Lava fountains are spectacular eruptive manifestations of basaltic and other low-viscosity magmas. Sustained jetting of magmatic gas and molten lava clots can reach hundreds of meters high and last for tens of minutes to days. Lava fountain genesis is still debated and has been attributed to either i) the effervescent disruption of fast-rising undegassed magma [1,2] or ii) the violent emptying of a bubble-melt foam layer previously accumulated at depth, on top of a reservoir or a conduit discontinuity [3,4].
DCO’s Patrick Allard and colleagues  first demonstrated in 2005 that by measuring the chemical composition of the driving magmatic gas phase they could discriminate between these two potential mechanisms. In a new publication  in Earth and Planetary Science Letters, Alessandro La Spina, Mike Burton, Patrick Allard, Salvatore Alparone, and Filippo Muré, volcanologists from INGV (Italy) and IPGP (France), report on the largest data set ever obtained for the magmatic gas phase powering lava fountains. Using open-path Fourier transform infrared spectroscopy and absorption spectra of the radiation emitted by hot lava fragments, they were able to remotely measure the degassing processes during a series of eight lava fountain events (up to 500 meters high) at the Southeast summit crater (SEC) of Mount Etna in June–July 2001. They show that each of these events was characterized by increasingly CO2-rich gas release, with CO2/SO2 and CO2/HCl ratios peaking in coincidence with maxima in seismic tremor and fountain height. Furthermore, the peak values in both CO2/SO2 ratio and tremor energy were found to increase linearly with the repose interval (2–6 days) between the events.
From these observations and a model of volatile degassing at Etna, La Spina and colleagues infer that the measured lava fountains resulted from the growth and periodic collapse of CO2-rich bubble foams in the approximately 2 km deep SEC reservoir, which at that time received a separate influx of deeper-derived CO2 from a batch of primitive basalt. This primitive basalt violently erupted onto the southern volcano flank immediately after the lava fountain series. In addition, the authors report an inverse behavior of the SO2/HCl ratio with respect to eruption intensity and CO2/SO2 ratio during the most powerful paroxysms, likely due to enhanced release of more soluble chlorine from finer particles during more intense magma fragmentation. Therefore, this study provides geochemical evidence that both pre- and syn-eruptive magma degassing can contribute to the dynamics of a lava fountain.
Image: Patrick Allard