Stability of volcanic conduits: Critical mechanical parameters

Several geological evidences support the occurrence of volcanic conduit enlargement during explosive events (e.g. presence of lithic fragments in most pyroclastic deposits), with significant effects on the eruptive dynamics, particularly on the mass discharge rate. Conduit wall collapse is supposed to be a relevant enlargement process, being more intense near and above the fragmentation level. Nonetheless, the influence of country rock conditions has never been addressed, and its implications on the eruptive dynamics are still unclear. This work focuses on the effects of the country rock mechanical parameters and the presence of unconfined aquifers on conduit stability, using a 1D steady-state model and the application of two collapse criteria. For given magma properties and conduit dimensions, it emerges that conduit stability is mainly controlled by the friction angle of the country rocks and, to a lesser extent, by the cohesion. The horizontal stress gradients are only significant when the Mohr-Coulomb criterion is employed, whereas the variability in the vertical stress gradient has a minor effect on conduit stability. Moreover, the presence of unconfined aquifers is an important destabilizing factor, which is consistent with the ejection of significant amounts of lithic fragments in many phreatomagmatic eruptions