Tracking the permeable porous network during strain-dependent magmatic flow
Résumé
Rheological variations have been postulated as the cause of transitions from effusive to explosive volcanic eruption style. Rheology is integrally linked to the composition and textural state (porosity, crystallinity) of magma as well as the stress, temperature and strain rate operative during flow. This study characterises the rheological behaviour and, importantly, the evolution of physical properties of two magmas (with different crystallinity and porosity) from Volcán de Colima (Mexico) -- a volcanic system known for its rapid fluctuations in eruption style. Magma samples deformed in a uniaxial press at a constant stress of 2.8, 12 or 24 MPa, a constant temperature of 940-945 °C (comparable to upper conduit or lava dome conditions) to strains of 20 or 30% displayed different mechanical behaviour and significant differences in measured strain rates (10− 2-10− 5 s− 1). The evolution of porosity, permeability, dynamic Young's modulus and dynamic Poisson's ratio illustrate a complex evolution of the samples manifested as strain-hardening, visco-elastic, constant-rate and strain-weakening deformation. Both magmas behave as shear-thinning non-Newtonian liquids and viscosity decreases as a function of strain. We find that strain localisation during deformation leads to the rearrangement and closure of void space (a combination of pores and cracks) followed by preferentially aligned fracturing (in the direction of the maximum principal stress) to form damage zones as well as densification of other areas. In a dome setting, highly viscous, low permeability magmas carry the potential to block volcanic conduits with a magma plug, resulting in the build-up of pressures in the conduit. Above a certain threshold of strain (dependent upon stress/strain rate), the initiation, propagation and coalescence of fractures leads to mechanical degradation of the magma samples, which then supersedes magmatic flow and crystal rearrangement as the dominant form of deformation. This results in lower apparent viscosities than those anticipated for magma of such crystallinity, especially at high strain rates. In a lava dome, this could result in dome collapse and the concomitant depressurisation could trigger an explosive eruption.