Gas content, eruption rate and instabilities of eruption regime in silicic volcanoes

Abstract

In silicic volcanoes, eruptions commonly begin with violent explosive phases and evolve towards a regime of dome formation. This transition is characterized by a decrease of gas volume fraction, which has usually been attributed to chemical gradients in the volcano chamber. Petrological and geochemical studies suggest that this interpretation may be oversimplified. A critical observation is that the eruption rate decreases with time and is markedly smaller during dome growth than during explosive activity. Following Eichelberger et al. [1], we suggest that the transition from explosive activity to dome formation is due to gas loss through permeable conduit walls. Further, in some cases, the same process may be responsible for the transition from the ash fall regime to pyroclastic flows. Both transitions are a direct consequence of a decrease of eruption rate at constant conduit radius. The gas content of lava rising towards the Earth's surface is determined by two competing processes: pressure release leading to gas exsolution and expansion, and gas loss to the country rock. The amount of gas lost is inversely proportional to the eruption rate and proportional to the pressure difference between conduit and country rock. The critical variable is the pressure in the volcano chamber. This pressure steadily decreases with time as the chamber empties, implying a decrease of eruption rate. In turn, this decrease acts to increase the fraction of gas lost to the country rock and hence to reduce the gas content of the erupted material. The model therefore predicts that, with time, the eruption undergoes a transition from explosive to non-explosive conditions. These transitions occur as bifurcations in the evolution of gas volume fraction with height in the conduit. We find that very small pressure fluctuations of the order of one bar in the chamber lead to large changes of gas content at the vent. This suggests that the transitions of eruptive regime are unstable, and provides an explanation for observed alternations between explosive phases and dome formation. With time, such pressure fluctuations occur when the average vesicularity is smaller and may manifest themselves by extrusion events through an existing dome. In Appendix B, we use data on the height of the Montagne Pelée spine in 1902 and 1903 to show that the pressure driving this eruption decreased by about 2 MPa in a year.