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Pressure and stress perturbations associated with volcanic activity and geothermal production can modify the porosity and permeability of volcanic rock, influencing hydrothermal convection, the distribution of pore fluids and pressures, and the ease of magma outgassing. However, porosity and permeability data for volcanic rock as a function of pressure and stress are rare. We focus here on three porous tuffs from Krafla volcano (Iceland). Triaxial deformation experiments showed that, despite their very similar porosities, the mechanical behavior of the three tuffs differs. Tuffs with a greater abundance of phyllosilicates and zeolites require lower stresses for inelastic behavior. Under hydrostatic conditions, porosity and permeability decrease as a function of increasing effective pressure, with larger decreases measured at pressures above that required for cataclastic pore collapse. During differential loading in the ductile regime, permeability evolution depends on initial microstructure, particularly the initial void space tortuosity. Cataclastic pore collapse can disrupt the low‐tortuosity porosity structure of high‐permeability tuffs, reducing permeability, but does not particularly influence the already tortuous porosity structure of low‐permeability tuffs, for which permeability can even increase. Increases in permeability during compaction, not observed for other porous rocks, are interpreted as a result of a decrease in void space tortuosity as microcracks surrounding collapsed pores connect adjacent pores. Our data underscore the importance of initial microstructure on permeability evolution in volcanic rock. Our data can be used to better understand and model fluid flow at geothermal reservoirs and volcanoes, important to optimize geothermal exploitation and understand and mitigate volcanic hazards. The volcanic rocks within a volcano or geothermal reservoir are frequently subject to changes in pressure and stress. Understanding whether pressures and stresses increase or decrease permeability, and by how much, is important for understanding volcano behavior and to maximize the efficiency of a geothermal reservoir. We performed experiments in which we measured the permeability evolution of porous volcanic rocks, three tuffs, as a function of pressure and stress. We also characterized their mechanical behavior. We first show that their mechanical behavior depends on composition: the more clay minerals and chlorite present, the weaker the tuff. We then show that increasing hydrostatic pressure (i.e., depth) decreases their permeability, but that differential loading in the ductile regime can either decrease or increase permeability. Rock permeability usually decreases during deformation in the ductile regime, and so the latter result is surprising. The observed differences in permeability evolution during ductile deformation is attributed to microstructural differences between the tuffs. However, changes in permeability as a function of pressure and stress are small when compared to other rocks, suggesting that tuff may help to maintain hydrothermal circulation in volcanoes and geothermal reservoirs, with attendant consequences for volcanic hazards and geothermal energy exploitation. Tuffs are likely weakened by the presence clay minerals, chlorite, and zeolites due to a reduction in fracture toughnessPermeability of tuff decreases during hydrostatic pressurization, particularly following the onset of inelastic hydrostatic compactionPermeability of tuff can increase during ductile deformation due to flow‐parallel microcracks that form surrounding collapsed pores Tuffs are likely weakened by the presence clay minerals, chlorite, and zeolites due to a reduction in fracture toughness Permeability of tuff decreases during hydrostatic pressurization, particularly following the onset of inelastic hydrostatic compaction Permeability of tuff can increase during ductile deformation due to flow‐parallel microcracks that form surrounding collapsed pores

Outgassing of volcanic systems is a ubiquitous phenomenon. Yet the mechanisms facilitating gas escape from vesiculating magmas and lavas remain poorly understood. Pervasive outgassing is thought to depend on the efficient and abundant formation of permeable pathways. Here we present results from experiments designed to identify the conditions and mechanisms needed to form such permeable pathways. We use a foamed silicate melt (FOAMGLAS®) in our experiments as a proxy for natural vesicular melts. FOAMGLAS® cores are compressed under a range of temperatures and strain rates, and results are evaluated against the state of melt relaxation (parameterized using the Deborah number). We find that foam microstructure and rheological and outgassing behaviors evolve with strain and as a function of melt relaxation state. Relaxed melt foams harden during deformation but remain impermeable. As foams become less relaxed at higher strain rate and/or melt viscosity, they show complex responses to deformation (strain weakening and hardening) yet remain impermeable. Strain localization and formation of high‐permeability bands occur only in highly strained, unrelaxed foams. However, these bands are thin and oriented perpendicular to the principal stress, resulting in limited outgassing. Permeable pathways do not readily form in foams; rather, high‐porosity melts are “persistently impermeable.” Our results imply that vesicular silicic lavas may not outgas as efficiently as previously thought. Instead, sustained impermeability allows the lava to maintain low effective viscosities, and to flow extended distances. In addition, pore pressures may rise in deforming impermeable lavas, perhaps priming the lava for later explosive behavior. High‐temperature deformation experiments on cores of FOAMGLAS® explore melt foam rheological and outgassing behaviorsDeforming melt foams show strain weakening, hardening, and localization, but undergo little or no outgassing (i.e., are impermeable)Crystal‐free bubble‐rich silicic lavas outgas inefficiently, allowing for sustained low effective viscosities and efficient transport

Volcanic systems often host crater lakes, flank aquifers, or fumarole fields that are strongly acidic. In order to explore the evolution of the physical and mechanical properties of an andesite under these reactive chemical conditions, we performed batch reaction experiments over timescales from 1 day to 4 months. The experiments involved immersion of a suite of samples in a solution of 0.125 M sulfuric acid (pH ∼0.6). Periodically, samples were removed and their physical and mechanical properties measured. We observe a progressive decrease in mass, coincident with a general increase in porosity, which we attribute to plagioclase dissolution accompanied by the generation of a microporous diktytaxitic groundmass due to glass dissolution. Plagioclase phenocrysts are seen to undergo progressive pseudomorphic replacement by an amorphous phase enriched in silica and depleted in other cations (Na, Ca, and Al). In the first phase of dissolution (t= 24–240 hr), this process appears to be confined to preexisting fractures within the plagioclase phenocrysts. However, ultimately these phenocrysts tend toward entire replacement by amorphous silica. We propose that the dissolution process results in the widening of pore throats and the improvement of pore connectivity, with the effect of increasing permeability by over an order of magnitude relative to the initial measurements. Compressive strength of our samples was also modified, insofar as porosity tends to increase (associated with a weakening effect). We outline broader implications of the observed permeability increase and strength reduction for volcanic systems including induced flank failure and related hazards, improved efficiency of volatile migration, and attendant eruption implications. Where water is present in volcanic environments, it is often strongly acidic due to the presence of dissolved gases such as sulfur dioxide (involving similar process to that which forms acid rain). Indeed, it is estimated that almost 800 acidic volcanic lakes exist worldwide. In terms of volcanic hazard it is important to understand the influence of such acids on volcanic rocks. We performed experiments where samples of volcanic rock were submerged in an acid solution for varying lengths of time (we used a strong concentration of sulfuric acid in order to mimic the chemistry of a natural acid lake). We find that some of the minerals in our samples dissolve over time when in contact with the acid. Ultimately, this makes the rock weaker and more porous and increases the ability for fluid to flow through the rock. These results indicate that certain large‐scale mechanisms might occur more frequently in nature when there are acidic conditions, such as the collapse of part of the side of the volcano or the rim of its crater. In this paper we describe the processes occurring on the microscale and outline broad implications for our results in the context of volcanic areas. Volcanic environments often host strongly acidic lakes or aquifersProlonged exposure of andesite to sulfuric acid induces mineral dissolutionAs a consequence, permeability and porosity increase, while mass decreases