Abstract

The causes of volcanic instability and failure are numerous, but rarely if ever confined to the effects of one process. While the triggering mechanism of a volcanic collapse event may manifest almost instantaneous with collapse, the processes generating the underlying instability may have been active for thousands of years, only a few weeks, or in some cases even shorter. One of the greatest hazards posed from volcanic edifice collapse events is the ability for them to affect both volcanic edifices that had exhibited no signs of magmatic activity and edifices that are physically erupting at the time of collapse. This poses a problem when identifying the collapse mechanisms, as obviously there are different processes at work within active and non-active volcanoes. However there is at least one mechanism of generating instability that may be present at all volcanic collapse locations. Increases of edifice pore-pressures from internal sources can be generated at both obviously active volcanoes through thermal and mechanical methods associated with intrusion directly into the edifice, and also at volcanic edifices not considered active through far-field transient pressure increases resulting from remote intrusion. Using FLAC3D, a three-dimensional explicit finite-difference modelling program, we model the effects on internal pressurization in true 3-D. When considering the source of edifice pore-pressure increases, deeper failures are shown to result from deeper sources. If we consider de-gassing magma bodies and the boiling of hydrothermal systems as a source for internal gas pore-fluid pressurisation, there is no limit to the internal pressures that can build up other than the physical strength of the edifice as long as the source persists. Strangely, while the emission of volcanic gasses and their compositions are routinely monitored, the mechanical effects of the gas phase on the shear strength and structural integrity of the edifice have been mostly overlooked. Even small localized internal pore pressures of 1MPa are predicted to sufficiently stress the system, leading to lower magnitude trigger events required to initiate a large-scale collapse event.