Failure of volcano slopes

Magmatic intrusions can initiate and sustain massive and catastrophic volcano collapse. Their role is towfold, involving both driving and resistiag forces. First, flank stability is diminished by magmasatic and magama overpressures, and steepeaed slopes, that accompany intrusion. Second, excess pore pressures in potenatial failure zones can be generated as a resalt of ing of the rock-fluid mechanical or thermal straining of the rock-fluid medium. pressurized rtrograde boiling in high level magma chamberrs, or hydrothermal fluid circulation. Also, earthqeakes may aid collapse through inertial forces and shaking-induced pore pressure generation. These excess pore pressures reduce the sliding resistance, as shown for wedge-shaped sliding blocks for selected cases. The destabilizing influence of mechnically inducwd pore pressures is maximized as the intruded width, or corrsponding overpressure, of the intrusion is increased. the destabilizing influence of thermally induced pore pressures is conditionned by the severity of thermal forcing racing of therMAl aNd hudraulic diffusiities, and the time required for the fluid perssure disturbance to propagate outwards from the intrusion. Retrograde boiling and hydrothermal circulation overpressure mechanisms may be ealuated by similar models. Failure initiation does not imply susmilar failure; in some cases, enhancement of pore pressures through deviatoric shearing, frictional heating, or runout over compreessible saturated alluvium or marine sediments may be necessary following slide initiation to maintain the impetus of flank failure for long runout. Models are examined for oceanic volcanoes of shallow flank inclination and for terrestrial composite volcanoes with considerably sleeper flanks.