Quantification of Volcano Deformation Caused by Volatile Accumulation and Release

Abstract

Volcano deformation is most frequently interpreted in terms of models of surface deformation due to processes in magma bodies of various geometries. The most widely applied model is that of a point source of pressure embedded within a uniform elastic half space (Mogi, 1958), but a range of more advanced models and approaches exist (e.g., Fialko, Khazan, et al., 2001;Hickey et al., 2016). As liquid magma flows in/out of the these "deformation sources," they expand/contract. Most often, such magma flow is considered to cause uniform pressure change on the boundary of the magma body, and the density difference between magma and host rock is not considered specifically. It has, however, been demonstrated in a number of studies that magma buoyancy can cause significant stresses in volcano roots and contribute to failure of magma bodies (e.g., Sigmundsson et al., 2020). A particular phenomenon not considered by traditional volcano deformation models is the effect of accumulated exsolved volatiles in volcano roots and their release during eruptions.During major explosive eruptions an excess of gas may be observed, beyond that which can be explained by a petrological calculation of the original dissolved volatile amounts and the volume of erupted lavas. Excess gas was observed in the 1991 eruption of Pinatubo, Philippines and an analysis from Wallace and Gerlach (1994) showed that this could be explained by a pre-existing gas/volatile phase representing 0.7-1.3 wt% of the erupted magma. Volatile accumulation was proposed to occur in the roof zone of the system.

Pre-eruptive excess volatiles and their relationship to effusive and explosive eruption styles in semi-plugged volcanoes

“…On the contrary, monitoring signals that could be related to fluid accumulation and magma degassing, or magma transfer within the plumbing system, could be useful in anticipating explosive eruptions. For example, identification of deep-long period seismicity using broadband seismometers, which could come from degassing of volatile-rich magma at depth (Melnik and Sparks, 2002) or deformation signals before and after the eruption (Spang et al, 2022). Finally, microgravity surveys or continuous gravity monitoring could be beneficial because we expect that significant fluid accumulation would lead to subtle changes in density of the plumbing system over time, which was demonstrated at Masaya volcano (Nicaragua) (Williams-Jones et al, 2003).…”