The effects of outgassing on the transition between effusive and explosive silicic eruptions

Abstract

International audienceThe eruption style of silicic magmas is affected by the loss of gas (outgassing) during ascent. We investigate outgassing using a numerical model for one-dimensional, two-phase, steady flow in a volcanic conduit. By implementing Forchheimer's equation rather than Darcy's equation for outgassing we are able to investigate the relative influence of Darcian and inertial permeability on the transition between effusive and explosive eruptions. These permeabilities are defined by constitutive equations obtained from textural analysis of pyroclasts and determined by bubble number density, throat-bubble size ratio, tortuosity, and roughness. The efficiency of outgassing as a function of these parameters can be quantified by two dimensionless quantities: the Stokes number, the ratio of the response time of the magma and the characteristic time of gas flow, and the Forchheimer number, the ratio of the viscous and inertial forces inside the bubble network. A small Stokes number indicates strong coupling between gas and magma and thus promotes explosive eruption. A large Forchheimer number signifies that gas escape from the bubble network is dominated by inertial effects, which leads to explosive behaviour. To provide context we compare model predictions to the May 18, 1980 Mount St. Helens and the August-September 1997 Soufrière Hills eruptions. We show that inertial effects dominate outgassing during both effusive and explosive eruptions, and that in this case the eruptive regime is determined by a new dimensionless quantity defined by the ratio of Stokes and Forchheimer number. Of the considered textural parameters, the bubble number density has the strongest influence on this quantity. This result has implications for permeability studies and conduit modelling

Permeability measurements of Campi Flegrei pyroclastic products: An example from the Campanian Ignimbrite and Monte Nuovo eruptions

“…4b; Rust and Cashman, 2004). The correlation found between k 1 and k 2 can be reproduced by a simplified parameterization of these quantities proposed by Degruyter et al (2012),…”

“…In the case inertial effects become dominant, explosive eruptions become more likely (Degruyter et al, 2012). From the permeability measurements we can obtain some insight into the onset of inertial effects inside magmatic foams during outgassing.…”

“…Previous studies have indicated that the region where degassing vesicle pathways are most likely to occur is near and/ or at the conduit walls (Cashman et al, 2000;Llewellin et al, 2002; Mueller et al (2005), and obsidian flow samples from Rust and Cashman (2004). Dashed line is parameterization from Degruyter et al (2012). See text for further explanation.…”

“…During volcanic eruptions the gas and magma phase are driven by the local pressure gradient across the conduit, which creates a difference in their velocity (Kozono and Koyaguchi, 2009a,b;Degruyter et al, 2012). This velocity difference creates an internal friction F mg (Pa/m) between the two phases, which transfers momentum from the gas to the magma phase.…”

“…Permeability results from the combination of various conduit processes related to magma vesiculation and crystallization that accompany magma rise from chamber/dykes along the conduit to the surface and/or to the formation of fractures due to shear fragmentation (Gonnermann and Manga, 2003;Tuffen and Dingwell, 2005). It is a key parameter in the transition from effusive to explosive volcanism (Eichelberger et al, 1986;Jaupart and Allegre, 1991;Woods and Koyaguchi, 1994;Kozono and Koyaguchi, 2009a,b;Degruyter et al, 2012), for example in the catastrophic failure from dome-forming eruptions to Vulcanian/Plinian behaviour (Lipman and Mullineaux, 1981;Herd et al, 2005). Permeability is also responsible for the quiescent gas loss from persistently degassing volcanoes (Oppenheimer et al, 2003), through formation of networks of continuously connected vesicles , where gas percolates and can exit the volcanic system non-explosively (Burton et al, 2007).…”