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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

Microcracking due to thermal stresses affects the mechanical and flow properties of rocks, which is significant for thermally dynamic environments such as volcanoes and geothermal reservoirs. Compared with other crustal rocks like granite, volcanic rocks have a complex and variable response to temperature; it remains unclear how thermal microcracks form and how they are affected by temperature. We heated and cooled samples of low‐porosity basalts containing different amounts of microcracks and a porous andesite over three cycles, whilst monitoring microstructural changes by acoustic emission (AE) monitoring and measurement of P‐wave velocity (vP; up to 450°C) and thermal expansion coefficient (TEC; up to 700°C). During the second and third cycles, the TEC was positive throughout and the rate of detected AE was low. In contrast to studies on granite, we measured a strong and reversible increase in vPwith increasing temperature (by 15%–40% at 450°C), which we interpret as due to microcrack closure. During the first cycle, AE and vPmeasurements indicated thermal microcracking within the andesite and the basalt with a low initial microcrack density. For these samples, strong inflexions in the TEC indicated stress relaxation during heating, preceding significant thermal microcracking during cooling. The basalt with a high initial microcrack density underwent little microcracking throughout all cycles. Our results and a review of the literature relate the initial microstructure to the occurrence of thermal microcracking and explore the potentially significant influence of temperature on volcanic rock properties. Microcracking due to thermal stresses affects the mechanical and flow properties of rocks, playing an important role within volcanoes and geothermal reservoirs. Volcanic rocks exhibit a complex response to temperature, and it remains unclear how thermal microcracks form and how they are affected by temperature. Here, we repeatedly heated and cooled samples of basalt and andesite with different initial porosities and microcrack content over three heating/cooling cycles, whilst monitoring for laboratory‐scale seismicity and changes in acoustic wave velocity (up to 450°C) and sample length (up to 700°C). During the second and third cycles, we measured a strong, reversible increase in wave velocity with increasing temperature (by up to 40% at 450°C): opposite to measurements on granite, and which we interpret as due to crack closure during heating. During cycle one, we detect thermal microcracking within the andesite and the basalt with a low initial microcrack population. For these samples, anomalous thermal expansion during heating is linked to the significant thermal microcracking during cooling. In contrast, the initially highly‐microcracked basalt underwent little microcracking throughout. We relate the initial microstructure to the occurrence of thermal microcracking, and explore how rock properties may significantly change with temperature. One basalt cracked significantly during cooling, following an anomalous thermal expansion, indicative of stress relaxation, when heatedAll samples see an increase in wave velocity with temperature during repeated heating, attributed to microcrack closure during heatingOur data literature study suggest low‐porosity volcanic rocks with high P‐wave velocities are most susceptible to thermal microcracking One basalt cracked significantly during cooling, following an anomalous thermal expansion, indicative of stress relaxation, when heated All samples see an increase in wave velocity with temperature during repeated heating, attributed to microcrack closure during heating Our data literature study suggest low‐porosity volcanic rocks with high P‐wave velocities are most susceptible to thermal microcracking

The evolution of permeability and its effective stress behavior is related to inelastic deformation and failure mode. This was systematically investigated in Indiana and Purbeck limestones with porosities of 16% and 14%, respectively. High‐pressure compression tests were conducted at room temperature on water‐saturated samples. At relatively high confinement shear‐enhanced compaction was observed to initiate at a critical stress, accompanied by significant permeability reduction of up to a factor of ~3. Overall, the permeability reduction due to inelastic compaction in our limestones is smaller than that observed in sandstones. At relatively low confinement, dilatant failure was observed, which was accompanied by a decrease and increase of permeability in Indiana and Purbeck limestones, respectively. There seems to be a trend for the correlation between porosity and permeability changes to switch from positive to negative with increasing porosity. The void space of both limestones has significant proportions of macropores and micropores. The effective stress behavior of such a limestone with dual porosity has been documented to be different from the prediction for a microscopically homogeneous assemblage, in that its effective stress coefficients for permeability and pore volume change may attain values significantly >1. In contrast, our investigation of damaged samples consistently showed effective stress coefficients for both permeability and pore volume change with values <1. This suggests that the behavior in the damaged samples is akin to that of a microscopically homogeneous assemblage, possibly due to pervasive collapse of macropores that would effectively homogenize the initially bimodal pore size distribution. With the development of shear‐enhanced compaction, a permeability reduction by up to a factor 3 was observed in two porous limestonesDilatant failure was accompanied by modest variations of permeability in two porous limestonesInelastic compaction changed significantly the effective stress behavior of limestones with dual porosity

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

A fundamental question in rock physics is how the coupling of confining stress and pore pressure influences geophysical properties, which is manifested by the effective stress behavior of the porous rock. We investigated the effective stress behavior of four water‐saturated limestones with porosities ranging from 13% to 30%. Unlike previous experimental studies limited to the permeability, we also characterized the effective stress coefficients for pore volume change and bulk strain. The pore spaces of three of the limestones (two allochemical oolitic and one micritic) have significant fractions of macropores and micropores. In these three limestones with dual porosity the effective stress coefficients for permeability and pore volume change were observed to be consistently greater than 1, even though that for axial strain was less than 1. In a microscopically homogeneous assemblage, the effective stress coefficients for permeability, bulk volumetric strain, and pore volume change are predicted to be equal to or less than unity. Our data therefore show that these limestones cannot be modeled as microscopically homogeneous. Berryman (1992a, https://doi.org/10.1029/92JB01593) and Berryman (1992b, https://doi.org/10.1103/PhysRevA.46.3307) analyzed theoretically a rock made up of two porous constituents, and our experimental data are in agreement with inequalities he derived for effective stress coefficients of such an assemblage. For comparison, we studied the Leitha limestone that is made up predominately of macropores. Our data showed that all three effective stress coefficients in this case were less than unity, as predicted for a microscopically homogeneous assemblage. Our study is the first integrated investigation of the effective stress behaviors for both permeability and deformation in limestonesFour limestones with a diversity of microstructural attributes were investigatedFor limestones with dual porosity effective stress coefficients for permeability and pore volume change were observed to be consistently greater than 1

We performed triaxial deformation experiments in Purbeck limestone (13.8% average porosity) across the brittle‐ductile transition and monitored the evolution of permeability and wave velocities as a function of strain. In the brittle regime, the rock yields in dilation. In the ductile regime, the rock first yields in compaction and then undergoes net dilation at some critical level of strain. The permeability increases after failure in the brittle regime and decreases with increasing compaction in the ductile regime. The wave velocities decrease with increasing strain, and the material becomes transversely isotropic. At axial strains of the order of 5%, the anisotropy parameters (from Thomsen, 1986) are around ε≈ −0.2 and δ≈ decrease with increasing axial strain beyond the yield point−0.3. Under hydrostatic conditions, the rock also yields in compaction. The hydrostatic yield point is not marked by any significant drop or increase in wave velocity during loading, but wave velocities decrease (and therefore crack density increases) significantly upon unloading. In all our tests, the permeability change is proportional to the initial porosity change until the point of net dilation is reached. The strain at that point also acts as a scaling factor for the relative drop in Pand Swave velocities during deformation. All our experimental data point to a disconnection between the evolution of permeability, porosity, and wave velocities during deformation in the ductile regime: permeability is controlled by a fraction of the micropore network, while wave velocities are mostly influenced by microcracks that do not contribute significantly to either the total rock porosity or fluid flow properties. Compaction deformation produces a moderate permeability reduction in Purbeck limestone (13% porosity)Deformation induces strong decreases in Pand Swave speeds, as well as a strong anelliptic anisotropyIn porous limestones, porosity, permeability, and wave velocities evolve during deformation but in response to distinct microscale processes

To monitor both the permanent (thermal microcracking) and the nonpermanent (thermo‐elastic) effects of temperature on Westerly Granite, we combine acoustic emission monitoring and ultrasonic velocity measurements at ambient pressure during three heating and cooling cycles to a maximum temperature of 450°C. For the velocity measurements we use both Pwave direct traveltime and coda wave interferometry techniques, the latter being more sensitive to changes in Swave velocity. During the first cycle, we observe a high acoustic emission rate and large—and mostly permanent—apparent reductions in velocity with temperature (Pwave velocity is reduced by 50% of the initial value at 450°C, and 40% upon cooling). Our measurements are indicative of extensive thermal microcracking during the first cycle, predominantly during the heating phase. During the second cycle we observe further—but reduced—microcracking, and less still during the third cycle, where the apparent decrease in velocity with temperature is near reversible (at 450°C, the Pwave velocity is decreased by roughly 10% of the initial velocity). Our results, relevant for thermally dynamic environments such as geothermal reservoirs, highlight the value of performing measurements of rock properties under in situ temperature conditions. We performed heating and cooling cycles of Westerly Granite to a maximum temperature of 450°C at ambient pressureWe monitored acoustic emissions and changes in velocity with temperature using direct traveltime and coda wave interferometry techniquesWe quantified both the reversible and irreversible effects of thermal expansion of crystals on ultrasonic velocity

We performed a systematic investigation of mechanical compaction, strain localization, and permeability in Leitha limestone. This carbonate from the area of Vienna (Austria) occurs with a broad range of grain sizes and porosity, due to changes in depositional regime and degree of cementation. Our new mechanical data revealed a simple relation between porosity and mechanical strength in both the brittle and ductile regimes. Increasing cementation and decreasing porosity led to a significant increase of the rock strength in both regimes. Micromechanical modeling showed that the dominant micromechanisms of inelastic deformation in Leitha limestone are pore‐emanated microcracking in the brittle regime, and grain crushing and cataclastic pore collapse in the ductile regime. Microstructural analysis and X‐ray computed tomography revealed the development of compaction bands in some of the less cemented samples, while more cemented end‐members failed by cataclastic flow in the compactant regime. In contrast to mechanical strength, permeability of Leitha limestone was not significantly impacted by increasing cementation and decreasing porosity. Our microstructural and tomography data showed that this was essentially due to the existence of a backbone of connected large macropores in all our samples, which also explained the relatively high permeability (in the range of 2–5 darcies) of Leitha limestone in comparison to other carbonates with significant proportion of micropores. New triaxial experiments, petrophysical and CT data on Leitha limestone of porosity ranging from 18 to 31%Simple relation between porosity and strength over a wide range of pressuresCompaction bands developed in less cemented samples of Leitha limestone

The modes of formation of clastic rocks result in a wide variety of microstructures, from poorly‐sorted heterogeneous rocks to well‐sorted and nominally homogeneous rocks. The mechanical behavior and failure mode of clastic rocks is known to vary with microstructural attributes such as porosity and grain size. However, the influence of the grain size distribution, in particular the degree of polydispersivity or modality of the distribution, is not yet fully understood, because it is difficult to study experimentally using natural rocks. To better understand the influence of grain size distribution on the mechanical behavior of porous rocks, we prepared suites of synthetic samples consisting of sintered glass beads with polydisperse grain size distributions. We performed hydrostatic compression experiments and found that, all else being equal, the onset of grain crushing occurs much more progressively and at lower pressure in polydisperse synthetic samples than in monodisperse samples. We conducted triaxial experiments in the regime of shear‐enhanced compaction and found that the stress required to reach inelastic compaction was lower in polydisperse samples compared to monodisperse samples. Further, our microstructural observations show that compaction bands developed in monomodal polydisperse samples while delocalized cataclasis developed in bimodal polydisperse samples, where small grains were systematically crushed while largest grains remained intact. In detail, as the polydispersivity increases, microstructural deformation features appear to transition from localized to delocalized through a hybrid stage where a compaction front with diffuse bands propagates from both ends of the sample toward its center with increasing bulk strain. In nature, sediments like sands and gravels in rivers, are composed of particles which can be all the same size or, more commonly, can be a mixture of lots of different sizes. Once the particles all hold together and the sediments become sedimentary rocks, the range of sizes of the particles can have an impact on how the rock behaves macroscopically under the pressure conditions of the Earth's crust. To explore this effect, we prepared synthetic rocks by sticking together particles of glass in mixture of different sizes which we then deformed under high pressure in the laboratory. Our results show that rocks made of mixture of sizes are systematically weaker than rocks with particles of a single size. Moreover, we found that, under a certain load, rocks deformed in different places in the microstructure depending on the range of sizes of the particles: in rock samples of single‐size particles, the deformation is concentrated into discrete bands while the rest of the rock remain intact whereas in samples with particles of two different sizes, the deformation is distributed within the entire volume of the rock. Increasing the polydispersivity of the grain size distribution decreases the stress required for inelastic compactionCompaction localization is inhibited in synthetic samples with a bimodal polydisperse grain size distributionA transition from compaction localization to delocalized cataclasis is observed for samples with very polydisperse grain size distributions Increasing the polydispersivity of the grain size distribution decreases the stress required for inelastic compaction Compaction localization is inhibited in synthetic samples with a bimodal polydisperse grain size distribution A transition from compaction localization to delocalized cataclasis is observed for samples with very polydisperse grain size distributions

We performed a systematic investigation of mechanical compaction and strain localization in Bleurswiller sandstone. Our data show that the effective pressure principle can be applied in both the brittle faulting and cataclastic flow regimes, with an effective pressure coefficient close to but somewhat less than 1. Under relatively high confinement, the samples typically fail by development of compaction bands. X‐ray computed tomography (CT) was used to resolve preexisting porosity clusters, as well as the initiation and propagation of the compaction bands in deformed samples. Synthesis of the CT and microstructural data indicates that there is no casual relation between collapse of the porosity clusters in Bleurswiller sandstone and nucleation of the compaction bands. Instead, the collapsed porosity clusters may represent barriers for the propagation of compaction localization, rendering the compaction bands to propagate along relatively tortuous paths so as to avoid the porosity clusters. The diffuse and tortuous geometry of compaction bands results in permeability reduction that is significantly lower than that associated with compaction band formation in other porous sandstones. Our new data confirm that Bleurswiller sandstone stands out as the only porous sandstone associated with a compactive cap that is linear, and our CT and microstructural observation show that it is intimately related to collapse of the porosity clusters. We demonstrate that the anomalous linear caps and their slopes are in agreement with a micromechanical model based on the collapse of a spherical pore embedded in an elastic‐plastic matrix that obeys the Coulomb failure criterion. Effective pressure law for sandstoneAnalysis of compaction band nucleation and propagation using CTInterpretation of the linear compactive envelope of Bleurswiller sandstone

Compaction bands in sandstone are laterally extensive planar deformation features that are characterized by lower porosity and permeability than the surrounding host rock. As a result, this form of localization has important implications for both strain partitioning and fluid flow in the Earth's upper crust. To better understand the time dependency of compaction band growth, we performed triaxial deformation experiments on water‐saturated Bleurswiller sandstone (initial porosity = 0.24) under constant stress (creep) conditions in the compactant regime. Our experiments show that inelastic strain accumulates at a constant stress in the compactant regime, manifest as compaction bands. While creep in the dilatant regime is characterized by an increase in porosity and, ultimately, an acceleration in axial strain rate to shear failure, compaction creep is characterized by a reduction in porosity and a gradual deceleration in axial strain rate. The global decrease in the rates of axial strain, acoustic emission energy, and porosity change during creep compaction is punctuated at intervals by higher rate excursions, interpreted as the formation of compaction bands. The growth rate of compaction bands formed during creep is lower as the applied differential stress, and hence, background creep strain rate, is decreased. However, the inelastic strain associated with the growth of a compaction band remains constant over strain rates spanning several orders of magnitude (from 10−8to 10−5s−1). We find that despite the large differences in strain rate and growth rate (from both creep and constant strain rate experiments), the characteristics (geometry and thickness) of the compaction bands remain essentially the same. Several lines of evidence, notably the similarity between the differential stress dependence of creep strain rate in the dilatant and compactant regimes, suggest that as for dilatant creep, subcritical stress corrosion cracking is the mechanism responsible for compactant creep in our experiments. Our study highlights that stress corrosion is an important mechanism in the time‐dependent porosity loss, subsidence, and permeability reduction of sandstone reservoirs. Inelastic strain can accumulate at a constant stress in the compactant regimeCreep compaction can lead to the development of time‐dependent compaction bandsStress corrosion cracking is the mechanism responsible for creep compaction

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. Pressurized fluids within the pores and bubbles of volcanic rocks and magma are thought to promote explosive volcanic behavior and drive volcano deformation. Since deformation can influence porosity and permeability, factors controlling the distribution of pore fluids and pore fluid pressure, we deformed samples of porous volcanic rock in the laboratory and measured their porosity and permeability during the deformation. We find that deformation at low‐ and high‐pressure (analogous to shallow and deep rock, respectively) decreases the permeability of the volcanic rock. Although this behavior is qualitatively similar to other porous rocks, such as sandstones and limestones, there are quantitative differences in the way porosity and permeability evolve during deformation. Understanding how porosity and permeability of volcanic rock changes as a function of deformation in the brittle and ductile regimes is important for understanding the distribution of pore fluids and pore fluid pressure. Since high pore fluid pressures can promote explosive volcanic behavior and drive volcano deformation, these data are therefore important for robust hazard assessments at active volcanoes worldwide. In the brittle regime, permeability decreases by a factor of 2–6, the result of pore throat closure prior to the peak stressIn the ductile regime, permeability of decreases by an order of magnitude, the result of localized cataclastic pore collapseThe porosity sensitivity exponent of permeability decreases from elastic to inelastic deformation, indicating granulation In the brittle regime, permeability decreases by a factor of 2–6, the result of pore throat closure prior to the peak stress In the ductile regime, permeability of decreases by an order of magnitude, the result of localized cataclastic pore collapse The porosity sensitivity exponent of permeability decreases from elastic to inelastic deformation, indicating granulation

We performed triaxial deformation experiments on a water‐saturated porous limestone under constant strain rate and constant stress (creep) conditions. The tests were conducted at room temperature and at low effective pressures Peff=10 and Peff=20 MPa, in a regime where the rock is nominally brittle when tested at a constant strain rate of 10−5s−1. Under these conditions and at constant stress, the phenomenon of brittle creep occurs. At Peff=10 MPa, brittle creep follows similar trends as those observed in other rock types (e.g., sandstones and granites): only small strains are accumulated before failure, and damage accumulation with increasing strain (as monitored by Pwave speeds measurements during the tests) is not strongly dependent on the applied stresses. At Peff=20 MPa, brittle creep is also macroscopically observed, but when the creep strain rate is lower than ≈10−7s−1, we observe that (1) much larger strains are accumulated, (2) less damage is accumulated with increasing strain, and (3) the deformation tends to be more compactant. These observations can be understood by considering that another deformation mechanism, different from crack growth, is active at low strain rates. We explore this possibility by constructing a deformation mechanism map that includes both subcritical crack growth and pressure solution creep processes; the increasing contribution of pressure solution creep at low strain rates is consistent with our observations. Brittle creep occurs in porous limestoneSubcritical cracking is dominant at high stress and low confining pressureA switch in deformation mechanism is observed at low creep strain rate

The reliable assessment of volcanic unrest must rest on an understanding of the rocks that form the edifice. It is their microstructure that dictates their physical properties and mechanical behavior and thus the response of the edifice to stress perturbations during unrest. We evaluate the interplay between microstructure and rock properties for a suite of edifice‐forming rocks from Volcán de Colima (Mexico). Microstructural analyses expose (1) a pervasive, isotropic microcrack network, (2) a high, subspherical vesicle density, and (3) a wide vesicle size distribution. This complex microstructure severely impacts their physical and mechanical properties. In detail, porosities are high and range from 8 to 29%. As a consequence, elastic wave velocities, Youngs moduli, and uniaxial compressive strengths are low, and permeabilities are high. All of the rock properties demonstrate a wide range. For example, strength decreases by a factor of 8 and permeability increases by 4 orders of magnitude over the porosity range. Below a porosity of 11–14%, the permeability‐porosity trend follows a power law with a much higher exponent. Microstructurally, this represents a critical vesicle content that efficiently connects the microcrack population and permits a much more direct path through the sample, rather than restricting flow to long and tortuous microcracks. Values of tortuosity inferred from the Kozeny‐Carman permeability model support this hypothesis. However, we find that the complex microstructure precludes a complete description of their mechanical behavior through micromechanical modeling. We urge that the findings of this study be considered in volcanic hazard assessments at andesitic stratovolcanoes. Andesites from Volcán de Colima are microstructurally complexTheir microstructural complexity influences their propertiesMicromechanical modeling is therefore a challenge

Volcanoes are unstable structures that deform laterally and frequently experience mass wasting events. Hydrothermal alteration is often invoked as a mechanism that contributes significantly to volcano instability. We present a study that combines laboratory experiments, geophysical data, and large‐scale numerical modeling to better understand the influence of alteration on volcano stability, using La Soufrière de Guadeloupe (Eastern Caribbean) as a case study. Laboratory experiments on variably altered (advanced argillic alteration) blocks show that uniaxial compressive strength, Young's modulus, and cohesion decrease as a function of increasing alteration, but that the internal friction angle does not change systematically. Simplified volcano cross sections were prepared (a homogenous volcano, a volcano containing the alteration zone identified by a recent electrical survey, and a volcano with an artificially enlarged area of alteration) and mechanical properties were assigned to zones corresponding to unaltered and altered rock. Numerical modeling performed on these cross sections, using a hydro‐thermo‐mechanical modeling code, show (a) the importance of using upscaled values in large‐scale models and (b) that alteration significantly increases volcano deformation and collapse volume. Finally, we combined published muon tomography data with our laboratory data to create a 3D strength map, exposing a low‐strength zone beneath the southern flank of the volcano coincident with the hydrothermal system. We conclude that hydrothermal alteration decreases volcano stability and thus expedites volcano spreading and increases the likelihood of mass wasting events and associated volcanic hazards. Hydrothermal alteration, and its evolution, should therefore be monitored at active volcanoes worldwide. The rocks forming a volcanic edifice can be altered by circulating hydrothermal fluids. This alteration can influence the physical and mechanical properties of these rocks, which could jeopardize volcano stability. The stability of a volcanic edifice is an important consideration in volcanic hazards and risk assessments due to the potentially dire consequences of partial volcanic flank collapse. Using a combination of experimental data, geophysical data, and modeling, and La Soufrière de Guadeloupe (Eastern Caribbean, France) as a case study, we find that hydrothermal alteration decreases volcano stability and thus promotes volcano instability and associated volcanic hazards. As a result, we conclude that hydrothermal alteration, and its evolution, should be monitored at active volcanoes worldwide. Laboratory experiments show that hydrothermal alteration reduces the strength of volcanic rock from La SoufrièreNumerical modeling shows that hydrothermal alteration significantly increases volcano deformation and collapse volumeWe provide a 3D strength map of La Soufrière that exposes a low‐strength zone coincident with the hydrothermal system Laboratory experiments show that hydrothermal alteration reduces the strength of volcanic rock from La Soufrière Numerical modeling shows that hydrothermal alteration significantly increases volcano deformation and collapse volume We provide a 3D strength map of La Soufrière that exposes a low‐strength zone coincident with the hydrothermal system

Understanding of the mechanics of the brittle‐ductile transition (BDT) in porous limestone is significantly more challenging than for sandstone because of the lack of consistent acoustic emission activity in limestone, meaning that one must rely on alternative techniques. In this paper, we investigate systematically the failure modes in Indiana limestone using X‐ray microComputed Tomography imaging (μCT) and Digital Volume Correlation (DVC). Our new mechanical data show that the envelope for the onset of shear‐enhanced compaction can be well approximated by an elliptical cap. The DVC analysis revealed the development of shear bands through the BDT, but no evidence of compaction bands. The shear band angles were between 29° and 46° with respect to the maximum principal stress. Compiling these new results with published data on Purbeck and Leitha limestones, we showed that inelastic compaction in each of these dual porosity allochemical limestones was in a good agreement with the normality condition, as defined in plasticity theory. Comparison of the observed failure modes with predictions based on bifurcation analysis showed that the shear band angles are consistently smaller than the theoretical predictions. A fundamental understanding of the brittle‐ductile transition (BDT) in porous sedimentary rocks is important in many geophysical and geological applications. In porous sandstone, a lot of our current understanding of both brittle and ductile behaviors is based on acoustic monitoring. In limestone however, the lack of consistent acoustic emission activity obliges us to rely on alternative techniques. In this paper, we investigated systematically deformation and failure in compression in Indiana limestone using X‐ray Computed Tomography imaging, a technique used to image density contrasts. Samples were scanned pre‐ and post‐deformation and image correlation was used to study the various failure modes. Our analysis revealed the development of shear bands through the BDT but no evidence of compaction bands. The shear band angles were between 29° and 46° with respect to the maximum principal stress. Compiling our new results with published data on Purbeck limestone, we showed that inelastic compaction in these allochemical limestones with double porosity (micro‐ and macro‐pores) was in a good agreement with the normality condition, as defined in plasticity theory. Comparison of the observed failure modes with theoretical predictions showed that the shear band angles are consistently smaller than the predicted ones. New triaxial and CT data are presented on Indiana limestone of porosity 16%Digital Volume Correlation reveals several complex failure modes involving shear bands through the brittle‐ductile transitionThe onset of inelastic compaction in dual porosity allochemical limestones is in agreement with the normality condition New triaxial and CT data are presented on Indiana limestone of porosity 16% Digital Volume Correlation reveals several complex failure modes involving shear bands through the brittle‐ductile transition The onset of inelastic compaction in dual porosity allochemical limestones is in agreement with the normality condition

The fundamentals of our understanding of the mechanical compaction of porous rocks stem from experimental studies. Yet, many of these studies use natural materials for which microstructural parameters are intrinsically coupled, hampering the diagnosis of relationships between microstructure and bulk sample behavior. To probe the influence of porosity and grain size on the mechanical compaction of granular rocks, we performed experiments on synthetic samples prepared by sintering monodisperse populations of glass beads, which allowed us to independently control porosity and grain diameter. We conducted hydrostatic and triaxial compression tests on synthetic samples with grain diameters and porosities in the ranges 0.2–1.15 mm and 0.18–0.38 mm, respectively. During hydrostatic compaction, sample porosity decreased suddenly and substantially at the onset of inelastic compaction due to contemporaneous and extensive grain crushing, a consequence of the monodisperse grain size. During triaxial tests at high confining pressure, our synthetic samples failed by shear‐enhanced compaction and showed evidence for the development of compaction bands. Critical stresses at the onset of inelastic compaction map out linear‐shaped yield caps for the porosity‐grain diameter combinations for which the critical stress for inelastic hydrostatic compaction is known. Our yield caps reinforce the first‐order importance of porosity on the compactive yield strength and show, all else being equal, that grain size also exerts a first‐order control and should therefore be routinely measured. Our study further reveals the suitability of sintered glass beads as analogs for crustal rocks, which facilitate the study of the deconvolved influence of microstructural parameters on their mechanical behavior. Porous rocks that form the shallow part of the Earth's crust are submitted to pressure conditions under which they deform by compaction, that is, their porosity is reduced. Our understanding of the compaction stems primarily from laboratory experiments conducted on samples of natural rocks. Yet, because the internal structure of natural materials is complex, studying the influence of structural parameters in isolation, such as porosity and grain size, on the way rock compacts is limited. To tackle this issue, we conducted compression tests on synthetic samples made of fused glass beads for which we could control porosity and grain size. When the pressure on the synthetic rocks is increased uniformly in all directions, a threshold pressure, characteristic of the rock strength, is reached beyond which they compact suddenly and substantially. When a differential stress is imposed on the synthetic rocks, discrete bands of lower porosity are observed. We show that porosity and grain size exert a first‐order control on the ability of the synthetic rocks to resist compaction. Further, our synthetic samples are good analogs for natural rocks such as sandstones and tuffs and our results therefore have broad applications, from reservoir compaction and subsidence to the destabilization of volcanoes. The mechanical compaction behavior of monodisperse sintered glass bead samples is similar to that of well‐sorted monomineralic sandstonesDuring triaxial compression at high confining pressure, discrete compaction bands developed in a synthetic sample with a porosity of 0.35All else being equal, increasing grain diameter from 0.2 to 1.15 mm decreases the stress to reach C* by more than a factor of two The mechanical compaction behavior of monodisperse sintered glass bead samples is similar to that of well‐sorted monomineralic sandstones During triaxial compression at high confining pressure, discrete compaction bands developed in a synthetic sample with a porosity of 0.35 All else being equal, increasing grain diameter from 0.2 to 1.15 mm decreases the stress to reach C* by more than a factor of two

The influence of confining and pore pressures on permeability and pore volume change was studied in two clayey sandstones. The coupling effect is characterized by the effective stress coefficients κ and β, respectively. Hydrostatic compression of a porous rock typically involves an initial nonlinear stage of elastic crack closure and a subsequent linear stage of pore deformation. To elucidate the influence of microcracks on the behavior, we measured the effective stress coefficients in these two stages separately. Our data show that the coefficients κ for permeability of both clayey sandstones are uniformly greater than 1, in agreement with published data and theoretical prediction for a microscopically inhomogeneous assemblage. Our data show that the presence of microcracks may homogenize the pore space, as reflected by a significantly lower κ value during the initial stage of nonlinear compression. We obtained some of the first measurements of the effective stress coefficient β for pore volume change in sandstones, which show values close to but uniformly less than 1. Synthesizing our sandstone data, recently published data on limestones, and theoretical analyses, we propose to define in the β‐κ space two fundamentally different regimes for the effective stress behavior. A comparison of our data with Berryman's model, the clay shell model, and clay particle model indicates apparent discrepancies, which we propose to resolve with a dual equivalent‐channel model based on the spatial partitioning of clayey and clay‐free pore volumes. Our study is the first integrated investigation of the effective stress behaviors for permeability and pore volume change in sandstonesThe presence of microcracks will influence the effective stress behavior, decreasing the effective stress coefficient for permeabilityWe propose a dual equivalent‐channel model to resolve apparent discrepancies between experimental data and existing models

We performed a systematic investigation of mechanical compaction and strain localization in Saint‐Maximin limestone, a quartz‐rich, high‐porosity (37%) limestone from France. Our new data show that the presence of a significant proportion of secondary mineral (i.e., quartz) did not impact the mechanical strength of the limestone in both the brittle faulting and cataclastic flow regimes, but that the presence of water exerted a significant weakening effect. In contrast to previously published studies on deformation in limestones, inelastic compaction in Saint‐Maximin limestone was accompanied by abundant acoustic emission (AE) activity. The location of AE hypocenters during triaxial experiments revealed the presence of compaction localization. Two failure modes were identified in agreement with microstructural analysis and X‐ray computed tomography imaging: compactive shear bands developed at low confinement and complex diffuse compaction bands formed at higher confinement. Microstructural observations on deformed samples suggest that the recorded AE activity associated with inelastic compaction, unusual for a porous limestone, could have been due to microcracking at the quartz grain interfaces. Similar to published data on high‐porosity macroporous limestones, the crushing of calcite grains was the dominant micromechanism of inelastic compaction in Saint‐Maximin limestone. New Pwave velocity data show that the effect of microcracking was dominant near the yield point and resulted in a decrease in Pwave velocity, while porosity reduction resulted in a significant increase in Pwave velocity beyond a few percent of plastic volumetric strain. These new data highlight the complex interplay between mineralogy, rock microstructure, and strain localization in porous rocks. Presence of significant quartz (20 wt. %) did not impact the mechanical behavior of Saint‐Maximin limestone (porosity 37%)Inelastic compaction in Saint‐Maximin limestone was accompanied by abundant acoustic emission (AE) activity, capturing strain localizationMicrocracking reduced Pwave velocity near the yield point, while porosity reduction resulted in a significant increase in Pwave velocity