The strength and permeability of tuffisite-bearing andesite in volcanic conduits

Abstract

Tuffisites result from volcanically-induced subsurface fragmentation, transport and de- position, and are common in explosive volcanic environments. Their study provides direct insight to explosive volcanic processes operating within volcanic conduits. Here we report the influence of tuffisite veins on the fundamental physical properties of an- desitic rocks. We find that: (1) strength is unaffected by the presence and/or orien- tation of tuffisites, (2) permeability doubles when tuffisites are oriented favorable (45 degrees to fluid flow), and (3) seismic wave velocities show a continuous increase with depth, independent of vein orientation. Although the influence of tuffisites on 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 tuffisites become compacted and lithified on relatively short time scales, restoring the strength of the rock to their initial host rock values.