| In this presentation I explore magma chamber stress conditions prior to a large-magnitude
andesitic eruption and associated pre-eruptive ground deformation and gravity field
perturbations. The analyis is informed by constraints from multi-disciplinary investigations of
the data-rich and ongoing Soufrière Hills volcano (SHV) eruptive episode and the data-poor
magnitude(M) 7 Tambora (TMB) 1815 eruption. Using analytical and numerical mechanical
models accounting for self-gravitation and (to start with) mechanical elasticity I
derive a set of conditions that simulate an andesitic magma chamber prior to a
M7 eruption to deduce uniform chamber pressures upon failure, resultant ground
displacements and Bouguer gravity anomalies. The results demonstrate that although a small
and shallow-seated (SHV-type) chamber is more prone to pre-mature rupture upon
pressurisation compared to a large TMB-type chamber, deduced uniform chamber
pressures upon failure at SHV are unrealistically high. This implies that even for
a SHV-type chamber, assuming elastic mechanical behaviour of crustal rocks is
problematic to constrain magma chamber dynamics. I show that time-dependent stress
dissipation is first-order in the evolution of small and, by extrapolation to pre-eruptive
conditions for TMB 1815, also for large andesitic chambers even if topography, edifice
load, and crustal mechanical heterogeneity are considered. A static failure criterion
appears inadequate for system characterisation, necessitating a dynamic criterion,
which is proposed to scale with volumetric strain rate upon chamber pressurisation.
Informed by petrological and thermal constraints, a revised set of models predicts
ground inflation of -ª1 cm/year from a large mid-crustal TMB-type chamber upon
non-failure pressurisation at strain rates |