掲載種別：研究論文（学術雑誌）  

Silicic volcanic eruptions range in style from gently effusive to highly explosive, and may switch style unpredictably during a single eruption. Direct observations of subaerial rhyolitic eruptions (Chaiten 2008, Cordón Caulle 2011–2012, Chile) challenged long-standing paradigms of explosive and effusive eruptive styles and led to the formulation of new models of hybrid activity. However, the processes that govern such hybrid explosive–effusive activity remain poorly understood. Here, we bring together observations of the well-studied 2011–2012 Cordón Caulle eruption with new textural and petrologic data on erupted products, and video and still imagery of the eruption. We infer that all of the activity – explosive, effusive, and hybrid – was fed by explosive fragmentation at depth, and that effusive behaviour arose from sticking and sintering, in the shallow vent region, of the clastic products of deeper, cryptic fragmentation. We use a scaling approach to determine that there is sufficient time available, during emplacement, for diffusive pyroclast degassing and sintering to produce a degassed plug that occludes the shallow conduit, feeding clastogenic, apparently effusive, lava-like deposits. Based on evidence from Cordón Caulle, and from other similar eruptions, we further argue that hybrid explosive–effusive activity is driven by episodic gas-fracking of the occluding lava plug, fed by the underlying pressurized ash- and pyroclast-laden region. The presence of a pressurized pocket of ash-laden gas within the conduit provides a mechanism for generation of harmonic tremor, and for syn-eruptive laccolith intrusion, both of which were features of the Cordón Caulle eruption. We conclude that the cryptic fragmentation models is more consistent with available evidence than the prevailing model for effusion of silicic lava that assume coherent non-fragmental rise of magma from depth to the surface without wholesale explosive fragmentation.

DOI： 10.1016/j.jvolgeores.2022.107672

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掲載種別：研究論文（学術雑誌）  

Breadcrust bombs are pyroclasts displaying fractured, dense surfaces enveloping expanded interiors, and are associated with Vulcanian explosions. We document pyroclasts from the 2008–2009 CE eruption of Chaitén (Chile) that are internally as well as externally breadcrusted. The pyroclasts are cut by intersecting micrometer to millimeter-thick tuffisites with dense glassy walls, which grade into strongly inflated pumiceous material. We find H2O diffusion gradients proximal to the breadcrusted surfaces, such that H2O is depleted from far-field magma (0.68 ± 0.04 wt%) into dense, fractured vein walls (0.2–0.3 wt%), indicating a spatial association between H2O mass transfer within the pyroclast interior and both suppressed vesiculation and breadcrusting. We experimentally confirm that diffusive H2O depletion suppresses bubble growth at shallow conduit conditions. Therefore, we interpret the breadcrust formation to be induced by H2O diffusion and the associated rise in viscosity rather than by cooling in the classical breadcrust-formation models. We posit that a “dehydration quench” is important as degassing continues to very low H2O contents in shallowconduit magma that continues to vesiculate.

DOI： 10.1130/G49959.1

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掲載種別：研究論文（学術雑誌）  

The impacts of volcanic eruptions on climate are increasingly well understood, but the mirror question of how climate changes affect volcanic systems and processes, which we term “climate-volcano impacts”, remains understudied. Accelerating research on this topic is critical in view of rapid climate change driven by anthropogenic activities. Over the last two decades, we have improved our understanding of how mass distribution on the Earth’s surface, in particular changes in ice and water distribution linked to glacial cycles, affects mantle melting, crustal magmatic processing and eruption rates. New hypotheses on the impacts of climate change on eruption processes have also emerged, including how eruption style and volcanic plume rise are affected by changing surface and atmospheric conditions, and how volcanic sulfate aerosol lifecycle, radiative forcing and climate impacts are modulated by background climate conditions. Future improvements in past climate reconstructions and current climate observations, volcanic eruption records and volcano monitoring, and numerical models all have a role in advancing our understanding of climate-volcano impacts. Important mechanisms remain to be explored, such as how changes in atmospheric circulation and precipitation will affect the volcanic ash life cycle. Fostering a holistic and interdisciplinary approach to climate-volcano impacts is critical to gain a full picture of how ongoing climate changes may affect the environmental and societal impacts of volcanic activity.

DOI： 10.1007/s00445-022-01562-8

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media {LLC}  

DOI： 10.1038/s41586-021-04164-0

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掲載種別：研究論文（学術雑誌）  

The French press is a popular device for brewing coffee, comprising a cylindrical beaker—or “jug”—fitted with a lid and plunger with a fine wire mesh filter. The plunger is used to drive the solid coffee particles to the bottom of the jug, separating these grounds from hot liquid above. When using the French press in this way, a growing permeable pack of ground coffee is pushed through hot water by applying force to the plunger. We use a combination of kitchen-based and laboratory experiments to determine the force required to push on the plunger as a function of the speed of the plunger and the mass of coffee used. We calculate that for the recommended preparation method, the maximum force is 32 N to complete the pressing action in 50 s. We propose that home coffee preparation provides a fun, low-cost, and relatable learning opportunity for students and for those who are interested in coffee science.

DOI： 10.1119/10.0004224

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掲載種別：研究論文（学術雑誌）  

DOI： 10.30909/vol.04.S1.ivi

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掲載種別：研究論文（学術雑誌）  

The Soultz-sous-Forêts geothermal reservoir (France)—in particular, the GPK-4 well—has been proposed as a target for chemical stimulation under the DESTRESS Horizon-2020 framework. With a combination of batch reaction tests and acid flow-through experiments using hydrochloric acid (HCl) at different molarities and temperatures, we investigate the potential for acid-induced permeability enhancement of a granite analogous to the Soultz-sous-Forêts reservoir rock, by means of acid stimulation. In the batch reaction experiments, we find that the propensity for increase or decrease in porosity and permeability depends on the physico-chemical properties of the starting material: unaltered granite underwent a significant increase in both porosity and permeability relative to its initial state, altered granite exhibited a moderate increase in both porosity and permeability (modulated slightly by HCl molarity), whereas initially more porous and permeable thermally and naturally fractured granite exhibited an increase in porosity accompanied by a relative decrease in permeability. The extent to which permeability increased or decreased appears to be tied to the initial fluid-flow characteristics of the material. Using a new, custom-built acid permeameter, flow-through tests were performed on unaltered granite, while the acid was sampled at regular time intervals. Element release into solution recorded throughout the experiments, indicated dissolution of granite minerals. Despite this operative micromechanism, however, the absolute change in sample permeability is limited, both at room temperature and at 100∘C. Ultimately, these data suggest that the potential for geothermal reservoir enhancement using HCl is low at Soultz-sous-Forêts. Nevertheless, the possibility remains that a more targeted thermal or chemical stimulation approach—or hybrid thereof—could prove effective in the future.

DOI： 10.1186/s40517-020-00168-7

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掲載種別：研究論文（学術雑誌）  

The accuracy of elastic analytical solutions and numerical models, widely used in volcanology to interpret surface ground deformation, depends heavily on the Young's modulus chosen to represent the medium. The paucity of laboratory studies that provide Young's moduli for volcanic rocks, and studies that tackle the topic of upscaling these values to the relevant lengthscale, has left volcano modellers ill-equipped to select appropriate Young's moduli for their models. Here we present a wealth of laboratory data and suggest tools, widely used in geotechnics but adapted here to better suit volcanic rocks, to upscale these values to the scale of a volcanic rock mass. We provide the means to estimate upscaled values of Young's modulus, Poisson's ratio, shear modulus, and bulk modulus for a volcanic rock mass that can be improved with laboratory measurements and/or structural assessments of the studied area, but do not rely on them. In the absence of information, we estimate upscaled values of Young's modulus, Poisson's ratio, shear modulus, and bulk modulus for volcanic rock with an average porosity and an average fracture density/quality to be 5.4 GPa, 0.3, 2.1 GPa, and 4.5 GPa, respectively. The proposed Young's modulus for a typical volcanic rock mass of 5.4 GPa is much lower than the values typically used in volcano modelling. We also offer two methods to estimate depth-dependent rock mass Young's moduli, and provide two examples, using published data from boreholes within Kīlauea volcano (USA) and Mt. Unzen (Japan), to demonstrate how to apply our approach to real datasets. It is our hope that the data and analysis presented herein will assist in the selection of elastic moduli for volcano modelling. To this end, we provide a Microsoft Excel© spreadsheet containing the data and necessary equations to calculate rock mass elastic moduli that can be updated when new data become available. The selection of the most appropriate elastic moduli will provide the most accurate model predictions and therefore the most reliable information regarding the unrest of a particular volcano or volcanic terrain.

DOI： 10.1016/j.jvolgeores.2019.106684

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掲載種別：研究論文（学術雑誌）  

Volcanoes often host hydrothermal systems that alter the host rock. To understand the influence of alteration on mechanical behaviour of edifice-forming rock, we performed a series of triaxial deformation experiments on variably altered andesite from Mt. Ruapehu (New Zealand) under constant effective pressure. Under the imposed conditions, andesite with intermediate argillic alteration deforms in a brittle manner forming fractures. By contrast, andesite with advanced argillic alteration deforms in a ductile manner, with sample failure driven by distributed cataclastic pore collapse. We consider this the result of an increase in porosity and clay content with increasing alteration. Ancillary experiments highlight that the brittle-ductile transition occurs at lower effective pressure (i.e. at shallower depths) in andesites with advanced argillic alteration relative to unaltered andesites of comparable porosity. We conclude that advanced argillic alteration can create an anomalous shallow ductile zone, which has important implications for fluid flow and pre-eruptive seismicity.

DOI： 10.1007/s00445-019-1306-9

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掲載種別：研究論文（学術雑誌）   出版者・発行元：American Geophysical Union (AGU)  

The EarthArXiv preprint archive, in operation for almost a year and a half, makes the latest Earth science research available to a wider community.

DOI： 10.1029/2019eo121347

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掲載種別：研究論文（学術雑誌）  

Volcanic systems often host crater lakes, flank aquifers, or fumarole fields that are strongly acidic. In order to explore the evolution of the physical and mechanical properties of an andesite under these reactive chemical conditions, we performed batch reaction experiments over timescales from 1 day to 4 months. The experiments involved immersion of a suite of samples in a solution of 0.125 M sulfuric acid (pH ∼0.6). Periodically, samples were removed and their physical and mechanical properties measured. We observe a progressive decrease in mass, coincident with a general increase in porosity, which we attribute to plagioclase dissolution accompanied by the generation of a microporous diktytaxitic groundmass due to glass dissolution. Plagioclase phenocrysts are seen to undergo progressive pseudomorphic replacement by an amorphous phase enriched in silica and depleted in other cations (Na, Ca, and Al). In the first phase of dissolution (t = 24–240 hr), this process appears to be confined to preexisting fractures within the plagioclase phenocrysts. However, ultimately these phenocrysts tend toward entire replacement by amorphous silica. We propose that the dissolution process results in the widening of pore throats and the improvement of pore connectivity, with the effect of increasing permeability by over an order of magnitude relative to the initial measurements. Compressive strength of our samples was also modified, insofar as porosity tends to increase (associated with a weakening effect). We outline broader implications of the observed permeability increase and strength reduction for volcanic systems including induced flank failure and related hazards, improved efficiency of volatile migration, and attendant eruption implications.

DOI： 10.1029/2018JB016130

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掲載種別：研究論文（学術雑誌）  

Geothermal exploitation in the Upper Rhine Graben increasingly targets the interface between the granitic basement and the overlying Buntsandstein unit. Results from deformation experiments are combined with structural assessments to provide reservoir-scale wet and dry strength and elastic modulus profiles for the Buntsandstein at Soultz-sous-Forêts (France). Our analysis finds five zones characterised by low strength and elastic modulus. The strength and elastic modulus of “massive” zones are lower when the rock is wet (i.e. water-saturated), highlighting the importance of performing wet deformation experiments for geothermal rock mass assessments. These data and methods can be used to provide assessments of other geothermal sites within the region to assist prospection, stimulation, and optimisation strategies.

DOI： 10.1016/j.geothermics.2018.10.003

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掲載種別：研究論文（学術雑誌）  

Volcanic edifices exhibit spatially variable physical and mechanical properties. Magmatic intrusions are common at shallow depths within the volcanic edifice and are a poorly-understood contributor to this spatial variability. Intrusion-related alteration has been found to weaken rock mass strength through the development of joints and fractures; however, there is a paucity of research investigating how intrusions affect rock mass strength specific to the geotechnical units that define the rock masses. In this study, we employ a range of field techniques—field permeametry, rock hardness assessment, rock mass classification, and discontinuity mapping—to characterise an exposed fossil geothermal system produced by a shallow intrusion at Pinnacle Ridge, Mt. Ruapehu (New Zealand). We find that intrusions detrimentally affect the rock mass characteristics of altered brecciated lava margins. The resulting change in rock mass strength may be offset by an increase in intact rock strength as a product of alteration mineral precipitation in microfractures. Consequently, the final strength of the rock mass of the altered brecciated lava margins has the potential to be lowest of any of the geotechnical units in the volcanic edifice. We also conclude that these discontinuities increase permeability of the host rock at distances from the intrusion roughly proportional to 1–2 times the thickness of the intrusion itself under near-surface conditions. The data and conclusions presented in this study help to bridge the gap between the lab- and the field-scale and have immediate relevance to engineering geology and geothermal applications worldwide, and to rock mass classification assessments in volcanic environments.

DOI： 10.1016/j.jvolgeores.2018.09.012

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掲載種別：研究論文（学術雑誌）  

Permeability is an important input to models of shallow magma ascent. It is a property that can exhibit anisotropy in volcanic magmas, rocks and edifices. Here we show that some important features of permeability anisotropy can be captured by a simple approach. The permeability of a layered medium can be described by a function that takes into account the angle between the direction in which pressure gradient acts, and the layering orientation. In the end-member case of flow parallel or perpendicular to the layering, the permeability of the whole system reduces to the arithmetic or harmonic means of the permeabilities of the constituent units, respectively. This implies that laboratory-scale measurements on homogeneous constituent layers can be upscaled to an effective permeability of a larger, multi-layered unit or edifice, including fractured systems. We outline the theoretical underpinning to these formulations, and provide experimental permeability data measured on anisotropic volcanic materials in order to validate this result. We show that this result implies that permeability parallel to layering or bedding must always be higher than that measured perpendicular to layering. Moreover, we emphasise that the choice of averaging method used to upscale permeability data on individual rock samples has important consequences for the validity of the derived values. We anticipate that these points will help move towards more realistic models of pressure evolution behaviour in volcanoes, and increase the utility of laboratory-derived data for volcano-scale modelling.

DOI： 10.1016/j.jvolgeores.2018.09.002

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掲載種別：研究論文（学術雑誌）  

Shallow magmatic intrusions are prevalent in volcanic settings worldwide. Understanding how these intrusions interact and influence their volcanic host rocks is therefore relevant to many engineering geology, geothermal, and volcanological applications. In this study, we present the most comprehensive dataset for a shallow intrusion and its host rock in a volcanic setting to date, detailing the mechanical and physical properties of volcanic rocks from Pinnacle Ridge, Mt. Ruapehu, New Zealand. Based on the geomechanical properties of 194 measured samples, we identify seven geotechnical units: (1) unaltered dense coherent lava, (2) altered dense coherent lava, (3) unaltered brecciated lava margin, (4) altered brecciated lava margin, (5) unaltered intrusion, (6) altered intrusion, and (7) hydrothermal veining. We detail the mineralogy (andesite compositions ranging from primary to an advanced argillic alteration assemblage), porosity (0.7–31%), permeability (10−21–10−12 m2), elastic wave velocities (1994–5615 m/s), uniaxial compressive strength (1–332 MPa) of these geotechnical units. Our laboratory analyses indicate that primary lithology is the predominant control on the physical and mechanical properties of the geotechnical units. Additionally, the data suggest that there is a correlation between distance to the largest intrusion; this is particularly evident for the measurements on the brecciated lava margin samples. Towards the largest intrusion, this breccia shows decreasing porosity (30.92 to 5.49%) and permeability (10−12 to 10−17 m2) and increasing elastic wave velocities (1994 to 4157 m/s) and uniaxial compressive strength (3 to 61 MPa). Thin-section analysis suggests that these correlations are due to mineral precipitation within fractures and pores in the brecciated lava margins. These correlations with distance to the largest intrusion are not shared by the altered intrusions or dense coherent lavas. We suggest that the high primary permeability of the unaltered breccia facilitated efficient hydrothermal fluid circulation and mineral precipitation adjacent to the intrusion. The other geotechnical units are less affected because hydrothermal fluid flow, alteration, and mineral precipitation were limited due to low initial permeability (10−21–10−16 m2). Our study shows that the initial properties of the host rock (i.e. porosity and permeability) control the extent of hydrothermal alteration and the susceptibility to modifications of rock geomechanical properties. Modifications to porosity and permeability can influence edifice-scale behaviour; for example, a reduction in permeability can result in pore pressure augmentation, which exerts a primary control on volcanic slope stability, seismicity, and eruptive behaviour. This study provides the most comprehensive and complete geomechanical properties data suite on a shallow intrusion in volcanic host rock to date and will support monitoring and modelling of volcanic hazards associated with shallow igneous intrusions.

DOI： 10.1016/j.jvolgeores.2018.05.020

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掲載種別：研究論文（学術雑誌）  

Neapolitan Yellow Tuff (NYT) has been used in construction in Naples (Italy) since the Greeks founded the city—then called Neapolis—in the sixth century BCE. We investigate here whether this popular building stone is weaker when saturated with water, an issue important for assessments of weathering damage and monument preservation. To this end, we performed 28 uniaxial compressive strength measurements on dry and water-saturated samples cored from a block of the lithified Upper Member of the NYT. Our experiments show that the strength of the zeolite-rich NYT is systematically reduced when saturated with water (the ratio of wet to dry strength is 0.63). Complementary experiments show that two other common Neapolitan building stones—Piperno Tuff and the grey Campanian Ignimbrite (both facies of the Campanian Ignimbrite deposit devoid of zeolites)—do not weaken when wet. From these data, and previously published data for tuffs around the globe, we conclude that the water-weakening in NYT is a consequence of the presence of abundant zeolites (the block tested herein contains 46 wt.% of zeolites). These data may help explain weathering damage in NYT building stones (due to rainfall, rising damp, and proximity to the sea or water table) and the observed link between rainfall and landslides, rock falls, and sinkhole formation in Naples, and the weathering of other buildings built from zeolite-rich tuffs worldwide.

DOI： 10.1007/s00445-018-1225-1

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掲載種別：研究論文（学術雑誌）  

The phase (gas or liquid) of the fluids within a porous volcanic system varies in both time and space. Laboratory experiments have shown that gas and water permeabilities can differ for the same rock sample, but experiments are biased towards rocks that contain minerals that are expected react with the pore fluid (such as the reaction between liquid water and clay). We present here the first study that systematically compares the gas and water permeability of volcanic rocks. Our data show that permeabilities to argon gas and deionised water can differ by a factor between two and five in two volcanic rocks (basalt and andesite) over a confining pressure range from 2 to 50 MPa. We suggest here that the microstructural elements that offer the shortest route through the sample—estimated to have an average radius ~0.1–0.5 μm using the Klinkenberg slip factor—are accessible to gas, but restricted or inaccessible to water. We speculate that water adsorption on the surface of these thin microstructural elements, assumed here to be tortuous/rough microcracks, reduces their effective radius and/or prevents access. These data have important implications for fluid flow and therefore the distribution and build-up of pore pressure within volcanic systems.

DOI： 10.1016/j.jvolgeores.2018.02.002

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掲載種別：研究論文（学術雑誌）  

Volcanic eruptions are driven by the ascent of volatile-laden magma. The capacity of a volcano system to outgas these volatiles—its permeability—controls the explosive potential, and fractures at volcanic conduit margins play a crucial role in tempering eruption explosivity by acting as outgassing pathways. However, these fractures are often filled with hot volcanic debris that welds and compacts over time, meaning that these permeable pathways have a finite lifetime. While numerous studies emphasize that permeability evolution is important for regulating pressure in shallow volcanic systems, how and when this occurs remains an outstanding question in volcanology. In this contribution, we show that different pressure evolution regimes can be expected across a range of silicic systems as a function of the width and distribution of fractures in the system, the timescales over which they can outgas (a function of depth and temperature), and the permeability of the host material. We define outgassing, diffusive relaxation, and pressure increase regimes, which are distinguished by comparing the characteristic timescales over which they operate. Moreover, we define a critical permeability threshold, which determines (in concert with characteristic timescales of diffusive mass exchange between the pore and melt phases) whether systems fracture and outgas efficiently, or if a volcano will be prone to pressure increases, incomplete healing, and explosive failure.

DOI： 10.1016/j.jvolgeores.2017.04.025

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掲載種別：研究論文（学術雑誌）  

Active volcanoes are mechanically dynamic environments, and edifice-forming material may often be subjected to significant amounts of stress and strain. It is understood that porous volcanic rock can compact inelastically under a wide range of in situ conditions. In this contribution, we explore the evolution of porosity and permeability-critical properties influencing the style and magnitude of volcanic activity- A s a function of inelastic compaction of porous andesite under triaxial conditions. Progressive axial strain accumulation is associated with progressive porosity loss. The efficiency of compaction was found to be related to the effective confining pressure under which deformation occurred: At higher effective pressure, more porosity was lost for any given amount of axial strain. Permeability evolution is more complex, with small amounts of stress-induced compaction ( < 0.05, i.e. less than 5% reduction in sample length) yielding an increase in permeability under all effective pressures tested, occasionally by almost 1 order of magnitude. This phenomenon is considered here to be the result of improved connectivity of formerly isolated porosity during triaxial loading. This effect is then overshadowed by a decrease in permeability with further inelastic strain accumulation, especially notable at high axial strains ( > 0.20) where samples may undergo a reduction in permeability by 2 orders of magnitude relative to their initial values. A physical limit to compaction is discussed, which we suggest is echoed in a limit to the potential for permeability reduction in compacting volcanic rock. Compiled literature data illustrate that at high axial strain (both in the brittle and ductile regimes), porosity φ and permeability k tend to converge towards intermediate values (i.e. 0.10 ≤ φ ≤ 0.20; 10-14 ≤ k ≤ 10-13m2). These results are discussed in light of their potential ramifications for impacting edifice outgassing- A nd in turn, eruptive activity-in active volcanoes.

DOI： 10.5194/se-8-561-2017

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掲載種別：研究論文（学術雑誌）  

Our multidisciplinary study aims to better understand the permeability of active volcanic hydrothermal systems, a vital prerequisite for modelling and understanding their behaviour and evolution. Whakaari/White Island volcano (an active stratovolcano at the north-eastern end of the Taupo Volcanic Zone of New Zealand) hosts a highly reactive hydrothermal system and represents an ideal natural laboratory to undertake such a study. We first gained an appreciation of the different lithologies at Whakaari and (where possible) their lateral and vertical extent through reconnaissance by land, sea, and air. The main crater, filled with tephra deposits, is shielded by a volcanic amphitheatre comprising interbedded lavas, lava breccias, and tuffs. We deployed field techniques to measure the permeability and density/porosity of (1) > 100 hand-sized sample blocks and (2) layered unlithified deposits in eight purpose-dug trenches. Our field measurements were then groundtruthed using traditional laboratory techniques on almost 150 samples. Our measurements highlight that the porosity of the materials at Whakaari varies from ∼ 0.01 to ∼ 0.7 and permeability varies by eight orders of magnitude (from ∼ 10−19 to ∼ 10−11 m2). The wide range in physical and hydraulic properties is the result of the numerous lithologies and their varied microstructures and alteration intensities, as exposed by a combination of macroscopic and microscopic (scanning electron microscopy) observations, quantitative mineralogical studies (X-ray powder diffraction), and mercury porosimetry. An understanding of the spatial distribution of lithology and alteration style/intensity is therefore important to decipher fluid flow within the Whakaari volcanic hydrothermal system. We align our field observations and porosity/permeability measurements to construct a schematic cross section of Whakaari that highlights the salient findings of our study. Taken together, the alteration typical of a volcanic hydrothermal system can result in increases (due to alteration-induced dissolution and fracturing) and decreases (due to hydrothermal precipitation) to permeability. Importantly, a decrease in permeability—be it due to fracture sealing in lava, pore-filling alunite precipitation in tuff, near-vent cementation by sulphur, and/or well-sorted layers of fine ash—can result in pore pressure augmentation. An increase in pore pressure could result in ground deformation, seismicity, jeopardise the stability of the volcanic slopes, and/or drive the wide variety of eruptions observed at Whakaari. Our systematic study offers the most complete porosity-permeability dataset for a volcanic hydrothermal system to date. These new data will inform and support modelling, unrest monitoring, and eruption characterisation at Whakaari and other hydrothermally modified volcanic systems worldwide.

DOI： 10.1016/j.jvolgeores.2016.12.004

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掲載種別：研究論文（学術雑誌）  

The extrusion of dense, viscous magma typically occurs along pronounced conduit-parallel faults. To better understand the evolution of fault permeability with increasing strain, we measured the permeability of low-porosity volcanic rock samples (basalt and andesite) that were deformed in the brittle regime to various levels of inelastic strain. We observed a progressive increase in sample permeability with increasing inelastic strain (i.e., with continued sliding on the fault plane). At the maximum imposed inelastic strain (0.11), sample permeability had increased by 3 orders of magnitude or more for all sample sets. Microstructural observations show that narrow shear fractures evolve into more complex fracture systems characterized by thick zones of friction-induced cataclasis (gouge) with increasing inelastic strain. These data suggest that the permeability of conduit-parallel faults hosted in the rock at the conduit-wall rock interface will increase during lava extrusion, thus facilitating outgassing and hindering the transition to explosive behavior.

DOI： 10.1002/2016GL071540

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掲載種別：研究論文（学術雑誌）  

The ease at which exsolving volatiles can migrate though magma and outgas influences the explosivity of a volcanic eruption. Volcanic rocks often contain discrete discontinuities, providing snapshots of strain localisation processes that occur during magma ascent and extrusion.Whether these features comprise pathways for or barriers to fluid flow is thus of relevance for volcanic eruption and gas emission modelling. We report here on nine discontinuity-bearing andesite blocks collected fromVolcán de Colima, Mexico.Wepresent a systematic porosity and permeability study of fifty cores obtained fromthe blocks collected, and interpret the genetic processes of the discontinuities through detailed microstructural examination. Bands in pumiceous blockswere inferred to be relicts of inhomogeneous bubble expansion which, despite significantly increasing porosity, do not markedly affect permeability. Other discontinuities in our blocks are interpreted to be shear strain-induced flowbanding, cavitation porosity, and/or variably healed fractures. In each of these cases, an increase in permeability (up to around three orders of magnitude) was measured relative to the host material. A final sample contained a band of lower porosity than the host rock, characterised by variably infilled pores. In this case, the band was an order of magnitude less permeable than the host rock, highlighting the complex interplay between dilatant and densifying processes in magma. We therefore present evidence for significant permeability anisotropy within the conduit and/or dome of a volcanic system.We suggest that the abundance and distribution of strain localisation features will influence the escape or entrapment of volatiles and therefore the evolution of pore pressure within active volcanic systems. Using a simple upscaling model, we illustrate the relative importance of permeable structures over different lengthscales. Strain localisation processes resulting in permeability anisotropy are likely to play an important role in the style, magnitude, and recurrence interval of volcanic eruptions.

DOI： 10.1016/j.jvolgeores.2016.05.007

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掲載種別：研究論文（学術雑誌）  

The failure mode of porous rock in compression—dilatant or compactant—is largely governed by the overlying lithostatic pressure and the pressure of pore fluids within the rock (Wong, Solid Earth 102:3009–3025, 1997), both of which are subject to change in space and time within a volcanic edifice. While lithostatic pressure will tend to increase monotonously with depth due to the progressive accumulation of erupted products, pore pressures are prone to fluctuations (during periods of volcanic unrest, for example). An increase in pore fluid pressure can result in rock fracture, even at depths where the lithostatic pressure would otherwise preclude such dilatant behaviour—a process termed pore fluid-induced embrittlement. We explore this phenomenon through a series of targeted triaxial experiments on typical edifice-forming andesites (from Volcán de Colima, Mexico). We first show that increasing pore pressure over a range of timescales (on the order of 1 min to 1 day) can culminate in brittle failure of otherwise intact rock. Irrespective of the pore pressure increase rate, we record comparable accelerations in acoustic emission and strain prior to macroscopic failure. We further show that oscillating pore fluid pressures can cause iterative and cumulative damage, ultimately resulting in brittle failure under relatively low effective mean stress conditions. We find that macroscopic failure occurs once a critical threshold of damage is surpassed, suggesting that only small increases in pore pressure may be necessary to trigger failure in previously damaged rocks. Finally, we observe that inelastic compaction of volcanic rock (as we may expect in much of the deep edifice) can be overprinted by shear fractures due to this mechanism of embrittlement. Pore fluid-induced embrittlement of edifice rock during volcanic unrest is anticipated to be highest closer to the conduit and, as a result, may assist in the development of a fractured halo zone surrounding the conduit, potentially explaining commonly observed near-conduit outgassing at many active volcanoes. Further, rock embrittlement at depth may create transient outgassing pathways by linking fracture networks near the edifice to larger-scale regional fault systems. Our experimental results affirm that pore pressure fluctuations associated with volcanic unrest may play a crucial role in dictating the evolution of a volcanic system.

DOI： 10.1007/s00445-015-0997-9

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掲載種別：研究論文（学術雑誌）  

Transitions between effusive and explosive behaviour are routine for many active volcanoes. The permeability of the system, thought to help regulate eruption style, is likely therefore in a state of constant change. Viscous densification of conduit magma during effusive periods, resulting in physical and textural property modifications, may reduce permeability to that preparatory for an explosive eruption. We present here a study designed to estimate timescales of permeability reduction and strength recovery during viscous magma densification by coupling measurements of permeability and strength (using samples from a suite of variably welded, yet compositionally identical, volcanic deposits) with a rheological model for viscous compaction and a micromechanical model, respectively. Bayesian Information Criterion analysis confirms that our porosity-permeability data are best described by two power laws that intersect at a porosity of 0.155 (the "changepoint" porosity). Above and below this changepoint, the permeability-porosity relationship has a power law exponent of 8.8 and 1.0, respectively. Quantitative pore size analysis and micromechanical modelling highlight that the high exponent above the changepoint is due to the closure of wide (~200-300 μm) inter-granular flow channels during viscous densification and that, below the changepoint, the fluid pathway is restricted to narrow (~50 μm) channels. The large number of such narrow channels allows porosity loss without considerable permeability reduction, explaining the switch to a lower exponent. Using these data, our modelling predicts a permeability reduction of four orders of magnitude (for volcanically relevant temperatures and depths) and a strength increase of a factor of six on the order of days to weeks. This discrepancy suggests that, while the viscous densification of conduit magma will inhibit outgassing efficiency over time, the regions of the conduit prone to fracturing, such as the margins, will likely persistently re-fracture and keep the conduit margin permeable. The modelling therefore supports the notion that repeated fracture-healing cycles are responsible for the successive low-magnitude earthquakes associated with silicic dome extrusion. Taken together, our results indicate that the transition from effusive to explosive behaviour may rest on the competition between permeability reduction within the conduit and outgassing through fractures at the conduit margin. If the conditions for explosive behaviour are satisfied, the magma densification clock will be reset and the process will start again. The timescales of permeability reduction and strength recovery presented in this study may aid our understanding of the permeability evolution of conduit margin fractures, magma fracture-healing cycles, surface outgassing cycles, and the timescales required for pore pressure augmentation and the initiation of explosive eruptions.

DOI： 10.1016/j.epsl.2015.07.053

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掲載種別：研究論文（学術雑誌）  

During the 2011-2012 eruption at Cordón-Caulle, Chile, an extensive rhyolitic flowfield was created (in excess of 0.5km3 in volume), affording a unique opportunity to characterise rhyolitic lava advance. In 2012 and 2013, we acquired approximately 2500 digital photographs of active flowfronts on the north and east of the flowfield. These images were processed into three-dimensional point clouds using structure-from-motion and multi-view stereo (SfM-MVS) freeware, from which digital elevation models were derived. Sequential elevation models-separated by intervals of three hours, six days, and one year-were used to reconstruct spatial distributions of lava velocity and depth, and estimate rheological parameters. Three-dimensional reconstructions of flowfronts indicate that lateral extension of the rubbly, 'a'ā-like flowfield was accompanied by vertical inflation, which differed both spatially and temporally as a function of the underlying topography and localised supply of lava beneath the cooled upper carapace. Compressive processes also drove the formation of extensive surface ridges across the flowfield. Continued evolution of the flowfield resulted in the development of a compound flowfield morphology fed by iterative emplacement of breakout lobes. The thermal evolution of flow units was modelled using a one-dimensional finite difference method, which indicated prolonged residence of magma above its glass transition across the flowfield. We compare the estimated apparent viscosity (1.21-4.03×1010Pas) of a breakout lobe, based on its advance rate over a known slope, with plausible lava viscosities from published non-Arrhenian temperature-viscosity models and accounting for crystallinity (~50vol.%). There is an excellent correspondence between viscosity estimates when the lava temperature is taken to be magmatic, despite the breakout being located >3km from the vent, and advancing approximately nine months after vent effusion ceased. This indicates the remarkably effective insulation of the lava flow interior, providing scope for significant evolution of rhyolitic flow fields long after effusive activity has ceased.

DOI： 10.1016/j.jvolgeores.2015.09.004

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掲載種別：研究論文（学術雑誌）  

The failure mode of lava—dilatant or compactant—depends on the physical attributes of the lava, primarily the porosity and pore size, and the conditions under which it deforms. The failure mode for edifice host rock has attendant implications for the structural stability of the edifice and the efficiency of the sidewall outgassing of the volcanic conduit. In this contribution, we present a systematic experimental study on the failure mode of edifice-forming andesitic rocks (porosity from 7 to 25 %) from Volcán de Colima, Mexico. The experiments show that, at shallow depths (<1 km), both low- and high-porosity lavas dilate and fail by shear fracturing. However, deeper in the edifice (>1 km), while low-porosity (<10 %) lava remains dilatant, the failure of high-porosity lava is compactant and driven by cataclastic pore collapse. Although inelastic compaction is typically characterised by the absence of strain localisation, we observe compactive localisation features in our porous andesite lavas manifest as subplanar surfaces of collapsed pores. In terms of volcano stability, faulting in the upper edifice could destabilise the volcano, leading to an increased risk of flank or large-scale dome collapse, while compactant deformation deeper in the edifice may emerge as a viable mechanism driving volcano subsidence, spreading and destabilisation. The failure mode influences the evolution of rock physical properties: permeability measurements demonstrate that a throughgoing tensile fracture increases sample permeability (i.e. equivalent permeability) by about a factor of two, and that inelastic compaction to an axial strain of 4.5 % reduces sample permeability by an order of magnitude. The implication of these data is that sidewall outgassing may therefore be efficient in the shallow edifice, where rock can fracture, but may be impeded deeper in the edifice due to compaction. The explosive potential of a volcano may therefore be subject to increase over time if the progressive compaction and permeability reduction in the lower edifice cannot be offset by the formation of permeable fracture pathways in the upper edifice. The mode of failure of the edifice host rock is therefore likely to be an important factor controlling lateral outgassing and thus eruption style (effusive versus explosive) at stratovolcanoes.

DOI： 10.1007/s00445-015-0938-7

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掲載種別：研究論文（学術雑誌）  

Volcanic hydrothermal systems host a prodigious variety of physico-chemical conditions. The physico-chemical state and mechanical behaviour of rocks within is correspondingly complex and often characterised by vast heterogeneity. Here, we present uniaxial and triaxial compression experiments designed to investigate the breadth of mechanical behaviour and failure modes (dilatant or compactant) for hydrothermally-altered lava and ash tuff deposits from Whakaari (White Island volcano) in New Zealand, a volcano with a well-documented and very active hydrothermal system. Our deformation experiments show that the failure mode of low porosity lava remains dilatant over a range of depths (up to pressures corresponding to depths of about 2. km). Upon failure, shear fractures, the result of the coalescence of dilatational microcracks, are universally present. The high porosity ash tuffs switch however from a dilatant to a compactant failure mode (driven by progressive distributed pore collapse) at relatively low pressure (corresponding to a depth of about 250. m). We capture the salient features of the dynamic conditions (e.g., differential stress, effective pressure) in a schematic cross section for the Whakaari hydrothermal system and map, for the different lithologies, areas susceptible to either dilatant vs. compactive modes of failure. The failure mode will impact, for example, the evolution of rock physical properties (e.g., porosity, permeability, and elastic wave velocity) and the nature of the seismicity accompanying periods of unrest. We outline accordingly the potential implications for the interpretation of seismic signals, outgassing, ground deformation, and the volcanic structural stability for Whakaari and similar hydrothermally-active volcanoes worldwide.

DOI： 10.1016/j.jvolgeores.2015.02.012

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掲載種別：研究論文（学術雑誌）  

Welding of pyroclastic deposits to reform a coherent rock mass is a common phenomenon, especially for pumiceous pyroclastic density current deposits (i.e., ignimbrites). However, and despite the pervasive abundance of block-and-ash flow (BAF) deposits in the geological and modern record, instances of strongly welded BAF deposits are few. Here, we present a series of high-temperature (800-900. °C) compaction experiments designed to map the conditions (deposit thickness/stress and temperature/viscosity) and timescales that permit or inhibit the welding of BAF deposits. Our experiments were performed on unconsolidated aggregates (containing an ash and lapilli component) derived from crushed and sieved lava blocks (containing 25% crystals) taken from the well-documented welded BAF deposit at Mount Meager volcano (British Columbia, Canada). The experiments demonstrate that welding efficiency increases with increasing time and temperature. Progressive welding is expressed by increasing axial strain, porosity loss, and bulk density. The rate of change of each of these physical properties reduces as welding progresses. Microstructural analysis of the experimental products shows that the loss of interclast porosity during welding results from the progressive sintering and amalgamation of vitric fragments, and that the pore shape changes from sub-equant pores to stretched lenses sandwiched between vitric and crystal fragments. The coincidence between the microstructure and rock physical properties of the natural and experimental samples highlight that we have successfully reproduced welded BAF in the laboratory. Furthermore, our permeability measurements highlight a hysteresis in the return journey of the ". there-and-back-again" volcanic permeability cycle (expressed by an increase in permeability due to vesiculation and fragmentation followed by a decrease due to welding). This hysteresis cannot be described by a single porosity-permeability power law relationship and reflects the change in pore shape and connectivity during welding. Finally, we show that a simple model for welding can accurately forecast the welding timescales of the BAF deposit at Mount Meager (as reconstructed from the collapse of the Lillooet River valley dam) using our experimental data. We use this validation as a platform to provide a universal window for the welding of BAF deposits, also applicable for comparable welded deposits (e.g., welded autobreccias in block-lavas and lava domes), for a broad range of deposit thickness (or stress) and effective viscosity.

DOI： 10.1016/j.jvolgeores.2014.11.010

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