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The geomechanical characterization of volcanic material has important implications for geothermal and mineral exploration, engineering design, geophysical signals of volcano unrest, and models of instability and mass flows. Chemical weathering and hydrothermal systems can alter the host rock, leading to changes in mechanical behavior and failure mode. Here, we compare the physical and mechanical properties of lava, autoclastic breccia, and pyroclastic (scoria) rocks from Mount Ruapehu volcano (Ruapehu) in New Zealand to mineralogical composition determined via infrared spectroscopy and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). We use correlation matrices, principal component analysis, and parametric analysis to determine which parameters can be used to predict physical and mechanical properties and form the basis for transfer functions. Laboratory-based spectroscopy shows that the samples contain absorption features indicative of Al- and Mg-rich hydrous phyllosilicates (e.g., kaolinite, halloysite, montmorillonite), Fe- oxides (e.g., goethite), and sulfates attributed to surface weathering, supergene, and steam-heated alteration. We find that porosity and primary lithology are the predominant control on physical and mechanical properties, followed by the pervasiveness of weathering/alteration, and then mineralogical composition. Several properties, such as porosity, uniaxial compressive strength, P-wave seismic velocity, density, and Young's modulus, show strong correlations with other properties, indicating the potential for transfer functions between these properties. Hydrothermally altered rocks near the vent complex (up to ~ 400 m depth beneath the crater lake) with high-intensity hydrothermal alteration do not follow typical physical and mechanical property trends due to high clay content, low permeability, and low strength. The presence of these rocks within the edifice at Ruapehu implies local barriers to fluid flow and subsequent pore pressure variations. Additionally, they may have less than half the strength than would be dictated by typical porosity-strength trends for surface rocks, increasing the likelihood of structural failure. Trends in the pervasiveness of weathering with physical and mechanical properties, along with shifts in the position of spectral absorption peaks as hydrothermal/weathering alteration increases, suggest that it may be possible to extrapolate properties from imaging spectroscopy. [ABSTRACT FROM AUTHOR]

Composite volcanoes can progressively weaken through hydrothermal alteration, which may lead to volcano collapse, forming far-reaching debris avalanches. This work reviews and synthesises the type and extent of hydrothermal alteration on Mt. Ruapehu, New Zealand, from rock mechanical, mineralogical, hyperspectral imaging and aero-magnetic studies for input into the first debris avalanche models of Mt. Ruapehu. Mt. Ruapehu shows surface weathering processes, forming goethite, hematite and phyllosilicate assemblages, and hydrothermally altered rock, with phyllosilicate, Fe- oxides and sulphates, pyrite, jarosite, alunite, anhydrite and sulphur. The surface alteration mapped by hyperspectral remote sensing is linked to the deep-seated (≤500 m) alteration by aero-magnetic inversion modelling. Textural analysis and mapping show that alteration weathering and porosity distribution are a result of the both the primary lithology, age, and location relative to active or fossil hydrothermal systems. The synthesis of the aero-magnetic data, previous magnetotelluric models, hyperspectral and hydrothermal mapping, and rock mechanics studies enabled the construction of a schematic model to provide inputs for modelling potential flank failure zones. These models suggest that the southwest and west flanks of Mt. Ruapehu have the highest probability of collapsing and generating debris avalanches. [ABSTRACT FROM AUTHOR]

Highlights • Airborne hyperspectral imagery was integrated with topographic information from LiDAR for surface geological mapping. • Hyperspectral imaging can provide insights into depositional processes. • Hydrothermal alteration and debris flow source zones can be located using texture-enabled hyperspectral imagery. Abstract Optical and laser remote sensing provide resources for monitoring volcanic activity and surface hydrothermal alteration. In particular, multispectral and hyperspectral imaging can be used for detecting lithologies and mineral alterations on the surface of actively degassing volcanoes. This paper proposes a novel workflow to integrate existing optical and laser remote sensing data for geological mapping after the 2012 Te Maari eruptions (Tongariro Volcanic Complex, New Zealand). The image classification is based on layer-stacking of image features (optical and textural) generated from high-resolution airborne hyperspectral imagery, Light Detection and Ranging data (LiDAR) derived terrain models, and aerial photography. The images were classified using a Random Forest algorithm where input images were added from multiple sensors. Maximum image classification accuracy (overall accuracy = 85%) was achieved by adding textural information (e.g. mean, homogeneity and entropy) to the hyperspectral and LiDAR data. This workflow returned a total surface alteration area of ∼0.4 km2 at Te Maari, which was confirmed by field work, lab-spectroscopy and backscatter electron imaging. Hydrothermal alteration on volcanoes forms precipitation crusts on the surface that can mislead image classification. Therefore, we also applied spectral matching algorithms to discriminate between fresh, crust altered, and completely altered volcanic rocks. This workflow confidently recognized areas with only surface alteration, establishing a new tool for mapping structurally controlled hydrothermal alteration, evolving debris flow and hydrothermal eruption hazards. We show that data fusion of remotely sensed data can be automated to map volcanoes and significantly benefit the understanding of volcanic processes and their hazards. [ABSTRACT FROM AUTHOR]

• Silicification reduces matrix permeability by up to four orders of magnitude. • Alteration to clay only reduces matrix permeability by up to an order of magnitude. • Only silicification increases propensity for permeability enhancing fracture formation. • Silicified rock-masses (reservoir) are more permeable than those altered to clay (cap). • We show how alteration creates geothermal and epithermal resources from a petrophysical standpoint. Hydrothermal fluids can alter the chemical and physical properties of the materials through which they pass and can therefore modify the efficiency of fluid circulation. The role of hydrothermal alteration in the development of geothermal and epithermal mineral resources, systems that require the efficient hydrothermal circulation provided by fracture networks, is investigated here from a petrophysical standpoint using samples collected from a well exposed and variably altered palaeo-hydrothermal system hosted in the Ohakuri ignimbrite deposit in the Taupō Volcanic Zone (New Zealand). Our new laboratory data show that, although quartz and adularia precipitation reduces matrix porosity and permeability, it increases the uniaxial compressive strength, Young's modulus, and propensity for brittle behaviour. The fractures formed in highly altered rocks containing quartz and adularia are also more planar than those formed in their less altered counterparts. All of these factors combine to enhance the likelihood that a silicified rock-mass will host permeability-enhancing fractures. Indeed, the highly altered silicified rocks of the Ohakuri ignimbrite deposit are much more fractured than less altered outcrops. By contrast, smectite alteration at the margins of the hydrothermal system does not significantly increase strength or Young's modulus, or significantly decrease permeability, and creates a relatively unfractured rock-mass. Using our new laboratory data, we provide permeability modelling that shows that the equivalent permeability of a silicified rock-mass will be higher than that of a less altered rock-mass or a rock-mass characterised by smectite alteration, the latter of which provides a low-permeability cap required for an economically viable hydrothermal resource. Our new data show, using a petrophysical approach, how hydrothermal alteration can produce rock-masses that are both suitable for geothermal energy exploitation (high-permeability reservoir and low-permeability cap) and more likely to host high-grade epithermal mineral veins, such as gold and silver (localised fluid flow). [ABSTRACT FROM AUTHOR]

Steam-driven eruptions are caused by explosive vaporization of water within the pores and cracks of a host rock, mainly within geothermal or volcanic terrains. Ground or surface water can be heated and pressurized rapidly from below (phreatic explosions), or already hot and pressurized fluids in hydrothermal systems may decompress when host rocks or seals fail (hydrothermal eruptions). Deposit characteristics and crater morphology can be used in combination with knowledge of host-rock lithology to reconstruct the locus, dynamics, and possible triggers of these events. We investigated a complex field of >30 craters formed over three separate episodes of steam-driven eruptions at Lake Okaro within the Taupo volcanic zone, New Zealand. Fresh unaltered rock excavated from initially >70 m depths in the base of phase I breccia deposits showed that eruptions were deep, "bottomup" explosions formed in the absence of a preexisting hydrothermal system. These phreatic explosions were likely triggered by sudden rise of magmatic fluids/gas to heat groundwater within an ignimbrite 70 m below the surface. Excavation of a linear set of craters and associated fracture development, along with continued heat input, caused posteruptive establishment of a large hydrothermal system within shallow, weakly compacted, and unconsolidated deposits, including the phase I breccia. After enough time for extensive hydrothermal alteration, erosion, and external sediment influx into the area, phase II occurred, possibly triggered by an earthquake or hydrological disruption to a geothermal system. Phase II produced a second network of craters into weakly compacted, altered, and pumice-rich tuff, as well as within deposits from phase I. Phase II breccias display vertical variation in lithology that reflects top-down excavation from shallow levels (10-20 m) to >70 m. After another hiatus, lake levels rose. Phase III hydrothermal explosions were later triggered by a sudden lake-level drop, excavating into deposits from previous eruptions. This case shows that once a hydrothermal system is established, repeated highly hazardous hydrothermal eruptions may follow that are as large as initial phreatic events. [ABSTRACT FROM AUTHOR]

Volcano slope stability analysis is a critical component of volcanic hazard assessments and monitoring. However, traditional methods for assessing rock strength require physical samples of rock which may be difficult to obtain or characterize in bulk. Here, visible to shortwave infrared (350–2500 nm; VNIR–SWIR) reflected light spectroscopy on laboratory-tested rock samples from Ruapehu, Ohakuri, Whakaari, and Banks Peninsula (New Zealand), Merapi (Indonesia), Chaos Crags (USA), Styrian Basin (Austria) and La Soufrière de Guadeloupe (Eastern Caribbean) volcanoes was used to design a novel rapid chemometric-based method to estimate uniaxial compressive strength (UCS) and porosity. Our Partial Least Squares Regression models return moderate accuracies for both UCS and porosity, with R2 of 0.43–0.49 and Mean Absolute Percentage Error (MAPE) of 0.2–0.4. When laboratory-measured porosity is included with spectral data, UCS prediction reaches an R2 of 0.82 and MAPE of 0.11. Our models highlight that the observed changes in the UCS are coupled with subtle mineralogical changes due to hydrothermal alteration at wavelengths of 360–438, 532–597, 1405–1455, 2179–2272, 2332–2386, and 2460–2490 nm. These mineralogical changes include mineral replacement, precipitation hydrothermal alteration processes which impact the strength of volcanic rocks, such as mineral replacement, precipitation, and/or silicification. Our approach highlights that spectroscopy can provide a first order assessment of rock strength and/or porosity or be used to complement laboratory porosity-based predictive models. VNIR-SWIR spectroscopy therefore provides an accurate non-destructive way of assessing rock strength and alteration mineralogy, even from remote sensing platforms. • VNIR-SWIR reflectance spectroscopy can predict rock mechanical properties (e.g., UCS with R2 of 0.43-0.82). • Spectroscopy-based UCS predictions are most accurate when combined with measured porosity. • Mineralogical changes and metal cation substitution exert a strong control on UCS and porosity. • Spectroscopy is an appropriate new tool for assessing in-situ volcanic rock masses. [ABSTRACT FROM AUTHOR]

Diagnostic absorption features in hyperspectral data can be used to identify a specific mineral or mineral associations. However, it is unknown how accurate hyperspectral mapping can be for identifying alteration mineral compositions at the resolution required to describe structures such as fossil intrusions, or whether it can accurately quantify the alteration present. This study compared petrographic observation with visible, near-infrared (VNIR), and shortwave infrared (SWIR) hyperspectral remote sensing at laboratory- (centimetre-scale) and aerial- (metre-scale) scales to characterise the abundance of surface hydrothermal rock alteration in and around a shallow fossil intrusion on Pinnacle Ridge, Mt. Ruapehu, New Zealand. We classified a high-resolution aerial hyperspectral image to develop a new surface alteration map using Spectral Angle Mapper (SAM) algorithm. The petrographic thin-section and the laboratory and aerial hyperspectral imaging revealed a spectrum of hydrous alteration phases as indicated by the presence of an absorption feature at 2207 nm. Moderate correlation exists between the depth of the absorption feature at 2207 nm and the point counting-derived alteration percent values, indicating reliability of laboratory-based hyperspectral analytical methods. In contrast, aerial hyperspectral data failed to provide any clear correlations to field-mapped alteration using a band-depth approach, and we interpret this due to 'oversampling' of surface (supergene) alteration, spectral mixing, and sensor limitations (e.g., bandwidth, signal-to-noise ratio). The hyperspectral image-derived alteration map, created using supervised image classification, can loosely be translated to a geotechnical map where porosity and permeability play a major role in localizing hydrothermal fluid flow and the formation of alteration mineral associations. • Laboratory SWIR hyperspectral scanning accurately quantified hydrothermal alteration. • Airborne hyperspectral imaging can detect presence of hydrothermal alteration minerals. • Intrusion related hydrothermal alteration was mapped using Spectral Angle Mapper. [ABSTRACT FROM AUTHOR]

Vigorous hydrothermal activity interspersed by sequences of phreatic and phreatomagmatic eruptions occur at Whakaari (White Island volcano), New Zealand. Here, we investigate the influence of sample type (hydrothermally altered cemented ash tuffs and unconsolidated ash/lapilli) and fragmentation mechanism (steam flashing versus gas expansion) on fragmentation and ejection velocities as well as on particle-size and shape. Our rapid decompression experiments show that fragmentation and ejection speeds of two ash tuffs, cemented by alunite and amorphous opal, increase with increasing porosity and that both are significantly enhanced in the presence of steam flashing. Ejection speeds of unconsolidated samples are higher than ejection speeds of cemented tuffs, as less energy is consumed by fragmentation. Fragmentation dominated by steam flashing results in increased fragmentation energy and a higher proportion of fine particles. Particle shape analyses before and after fragmentation reveal that both steam flashing and pure gas expansion produce platy or bladed particles from fracturing parallel to the decompression front. Neither fragmentation mechanisms nor sample type show a significant influence on the shape. Our results emphasize that, under identical pressure and temperature conditions, eruptions accompanied by the process of liquid water flashing to steam are significantly more violent than those driven simply by gas expansion. Therefore, phase changes during decompression and cementation are both important considerations for hazard assessment and modeling of eruptions in hydrothermally active environments. [ABSTRACT FROM AUTHOR]

There are currently no reliable methods to determine rock physical and mechanical properties that are not labor or resource intensive, especially at the scale of volcanoes. Using mineralogical-physical-mechanical relationships, we suggest it is possible to derive rock properties from rapid, non-invasive reflectance spectroscopy measurements. To demonstrate this potential, we correlate the physical and mechanical properties of variously altered andesitic volcanic rocks to laboratory reflectance spectroscopy using statistical analysis. Several rock properties, including density, connected porosity, strength, magnetic susceptibility, and elasticity, correlate with reflectance spectroscopy in both the visible and short-wave infrared parts of the electromagnetic spectrum. We attribute these correlations to the presence and degradation (i.e. weathering or hydrothermal alteration) of iron-bearing minerals such as pyroxene, magnetite, and pyrite, which reflect changes to both rock properties and reflectance spectroscopy measurements. Results support the use of transfer functions to estimate rock properties directly from reflectance spectroscopy. Ultimately, aerial or satellite imaging spectroscopy could be used to create geotechnical maps at volcano scale. • Laboratory reflectance spectroscopy correlates to many physical and mechanical rock properties. • Iron-bearing minerals are sensitive to weathering and hydrothermal alteration. • Their reflectance correlates well to rock properties such as strength and porosity. • Material property-reflectance correlations associated with clay minerals are poor. • Spectroscopy shows potential for predicting rock properties at large spatial scales. [ABSTRACT FROM AUTHOR]

The thermal properties of volcanic rocks are crucial to accurately model heat transfer in volcanoes and in geothermal systems located within volcanic deposits. Here we provide laboratory measurements of thermal conductivity and thermal diffusivity for variably porous andesites from Mt. Ruapehu (New Zealand) and variably altered basaltic-andesites from Merapi volcano (Indonesia) measured at ambient laboratory pressure and temperature using the transient hot-strip method. The specific heat capacity of each sample was then calculated using these measured values and the bulk sample density. Thermal conductivity and thermal diffusivity decrease as a function of increasing porosity, but specific heat capacity does not vary systematically with porosity. For a given porosity, saturation with water increases thermal conductivity and specific heat capacity, but decreases thermal diffusivity. Measurements on samples from Merapi volcano show that, compared to the unaltered samples from Mt. Ruapehu, hydrothermal alteration deceases thermal conductivity and thermal diffusivity, and increases specific heat capacity. We use an effective medium approach to parameterise these data, showing that when the porosity and pore-fluid properties are scaled for, the measured values agree well with theoretical predictions. We find that despite the microstructural complexity of the studied andesites, porosity is the principal parameter dictating their thermal properties. To understand whether the measured changes in thermal properties are sufficient to influence natural processes, we model heat transfer from magma to the surrounding host-rock by solving Fick's second law cast in 1D Cartesian (dyke geometry) and cylindrical (conduit geometry) coordinates. We provide models for different host-rock porosities (0–0.6), different initial magmatic temperatures (800–1200 °C), and different levels of host-rock alteration. Our modelling shows how the cooling of a dyke and conduit is slowed by a higher host-rock porosity and by increased hydrothermal alteration. The thermal properties provided herein can help improve modelling designed to inform on volcanic and geothermal processes. • Thermal conductivity decreases from 1.5 to 0.4 W·m−1·K−1 as porosity increases from 0.05 to 0.6. • Thermal diffusivity decreases from 0.7–0.8 to 0.5–0.55 mm2·s−1 as porosity increases from 0.05 to 0.6. • Specific heat capacity is 0.591–0.856 kJ·kg−1·K−1 and does not vary with porosity. • Porosity plays a first-order role in dictating thermal properties. • Cooling of a dyke/conduit is slowed by higher host-rock porosity and hydrothermal alteration. [ABSTRACT FROM AUTHOR]