Inelastic compaction and permeability evolution in volcanic rock
Jamie I. Farquharson, Patrick Baud, and Michael J. Heap
Institut de Physique de Globe de Strasbourg (UMR 7516 CNRS), EO ST, Université de Strasbourg, France
Received: 01 Dec 2016 – Accepted for review: 16 Dec 2016 – Discussion started: 19 Dec 2016
Abstract. Active volcanoes are mechanically dynamic environments, and edifice-forming material may often be subjected to significant amounts of stress and strain. It is understood that porous volcanic rock can compact inelastically under a wide range of in situ conditions. In this contribution, we explore the evolution of porosity and permeability – critical properties influencing the style and magnitude of volcanic activity – as a function of inelastic compaction of porous andesite under triaxial conditions. Progressive strain accumulation is associated with progressive porosity loss. The efficiency of compaction was found to be related to the effective confining pressure under which deformation occurred: at higher effective pressure, more porosity was lost for any given amount of strain. Permeability evolution is more complex, with small amounts of stress-induced compaction (< 0.05, i.e. less than 5 % reduction in sample length) yielding an increase in permeability under all effective pressures tested, occasionally by almost one order of magnitude. This phenomenon is considered here to be the result of improved connectivity of formerly isolated porosity during triaxial loading. This effect is then overshadowed by a decrease in permeability with further inelastic strain accumulation, especially notable at high strains (> 0.20) where samples may undergo a reduction in permeability by two orders of magnitude relative to their initial values. A physical limit to compaction is discussed, which we suggest is echoed in a limit to the potential for permeability reduction in compacting volcanic rock. Compiled literature data illustrate that at high strain (both in the brittle and ductile regimes), porosity ϕ and permeability k tend to converge towards intermediate values (i.e. 0.10 ≤ ϕ ≤ 0.20; 10–14 ≤ k ≤10–13 m2). These results are discussed in light of their potential ramifications for impacting edifice outgassing – and in turn, eruptive activity – at active volcanoes.
Farquharson, J. I., Baud, P., and Heap, M. J.: Inelastic compaction and permeability evolution in volcanic rock, Solid Earth Discuss., doi:10.5194/se-2016-166, in review, 2016.