Exploring the mechanical influence of mush poroelasticity on volcanic surface deformation

Understanding the mechanical behaviour of melt reservoirs is vital for advancing geophysical models that aim to constrain the evolution of subvolcanic systems and inform hazard monitoring and mitigation. From geophysical and petrological studies, large melt-dominated (magma) reservoirs are difficult to sustain over long periods of time. Melt is more likely to reside within reservoirs which consist of variably packed frameworks of crystals, so-called crystal mush, as well as in pockets of magma, in changing proportions over time. The behaviour of crystal mush, in particular, is emerging as a vital consideration in understanding how magmatic systems evolve. In addition, current models for volcano deformation often consider static magma sources and thus provide little insight into the internal dynamics of melt reservoirs; and these models ignore the presence of crystals and therefore the likely poroelastic mechanical response to melt intrusion or withdrawal. Our study considers the melt reservoir to be partly crystalline (> 50% crystal fraction), with melt residing between crystals. We examine the influence of poroelastic mechanical behaviour on the evolution of reservoir pressure and the resultant surface deformation. From our results, the modelling of a crystal mush rather than a 100% melt magma reservoir can significantly modify the resulting spatial and temporal mechanical evolution of the system. Specifically, the poroelastic behaviour of a mush reservoir will continue to develop following the end of a melt injection period, generating further time-dependent surface displacements. Post-injection and post-eruption inflation can occur, which are linked to a poroelastic response associated with continuous melt diffusion. Following an injection/eruption, a steady-state point is eventually achieved when the fluid pressure reaches a uniform value throughout the reservoir. This process is controlled by the poroelastic diffusivity. Increasing the reservoir crystal fraction from 50% to 90% reduces the mobility of melts, decreases permeability, and leads to a slow rate of melt diffusion. Our study confirms that volcanic surface deformation can occur without continued intrusion or withdrawal of melt.