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

A simple formula for calculating porosity of magma in volcanic conduits during dome-forming eruptions

“…We suggest that the high flow rate immediately prior to the explosive activity induced the increase in the subsurface porosity. Equation (10) can be extended to the case where the effect of lateral gas escape from magma (e.g., Jaupart and Allegre, 1991;Woods and Koyaguchi, 1994;Diller et al, 2006) is taken into account (see Appendix). When this effect is considered, the isopleths of the porosity in Fig.…”

“…The porosity changes with depth owing to the competition between the vesiculation and escape of gas from the magma. When gas escape occurs efficiently, the porosity decreases, which may lead to an effusion of a lava dome with a low porosity (Eichelberger et al, 1986;Jaupart and Allegre, 1991;Woods and Koyaguchi, 1994). Recent numerical studies have revealed that the porosity critically depends on magma properties such as viscosity and permeability in dome-forming eruptions and that complex porosity profiles may result as viscosity, permeability, or both change with depth; the porosity increases in the subsurface region, and then decreases near the surface (e.g., Diller et al, 2006).…”

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

“…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).…”

Petrologic determination of ascent rates for the 1995–1997 Soufriere Hills Volcano andesitic magma

“…Volatiles may be variably lost from the magma, depending on the rate of ascent, the permeability of the wall rocks, and the physical and chemical properties of the magma. If the volatiles escape gradually, the magma will erupt quiescently [Eichelberger, et al, 1986;Jaupart and Allegre, 1991 ]. If the magma degasses very rapidly, however, then trapped gas will tend to fragment the magma and produce an explosive eruption [Sparks, 1978;Sparks et al, 1994;Gardner et al, 1996;Klug and Cashman, 1996].…”