Strain‐induced permeability increase in volcanic rock

Abstract

The extrusion of dense, viscous magma typically occurs along pronounced conduit‐parallel faults. To better understand the evolution of fault permeability with increasing strain, we measured the permeability of low‐porosity volcanic rock samples (basalt and andesite) that were deformed in the brittle regime to various levels of inelastic strain. We observed a progressive increase in sample permeability with increasing inelastic strain (i.e., with continued sliding on the fault plane). At the maximum imposed inelastic strain (0.11), sample permeability had increased by 3 orders of magnitude or more for all sample sets. Microstructural observations show that narrow shear fractures evolve into more complex fracture systems characterized by thick zones of friction‐induced cataclasis (gouge) with increasing inelastic strain. These data suggest that the permeability of conduit‐parallel faults hosted in the rock at the conduit‐wall rock interface will increase during lava extrusion, thus facilitating outgassing and hindering the transition to explosive behavior.

Inelastic compaction and permeability evolution in volcanic rock

“…1b). In volcanic rock -at the sample scale -this is characterised by significant shortening and barrelling of the sample (a consequence of the high strains required to achieve and exceed C * ) and the generation of a dilatant shear zone (Heap et al, 2015a), characteristically similar to highly strained fault zones observed in volcanic rock (Farquharson et al, 2016b). Figure 4 illustrates these separate mechanisms, with evidence of cataclastic pore collapse being shown in Fig.…”

“…7 and suggests that for a volcanic rock of given initial porosity, there is a limited range of strain-induced subsolidus k-φ states in which it can exist: one cannot compact indefinitely without promoting dilatant mechanisms. Figure 7 compiles data from this study with those of Farquharson et al (2016b) and Heap et al (2015a). Farquharson et al (2016b) performed triaxial experiments on lowand intermediate-porosity volcanic rocks (basalt from Mount Etna, Italy, and andesites from Volcán de Colima and Kumamoto, Japan), exploring the evolution of physical properties as a function of stress-induced dilation.…”

“…Figure 7 compiles data from this study with those of Farquharson et al (2016b) and Heap et al (2015a). Farquharson et al (2016b) performed triaxial experiments on lowand intermediate-porosity volcanic rocks (basalt from Mount Etna, Italy, and andesites from Volcán de Colima and Kumamoto, Japan), exploring the evolution of physical properties as a function of stress-induced dilation. Heap et al (2015a) performed compaction experiments comparable to those described in this study, with pre-and post-deformation permeability being assessed for two samples of San Antonio (C8) andesite from Volcán de Colima.…”

“…With a diameter of 20 mm, 10 sample cores were prepared and were ground flat and parallel to a nominal length of 40 mm. Samples were dried in a vacuum oven for at least 48 h, and the following steps were carried out (adopting the protocol of Farquharson et al, 2016b): 1. Physical properties (porosity, permeability) were measured.…”

“…However, a recent study explored brittle failure in compression of low-to-intermediate-porosity volcanic rock and the J. I. Farquharson et al: Inelastic compaction and permeability evolution in volcanic rock 563 influence of progressive stress-induced dilation (Farquharson et al, 2016b). Permeability was found to increase with ongoing strain accumulation under triaxial conditions.…”