very viscous magma (more than a billion time larger than the viscosity of water!) whereas less viscous magma (only ten thousand time larger) produces at the surface a volcanic activity of a lesser intensity, both in term of the velocity of the fragments of magma and of the sound production.

Despite the long-lasting observations of eruptions, the interest in studying the of their sound waves has only been renewed in the last fifteen years. The sound of volcanic eruptions has been used to unravel the properties of the magma column close to the vent (Buckingham and Garcès, 1996) or to detect eruptions as they can be recorded on infrasonic networks at several hundreds of kilometers away from the volcano (LePichon et al., 2005). But the sound waves have also been used at Stromboli (Italy) as a remote sensing tool to estimate, at a safe distance, the volume of the gigantic bubbles regularly bursting at the top of the magma column (Vergniolle and Brandeis, 1994). We have pursued in this line of thought and generalised the approach to several volcanoes because of the crucial role played by the gas in driving eruptions. The gas, exsolved at depth, carries physical information about the the eruption dynamics, which in turn may lead to a better understanding of volcanic systems.

Modelling the source of the sound produced by these gigantic volcanic bubbles give access to the gas overpressure, whose value is a measure of the danger faced by people as the pressure drives the ejection of the fragments of magma. When the gas pressure is only slightly in excess compared to atmospheric pressure, the sound is produced by the gas escaping through a small hole located on the bubble nose. In that case, the droplets of magma only land at less than one hundred meters from the vent. However, when the difference between the inner and outer pressure around the bubble is not tiny, the gas phase can strongly oscillate and change volume prior to its breaking. And the change in radius is so strong and fast, that sound can be produced! Although its main frequency is below the range of human hearing, these bubble oscillations can be heard by people via the sound produced by the motion of the fragments of magma in air and their landing on the ground at a much greater distance than in the previous case. However our infrasonic records can keep an accurate memory of all the physical processes at work during the bubble arrival at the surface and its breaking even if the strong motions are too slow to be detectable by human hearing.

An example of relatively quiet bursting of a gigantic bubble has been provided by the recent eruption (November to December 2006) of Piton de la Fournaise volcano in Reunion Island (Indian ocean, France). Video recordings show that the fragments of magma are located on a hemisphere, a shape characteristic of a bubble bursting, and that the gas expansion pushes the droplets of magma outwards (joined video shot by A. Dupont at one hundred meters from the vent).

WATCH VIDEO of Piton de la Fournaise volcano in Reunion Island

The first fragments of magma to appear are the smallest and they correspond to the film of magma initially above the bubble prior to its breaking. A second generation of magma droplets is produced less than a second later and these fragments are larger than before. This second phase is related to the violent motion, generated at the bottom of the bubble during its breaking, which tears apart the magma and produce fragments. Although the sizes of the bubble (a few meters) and of the asssociated fragments of magma (millimeters to several centimeters) are extremely different to those generated by the bursting of a submillimetric bubble in the ocean, the processes are exactly the same! However visual observations on volcanoes are not sufficient to unravel the physical processes because it is extremely difficult to have a direct and safe view of the top of the magma column. Recording in continuous the music played by the Piton de la Fournaise volcano has shown that more than one million of gigantic bubbles have been expelled during the two months of an eruption for a total gas volume of more than one hundred millions of cubic meters.

But the standart eruptions at Piton de la Fournaise are among quietest ones due to the relatively fluid magma. The sound and the pressure, released by the gigantic volcanic bubbles when they break at the top of a much more viscous magma, can be of several orders of magnitude above what is displayed by the attached video. Since nature has provided us with great singers of volcanic mood, more is to come in the future, from recording the sound waves, to unraveling the forces driving the eruption dynamics.

References:

Buckingham, M.J., and M.A. Garcès, A canonical model of volcano acoustics, J. Geophys. Res., 101, 8129--8151, 1996.

LePichon, A., E. Blanc, D. Drob, S. Lambotte, J.X. Dessa, M. Lardy, P. Bani and S. Vergniolle, Continuous infrasound monitoring of volcanoes to probe high-altitude winds, J. Geophys. Res., 110, D13106, doi: 1029/2004JD005587, 2005.

Vergniolle, S., and G. Brandeis, Origin of the sound generated by strombolian explosions, Geophys. Res. Lett., 21, 1959--1962, 1994.