The volume of gas contained within a silicic magma, dissolved and subsequently exsolved, greatly influences the behaviour of a volcano. There is a marked contrast between the behaviour of a volcano "open" to degassing, compared to one which is "closed". It is, therefore, essential to understand the entire degassing process of gas transport, storage and loss.
The particular focus of this study is the effect different permeabilities and pressure gradients within a volcanic edifice have on the degree and pattern of the gas velocity. Gas loss is modelled numerically in two- and three-dimensions using a finite element approach. By combining the time-dependent continuity equation and Darcy's law, a partial differential equation is derived and solved for the pressure. The associated pressure gradient is used in Darcy's law to determine the corresponding gas velocity distribution. The momentum equation is also used to determine the surface displacement pattern resulting from the movement and storage of gas within the system.
The model framework is applied to numerous volcanic scenarios including cracks and sealing within the dome structure and shear fractures at the margin between the conduit and country rock. Two case studies are investigated: Ash venting at Soufriere Hills volcano in March 2012, and persistent, repetitive ring-shaped degassing at Santiaguito. Quantitative estimates regarding gas emissions and deformation provide the link to constraining observations.
The results show the country rock and dome are important and it is the relative permeabilities, rather than the actual values which determine the pressurisation. A decrease of just two orders of magnitude in the surrounding permeability could switch behaviour from effusive to explosive. For efficient gas storage within a volcano, a high permeability is required to hold the gas, whilst a low permeability is necessary to trap it. From the modelled surface displacement patterns and gas emissions at the surface, it may be possible to track the migration of large volumes of gas, particularly if used in conjunction with real-time monitoring of active volcanoes.
