The Permeability of Porous Volcanic Rock Through the Brittle-Ductile Transition

The permeability of volcanic rock controls the distribution of pore fluids and pore fluid pressure within a volcanic edifice, and is therefore considered to influence eruptive style and volcano deformation. We measured the porosity and permeability of a porous volcanic rock during deformation in the brittle and ductile regimes. In the brittle regime, permeability decreases by a factor of 2-6 up to the peak stress due the closure of narrow pore throats but, following shear fracture formation, remains approximately constant as strain is accommodated by sliding on the fracture. In the ductile regime, permeability continually decreases, by up to an order of magnitude, as a function of strain. Although compaction in the ductile regime is localized, permeability is not reduced substantially due to the tortuous and diffuse nature of the compaction bands, the geometry of which was also influenced by a pore shape preferred orientation. Although the evolution of the permeability of the studied porous volcanic rock in the brittle and ductile regimes is qualitatively similar to that for porous sedimentary rocks, the porosity sensitivity exponent of permeability in the elastic regime is higher than found previously for porous sedimentary rocks. This exponent decreases during shear-enhanced compaction toward a value theoretically derived for granular media, suggesting that the material is effectively granulating. Indeed, cataclastic pore collapse evolves the microstructure to one that is more granular. Understanding how permeability can evolve in a volcanic edifice will improve the accuracy of models designed to assist volcano monitoring and volcanic hazard mitigation.