The strength and stiffness of lavas, as well as their propensity to fracture and develop permeable flow networks, are all governed by microstructural heterogeneity. In vesicular andesitic–dacitic lavas from the Nevados de Chillán Volcanic Complex (NChVC) in Chile, we show that uniaxial compressive strength (UCS) and Young’s modulus (E) depend not only on connected porosity but also on the distribution, orientation, size and aspect ratio of pores. Pore fabrics were first quantified via 2D analyses (transparent sections and surface imaging) and 3D micro–computed tomography (micro-CT). Then, using cylinders with pores aligned parallel, perpendicular, and inclined to the loading direction, we performed UCS tests instrumented with acoustic emission (AE) monitoring. Loading perpendicular to the pore major axis presents the lowest strength and stiffness and triggers AE output at low stress, whereas loading parallel to the pore major axis initiates damage at higher stress, attains relatively higher strength, and culminates in macroscopic failure with higher rates of AE concentrated near the peak stress. Mixed AE patterns are observed when loading at an inclined angle to the pore major axis. These observations are consistent with the theory of microcrack nucleation and propagation controlled by stress concentration around elliptical pores. While porosity exerts a first-order control on the overall magnitude of strength and stiffness, we conclude that pore alignment, aspect ratio and directional variability fundamentally modulate failure style, damage evolution, and rock mechanical properties. Operationally, this framework anticipates preferential directions of weakness and damage evolution prior to failure, thereby supporting assessments of the mechanical stability of lava domes, levees, and scoria cones composed of high porosity materials.

Determining the contributing factors, distribution, and volume of unstable regions in volcanic flanks is critical for constraining the susceptibility of volcanic debris avalanche generation. These are rapid-onset catastrophic events that can cause substantial damage to infrastructure and loss of life within minutes to hours. Antuco, a basaltic stratovolcano located in the Southern Andean Volcanic Zone of Chile, catastrophically collapsed at ca. 7.1 ka BP, producing a 6.4 km(3) debris avalanche deposit. Rapid post-collapse edifice growth has substantially regenerated the central cone. In this contribution, we seek to describe the lithotechnical units and rock mechanical properties of both pre- and post-collapse units of Antuco. We performed in-situ geotechnical characterization and rebound hardness measurements, complemented by sampling representative blocks of lithological units for petrological, geochemical, and geomechanical characterization. Using a geological profile, we constructed a 3D model of the present-day volcano. We incorporated empirical data into a limit-equilibrium model to simulate unstable regions and potential failure planes within the flank under static and pseudostatic conditions. Results indicate that the upper portion of the western flank is stable under static conditions, but a volume up to 0.59 km(3) could slip with horizontal peak ground accelerations of >0.58 g. In addition, dyke intrusions have the potential to increase flank instability to the point of failure, whereas changes to the water table appears insufficient.

Hydrothermal alteration can modify the permeability of volcanic rocks, influencing the movement of fluids and volcanic hazard potential. Permeability increases could facilitate outgassing, promoting effusivity, and permeability decreases could result in overpressurisation, promoting explosivity and instability. We measured the permeability of 573 variably altered rocks from La Soufrie`re de Guadeloupe (Eastern Caribbean) using a calibrated field permeameter. Our data show a wide range of alteration (from very slightly to extremely altered) and permeability (10-18-10-11 m2), and that more altered rocks have a higher average permeability, and a wider permeability range, than less altered rocks. We also find that alteration and permeability are spatially heterogeneous. Microstructural analysis reveals evidence for dissolution in plagioclase phenocrysts and void-filling precipitation. For high-permeability rocks, such alteration did not affect the well-connected flow-path: these rocks were simply altered without modifying their permeability significantly. For low-permeability rocks, dissolution in isolated plagioclase phenocrysts did not decrease flow-path tortuosity, but precipitation blocked important pathways for flow: alteration reduced their permeability. Therefore, although the more altered rocks have a higher average permeability, we conclude that alteration reduces permeability. Highly altered zones on the dome are, therefore, sites of high permeability that are undergoing permeability reduction. Finally, downsampling our large dataset to test whether smaller datasets reproduce the observed trend highlights the problems associated with using small datasets to understand the influence of alteration on permeability. The results of our study can help improve volcano monitoring and hazard mitigation.

The Great Oolite Formation is a Middle Jurassic (upper Bajocian-lower Bathonian) carbonate Formation widespread across northern Europe, deposited in a shallow-marine epicontinental sea. This Formation is of great economic importance as a geo-resource, for example as a hydrocarbon reservoir in the UK and as a low-temperature geothermal reservoir in the Paris Basin. Comparable Jurassic oolitic carbonate Formations are also economically important worldwide (e.g. the Arab Formation in the Persian Gulf as a hydrocarbon reservoir, and the Smackover Formation in the US as an aquifer reservoir).In the Upper Rhine Graben (URG), the Great Oolite Formation has recently attracted new attention as a potential intermediate-depth geothermal reservoir. Despite its significance, geological knowledge of the Great Oolite Formation in the URG remains limited and is largely based on outdated mid-20th-century studies. Indeed, subsurface reservoir quality is poorly constrained from scattered data. An integrated analytical approach combining petrographic and facies analyses, with non-destructive micro–X-ray fluorescence (µ-XRF) mapping, were conducted on 64 core samples from the Chalampé well. This unique well represents a key reference well for the Great Oolite Formation in the URG, as it fully penetrates this Formation. The Great Oolite is only know at the surface from some limited outcrops, exposing incomplete sections of the Formation.Petrographic analysis identified the diverse skeletal and non-skeletal components, from which five distinct carbonate textures (Mudstone, Wackestone, Packstone, Grainstone, and Rudstone) have been identified. Various post-depositional features, including micritization, pyritization, bioturbation, fracture, and stylolite, are documented. While the Great Oolite Formation from the URG was formerly presented as a monolithic unit, these preliminary results reveal a much heterogeneous Formation (i.e. lithology, textural fabrics, allochems content; vertical variations), implying lateral variability within the Formation. A first detailed sedimentological log of the Great Oolite Formation is now proposed for the URG. Comparison with surrounding Formations equivalent to the Great Oolite shows comparable vertical stratigraphic organisation and diversity of textures, suggesting a common regional depositional story.Mineralogical composition maps derived from µ-XRF analysis are used to investigate the diagenetic story, related to the post-depositional processes and the geodynamic evolution of the URG.So far, this ongoing work allows to list some main characteristics of the Great Oolite in the URG:- a diversity of textures that reflects fluctuating depositional energy conditions, and controls variable primary porosity and permeability properties;- fine grained intervals (mudstone texture) suggest the presence of potential intra-formational seals, which need to be investigated (thickness, continuity);- a newly identified heterogeneity of the Formation, that suggests a possible layer cake model, and thus a more complex reservoir;- a clear diagenetic impact on the primary porosity and permeability;- the presence of fractures and stylolites which may modify fluid flow.

Under upper crustal conditions, rock pore space is often filled with fluids spanning various chemical compositions. The presence of fluids (water or others) was shown to affect the mechanical strength of porous rocks in both the brittle and ductile regimes. In the brittle regime, micromechanical models predict that porous rock strength is dictated by the frictional parameters and the fracture toughness of the material, while in the ductile regime, only by the fracture toughness of the material. Here, we investigate the influence of fluids and fluid composition on Adamswiller sandstone's mode-I fracture toughness and frictional strength. Different fluidsaturated conditions were tested: dry, water, 6 mol NaCl solution, 0.1 mol HCl solution and 0.1 mol NaOH solution. We found that the mode-I fracture toughness is significantly reduced in the presence of fluids. Most of the weakening occurs when comparing dry to saturated conditions. Additional reductions are observed for acidic and basic solutions. Basic solution allows for the largest toughness weakening. On the contrary, the measured static and peak friction coefficients are unaffected by fluid presence and fluid composition. Incorporating the measured toughness and frictional strength into micromechanical models successfully reproduces fluidweakening under uniaxial and triaxial conditions. The models capture the effective-pressure dependence of fluid-weakening in both the brittle and ductile regimes, reproducing the observed variations in strength under different fluid compositions. This experimental dataset provides new insight that constrains the micromechanical mechanisms relevant to porous rock deformation in the presence of natural and anthropogenic fluid-saturated environments, necessary for safe geo-reservoir exploitation.

The deformation behavior of volcanic rocks is governed by their microstructure (porosity, pore geometry, orientation) and external conditions such as pressure, temperature, and strain rate. At low confining pressures, porous rocks fail by shear fracturing, whereas higher pressures promote ductile processes, including cataclastic flow or compaction band formation. While these processes are well established in sedimentary rocks, compaction localization in volcanic rocks remains poorly understood. Here, we investigate how preferred pore orientation influences the mechanical behavior, compaction band development, and permeability of lava deformed in the ductile regime. We focus on a porous lava from Volvic, France, for which 3D imaging reveals a pronounced pore preferred orientation. Cylindrical samples were cored with the pore major axes parallel (VBY) and perpendicular (VBZ) to the maximum principal stress and permeability measurement direction. Triaxial deformation experiments show that VBY samples require significantly higher stress to initiate inelastic deformation. Post-deformation imaging reveals oblique compaction bands in VBY samples and diffuse, sub-perpendicular bands in VBZ samples, with microcracking being more abundant in VBZ samples, indicating that compaction localization is strongly influenced by pore preferred orientation. Despite similar porosities, initial permeability in VBY samples is an order of magnitude higher than in VBZ samples. Upon deformation, microcracking in VBZ samples reduces flow-path tortuosity and leads to an order-of-magnitude permeability increase, whereas permeability in VBY samples increases negligibly. Our findings demonstrate that preferred pore orientation affects strain localization, mechanical behavior, and fluid transport in lava, with implications for volcanic risk assessment and geothermal energy extraction.

A fundamental understanding of the transport properties of carbonate rocks is crucial in many geophysical contexts related to natural resources, energy transition, and environment. In this study, we investigated the applicability of the Katz-Thompson model, based on percolation theory and critical path analysis, to predict the transport properties of dual-porosity limestones. Laboratory measurements of permeability, formation factor, and specific surface area were conducted on five dual-porosity limestones, supplemented by Mercury Intrusion Capillary Pressure (MICP) data and published data for three additional limestones. The MICP curves for these rocks exhibit two inflection points, reflecting bimodal pore size distributions. We demonstrated that selecting the first inflection point to estimate the characteristic length scale yields permeability predictions that agreed with experimental measurements within a factor of two. Formation factor predictions based on the second characteristic length defined by Katz & Thompson also from MICP data showed good agreement for most studied rocks, with notable exception in Thala limestone. For this rock, several factors could explain that the model underestimates the formation factor, such as differences between micritic structure or diagenetic process of dolomitization. X-ray Computed Tomography imaging and Lattice Boltzmann flow simulations on Indiana and Purbeck limestones confirmed that macropores formed interconnected networks dominating fluid transport, justifying the percolation approach. Our findings suggest that despite pore complexity, the Katz-Thompson critical path model remains robust for allochemical and many dual-porosity limestones, provided the characteristic length is correctly chosen. However, its applicability to micritic limestones and rocks with narrow or poorly connected pore networks may be limited.

In the upper crust, rock pore spaces may be occupied by fluids of diverse chemical compositions. Pore spaces can be naturally filled with water, carbon dioxide, oil or gas, or artificially saturated with reactive fluid for geo-engineering purposes, including geothermal energy, wastewater disposal, carbon dioxide or hydrogen storage. The presence of water and other fluids modifies the mechanical strength of porous rocks in both the brittle (i.e., localised) and ductile (i.e., distributed) regimes. According to micromechanical models, the strength of porous rock in the brittle regime is controlled by both frictional parameters and fracture toughness of the material, while inelastic compaction by cataclastic pore collapse is governed exclusively by fracture toughness. Experimental studies indicate that the presence of fluid affects the fracture toughness and static friction of limestones and sandstones. Accordingly, for a given rock type, fluid-induced weakening of the rock strength should be explained entirely by a decrease in fracture toughness and/or frictional parameters.This interpretation is supported by measurements of the mode-I fracture toughness (KIc) and static friction (µs) of sandstones and limestones, under both dry and water-saturated conditions, which allow for the estimation of the uniaxial compressive strength and quantification of the degree of water-weakening. In this context, we investigate the influence of fluids and fluid composition on the mode-I fracture toughness and frictional strength of Adamswiller sandstone. This sandstone was selected because its mechanical behaviour is well-documented in the literature, and because both fluid presence and fluid composition have been shown to affect its response under uniaxial and triaxial compression. We tested a range of fluid-saturated conditions, including dry, deionised water, 6 mol NaCl solution, 0.1 mol HCl solution and 0.1 mol NaOH solution. For KIc, most of the weakening occurs between dry and fluid-saturated conditions, with additional reductions observed for acidic and basic solutions, with the greatest under basic conditions. For a saline solution, the extent of weakening relative to water-saturated conditions is unclear. In contrast, the measured static and peak friction coefficients are unaffected by either the fluid presence or the fluid composition. Incorporating the measured toughness and frictional strength into micromechanical models (wing crack model and pore collapse model) successfully reproduces fluid-weakening under uniaxial and triaxial conditions. The models capture the effective pressure dependence of fluid-weakening in both the brittle and ductile regimes, reproducing the observed strength variation associated with different fluid compositions. This experimental dataset provides new insight that constrains the micromechanical mechanisms governing porous rock deformation in natural and anthropogenic fluid-saturated environments, with direct implications for the safe exploitation of geo-reservoirs.

Abstract Porous volcanic rocks form volcanic edifices and host key geothermal reservoirs worldwide. When subjected to constant stress above dilatancy in the brittle regime, these materials undergo time‐dependent failure known as brittle creep. In the ductile regime, where deformation is compactive, time‐dependent compaction—compaction creep—has been documented in sandstones but never in lavas. Here, we present the first experimental investigation of compaction creep in porous lava from Volvic, France. We performed triaxial constant strain rate and compaction creep experiments at an effective pressure of 100 MPa. Our results show that lava accumulates inelastic strain and loses porosity continuously with decreasing rate during creep deformation. Creep strain rates weakly depend on the applied stress, and microstructural observations reveal that time‐dependent compaction localizes into compaction bands. These findings demonstrate that compaction bands can form slowly over extended periods in lava with implications for the long‐term evolution of volcanic systems and geothermal reservoirs.

Hydrothermal fluid circulation in active volcanoes can significantly modify the porosity, permeability, and mechanical strength of rocks, reshaping heat and mass flow in volcanic edifices. La Soufrière de Guadeloupe, in the Eastern Caribbean, is an active volcano characterized by an extensive hydrothermal system. A period of unrest has been ongoing since 1992, which manifests at the surface as increasing fumarolic activity and high ground temperatures in a delimited region of the volcano summit. Here, we present a high-resolution, 3D electrical conductivity model of the volcano summit, obtained using electrical resistivity tomography data. It is the highest resolution near-surface electrical conductivity model of an active volcano summit to date. To interpret our conductivity model, we measured the electrical conductivity of rock samples from the volcano in the laboratory. The combined laboratory and field data suggest that the hydrothermal system has altered an important volume of the surveyed summit region (volume of approximately 2.2 × 10^4 m^3 , out of 7.0 × 10^5 m^3 ). Furthermore, we infer the presence of several shallow pressurized gas pockets confined by low-permeability clay caps, located in the diffuse degassing zone of the summit, which is frequented by guided tours. Our findings help to assess risks, such as sudden explosions, ground subsidence and collapse, slope instability, and toxic gas emissions, especially in zones that are visited on a daily basis.

In many geotechnical and tectonic settings, a fundamental understanding of the inelastic behavior of porous rocks under polyaxial compression is necessary. In this study, we present new true triaxial compression data obtained in the ductile regime on Bleurswiller sandstone with the size of 100mm X 50mm X 50mm. The deformed samples show a range of failure modes qualitatively similar to what was reported by earlier experimental studies performed in conventional conditions (axisymmetric compression). In particular, visual inspection and X-ray Computed Tomography imaging reveal compaction localization in all our deformed samples. The pore collapse model of Zhu et al. (2010) is extended to include the role of the intermediate principal stress and our new data for the onset of shear-enhanced compaction are in basic agreement with this extended model that includes three stress invariants. At constant minimum principal stress, the onset of shear-enhanced compaction tends to decrease slightly with increasing intermediate principal stress.Published true triaxial data obtained in the brittle regime highlights the impact of the intermediate principal stress on the onset of dilatancy. The predictions of the conventional sliding wing crack model extended to true triaxial conditions are in poor agreement with these data. Our analysis suggests that the observed discrepancies are related to the influence of the intermediate principal stress on the effective shear stress on the wing cracks. Another energetic approach pioneered by Wiebols & Cook (1968) shows a better agreement with the experimental results, and predicts that at constant minimum principal stress, the onset of dilatancy would not be a monotonic function of the intermediate principal stress. Our new data and analysis will help the interpretation of inelastic deformation under polyaxial compression in various geotechnical and tectonic settings.

We present new true triaxial compression data obtained in the ductile regime on Bleurswiller sandstone. The deformed samples show a range of failure modes qualitatively similar to what was reported by earlier experimental studies performed in conventional conditions (axisymmetric compression). In particular, visual inspection and X-ray Computed Tomography imaging reveal compaction localization in all our deformed samples. The pore collapse model of Zhu et al.( 2010) 1 is extended to include the role of the intermediate principal stress and our new data for the onset of shear-enhanced compaction are in basic agreement with this extended model that includes three stress invariants. Published true triaxial data obtained in the brittle regime highlights the impact of the intermediate principal stress on the onset of dilatancy. The predictions of the conventional sliding wing crack model extended to true triaxial conditions are in poor agreement with these data. Another energetic approach pioneered by Wiebols & Cook shows a better agreement with the experimental results. Our new data and analysis will help the interpretation of inelastic deformation under polyaxial compression in various geotechnical and tectonic settings.

Steep slopes, excessive volcanic edifice volume, weathering and/or alteration generating weak substratum, and magmatic-volcanic or seismic activity are known factors that all control volcano flank instability. These factors are critical in generating volcanic debris avalanches, which are rapid-onset catastrophic events that can cause substantial damage to infrastructure and loss of life within minutes to hours. Despite this, volcano hazard maps of Andean volcanoes do not specifically incorporate analysis of zones or areas prone to flank collapse. In a new project (Fondecyt 11241126), we aim to determine the distribution and geometry of potential sites of flank instability, and their controlling factors, that may produce potential future volcanic debris avalanches at the Chillán Viejo (3,195 m asl) and Antuco (2,979 m asl) volcanoes. In 1883, the upper south-southeast flank of Chillan Viejo volcano collapsed after an eruptive cycle, producing a 600 m-length scar. Comparatively, at ca. 7 ka BP, the western flank of Antuco collapsed catastrophically, producing a 6.4 km3 debris avalanche deposit [1]. Rapid edifice growth and regeneration at both volcanoes demands the assessment of future flank collapse scenarios. In the field, we sampled representative lithological units of both fresh and hydrothermally altered rocks for petrological and geochemical characterisation. Simultaneously, we performed rebound tests in substratum units, lava flows, and intrusive bodies using N-type Schmidt test hammers. These measurements were complimented by the construction of three-dimensional outcrop models from Unmanned Aerial Vehicle surveys for structural analysis. The samples collected were analysed in the laboratory in order to constrain the physical and mechanical rock properties of both intact and hydrothermally altered blocks. Preliminary results are compared with textural and compositional features of the effusive products to better understand the mechanical behaviour of both volcanic edifices, identify potential sites of future collapse, and ascertain potential collapse drivers.This is a contribution to Fondecyt iniciación 11241126 and the European Research Council (ERC) SYNERGY grant 101118491 "ROTTnROCK". References:[1] Romero, J. E., Moreno, H., Polacci, M., Burton, M., & Guzmán, D. (2022). Mid-Holocene lateral collapse of Antuco volcano (Chile): debris avalanche deposit features, emplacement dynamics, and impacts. Landslides, 19(6): 1321-1338.

The analysis of deformation and failure in many sedimentary settings hinges upon a fundamental understanding of inelastic behaviour, failure mode of porous carbonate rocks and their implications on fluid flow at various scales. The mechanical compaction behaviour of carbonate rock of a broad range of porosity has been investigated in the laboratory over a wide range of stress conditions in the last decades. The phenomenology of brittle failure and inelastic compaction in this rock type with often bimodal pore size distribution was found similar to that of sandstone. Inelastic compaction in limestone involved primarily cataclastic pore collapse and micromechanical analysis showed the strong influence of the micropore size on the yield stress. Compaction experiments on porous limestones also revealed a broad spectrum of complex failure modes. In situ X-ray Computed Tomography imaging combined with Digital Volume Correlation provided the first observations of discrete compaction bands in a high porosity limestone. Permeability variations in carbonates associated with shear-enhanced compaction and these failure modes were found significantly smaller than variations previously reported in porous sandstones of comparable porosities. In geophysical applications such 4D reservoir monitoring and the production of geothermal reservoirs, an understanding of the mechanical and chemical effects of pore fluid is fundamental. The mechanical influence of pore fluid on different properties is characterized by effective pressure coefficients. For limestone with dual porosity, both effective stress coefficients for permeability and pore volume change were observed to be consistently greater than unity. This implies that microscopic homogeneity is not a valid approximation for a limestone with dual porosity, and a realistic model must explicitly differentiate between the macropores and micropores, as well as account for their interplay in controlling the hydromechanical behaviour. Recent data showed that a significant weakening effect of water could also be expected in most carbonates.

Reductions to the permeability of a volcanic system can increase pore fluid pressure and, in turn, promote volcanic hazards such as erratic explosive behaviour and slope failure. Hydrothermal alteration, ubiquitous at volcanoes worldwide, is one mechanism thought to reduce permeability and therefore increase volcanic hazard potential. Turrialba, Poás, and Rincón de Vieja, active stratovolcanoes in Costa Rica, are characterised by erratic explosive behaviour thought to be the result of the formation, maturation, and rupture of hydrothermal seals that clog the conduit. To better understand this process, we present here a systematic study in which we assessed the textural, mineralogical, and physical properties of hydrothermal seals from Turrialba, Poás, and Rincon de Vieja volcanoes, ejected as ballistics following recent explosive activity. We first document the type and intensity of the hydrothermal alteration preserved in the collected ballistics using X-ray powder diffraction and scanning electron microscopy. All samples are characterised by pervasive acid-sulphate alteration. Prior to sample preparation, the permeability of these ballistics was first estimated using a TinyPerm III, a portable handheld air permeameter. We find that the hydrothermal seal is characterised by low values of permeability, between 10−15 and 10−13 m2. Cylindrical samples were then prepared for a systematic physical property (porosity, permeability, P-wave velocity, thermal properties, and mechanical strength) characterisation in the laboratory. These laboratory data, together with fluid flow modelling, highlight how alteration can create a low-permeability hydrothermal seal that promotes cyclic, but erratic, explosive volcanic behaviour.

Rock physics theory and experimental data suggest that fracture growth in rock proceeds not only as a function of synchronous stress and environmental conditions but also as a function of past fracture growth in response to those conditions. ‘Stress memory’ or ‘fatigue-limit’ fracture mechanics phenomena such as the Kaiser effect epitomize this idea. Many questions exist, however, as to if and how these phenomena impact the growth of fractures under natural environmental conditions. For example, to what extent does the orientation of past experienced stresses manifest in a rock’s response to stresses of the same magnitude?Here we test for a memory of intergranular thermal stresses in two natural granite boulders of the same lithology for which we have 1 and 3 years of known temperature history, respectively. We hypothesize that cores extracted from the exterior portions of the boulders – that have necessarily experienced more and larger temperature fluctuations – will have more ‘memory’ of peak temperatures than those cores extracted from the boulder centers. In turn, we hypothesize that outer cores will crack less in response to temperature cycling than inner cores. For the first boulder, we measured P-wave velocities and connected porosities before and after 4 different oven heat treatments – heating up to 40, 45, 50 and 65 °C at a rate of at 20 °C/hr and cooling at an ambient rate over several cycles each. For two transects of cores extracted from the natural upward facing surface down, and the natural west-facing surface inward, we found that porosities increased after each subsequent heat treatment, but by larger amounts with distance away from the outer rock surface, as hypothesized. P-wave velocities, however, both increased and decreased with different heating cycles and positions. Therefore, for the second boulder, we extracted a top-down transect of 5 cores and, using a special-made rig, found that the samples exhibit significant P-wave velocity directional anisotropy. We subjected these cores to the same heat treatments as those of the first boulder, but this time orienting the samples identically in the oven with respect to their original positions in the boulder. Preliminary data show similar results as the first boulder, with the outermost core cracking the least (as interpreted from porosity changes) relative to the inner cores. Ongoing work will examine changes in P-wave velocity in different directions relative to measured anisotropy as a function of heat treatment cycles. This work has important implications for understanding if and how, with ongoing global warming, Earth’s rocks will respond to ‘new’ temperatures.

Volcanoes are unstable heterogeneous structures that can host hazardous mass movements. Hydrothermal alteration can create weak zones that promote instability. Volcanic instability can be assessed using large-scale numerical models, which require accurate and reliable physical and mechanical rock input parameters. Volcano stability models are often constructed using discrete zones that are assigned homogeneous parameters, rarely accounting for the heterogeneity of volcanoes. Given that the range and distribution of alteration and rock strength in volcanoes are likely highly variable, these factors should be carefully determined for accurate modelling. Here, we performed an integrated field and laboratory study. We examined a total of 544 variably altered andesites from seven sampling locations at La Soufrie`re de Guadeloupe (Eastern Caribbean). Based on a visual assessment, we assigned the rocks an alteration grade index, from 1 (least altered) to 5 (most altered), and measured the strength of rocks in the field using a point load tester. The alteration and strength distribution maps we provide highlight the extreme heterogeneity of a volcanic structure. We provide a method for direct on-site conversion from field to laboratory strength. We find that porosity and strength increase and decrease, respectively, as a function of increasing alteration. The most altered rocks were weak regardless of their porosity, suggesting that the alteration is the primary factor governing strength. We conclude that a volcano can be heterogeneous in terms of alteration and strength, between and within the discrete zones. Therefore, if possible, material property heterogeneity should be incorporated in future volcanic stability models.

We aimed to establish how permeability heterogeneities develop in relation to compaction deformation in sandstone. Three sandstones were tested in the compactant regime: Locharbriggs sandstone, which is initially heterogeneous with beds of lower initial permeability; a low porosity (22%) Bleurswiller sandstone, which is initially homogeneous and produces localized compaction bands; a high porosity (24%) Bleurswiller sandstone, also homogeneous but producing compaction in a more diffused pattern. We monitored acoustic emission locations and elastic wave speed variations throughout deformation. In addition, at regular stages during each test, a constant pore pressure difference was imposed at the boundaries of the samples, and steady-state flow was established. Internal pore pressure measurements at four locations allowed us to derive local permeability estimates. In all samples, progressive compaction produced overall reductions in permeability. In addition, localized compaction also produced internal reorganization of the permeability structure. Strong permeability reductions in the direction perpendicular to flow, by up to two orders of magnitude, are only observed when fully connected compaction bands grow across samples. Compaction and permeability reduction preferentially impacted the more porous and permeable regions of the samples, which lead to an overall homogenization of the transport properties of the samples during deformation. Compaction results from grain crushing, and is directly linked to progressive reductions in elastic wave speed. However, the impact of compaction on permeability depends strongly on the spatial connectivity of the compacted regions.

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 the Krafla geothermal system (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.

Hydrothermal alteration, which alters the chemical composition and physical state of volcanic rocks, can weaken a volcanic edifice, potentially leading to instability and collapse, thereby endangering the livelihoods of neighbouring residents. Instability scenarios can be modelled using large-scale numerical models, the accuracy of which depends on acquiring the physical and mechanical properties of volcanic rocks from laboratory measurements. Routinely, laboratory studies provide measurements for small sample suites (< 10) and, as a result, we do not fully grasp the range of rock properties that describe a particular unit or volume of the volcano, nor is it clear whether the few samples collected are representative. Here, we introduce a method that can help us select the most appropriate value, or range of values, for the physical and mechanical properties of volcanic rocks for large-scale numerical models.We collected nearly 550 rocks from seven different sampling sites at La Soufrière de Guadeloupe, an active andesitic stratovolcano in the Eastern Caribbean. The rocks were assigned an alteration grade index, from 1 (least altered) to 5 (most altered), based on a visual assessment of their alteration. Their bulk densities were then measured in the field using the Archimedes method and, lastly, their strengths were measured using a point load tester, a field strength measuring apparatus. Alteration grade index ranges from 1 to 5 , bulk densities range from less than 1000 to 2700 kg/m3, and point load strength values span from 0.012 to 8.53 MPa . Point load strength and alteration distribution maps for the seven sampling locations show not only that rock physical properties vary greatly at an individual location, but also that the distributions of alteration and strength vary from place to place, highlighting the large heterogeneity of the dome at La Soufrière de Guadeloupe.To calibrate these field data, we took small samples of the tested rocks back to the laboratory for porosity and uniaxial compressive strength measurements. Porosity measurements, ranging from 0.02 to 0.8, exhibited a strong correlation with field density values. The extremely altered rocks, deemed alteration grade index 5, despite having porosities ranging from 0.11 to 0.8, did not exceed point load strength of 2 MPa, suggesting that strength was not solely dependent on porosity. We also found that strength decreases as a function of increasing porosity and alteration grade index. Although uniaxial compressive strength tests revealed synchronicity with point load strength data, our current goal is to perform further tests to better constrain this relationship.The point load test is a field method that can simplify the logistic problems behind strength assessments of volcanic domes, and likewise enlarge the sampling suite of individual studies. Calibrating field results to uniaxial compressive strength laboratory data will allow our data to be used in volcano stability models and accommodate direct conversion, from point load values to uniaxial compressive strength values, on-site. The potential abundance of available data, and spatial strength distributions, could make the large-scale models more realistic, and consequently more accurate and reliable.