Kennedy, and 9 co-authors, reported in 2020 that breccia-filled eruption conduits are dynamic systems where pressures frequently exceed critical thresholds, generating earthquakes and transmitting fluids. To assess the dynamics of breccia-filled conduits, the authors examined lava, ash tuff, and hydrothermal breccia ballistics with varying alteration, veining, fractures, and brecciation ejected during the 27 April 2016 phreatic eruption of Whakaari/White Island. They measured connected porosity, strength, and permeability with and without tensile fractures at a range of confining pressures. Many samples are progressively altered with anhydrite, alunite, and silica polymorphs. The measurements show a large range of connected porosity, permeability, and strength.
In contrast, the cracked samples show a consistently high permeability. The cracked altered samples have a permeability more sensitive to confining pressure than the unaltered samples. The permeability of our altered ballistics is lower than surface rocks of equivalent porosity, illustrating that mineral precipitation locally blocked pores and cracks. The authors summarized that alteration within the conduit breccia allows cracks to form, and open and close, in response to pore pressure and confining pressure, providing a mechanism for frequent and variable fluid advection pulses to the surface. This produces temporally and spatially variable geophysical and geochemical observations and has implications for volcano monitoring for any volcano system with significant hydrothermal activity.
The authors concluded that the frequency of explosive processes is on the order of weeks to years, which is sufficiently long to allow minerals to precipitate in veins and between breccia clasts in the conduits, resealing them. Similar timescales for pressurization as a result of alteration-induced reductions to permeability were proposed by earlier. Similar processes have been envisaged at other volcanoes by others. However, here the authors offer an explanation for geophysical, geochemical, and visible changes that can occur in seconds to hours, and provide evidence for a breccia-filled conduit of altered lava and tuff clasts dominantly cemented by alunite and anhydrite. The permeability of this altered material is susceptible to rapid variation in effective pressure allowing highly time-variable fluid advection, out-gassing, and geophysical changes at the surface.
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