Tuning the permeability of beds of ground coffee is central to making good espresso. Here, we develop a theoretical framework for such a permeability model, which depends principally on the pore volume fraction that is connected between the grains phi p and the specific surface area of the interfaces between liquid and coffee grains, s. We test our theoretical predictions by using lattice-Boltzmann simulations of fluid flow through three-dimensional domains obtained via X-ray computed micro-tomography (XCT) of real packed ground coffee. Our tomography is performed on two different roasted coffees ('Tumba' from Rwanda and 'Guayac & aacute;n' from Colombia) ground to one of 11 grind settings using a Mahlk & ouml;nig grinder, ranging from very fine to coarse grinds. We find excellent agreement with our percolation theory, suggesting a practical way to relate grind size and packing fraction to the specific surface area to predict coffee permeability. We discuss this model in the context of optimal espresso recipe design and the effects of coffee dose to define a Forchheimer number that can be used to predict the onset of inertial flow in coffee.

Magma vesiculation controls the ability of gas to escape from a volcano to the atmosphere and strongly influences volcanic eruption style. In silicic magmas, vesiculation in the conduit is often accompanied by elements of pure and simple shear deformation at different scales. To constrain the impact of shearing on outgassing in volcanic conduits, we performed synchrotron-based, time-resolved X-ray tomographic imaging during a thermally-induced vesiculation experiment on an obsidian from Lipari volcano, Italy. We tracked the real time, 4D evolution of vesicularity, bubble size distribution and bubble number density, bubble shape, and bubble orientation during vesiculation and shearing. We employ numerical simulations of heat (diffusion), diffusive bubble growth and pressure and velocity (linearized compressible Navier-Stokes) to map the temperature, pressure, water content and shear conditions throughout the sample. Vesiculation occurred in two main domains, (i) a shear zone that rapidly established at the boundary with the silicon carbide crucible during vesiculation and underwent pure and simple shear and (ii) the interior, in which initial pure shear was followed by isotropic vesiculation. Shear bands first promoted outgassing due to system-spanning bubble coalescence creating permeable pathways, followed by collapse, densification and creation of impermeable domains. This evolution leaves behind only faint textural traces that could be overlooked during analysis of natural samples, or ex-situ experiments. Our study reveals, in 4D, the role of shearing on transient permeability evolution at the microscale, which may drive dynamic shifts from effusive to explosive behavior and regulate hybrid volcanic activity commonly observed at silicic volcanoes.

The conditions under which magma accumulates and is stored are fundamental to unraveling the processes of crust formation, planetary differentiation, geothermal heat recharge, and volcanic eruptions. Storage pressure and temperature are typically inferred from erupted volcanic products. However, changes during kilometers of magma ascent induce disequilibrium crystallization and vesiculation, and inverting back to storage conditions comes with arguably unresolvable uncertainties. Here, we explore opportunities arising from magma drilling at Krafla volcano (Iceland) to reconstruct real, in situ magmatic conditions for the first time. The findings show that over the ∼5 min in which the magma is quenched, vapor bubbles consisting of H2O and CO2 exsolve, grow, and resorb, but the changes can be accounted for by careful multiparametric inversion (for chemistry, vesicularity, and vitrification), and that the magma was stored under lithostatic conditions, unlike previous assertions based on classic methods1. This constraint provides us with the unique pairing of precisely measured depth and pressure on a single magma body, and thus a robust method to improve our understanding of magma storage conditions and evolution.

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.

Amphibole reaction rims record critical pre-eruptive magmatic processes, including storage and ascent dynamics. Although decompression and heating are traditionally viewed as key triggers for amphibole breakdown, variations in rim textures and mineralogy indicate that multiple or fluctuating processes often operate simultaneously. This study presents experimental results demonstrating significant impacts of CO2 flushing and redox (fO2) conditions (versus heating and decompression) on amphibole reaction rim formation in rhyolitic and rhyodacitic melts at shallow crustal conditions. In experiments at 830 degrees C and 120 MPa, the presence of a mixed XH2O:XCO2 fluid (XCO2 = 0.3-0.7) rapidly induced a reaction, with rim thickness, grain size, aspect ratio, nucleation rate and crystal number density all increasing with CO2 concentration. In contrast, rims produced by heating (880 degrees C for up to 48 h) had distinct characteristics, while decompression (120 MPa to 65 MPa over 120 h) from the same starting conditions did not produce reaction rims. Increasing oxygen fugacity (fO2 from NNO+1 to RRO [NNO+2]) led to rapid rim formation within 24 h, accompanied by distinct mineralogical changes that favoured the stability of Fe3+ phases. These findings demonstrate that CO2, fO2, heating and decompression each exert unique influences on amphibole breakdown; thus, quantitative textural analysis of amphibole rims can help differentiate the driving mechanisms. Recognising the full range of factors affecting amphibole stability and an understanding of the multi-parametric controls is essential for accurately interpreting pre-eruptive conditions, enhancing our ability to reconstruct magmatic histories, and for assessing volcanic hazards.

Volcanic deposits compact and deform following emplacement and burial. Here, we experimentally investigate the compaction of volcaniclastic material through gravitational loading (i.e. burial). Two lithologies (scoria and hyaloclastite) of different grain size (ash and lapilli) were held in a cylindrical container and compressed between two pistons to target stresses of 2, 5, 10, or 20 MPa, whilst monitoring axial displacement and acoustic emissions, enabling quantification of strain, densification, and comminution. In a second suite of experiments, samples were loaded and held at each stress to creep for six hours. The density and porosity of all samples were measured pre- and post-experiment. For all experiments, most deformation occurred during the early loading phase, then strain rates diminished with increasing compaction. During early loading, the hyaloclastite compacted faster than the scoria, but due to efficient early compaction, deformed more slowly at higher stresses and during creep. Grain size was also important for the amount of compaction; lapilli samples were initially less efficiently packed than ash samples and accumulated higher strain during the early part of loading. The strain experienced by all samples was substantial: even 2 MPa (equivalent to an overburden of ~180–230 m for our porous lithologies) caused volume reductions of 10–30 % due to grain rearrangement and crushing. Interpolation and extrapolation of the data were used to forecast instantaneous and time-dependent surface deformation of volcaniclastic deposits of different thicknesses. The findings yield important new constraints for the interpretation of ground deformation signals and development of models of volcanic flank instability.

In the Earth, the flow of crystal-bearing magma is thought to be non-Newtonian and shear thinning, but the physical origin for this is poorly understood. We use hydro-granular theory to show that the decoupled migration of crystals toward conduit cores during magma ascent is a tenable microphysical mechanism for plug flow, emergent in an otherwise purely Newtonian crystal-bearing magma. We use a numerical conduit model to define the flow development length beyond which crystal migration dominates and strain localises near conduit margins. Applied to magma ascent scenarios, we show that this crystal-migration strain localisation only develops in high crystallinity magmas or magmas ascending in very narrow cracks/conduits. In all other scenarios, crystals do not contribute to non-Newtonian behaviour and such magmas are usually strictly Newtonian. The ascent of very crystal-rich dome magma could be associated with strain localisation and crystal depletion at the conduit margins, lubricating ascent through the crust. Crystal migration can occur during magma ascent in volcanic eruptions and when this process occurs, it leads to non-Newtonian behaviour, according to a volcanic conduit flow model.

Changes in rhyolite melt viscosity during magma decompression and degassing exert a first order control on ascent through the crust and volcanic eruption style. These changes have as yet unknown hazard implications for geothermal drilling in pursuit of particularly hot fluids close to magma storage regions. Here, we exploit the situation at Krafla volcano in which rhyolite has both erupted at Earth's surface and been sampled at shallow storage depths via drilling of the 2009 IDDP-1 and 2008 KJ-39 boreholes. We use differential scanning calorimetry to constrain that the IDDP-1 magma quenched to glass at similar to 700 K, at a rate of between 7 and 80 K.min(-1). We measure the equilibrium viscosity of the IDDP-1 rhyolite at temperatures close to the glass transition interval and show that the rhyolite viscosity is consistent with generalized viscosity models assuming a dissolved H2O concentration of 2.12 wt%. We couple these results with micro-penetration and concentric cylinder rheometry over a range of potential magma storage temperatures to constrain the response of surficial Krafla rhyolites to stress. The surficial rhyolites at Krafla match the same viscosity model, assuming a lower dissolved H2O concentration of 0.12 wt%. Our results show that at a storage temperature of 1123-1193 K, the viscosity of the stored magma is similar to 3x10(5) Pa.s. At the same temperature, the viscosity following degassing during ascent to the surface rises to similar to 2x10(9) Pa.s. Finally, we use high-stress compression tests on the Hrafntinnuhryggur surface obsidian to determine the onset of unrelaxed behavior and viscoelastic melt rupture or fragmentation pertinent to understanding the melt response to rapid pressure changes that may be associated with further (near-) magma exploration at Krafla. Taken together, we characterize the relaxation and viscosity of these magmas from source-to-surface.

Granite formation and evolution are influenced by various physical and chemical processes in magmatic systems, resulting in diverse textures and compositions. While the generation of granitic rock through dehydration melting of hydrous minerals, which requires higher temperatures, is widely accepted, the formation and evolution of cold granite remains a topic of considerable debate. Our experiments aim to understand how small changes in granitic magma composition influence late-stage crystallization, providing insights into the chemical and textural evolution of granitic rocks and determining the significance of these conditions in shaping the diversity observed in nature. We conducted crystallization experiments at 1 and 2 kbar, between 680 and 815 degrees C, using two granitic compositions: a natural I-type granitoid (MA) and a slightly more mafic, synthetic analogue (FC). In these relatively low-pressure and low-temperature experiments, orthopyroxene remains stable at 1 kbar regardless of the starting material. At 2 kbar, its stability is limited to the more mafic sample FC when containing 3.7 wt% water. Hornblende only crystallizes in the more mafic rock FC at 2 kbar across all temperatures tested when water concentration exceeds 4 wt%. Plagioclase spans a broad temperature range (700-815 degrees C) at both 1 and 2 kbar in the FC material, whereas in the MA material at 2 kbar, it appears only at temperatures <725 degrees C. At 2 kbar few crystals were observed in the MA run at 750 degrees C, indicating that the liquidus temperature was nearly reached, contrasting with the FC starting material. Additionally, temperature cycling proved more efficient in promoting crystal growth in FC samples. Under low-temperature conditions, only local equilibrium was achieved, highlighting the significant role of late-stage crystallization processes and kinetic limitations in shaping the diversity observed in cold granites. Our experiments demonstrate that even small changes in CaO and FeO content can markedly alter the solidus/liquidus temperature of melts, significantly influence phase stability, melt production, and texture in granitic systems. This study surmises the need to further pursue systematic investigations of the influence of major elements on crystallization sequences in granitic systems, and we compare our findings to previous observations made on the Gentio metagranitoids of the Mineiro belt, Brazil.

Hot volcanic pyroclasts can sinter, vesiculate, and outgas in concert - a combination of processes which remains poorly constrained. And yet this combination of processes can occur coincidently during deposition from pyroclastic density currents, in conduit-filling pyroclastic debris, and in tuffisites. In many of these settings, it is the sintering-driven evolution of permeability that is key to gas transport through the evolving deposit. Here, we experimentally and theoretically investigate the evolution of the permeable networks during sintering of hot fragmental volcanic systems, which are hydrous and oversaturated at the experimental conditions. Firstly, we find that vesiculation results in shutting of the inter-granular porous network as bubble growth drives expansion of the particles into one another, destroying interconnected pores. Secondly, we observe that degassing by diffusion out of the particle edge results in contraction of the vesicular particles, re-opening pore spaces between them. Therefore, we find that vesiculation, and diffusive outgassing compete to determine both the intra-fragment vesicularity and the permeability during sintering. The development of intra-fragment vesicularity directly impacts the inter-fragment pore space and its connectivity, which decreases during vesiculation and subsequently increases during diffusive outgassing, prompting complex, non-linear permeability evolution. The relative dominance of these processes is fragment size dependent; proportionally, fine fragments lose gas at a higher rate than coarser fragments during diffusive outgassing due to larger surface area to volume ratios. As the systems progress, larger fragments retain a higher proportion of gas and so attain greater vesicularities than finer ones - and therefore, the coarse fragmental pyroclasts experience a greater, yet transient, reduction in connected porosity and permeability. We suggest that where vesiculation is sufficient, it can lead to the complete loss of connected porosity and the sealing of permeable pathways much earlier than in a sintering-only system. Our results suggest that classical sintering models must be modified to account for these vesiculation and diffusive degassing processes, and that only a combined vesiculation, sintering, and diffusive outgassing model can resolve the evolution of permeability in hot clastic volcanic systems.(c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/).

Cycles of stress build-up and release are inherent to tectonically active planets. Such stress oscillations impart strain and damage, prompting mechanically loaded rocks and materials to fail. Here, we investigate, under uniaxial conditions, damage accumulation and weakening caused by time-dependent creep (at 60, 65, and 70% of the rocks’ expected failure stress) and repeating stress oscillations (of ± 2.5, 5.0 or 7.5% of the creep load), simulating earthquakes at a shaking frequency of ~ 1.3 Hz in volcanic rocks. The results show that stress oscillations impart more damage than constant loads, occasionally prompting sample failure. The magnitudes of the creep stresses and stress oscillations correlate with the mechanical responses of our porphyritic andesites, implicating progressive microcracking as the cause of permanent inelastic strain. Microstructural investigation reveals longer fractures and higher fracture density in the post-experimental rock. We deconvolve the inelastic strain signal caused by creep deformation to quantify the amount of damage imparted by each individual oscillation event, showing that the magnitude of strain is generally largest with the first few oscillations; in instances where pre-existing damage and/or the oscillations’ amplitude favour the coalescence of micro-cracks towards system scale failure, the strain signal recorded shows a sharp increase as the number of oscillations increases, regardless of the creep condition. We conclude that repetitive stress oscillations during earthquakes can amplify the amount of damage in otherwise mechanically loaded materials, thus accentuating their weakening, a process that may affect natural or engineered structures. We specifically discuss volcanic scenarios without wholesale failure, where stress oscillations may generate damage, which could, for example, alter pore fluid pathways, modify stress distribution and affect future vulnerability to rupture and associated hazards.

Research Article| May 01, 2022 Strain Localization in Magmas Yan Lavallée; Yan Lavallée Department of Earth, Ocean and Ecological Science, University of Liverpool, Liverpool, L69 3GP, United Kingdom Search for other works by this author on: GSW Google Scholar Jackie E. Kendrick Jackie E. Kendrick School of Geosciences, University of Edinburgh, James Hutton Road, Edinburgh EH9 3FE, United Kingdom Search for other works by this author on: GSW Google Scholar Author and Article Information Yan Lavallée Department of Earth, Ocean and Ecological Science, University of Liverpool, Liverpool, L69 3GP, United Kingdom Jackie E. Kendrick School of Geosciences, University of Edinburgh, James Hutton Road, Edinburgh EH9 3FE, United Kingdom Publisher: Mineralogical Society of America First Online: 01 May 2022 Copyright © 2022 by the Mineralogical Society of AmericaMineralogical Society of America Reviews in Mineralogy and Geochemistry (2022) 87 (1): 721–765. https://doi.org/10.2138/rmg.2022.87.15 Article history First Online: 01 May 2022 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Yan Lavallée, Jackie E. Kendrick; Strain Localization in Magmas. Reviews in Mineralogy and Geochemistry 2022;; 87 (1): 721–765. doi: https://doi.org/10.2138/rmg.2022.87.15 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search One of the most intriguing and (from the point of view of modeling) poorly understood aspects of geomaterial mechanics is strain localization. Strain localization is a common feature of viscous, elastic and/or plastic materials undergoing non-homogeneous deformation. During the deformation of rocks or magmatic suspensions, strain may be variably partitioned (spatially and in magnitude) between phases or multi-scalar heterogeneities of variable strengths, which may promote the development of shear zones and/or shear bands (e.g., Wright and Weinberg 2009); as such, strain localization is a scale-dependent phenomenon that may range between discrete and pervasive deformation. Strain localization takes place in... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Magma ascending in the Earth’s crust can undergo oscillations in pressure, from ultra-low frequency changes associated with tectonics, to relatively higher frequency oscillations associated with seismicity. Seismic waves travelling through shallow magma bodies can lead to a range of unrest phenomena and potentially trigger volcanic eruptions. The mechanisms by which pressure oscillations can induce unrest or eruption remain debated. Here, we experimentally impose pressure oscillations on magma and study how they affect vesiculation processes. We use cylindrical samples (4.00 mm long, 4.85 mm diameter) of hydrous rhyolitic obsidian (0.11 ± 0.01 wt% H 2 O) placed in alumina (AL23) crucibles and vary pressure by the uniaxial loading of an alumina plunger in a thermo-mechanical analyzer. We monitor vesiculation at temperatures of 950–990°C and confining pressure of 177 kPa. We perform two types of experiment: 1) “static” experiments (at constant pressure) and 2) “oscillating” experiments in which we impose sinusoidal pressure oscillations of up to 71 kPa upon the static pressure (i.e., between 106 and 250 kPa). In both cases, we dilatometrically observe sample expansion driven by vesiculation. Post-experimental bubble textures reveal that bubbles formed preferentially at the sample margins. For the oscillating experiments, the sample expansion rate is lower than in the static experiments, and there are fewer vesicles at the sample margins. We examine the constituent processes of bubble formation (nucleation, growth, coalescence) and gas loss (diffusion, permeable flow) occurring during static experiments and with the added element of pressure oscillations. The most likely mechanism responsible for reduced sample expansion is that pressure oscillations drive the sample in and out of water saturation conditions and thus reduce the fraction of residence time over which bubble nucleation and/or growth are driven. Future work will be needed to confirm this hypothesis. These results are relevant to the study of earthquake-volcano interactions, where a magma body that sits close to volatile saturation is subject to pressure fluctuations.

The magmatic-hydrothermal system at Krafla Volcano, North-East Iceland, is an important source of fluids exploited for geothermal energy. Here, we employ laboratory measurements to constrain the porosity and permeability of the main lithologies forming the reservoir, and investigate the impacts of different thermal and mechanical stimulation practices to improve fluid flow. Six main rock types were identified and sampled: three basalts (a dense and a porous lava, and a surficial dyke); a hyaloclastite; an obsidian; an ignimbrite; a felsite; and a gabbro. Permeability measurements were made in a hydrostatic cell using the steady-state flow method at a range of confining pressures (1-100 MPa). The measurements show that permeability generally increases with porosity, but that permeability may vary significantly for a given porosity, depending on the presence of pore connectivity and micro-fractures. We note that an increase in effective pressure results in a decrease in permeability due to closure of pre-existing cracks, abundant in some rocks. When unloading, samples fail to recover pre-loading permeability, as cracks do not necessarily entirely reopen. To further examine the hysteresis imposed by crack closure, we cyclically loaded/unloaded a felsite sample ten times by varying pore pressure which resulted in a further nonlinear decreases in permeability with each pressurisation cycle; thus an understanding of the pressurisation path may be a requirement to constrain fluid flow variations in geothermal systems. To test the effects of thermal stimulation on fluid flow, samples of dense basalt and felsite were thermally stressed by heating to 450 degrees C and cooling at different rates (in air, in water and at a controlled rate of <5 *C. min(-1)). The results show that the permeability of originally highly fractured rocks is not affected by thermal stressing, but originally unfractured rocks show a nonlinear increase in permeability with each thermal stressing cycle, especially with the largest thermal shock imposed by quenching in water; thus thermal stimulation may not be expected to result in a similar magnitude of permeability creation along the length of a borehole. Finally, following the permeability measurements on intact rocks, the Brazilian tensile testing method was employed to impart one and two (orthogonal) macTo-fractures, and permeability was measured after each step. The creation of one macro-fracture strongly enhanced the permeability of the rock (especially dense rocks), resulting in a narrower range of permeability (as a function of porosity) for the fractured rocks. Imparting a second fracture had trivial additional impact on the permeability of the rock. Yet, the presence of fine fragments and possible minor offset of fracture interfaces was found to obstruct fracture closure, which resulted in higher permeability irrespective of effective pressure; thus hydraulic fracturing may locally increase fluid flow, especially when employing proppants to obstruct fracture closure and ensure a stable permeable network in a reservoir. We discuss the implications of the findings for a first order constraint on the permeability of the reservoir rock and the potential of thermal and mechanical stimulation methods on energy production in geothermal systems nested in active volcanic fields. Crown Copyright (C) 2018 Published by Elsevier B.V.

Volcanic ash particle properties depend upon their genetic fragmentation processes. Here, we introduce QEMSCAN Particle Mineralogical Analysis (PMA) to quantify the phase distribution in ash samples collected during activity at Santiaguito, Guatemala and assess the fragmentation mechanisms. Volcanic ash from a vulcanian explosion and from a pyroclastic density current resulting from a dome collapse were selected. The ash particles resulting from both fragmentation modes are dense and blocky, typical of open-vent dome volcanoes and have a componentry consistent with their andesitic composition. We use image analysis to compare the fraction of each phase at particle boundaries compared to the total particle fraction. Our results show that the explosion-derived ash has an even distribution of plagioclase and glass, but boundaries enriched in pyroxene and amphibole. In contrast, the ash generated during dome collapse has an increased fraction of glass and decreased fraction of plagioclase at particle boundaries, suggesting that fractures preferentially propagate through glass during abrasion and milling in pyroclastic flows. This study presents QEMSCAN PMA as a new resource to identify generation mechanisms of volcanic ash, which is pertinent to volcanology, aviation, respiratory health and environmental hazards, and highlights the need for further experimental constraints on the fragmentation mechanism fingerprint.

Cycles of fracture and healing in magma are important controls on outgassing time scales and repetitive seismicity at silicic volcanoes. Here, we experimentally drove silicate melts (at 109–1011 Pa·s) to tensile failure, measuring the strength during fracture of the otherwise liquid material. We then took the same melts with parallel contact surfaces and closed the fracture under compressive stress and recorded the evolution of tensile strength of the interface healed for different times. We provide a semi-empirical model for fracture healing time scales useful for volcanic applications. As the time available for healing is increased, strength nonlinearly recovers toward that of unfractured glass. We parameterized the healing kinetics as a three-stage process: (1) relaxation of the compressive stress, (2) fracture surface–surface wetting, and (3) diffusive removal of the interface. During welding of these surfaces in air, we observed that micropores are trapped along the fracture plane, which may inhibit complete healing and provide a textural record of relict fracture planes in volcanic glasses. We conclude that at magmatic conditions, fracture healing is efficient in crystal-poor melts, and it could rapidly seal outgassing pathways over eruptive time scales, contributing to cyclic behavior associated with recurring gas-and-ash explosions and outgassing events.

Changes in permeability can impact geological processes, geohazards, and geothermal energy production. In hydrothermal systems, high-temperature heat sources drive fluid convection through the pore network of reservoir rocks. Additionally, thermal fluctuations may induce microfracturing and affect the mineralogical stability of the reservoir rock, thus modifying the fluid pathways and affecting permeability and strength. This study describes the results of thermal heating events lasting several hours on a "moderately altered" plagioclase-clinochlore-calcite-quartz andesite and a "highly altered" plagioclase-clinozoisite-quartz-clinochlore andesite from the Rotokawa Geothermal Field, New Zealand. We use a low thermal gradient (similar to 1.2 degrees C/min) in an H2O-saturated, 20-MPa pressure environment to constrain changes in petrophysical properties associated with transitory thermal phenomena between 350 and 739 degrees C. As the treatment temperature increases, the mass reduces, while porosity and permeability increase. These effects were greater in the "moderately altered" andesite than in the "highly altered" andesite. Microfracturing is responsible for these changes at lower temperatures (e.g., <= 400 degrees C). At higher temperatures (e.g., >400 degrees C), microfracturing remains partially responsible for these rock property changes (e.g., higher permeability); however, these changes are also a product of clinochlore, quartz, and (when present) calcite reacting out of the altered andesite, and increasing porosity. We propose that at temperatures >400 degrees C, volumetric phase changes associated with heat-driven reactions in a wet environment can contribute to microcracking and porosity/permeability changes. Our data support observations where high-temperature conditions at the margins of magma bodies can be associated with substantial increased permeability and decreased strength.