The uniaxial compressive strength sigma c of rocks is a key material property in a wide range of applications. Models for sigma c typically either require numerical solutions, restricting their wide utility, or are empirical and therefore confined to a specific case. Here, we study the theoretical pore-emanated crack model and provide an analytical emulator function that matches the 2-D and 3-D solutions to a high degree of accuracy over all porosities, phi. A key input to both the full solution and to our emulator functions is the pore radius, assumed in the model to be circular or spherical, in a porous rock. In most porous lithologies, including sandstone, the notion of a pore radius is poorly defined since they are built from compacted or lithified grains. And so here we explore statistical methods to find a characteristic pore length scale, l2, from an initial particle radius; this method is provided as an easy-to-use supplementary tool. We advocate for the use of our 3-D function sigma(c )approximate to 1.57K(Ic)/(phi(0.43)pi l(2)), where K-Ic is the fracture toughness of the solid matrix. A compilation of K-Ic values for minerals and rocks allows us to explore the effect of this parameter and to make recommendations for appropriate values in the model. We compare our simple emulator function for sigma c with existing data sets across a wide range of sandstones to demonstrate the utility of this law for applied cases. We find that our function performs particularly well for relatively low porosity sandstones (phi less than or similar to 0.15) representative of mature basin systems from a diagenetic point of view; we discuss alternative models that are more appropriate for higher porosity sandstones.

Hydrothermal alteration is well recognized to change the physical and mechanical properties of volcanic rocks and promote instability and flank collapse. Here, we investigate La Fossa of Vulcano Island (Italy), the southernmost exposure of the Aeolian volcanic archipelago, and its associated regions of hydrothermal alteration. La Fossa’s accessibility, altered flanks, history of mass wasting events, and periods of escalating fumarole activity set an ideal environment for a natural laboratory. We used drone remote sensing methods coupled with field and ongoing laboratory rock property measurements to classify regions of hydrothermal alteration and assess their associated rock properties. Our results have (1) identified a heterogenous distribution of alteration intensity and alteration types, (2) distinguished a relationship between decreasing rock strength and increasing alteration, and (3) correlated regions with the weakest rock strength with hydrothermally altered flanks. This combined approach allows us to explore the relationships between hydrothermal alteration, rock strength, and flank instability of La Fossa.

Volcanoes are dynamic and complex natural systems, constantly changing through eruptions, alteration, and erosion. Hydrothermal systems are ubiquitous on volcanoes, causing physical and mechanical change to rock properties via hydrothermal alteration. The most commonly measured rock physical properties are porosity and uniaxial compressive strength (UCS) as they provide insight as to their history and potential mechanical behavior. Porosity and UCS are affected by the primary properties and emplacement history (e.g., volatile content, crystallinity, cooling rate, composition, fragmentation type) and the post-emplacement conditions (e.g., surface weathering and hydrothermal alteration). This creates highly heterogenous rock masses, with variation occurring across mm to m scales. Currently, destructive testing is required to measure UCS and porosity (destructive of the wider sample) accurately and needs a large volume of samples to capture the heterogeneity of volcanic rock masses. This testing is cost and time prohibitive, requiring large sample volume, in terms of sample size and number, which often require shipping to specialist labs. Schmidt hammers can be used to estimate UCS non-destructively, however, they produce inaccurate results on soft rocks such as hydrothermally altered rocks. Here, we present a new non-destructive method for predicting porosity and UCS across a range of volcanic rocks, from non-altered to highly altered. This study uses visible-near infrared (VNIR) to shortwave infrared (SWIR) wavelengths (350-2500 nm) reflectance spectroscopy to predict porosity and UCS via Partial Least Squares Regression (PLSR). Reflectance spectroscopy is a non-destructive method that is sensitive to both physical (surface roughness and crystal/particle size) and chemical (mineral species and abundance) properties of volcanic rocks. Because these rock attributes also influence the physical and mechanical properties of rock, reflectance spectroscopy could be used to quantitatively predict porosity and UCS. This study used experimentally deformed volcanic rocks from Ruapehu, Ohakuri, Whakaari, and Banks Peninsula (New Zealand), Merapi (Indonesia), Chaos Crags (USA), Styrian Basin (Austria), La Soufrière de Guadeloupe (Eastern Caribbean), Volvic (France), and Cracked Mountain (Canada) to evaluate the accuracy of PLSR-based predictions for porosity and UCS. The training samples encompass a wide range of volcanoes, alteration degree (non-altered, silicic, argillic, and phyllic alteration), mineralogical differences (initial composition from basalt to rhyolite and alteration products), and textural differences (original textures such as lava and pyroclastic, and alteration textures including veins). Model sensitivity is evaluated by adding randomly individual samples to the training database or performing leave one group out cross validation based on characteristics (e.g., alteration mineral types, textural features, or volcano location). From this analysis, specific alteration mineralogy can be evaluated for its effect on porosity and UCS predictions such as the role of phyllosilicate formation causing a reduction in UCS. The proposed non-destructive method via VNIR-SWIR spectroscopy can complement existing rock mechanical testing methods to better quantify highly heterogenous volcanic and hydrothermal systems and their rock successions.

A non-proportional coordinated strength reduction method is proposed to accurately capture the degradation of mechanical and deformation parameters in water-weakened rock slopes, effectively reflecting the observed weakening behavior of rocks under the influence of water. The proposed method is applied to strip-mining operations at the Huolinhe open-pit coal mine in China to investigate slope displacement and stability, establishing the relationships between excavation width, slope displacement, and stability. Laboratory rock experiments provide the parameters for the proposed strength reduction method, while site excavation data inform the numerical simulations. The failure process of strip-mining slopes under water-weakening conditions is categorized into four distinct stages. Numerical simulations demonstrate a nonlinear decrease in the safety factor for upper arching slope failure as excavation width increases. The arching failure height in the upper slope area follows a nonlinear trend, initially decreasing and then increasing as the excavation width increases. Furthermore, a displacement curve illustrating slope instability due to water weakening is obtained through numerical simulation. The research findings provide important insights into the displacement patterns and failure mechanisms of slopes influenced by water, providing valuable guidance for engineering practice and the formulation of effective risk mitigation strategies.

A more comprehensive understanding of the progressive, time-dependent deformation and fracturing of brittle rock is crucial for assessing the long-term integrity of rock masses surrounding engineering structures. In this study, we propose a three-dimensional numerical model that integrates the microplane model and subcritical crack growth to investigate the progressive, time-dependent deformation and fracturing of brittle rock. The model incorporates subcritical crack growth and time-dependent damage evolution constitutive laws into the microplanes. By following the trend of subcritical crack growth observed in previous studies, the model accurately captures the time-dependent propagation of virtual cracks. The cooperative interaction between strain and damage evolution on the microplanes ultimately leads to localized material degeneration over extended time. Moreover, this model effectively characterizes the temporal and spatial distribution of damaged elements during time-dependent deformation and fracturing of brittle rock. The numerical simulations successfully replicate phenomena observed in laboratory experiments performed on brittle rock. Specifically, they demonstrate how different stress levels influence creep strain rate and time-to-failure. Additionally, the simulations reveal that the microscale interaction of potential cracks (microplanes) can effectively describe the complex macroscopic time-dependent behavior of brittle rock. As a result, it becomes possible to predict time-to-failure and rupture patterns using the calibrated model based on laboratory tests. The proposed numerical model holds the potential to be further extended for predicting the long-term stability of larger rock masses.

Calderas are steep morphological and collapse basins that continue to reshape long after initial structural collapse. While large landslides are associated with caldera collapse and widen the basin, little is known about the morphological and structural changes that occur long after caldera formation. Here, we investigate the shape and slope of the Tambora caldera in Indonesia, which formed in 1815 during one of the most devastating eruptions of the past centuries. The release of over 150 cubic kilometers of volcanic ash created a caldera 6 kilometers wide and 1250 meters deep, causing climatic effects worldwide. Here we explore an ultra-high-resolution dataset we generated from Pleiades, a tri-stereo satellite, that now allows us to apply computer vision approaches to study the morphology and geometry of the Tambora caldera. We generated a 12 million pixel point cloud resampled to a 1 m resolution Digital Elevation Model and a 0.5 m orthomosaic. We explore the dimension, slope, and outline of the caldera and find localized open fissures, tension cracks, and morphological scars. We also apply an unsupervised image classification approach to the stereo multispectral data and find locations of fumarole activity and hydrothermal alteration in close proximity to these structural features. Hydrothermal alteration sites are commonly located in the caldera wall below the scars and open fissures. We explore this proximity of alteration, scarring, and faulting using newDistinct Element Method models, emphasizing that caldera morphology and structure is strongly influenced by hydrothermal weakening that causes flank instability, localized shedding of material, and large-scale morphological changes.

Volcanic and magmatic outgassing mechanisms can determine eruptive behavior of shallow silicic magma bodies. Most outgassing mechanisms proposed take place along conduit margins, where the highest strain rates drive ascending magma to brittle failure. However, these mechanisms do not account for outgassing large volumes of magma away from the conduit walls. Here, we present a continuum of porosity preserved in the microcrystalline rhyolitic Sandfell laccolith, Eastern Iceland. Three stages in the continuum are described: porous flow bands, pore channels, and fracture bands. These deformation features are present throughout the entire exposed volume of the Sandfell laccolith in meter-long band geometries, ranging from mm- to dm-scale thickness, and interlayered with coherent, undeformed rhyolite. Using microstructural analytical methods and drawing on the result of previous experimental studies, we show that emplacement-related deformation induced strain partitioning around a crystal content of 45 % that resulted in the segregation of melt-rich and melt-poorer flow bands. Subsequent deformation induced by continued magma emplacement caused strain partitioning in the melt-rich flow bands. Depending on strain rate, different types of deformation features developed, through dilation or porosity redistribution (porous flow bands), cavitation (pore channels), or tensile fracture (fracture bands). Porous flow bands have permeability values similar to 4 orders of magnitude higher than undeformed rhyolite. Pore channels and fracture bands have much larger length scales, and so permeability increases dramatically in those systems. Hence, the abundance and interconnectivity of deformation features preserved in the Sandfell laccolith provided an efficient outgassing mechanism for the bulk of the intrusion. Outgassing due to viscous-brittle magma deformation during magma emplacement should therefore be considered for crystal-rich magmas, e.g., during effusive lava dome extrusion.

Muography is a non-invasive geophysical method that relies on the detection of muons, which are subatomic particles generated by the interaction of cosmic rays with the Earth's atmosphere. The physical quantity estimated by this method is the opacity, which represents the amount of matter traversed by muons along their trajectories, resulting in energy loss and scattering for the particles. Thus, absorption muography consists of deploying a muon detector targeting the volcano and registering the muons traversing it per unit of time and trajectory. From these data, radiographs of average density of extensive rock volumes can be obtained using a single measuring instrument and from a singular measurement position. Copahue volcano is located in the Andes mountain range and is considered the highest-risk volcano in Argentina due to its proximity to two towns situated within an 8 km radius of the volcano's crater. Additionally, the region attracts a significant number of tourists, leading to a substantial increase in the population of both localities. The latest eruptive cycle, initiated in 2012, has maintained a near-continuous state of activity, marked by ash emissions, crater explosions, and seismic activity. In this work, we study the hydrothermal alteration at Copahue volcano through a combination of muography and laboratory measurements of the chemical and physical properties of rock samples. The muography dataset was acquired by installing a muon detector on the eastern flank of Copahue volcano, situated at an altitude of approximately 2500 meters above sea level. For the laboratory analyses, we collected rock blocks with the objective of representing a diverse spectrum of alteration stages within Copahue volcano. Through this selection process, we captured variations in mineralogical composition, geochemical signatures, and physical properties that correspond to different stages of hydrothermal alteration. We carried out a series of examinations on the rock samples extracted from the targeted flank, such as X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS) analyses that identified mineralogical compositions and geochemical signatures associated with hydrothermal processes. We also carried out additional measurements, including density, porosity, permeability, thermal properties, and uniaxial compressive strength, contributing to a comprehensive understanding of the physical properties of the samples. In addition, we performed microscopic examinations using a scanning electron microscope (SEM) to study the microstructural changes induced by hydrothermal alteration. This integrative approach, between muography and detailed laboratory measurements on rock samples, aims to reveal correlations between subsurface density variations and hydrothermal alteration observed at the microscopic and macroscopic scales.

Volcano unrest associated to ascent of magmatic fluids (magma, brines, gases) at shallow depths in the presence of a very active hydrothermal system, can promote or enhance extensive hydrothermal rock alteration and form fragile discontinuities within the edifice. A process that can favour flank instability and culminate in partial flank collapse, engendering significant risks to the surrounding population. In the MYGALE ANR project, we focus on hydrothermal alteration timescales of andesitic rocks to better assess the hazard of volcano flank instability at La Soufrière de Guadeloupe (Eastern Caribbean, France). The conditions and kinetics of hydrothermal alteration reactions of the volcanic rocks of La Soufrière lava dome are determined by three approaches: mineralogical, experimental, and by modelling. Firstly, we characterized the natural alteration sequence of 20 samples from the lava dome and lava flows showing different degrees of alteration and porosity. SEM and XRD analyses of the samples show that the plagioclases are replaced by secondary minerals such as kaolinite, natroalunite, and amorphous silica. Secondly, we performed time-series fluid flow-through experiments, in which fluids are circulated through a pristine and porous andesite core (representative of the unaltered state of the present-day lava dome at La Soufrière). We varied temperature (200-250 °C), pressure (100-150 bar), duration (from days to months), and fluid composition (H2O-HCl mixtures). Rock permeability is measured in-situ and the mineralogical changes are characterized by post-experiment using various methods (SEM, EDS, EMPA, X-ray microtomography, and XRD). A sharp decrease in permeability of four orders of magnitude (from 10-14 to 10-18 m2) during the first 3-6 days was observed when using pure water as the percolating pore fluid. The cause of this large and fast decrease in permeability is currently being investigated. Experiments using H2O-HCl fluids are in progress attempting to reproduce the alteration sequence of the natural samples. Thirdly, we performed thermodynamic and kinetic numerical modelling using Perple_X, Phreeqc, and GEM-Selektor codes, in order to better constrain the conditions (especially the fluid phase composition) for hydrothermal alteration and to explore the alteration kinetics for timescales longer than those attained experimentally.

Catastrophic lava dome collapse is considered an unpredictable volcanic hazard because the physical properties, stress conditions, and internal structure of lava domes are not well understood. To better explain the locations of recent dome instability events at Merapi volcano, Indonesia (1), we combined geochemical and mineralogical analyses, rock physical property measurements, drone-based photogrammetry, and numerical modelling. We show that a linear fissure and a horseshoe-shaped alteration zone that formed in 2014 was buried by lava extrusion in 2018. The linear fissure controlled the location of the new lava dome, while the horseshoe shaped zone influenced subsequent instability. Geomechanical, mineralogical, and geochemical data suggest that such alteration zones are characterised by mechanically weak, hydrothermally altered materials, and we show that the new lava dome is collapsing along this now-hidden horseshoe shaped and comparatively weak alteration zone (2). To derive an improved general understanding of this phenomenon, we then combined recent laboratory data for the mechanical behaviour of dome rocks with discrete element method models to show that the presence of weak zones within lava domes increases instability, which is exacerbated when the size of the zone increases or when the zone is positioned off-centre (3). Our results highlight that improved understanding of dome architecture and compositional variations due to hydrothermal alteration within domes is essential for assessing hazards associated with dome and edifice failure at volcanoes worldwide.

Understanding the influence of sedimentary processes on reservoir architectures can be essential for improving the prediction of permeability heterogeneity. The Upper Rhine Graben offers an ideal geological context for using outcrops as reservoir analogues, as the rocks cropping out at the Graben's shoulders belong to the same formations that host lithium-rich geothermal brine in the Graben. The interval of interest in this study, the upper section of the Lower Gre`s Vosgien Formation (LGV), consists of fluvio-aeolian deposits considered preferential zones for fluid migration within the Buntsandstein Group. In this study, the main factors controlling permeability heterogeneity distribution of these fluvio-aeolian deposits are presented, in different scales, by combining permeability and petrographic characteristics with quantitative sedimentological characterisation of the LGV. The fluvial channel facies association (FA), predominantly composed of cross-bedded sandstones deposited in braided channels, exhibits higher permeability compared to the wind- and water-laid FA, made up of sandstones deposited in a sand-sheet-dominated aeolian system. Vertical profiles reveal permeability contrasts of up to four orders of magnitude between these FAs. At the facies scale, the fluvial channel FA shows a distinction in permeability associated with facies formed under different flow-regime conditions. Trough cross-bedded sandstones, formed under lower flow-regime conditions, exhibit higher permeability than low-angle cross-bedded sandstones, formed mainly under upper flow-regime conditions. In the wind- and water-laid FA, permeability distribution is directly influenced by palaeoclimatic variations. Low-permeability hybrid sand sheets (HSS) were deposited under relatively humid conditions, while high-permeability aeolian dunes (AD) formed during more arid periods. Despite the high permeability of the AD, the architecture of the wind- and water-laid FA suggests that the AD have low connectivity potential. The lateral continuity of the wind- and water-laid FA, extending over hundreds of metres, combined with the dominant occurrence of low-permeability HSS, indicates that this facies association may act as flow baffles or barriers. Compaction is the main process influencing the permeability and porosity of the LGV. Samples with a higher proportion of lithoclasts and infiltrated clay recorded a more advanced degree of compaction. In contrast, samples with a higher percentage of quartz overgrowth were less compacted. The diagenetic overprint affected the distinct facies differently, indicating that primary sedimentary processes and architecture govern the distribution of permeability heterogeneity in the LGV. Permeability data from outcrops exhibited median values up to three orders of magnitude higher than those from subsurface data. However, the same facies and similar trends of relative porosity-permeability responses to the distinct sedimentary features are observed in both outcrops and borehole samples. This highlights sedimentology an essential resource for reservoir heterogeneity studies.

The modeling of crack growth in three-dimensional (3D) space poses significant challenges in rock mechanics due to the complex numerical computation involved in simulating crack propagation and interaction in rock materials. In this study, we present a novel approach that introduces a 3D numerical manifold method (3D-NMM) with a geometric kernel to enhance computational efficiency. Specifically, the maximum tensile stress criterion is adopted as a crack growth criterion to achieve strong discontinuous crack growth, and a local crack tracking algorithm and an angle correction technique are incorporated to address minor limitations of the algorithm in a 3D model. The implementation of the program is carried out in Python, using object-oriented programming in two independent modules: a calculation module and a crack module. Furthermore, we propose feasible improvements to enhance the performance of the algorithm. Finally, we demonstrate the feasibility and effectiveness of the enhanced algorithm in the 3D-NMM using four numerical examples. This study establishes the potential of the 3D-NMM, combined with the local tracking algorithm, for accurately modeling 3D crack propagation in brittle rock materials.

The ascent and advance of volcanic dome lava is non-linear and viscoelastic. There exists a mismatch between current theoretical approaches to dome lava rheology, which are based on rheological laws for viscous suspensions, and empirical experimental approaches to convolved viscous-brittle deformation, which show mixed evidence for simultaneous lava flow and fracturing. The missing requirement is a unified framework for understanding the transition between micro-mechanical flow mechanisms that are dominantly viscous, and those that include micro-cracking in multiphase suspensions such as magmas. Here, we use high-temperature compression rheology with sample-scale acoustic emission analysis to constrain the conditions under which crystal-rich volcanic dome lava can flow by mixed viscous and brittle fracturing processes at small scales, leading to 'crackling' acoustic signals, even at moderate shear stresses extant in nature. Using multi-directional permeability measurements on large 60 mm diameter quenched samples of natural magmas, we show that this micro-cracking flow mechanism leads to permeability anisotropy, localizing outgassing into pathways that are off-axis relative to the direction of flow. Finally, we use a scaling approach and a database of published observations from real eruptions to upscale our findings, and show that bulk, apparently ductile flow of lowporosity dome magma is likely to involve a local mixed-mode of micro-cracking and viscous flow during the shallowest portions of ascent and during emplacement on the Earth's surface. The micro-cracking involved in lava advance divorces real crystal-bearing lava emplacement from most current rheology models based on a purely viscous micro-mechanism and shows that a revised solution for the rheology of mixed brittle-viscous flow is required. By re-examining published numerical models for dome emplacement, we demonstrate that the viscous-brittle transition can be intercepted in spatially heterogeneous zones within the dome core.

ContextAs part of the Deep-HEAT-Flows project (https://deep-heat-flows.voog.com), we have collected a comprehensive geological and petrophysical dataset of crystalline reservoirs formed within fault zones and at the contact of igneous intrusions across Finland, evaluating their potential as deep geothermal reservoirs. Our investigations involve a range of laboratory-based experiments encompassing measurements of rock density, elastic wave velocity, electric resistivity, porosity, and permeability under various confining pressures, and the thermal properties of 120+ samples collected from diverse crystalline rocks. Additionally, we apply mineral and pore space caracterization techniques including petrography, micro-XRF spectrometry, SEM-EDS, hyperspectral imaging, and CT scans to understand the processes that control crystalline reservoir formation.FindingsOur findings highlight a common trend among various petrophysiscal parameters: rock density, resistivity, elastic wave velocity, thermal conductivity, and heat capacity typically reduce as the porosity increases, a characteristic observed across many sedimentary and volcanic rocks. Reservoir quality is primarily determined by the morphology of the pore network, encompassing fractures and interconnected moldic, sieve, and interparticle pores. The most promising reservoir properties were observed in rocks intersected by regional shear zones and therefore affected by intense brecciation, cataclasis, and hydrothermal alteration, leading to a notable porosity of ~20% and permeability in the order of 10−12 m2 (1 darcy). Moreover, the contact margin of rapakivi intrusions also include fractured and hydrothermally altered rocks that have significantly high porosity and permeability. In detail, rocks dominated by fractures typically have little porosity (

Deception Island is a composite volcano located in the Bransfield Strait, between the South Shetland Islands and the Antarctic Peninsula. The volcano is defined by a caldera-forming event, and the rocks forming the island-which range in composition from basalts to trachydacites-rhyolites-are typically classified as either pre-, syn-, or post-caldera. Here, we provide petrophysical properties (bulk density, porosity, P-wave velocity, permeability, thermal properties, Young's modulus, and uniaxial compressive strength) for representative lapilli tuffs (pyroclastic density current and fall deposits) from the pre-, syn-, and post-caldera volcanic activity. We find that dry bulk density varies from similar to 982 to similar to 1813 kgm(-3), connected porosity varies from 0.30 to 0.62, P-wave velocity varies from similar to 0.3 to similar to 1 kms(-1), permeability varies from similar to 10(-15) to similar to 10(-11) m(2), thermal conductivity varies from similar to 0.3 to similar to 0.65 Wm(-1)K-1, thermal diffusivity varies from similar to 0.35 to similar to 0.25 mm(2)s(-1), specific heat capacity varies from similar to 0.8 to similar to 1.3 Jkg(-1)K-1, Young's modulus varies from similar to 1 to similar to 9 GPa, and that uniaxial compressive strength varies from similar to 1 to similar to 25 MPa. Our data show that P-wave velocity, Young's modulus, uniaxial compressive strength, thermal conductivity, and thermal diffusivity decrease, permeability increases, and specific heat capacity does not change systematically as a function of increasing porosity. We also find that the fall tuffs are more porous than the pyroclastic density current tuffs, and therefore have a lower P-wave velocity, Young's modulus, uniaxial compressive strength, thermal conductivity, and thermal diffusivity, but a higher permeability. Our data expose the heterogeneity of the petrophysical properties of the lapilli tuffs at Deception Island, and are in good agreement with those for similar lapilli tuffs from Surtsey volcano (Iceland) and Cracked Mountain (Canada). Microscale models for permeability, thermal conductivity, and uniaxial compressive strength provide insight into the microscale factors controlling the petrophysical properties of the lapilli tuffs, and can be used to help predict their petrophysical properties when data are absent or laboratory experiments are not possible. Large-scale hydromechanical models that use our laboratory data provide the range of expected surface displacement at Deception Island following fluid injection at depth, and highlight the importance of choosing appropriate rock property input parameters for volcano modelling.

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.

Hydrothermal alteration gradually and imperceptibly changes the chemical and physical state of the rocks inside a volcano, creating a soft and unstable (or 'rotten') interior. However, the link between 'soft' volcanoes and unpredictable volcanic events remains poorly resolved. ROTTnROCK is a 6-year ERC project, running from 2024-2030, funded through the European Research Council's Synergy Call. The growing ROTTnROCK team will investigate the role of hydrothermal alteration in unpredictable volcanic hazards, such as volcanic instability, alteration-induced eruption triggering, and caldera fault (re-)activation. Specifically, we will use remote sensing and geophysics to identify where and at what scales alteration is occurring. Laboratory investigations will study the chemical fingerprint of alteration and its effects on rock mechanical properties and strength. These approaches will be coupled with 4D volcano stability simulations to produce an innovative and optimised hazard assessment workflow. We will work at selected target sites that show evidence of strong hydrothermal alteration, either associated with flank collapse, collapsing lava domes, crater lakes, or active collapse calderas. This project will transform our understanding of hydrothermal alteration and its impact on volcanic hazards, and will pave the way for strategies to predict and mitigate unexpected volcanic events caused by hydrothermal alteration.

Fractures are diverse geological features. Despite extensive research on their varied geometries and growth mechanisms, there has been relatively little focus on how acid–rock interactions, such as reactive transport and precipitation, influence crack growth. To understand the chemical corrosion of sandstone exclusively from chemical reaction, we have developed a chemical corrosion model that highlights that the dissolution of calcite grains within the sandstone, coupled with the diffusion of Ca2+ ions and precipitation of calcite, contributes to time-dependent crack formation and their subsequent filling. Based on this theory, we propose a heterogeneous grain-based phase-field method (PFM) model to analyze the failure pattern and changes in ion behavior in sandstone. Our model results are in good agreement with experimental data, validating the proposed chemical corrosion theory and the grain-based PFM model. Both numerical and experimental results reveal that the chemical corrosion of sandstone is a time-dependent deterioration process that progresses from the exterior to the interior of the sample. The cracks that form act as pathways for ion transport, leading to a gradual decrease of Ca2+ ions with increasing distance from the surface of calcite grains. Following the validation of our approach, we used the heterogeneous grain-based PFM model to analyze the effect of rock heterogeneity and to simulate chemical corrosion induced by calcite grains within a sandstone sample. The numerical results reveal that a lower homogeneity index leads to a larger damaged area and accelerates crack initiation. Additionally, we identify and categorize the acid–rock interactions into three distinct stages: initial surface erosion, subsequent crack propagation, and further deepening of etched features. The proposed chemical corrosion theory enhances our understanding of the degradation and fracture mechanics of sandstone in an acidic environment, and can be further extended to elucidate the sealing of hydraulic fractures and formation and dissolution of calcite veins.

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.

The thermal properties of lavas are required for modelling volcanic and hydrothermal processes, yet are scarce for submarine lavas. Laboratory experiments, using the transient hot-strip method, are used to understand the role of porosity, glass content, and pore fluid type on the thermal properties of submarine lavas from Havre volcano. Thermal diffusivity and specific heat capacity do not change systematically with porosity; however, thermal conductivity decreases as porosity increases. The thermal conductivity of the submarine lavas is lower than for subaerial lavas with the same porosity, a consequence of their higher glass contents. We show that, using our data and effective medium models, the thermal properties of any lava can be estimated as long as their porosity, glass content, and void-filling fluid phase is known. This approach can be used to estimate properties for multiphase models for submarine eruption dynamics, hydrothermal system hydrology, cooling timescales, and heat flux calculations.