Physical properties of tuffisite-bearing andesite and their implications for volcanology

Tuffisites result from volcanically-induced subsurface fragmentation, transport and deposition, and are common in explosive volcanic environments. The properties of these fragmental rocks provide direct evidence for the explosive volcanic processes operating within volcanic conduits. Here we report the influence of tuffisite veins on the fundamental physical properties of andesitic rocks. We find that: (1) compressive strength is unaffected by the presence and/or orientation of tuffisite veins, (2) permeability doubles when tuffisite veins are oriented sub-parallel to fluid flow and, (3) P- and S-wave velocities show a similar increase with depth, independent of vein orientation. Although our data demonstrate that, at these experimental conditions, the role of tuffisites on affecting andesitic rock properties is modest, we emphasize that the material tested represents the post-eruptive state of tuffisite. Thus, these results delineate the upper boundary of strength and lower boundary of permeability and porosity. All evidence suggests that tuffisite veins become compacted and lithified on relatively short time scales resulting in the host rock strengths being restored to their initial values.