Hydrothermal pore fluid pressure and the stability of porous, permeable volcanoes

Lateral collapses of large volcanoes are commonly associated with phreatic explosions and other evidence for the presence of pressurized hydrothermal pore fluids within the volcanoes prior to collapse. Furthermore, hydrothermal alteration of volcanic edifices is a major factor in increasing susceptibility to collapse. This is generally held to be because of a reduction in effective friction coefficient μ through alteration. However, this is inconsistent with an analysis of the factors affecting the strength of fluid-saturated rocks and volcanic debris in terms of the Rubey & Hubbert equations for shear failure of such materials. Instead, this analysis indicates that reductions in the strength of volcanic materials are mainly due to the effect of high pore pressure relative to confining pressure, expressed as the ratio A of the two. Consideration of field and seismic evidence, together with simple calculations, indicates that high values of, and large increases in, λ are produced by a variety of mechanisms: heating of confined pore water by intrusions; degassing of intrusions; discharges of highly pressurized fluids from depth through clastic dykes; and by deformation associated with faulting. The sensitivity of pore fluid pressures to perturbation by these mechanisms is however highly dependent upon the permeability of their host rocks, which may itself be subject to rapid changes by fracturing, faulting and other processes. The rapidity of temperature changes and other mechanisms for pressurization in volcanic edifices means that the resultant pore pressure changes are large even in quite highly permeable rocks, but the effects are unpredictable. Detection of the development and spreading of high pore pressures within active volcanoes may however be possible by careful monitoring of patterns of seismicity.