Abstract
Abstract. Tuffisites, the products of subsurface fragmentation, transport and deposition, are common in explosive volcanic environments. Their study provides direct insight to the mechanical processes operating within volcanic conduits. Here we document the influence of the presence of coherent tuffisite veins on the physical properties of andesitic rocks. We find that (1) compressive strength is unaffected by the presence and/or orientation of tuffisites, (2) permeability doubles when tuffisites are oriented favorably (at 45° to the fluid flow direction), and (3) ultrasonic wave velocities show a continuous increase with depth, independent of vein presence and orientation. Although the influence of tuffisites on andesitic rock properties determined here is modest, we emphasize that the material tested represents the post-eruptive state of tuffisite. Thus, these results likely delineate the upper and lower boundaries of strength vs. permeability and porosity, respectively. Our evidence suggests that, via compaction and lithification, tuffisites may restore the strength of the volcanic host-rocks to that of their pre-tuffisite values.
Highlights
Tuffisites are common in exhumed volcanic environments and may provide a direct record of the subsurface processes operating in a conduit during explosive eruptions (Lavallee et al, 2012a)
They were first defined by Cloos (1941) who described them as follows: “The host rock seems tuffisized, i.e. infiltrated by the tuff along its finest cracks and crevices and intimately mixed with it.”...“Appearance and color of such mixed rock types “tuffisites” is governed by the nature of the host rock”
At eruptive temperatures (940 ◦C), the tested tuffisitebearing andesites deform in a strictly brittle manner and their uniaxial compressive strength (UCS) is independent of presence and/or orientation of tuffisite veins
Summary
Tuffisites are common in exhumed volcanic environments and may provide a direct record of the subsurface processes operating in a conduit during explosive eruptions (Lavallee et al, 2012a). Tuffisites have been reported for a wide range of chemical compositions and diverse volcanic environments, including basaltic diatremes (Cloos, 1941), andesitic fossil conduits (Noguchi et al, 2008), and rhyolitic conduits (Tuffen et al, 2003) They were first defined by Cloos (1941) who described them as follows:. “All observations point towards this tuffisitation taking place during the main phase of volcanism through penetration of gases into the surrounding rock.” This makes them inherently different from rocks occurring within cataclastic shear zones enveloping lava domes and spines, such as described at Mt. St. One does not preclude the other; tuffisites can potentially be sourced from shear zones by later gas fluidization of the fine-grained cataclastic material
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