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Shear thinning and brittle failure of silicate melt control the dynamics of volcanic eruptions, but their molecular‐scale origin is still unclear. Here, we conducted tension and compression experiments on silicate melts, using time‐resolved X‐ray diffraction. Our experiments revealed that the intermediate‐range ordering of silicate structures, that is, the ring size formed by the SiO4 tetrahedra, demonstrated elastic and anisotropic dilation under tension and shrinkage under compression in the non‐Newtonian regime. In contrast, there were no significant changes in short‐range ordering, such as Si–O and Si–Si distances. Based on these findings, we inferred that shear thinning observed under high stress originates from the formation of anisotropically deformed large and small rings in silicate structures that are energetically unfavorable and unstable. Brittle failure occurred under high‐stress conditions, in both tension and compression. We propose a stress criterion as a necessary and sufficient condition for magma failure, rather than a strain rate criterion. Plain Language Summary: Explosive volcanic eruptions occur when magma fragments and volcanic gases are released to the surface. However, magma is originally a continuous fluid that behaves like a liquid in the crust. To understand how magma fails during volcanic eruptions, we need to uncover the underlying mechanisms. This challenge was identified over four decades ago, and researchers proposed conducting molecular‐scale experiments on magma deformation to shed light on its complex behavior, including brittle failure. However, this has been difficult to achieve experimentally. In this study, we used powerful X‐ray sources at SPring‐8 in Japan to address this challenge. Our findings reveal that the complex behavior of magma originates from previously unrecognized molecular‐scale elastic and anisotropic deformation. Finally, we propose a criterion for magma failure that can help determine whether an eruption will be explosive. Key Points: Molecular‐scale elastic and anisotropic deformation of the silicate melt was observed under tension (dilation) and compression (shrinkage)Formation of large and small rings by the SiO4 tetrahedra could be the origin of shear thinning and brittle failure in the silicate meltStress condition is "the necessary and sufficient condition" for the magma failure criterion [ABSTRACT FROM AUTHOR]

The eruption activity of Shinmoedake in the Kirishima volcanic group of Japan resumed in 2017–18, following a quiet period during 2011–17. Subplinian eruptions preceded lava effusion in 2011; however, no subplinian eruption occurred during 2017–18. Petrological studies and melt inclusion analyses were conducted to investigate the ascent and degassing of the magma to understand the cause of the different eruption styles. Chemical analysis of the melt inclusions from the 2011 eruption indicates that mafic magma with high volatile content (6.2 wt% H2O, 0.25–1.4 wt% CO2) ascended into the shallow felsic magma (1.9–3.7 wt% H2O, 0.025–0.048 wt% CO2) at depths of 5–6 km. Calculations indicate that the mafic magmas were of lower density (1717–1835 kg m−3) than the felsic magma (2264–2496 kg m−3) at 125 MPa and that the two magmas were mixed. The 2011 mixed magma with high volatile content (4.0 wt% H2O, 0.14–0.70 wt% CO2) had a bubble volume of approximately 50 vol% at 50 MPa, which is likely to have caused the subplinian eruption. The whole-rock and chemical compositions of the plagioclase, clinopyroxene, and orthopyroxene phenocryst cores from 2018 and 2011 were similar, suggesting that the 2018 magma was a remnant of the 2011 magma. Chemical analyses of the groundmass from 2018 and the MELTS calculation indicate that the magma approached chemical equilibrium during 2011–18. Melt inclusion analyses and volcanic gas observation noted a lower bulk volatile content in the 2018 magma (2.1–3.0 wt% H2O, 0.087–0.10 wt% CO2) than that in the 2011 magma. Comparison of the degassed-magma volumes estimated from the S and Cl contents of the melt inclusions, SO2 flux and volcanic gas composition, and erupted-magma volume indicates that excess degassing has been occurring in the magma due to convection since February 2011, which may have decreased the volatile content of the magma. The relatively low volatile content meant that the 2018 magma could not erupt explosively and lava was instead erupted via effusion. [ABSTRACT FROM AUTHOR]

Recent developments in unmanned aerial vehicle (UAV) technology have made it possible to measure gas compositions in volcanic plumes using lightweight compact gas sensors. However, the differences in the responses of each gas sensor can be critical in estimating gas compositions based on regression scatter plots, particularly for small plumes emitted during volcanic unrest and non-eruption periods. Based on the laboratory experiments, we show that air blowers easily adjust sensor responses and improve correlation on regression scatter plots, allowing quick composition estimates without the use of mathematical applications. Applying our measurement system, lightweight compact gas sensors for H2S, SO2, CO2, and H2O, with air blowers suspended from a UAV, were used to determine the compositions of a small plume at Io-yama, Kirishima volcano, Japan. The compositions of the plume estimated by our system were reasonably consistent with those obtained by laboratory analysis of volcanic gas collected at ground level near the vent, with fluctuations in CO2 ratios and lower H2O ratios, relative to other gases, being observed. For more accurate estimations of CO2 and H2O concentrations, low humidity conditions at a distance from the fumarole are preferable for analysis of plumes diluted by ambient dry air. Our measurement system is simple, easy to set up, and useful for estimating the compositions of small passive fumarolic gas plumes during volcanic unrest and non-eruption periods, without mathematical applications. [ABSTRACT FROM AUTHOR]

We modeled the vertical deformation detected from a leveling survey in Jigokudani valley, Tateyama volcano, central Japan. In Jigokudani valley, uplift of 4 cm/year was previously detected during the period from 2007 to 2010 by interferometric synthetic aperture radar (InSAR). To confirm whether this inflation has continued to present day, we have conducted leveling surveys in Jigokudani valley since 2015. Most bench marks showed a subsidence of up to 5.6 cm during the 4-year period from October 2016 to September 2020, while a bench mark located at the center of the leveling route uniquely showed an uplift of 1.6 cm. We applied a dislocation source model to the deformation using a grid search method. A crack with a length of 650 m, a width of 425 m, a strike of N18° E and a dip of 67° is located at a depth of 50 m near the center of Jigokudani valley (Koya jigoku and the new fumarolic area) where higher activity has been observed recently. Closing of the crack of 59 cm yields a volume decrease of 163,000 m3. The closing direction of the crack is parallel to the line of old explosion craters (Mikurigaike and Midorigaike ponds) and corresponds to the current maximum compressive stress field in the region of the Hida Mountains, including Tateyama volcano. The deformation source of the previous period from 2007 to 2010 detected from InSAR was estimated to be at a depth of 50 m and a gas chamber was correspondingly found in an audio-frequency magnetotelluric (AMT) survey. The AMT survey also revealed that thermal fluid is accumulating from a magma chamber and the location of our crack is similar to uppermost part of the thermal fluid path. During the period from 2015 to 2016, the crack opened and the inflation stopped during the next 1 year period from 2016 to 2017. During the period from 2017 to 2020, the crack began closing, probably because of the increase in emissions of volcanic fluid or gas with the formation of a new crater at the western side of Jigokudani valley during the period from 2017 to 2018. [ABSTRACT FROM AUTHOR]

Direct sampling and analysis of fumarolic gas was conducted at Ebinokogen Ioyama volcano, Japan, between December 2015 and July 2020. Notable changes in the chemical composition of gases related to volcanic activity included a sharp increase in SO2 and H2 concentrations in May 2017 and March 2018. The analyses in March 2018 immediately preceded the April 2018 eruption at Ioyama volcano. The isotopic ratios of H2O in fumarolic gas revealed the process of formation. Up to 49% high-enthalpy magmatic vapor mixed with 51% of cold local meteoric water to generate coexisting vapor and liquid phases at 100–160 °C. Portions of the vapor and liquid phases were discharged as fumarolic gases and hot spring water, respectively. The CO2/SO2 ratio of the fumarolic gas was higher than that estimated for magmatic vapor due to SO2 hydrolysis during the formation of the vapor phase. When the flux of the magmatic vapor was high, effects of hydrolysis were small resulting in low CO2/SO2 ratios in fumarolic gases. The high apparent equilibrium temperature defined for reactions involving SO2, H2S, H2 and H2O, together with low CO2/SO2 and H2S /SO2 ratios were regarded to be precursor signals to the phreatic eruption at Ioyama volcano. The apparent equilibrium temperature increased rapidly in May 2017 and March 2018 suggesting an increased flux of magmatic vapor. Between September 2017 and January 2018, the apparent equilibrium temperature was low suggesting the suppression of magmatic vapor flux. During this period, magmatic eruptions took place at Shinmoedake volcano 5 km away from Ioyama volcano. We conclude that magma sealing and transport to Shinmoedake volcano occurred simultaneously in the magma chamber beneath Ioyama volcano. [ABSTRACT FROM AUTHOR]

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The island of Miyakejima in Japan is subject to ongoing emissions of volcanic gases, including high concentrations of sulfur dioxide (SO2). Annual health checkups on the island, therefore, include the examination of respiratory system parameters. Here, we aimed to investigate the relationship between SO2 exposure and fractional exhaled nitric oxide (FeNO) concentration among children who received health checkups from 2008 to 2014. The subjects were 83 and 31 second-year junior high school students aged 13-14 years who resided on Miyakejima island and an SO2-free reference island, respectively. SO2 concentration in the air was measured at 6 fixed-point monitoring stations. FeNO was examined according to the American Thoracic Society guideline and European Respiratory Society recommendations for standardized procedures. Average concentrations of SO2 on Miyakejima over a period of 3 months prior to each health checkup from 2008 to 2014 were 22.2, 20.6, 8.9, 10.5, 10.7, 4.4 and 8.0 ppb, respectively. Among the Miyakejima students, geometric mean (GM) FeNO concentrations measured at each health checkup from 2008 to 2014 were 28.2, 18.2, 23.6, 35.5, 36.9, 28.1, and 32.1 ppb. The GM FeNO concentration measured from all Miyakejima students across the study period was 28.3 ppb. No clear dose-response relationship was observed. The GM FeNO concentration among the students from the reference island was 27.7 ppb in 2017. No significant difference was observed between the two populations, even when the data was stratified by sex and sensitivity. There was no clear significant difference in GM of FeNO concentration between Miyakejima and control students, when the average concentration of SO2 over a period of 3 months was 22 ppb or less. [ABSTRACT FROM AUTHOR]

Phreatic eruptions at Nakamachineshiri and Ponmachineshiri reflect the vigorous fumarolic and hydrothermal activities of Meakandake volcano. Thus, phreatic eruptions are likely to reoccur in the future, and hydrothermal systems should be understood to mitigate the risks of potential future hazards. Here, we reveal the results of long-term geochemical monitoring of hot spring waters and fumarolic gases since 1986 and discuss the chemical and temperature changes in hot spring waters, discharging at the foot, related to volcanic unrest. In Nakamachineshiri, related to the increase in seismic and fumarolic activity since 2019, the HCl concentration of the fumarolic gas has increased, and δD and δ18O values have also increased to approach those of the andesitic water. Simultaneously, with the increase in fumarolic and hydrothermal activity, the temperature and Cl concentration of the Meakan Onsen (MO) hot spring water discharged at the western foot of Nakamachineshiri have increased. The chemical characteristics of the fumarolic gas and MO water indicate that they were derived from a hydrothermal system in the shallow subsurface of Nakamachineshiri. Thus, we can consider that the temperature and chemical increases in the MO water represent activation of the system, and monitoring the MO water provides important information for understanding the present state of Nakamachineshiri. At Ponmachineshiri, phreatic eruptions occurred in 1988, 1996, 1998, 2006, and 2008, and seismic and fumarolic activity has sometimes increased since 1986 when we began our monitoring program. At the western foot of Ponmachineshiri, hot spring water is discharged at Yunotaki (YNT_U and YNT_L) and Lake Onneto (ONT). The results of our monitoring suggest that the YNT_U water is formed by mixing thermal water with a high HCO 3 molar ratio (HCO 3 -type) and thermal water containing Cl and SO 4 (Cl-SO 4 -type), which causes temperature and chemical changes related to the mixing ratio of these thermal waters related to volcanic unrest, especially increases in seismic activity. The Cl and SO 4 concentrations in the ONT water began to increase approximately two years before the phreatic eruptions and increase in fumarolic activity. Such chemical increases can be explained by the increase in the supply of Cl-SO 4 -type thermal water and are likely to indicate precursor changes in volcanic unrest. Therefore, we expect to utilize these hot spring waters to understand the present state of Ponmachineshiri and detect precursor changes to future volcanic unrest. • Long-term monitoring for hot spring waters and fumarolic gases has been performed at Meakandake volcano. • Chemical and thermal changes related to volcanic unrest were detected in hot spring waters discharged at the foot. • Monitoring for hot spring waters is useful to understand the present state of volcanic activity. [ABSTRACT FROM AUTHOR]

Continuous self-potential (SP) monitoring has been conducted at Izu-Oshima volcano to detect signals resulting from volcanic activity since the installation of an SP monitoring network in March 2006. Since the installation, annual variations of up to 100 mV have been recorded. If we exclude these annual variations, temporal variations in SP do not show notable changes. This is consistent with the volcano being in a state of quiescence during the measurement period. The annual variations have the different amplitudes and mean levels between stations. To investigate the causes of these annual variations, we carried out numerical simulations of SP generation associated with downward meteoric water flow through electrokinetic coupling in a ~ 550 m thick unsaturated layer. The results show that the vertical electric potential gradient varies with changes in liquid-phase saturation in the unsaturated layer. These changes are caused by variations in the rate of meteoric water percolation. This, in turn, correlates with fluctuations in daily precipitation, thus explaining the annual SP variation recorded at the ground surface. Differences in the amplitude and mean level of SP variation are shown to be associated with different rock properties, especially permeability, porosity, and electrical conductivity. Our results indicate that observable SP changes will appear at stations near the summit if the distributions of liquid-phase saturation and/or pertinent parameters controlling the electrokinetic coupling in the thick unsaturated layer are modified the upward flow of volcanic gas. [ABSTRACT FROM AUTHOR]

During 2014–2016, the Aso volcano, located in the center of the Kyushu Islands, Japan, erupted and emitted large amounts of volcanic gases and ash. Two episodes of the eruption were observed; firstly Strombolian magmatic eruptive episodes from 25 November 2014 to the middle of May 2015, and secondly phreatomagmatic and phreatic eruptive episodes from September 2015 to February 2016. Bulk chemical analyses on total mercury (Hg) and major ions in water soluble fraction in volcanic ash fall samples were conducted. During the Strombolian magmatic eruptive episodes, total Hg concentrations averaged 1.69 ± 0.87 ng g − 1 (N = 33), with a range from 0.47 to 3.8 ng g − 1 . In addition, the temporal variation of total Hg concentrations in volcanic ash varied with the amplitude change of seismic signals. In the Aso volcano, the volcanic tremors are always observed during eruptive stages and quiet interludes, and the amplitudes of tremors increase at eruptive stages. So, the temporal variation of total Hg concentrations could provide an indication of the level of volcanic activity. During the phreatomagmatic and phreatic eruptive episodes, on the other hand, total Hg concentrations in the volcanic ash fall samples averaged 220 ± 88 ng g − 1 (N = 5), corresponding to 100 times higher than those during the Strombolian eruptive episode. Therefore, it is possible that total Hg concentrations in volcanic ash samples are largely varied depending on the eruptive type. In addition, the ash fall amounts were also largely different among the two eruptive episodes. This can be also one of the factors controlling Hg concentrations in volcanic ash. [ABSTRACT FROM AUTHOR]

Since Japan has 111 active volcanoes, we living there need to be prepared for the volcanic disasters that we will likely encounter in our lifetime. The Integrated Program for Next Generation Volcano Research and Human Resource Development (INeVRH), a research project of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), started in 2016 and is now in its 7th year. It is approaching its final stage will be ready to make proposals that will improve our knowledge from both scientific and practical points of view. In theme A, the Japan Volcanic Data Network (JVDN) system started its function as a volcanic research platform, providing the observation and analysis data of many universities, governmental agencies, and institutes. The JVDN system promotes collaborative, multidisciplinary study. In theme B, many strategic observation techniques now deployed have revealed the characteristics of volcanoes in detail. For example, a dense seismic and magnetic survey has clarified the fine structure of the hydrothermal system beneath volcanoes, providing information that can be used to evaluate their phreatic eruption potential. In addition, brand-new volcanic gas observation techniques using drones now obtain in-situ information effectively. Theme C has obtained geologic and petrologic findings at many volcanoes and has come to the stage in which systematic compilation of this database would be useful to comparative studies of many volcanoes. These findings are also used in numerical simulations that combine geologic, petrologic, and geophysical formulations to produce more comprehensive models used to interpret volcanic activity. Theme D focuses on the development of practical technologies for volcanic disaster mitigation, mainly related to volcanic ash. A scheme for the quick detection and evaluation of volcanic ash is proposed through observation and numerical simulation, and an experiment is performed to reveal the thickness threshold of volcanic ash deposits on air-conditioners. This special issue reports on some up-to-date topics that could become the basis of dynamic and effective links between themes A, B, C, and D, which may serve as a base and direction for discussions that summarize and conclude this 10-year project. [ABSTRACT FROM AUTHOR]

The composition of volcanic gases discharged from Miyakejima volcano has been monitored during the intensive degassing activity that began after the eruption in 2000. During the 15 years from 2000 to 2015, Miyakejima volcano discharged 25.5 Mt of SO, which required degassing of 3 km of basaltic magma. The SO emission rate peaked at 50 kt/day at the end of 2000 and quickly decreased to 5 kt/day by 2003. During the early degassing period, the volcanic gas composition was constant with the CO/SO = 0.8 (mol ratio), HO/SO = 35, HCl/SO = 0.08, and SO/HS = 15. The SO emission rate decreased gradually to 0.5 kt/day by 2012, and the gas composition also changed gradually to CO/SO = 1.5, HO/SO = 150, HCl/SO = 0.15, and SO/HS = 6. The compositional changes are not likely caused by changes in degassing pressure or volatile heterogeneity of a magma chamber but are likely attributed to an increase of hydrothermal scrubbing caused by large decrease of the volcanic gas emission rate, suggesting a supply of gases with constant composition during the 15 years. The intensive degassing was modeled based on degassing of a convecting magma conduit. The gradual SO emission rate that decrease without changes in volcanic gas composition is attributed to a reduction of diameter of the convecting magma conduit. [ABSTRACT FROM AUTHOR]

The purpose of this study is to characterize in detail the temporal changes in Rn (radon-222) concentration in soil gases near fumaroles and clarify its correlation with volcanic earthquakes and temperatures in two geothermal reservoirs. Mt. Aso crater in southwest Japan, which has two reservoirs on its western side estimated by magnetotelluric survey to be at about 2km in depth, was selected for this study. For the long-term survey, the α scintillation counter method was used weekly for 12.5years at the three hot springs within a 2-km range. Rn concentrations were calculated using the CRAS method, a calculation method that considers radioactive equilibrium or nonequilibrium state of the soil gas. Rn concentrations generally showed similar fluctuation patterns among the sites. CRAS was used as a new indicator for evaluating the age of the soil gas. This age corresponds to the elapsed time determined from the generation of Rn based on the measurement of the numbers of atoms of Rn and its daughter 218Po at the start of measurement. In comparing the Rn data with the history of earthquakes in the Aso caldera, volcanic seismicity was identified as a major controlling factor in the sudden increase and decrease in Rn concentration as a function of age. For more precise detections of change, Rn concentrations were measured continuously at one site by pumping soil gas from a borehole and using an ionization chamber over 2.5years. Five chemical components (He, H2, N2, CH4, and CO2) were then measured by gas chromatography at 1-week intervals. Because Rn concentrations are affected strongly by atmospheric temperatures, the residual components were obtained by subtracting the trend of the components from the original data. Chemical component data were used to estimate the temperature and pressure in the reservoir at the site; temperatures ranged from 229 to 280°C, (average 265°C, average pressure 80MPa). Residual Rn concentrations showed a clear correlation with temporal changes in estimated temperature and, to a degree, with pressure. Considering the migration mechanism of Rn by molecular diffusion and advection under rapid gas upflow conditions, change in the diffusion coefficient is regarded as a possible factor to have induced temporal changes in Rn concentrations in conjunction with the temperature changes in the reservoirs. In addition, the increase of the amount of degassed Rn from the advective–convective hydrothermal fluids, which is associated with wall-rock fracturing, is another important factor. Earthquakes likely enhance the permeability of a reservoir by generating fractures, which in turn can cause an increase in the reservoir temperature, upflow fluid velocity, and gas flux. This provides one possible interpretation for the significant correlation of Rn concentrations with earthquakes. [ABSTRACT FROM AUTHOR]

Abstract: Global volatile fluxes from subaerial volcanoes at subduction zones were estimated based on a compilation of fluxes from various sources, including persistent degassing, hot and cold springs, soil degassing, and eruptions. Because worldwide comprehensive datasets are not available, especially for diffuse volatile discharges, volatile fluxes from Japan arcs were estimated based on detailed datasets, and the regional fluxes were extrapolated to the global flux with consideration of the regional characteristics of volcanic volatile compositions, which were estimated based on volcanic gas compositions of persistent degassing. The estimated global fluxes indicate that persistent degassing is the major source of volatiles, especially for S with a contribution of 80%. Diffuse discharges and persistent degassing are similarly important sources of H2O, CO2, and Cl, but the contribution of explosive eruptions is less than 15% for all the volatiles. The estimates of diffuse degassing fluxes include large errors due to limited data. However, the potential impact of these sources on the global flux indicates the importance of further studies to quantify these fluxes. The volatile budget of subduction zone volcanism was evaluated by comparing the estimated volatile fluxes, the volatile contents in the crust, and the primitive magma volatile contents. The contribution of volatiles remaining in the crust are not significant; however, consideration of lower crust foundering significantly alters the volatile budget estimate because the primitive magma supply rate should be significantly increased to account for the lower crust foundering. [Copyright &y& Elsevier]

We examine precursory signals of vulcanian eruptions at Suwanosejima volcano, southwest of Japan, by deploying seismic and tilt stations close to the active crater to understand the shallow conduit processes just before explosions. Vulcanian eruptions at Suwanosejima for the period from October 2009 to 2011 are often preceded by a sudden cessation of continuous tremor associated with continuous ash emissions. The amplitude of continuous tremor decreases within about 10 s, and tilt signal starts to show uplift of the active crater. Then, the pause of continuous tremor and uplift continue for about 1–2 min, and tilt amplitudes reach a few tens of nanoradians at a distance of 400 m from the active crater. As the pause time of continuous tremor before explosion becomes longer, the maximum amplitude of explosion earthquake and uplift signal become larger. Vulcanian eruptions are also observed during periods without continuous tremor. In these cases, tilt signals capture uplifts of the active crater for about 10 min before explosions and reach a few tens of nanoradians. Cessations of volcanic gas emissions are observed before some explosions during periods without continuous tremor (Mori et al. 2011 ). These observations suggest that pressurization in the shallow conduit is caused by an accumulation of volcanic gases and ash through the formation of ‘cap’ in the conduit, possibly composed of stiffened magma due to degassing, cooling, rheological changes, or other mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: We report here the development and application of a compact “geochemical nose” incorporating electrochemical sensors for gas measurements in volcanic plumes. A novel element of the instrument design is the arrangement of the sensors in a parallel array that enables near-simultaneous exposure and fast response. Data analysis methods were developed that utilise the multi-sensor output currents to extract gas mixing ratio abundances and eliminate cross-sensitivities. Use of filter methods is demonstrated to remove baseline drift or instrument noise. We introduce a new approach for analysis of measurements from sensors that have a slower response time (e.g. HCl), and apply this model to estimate HCl/SO2 ratios. We deployed the sensor system at Aso volcano, Japan, detecting emissions from its fumarole field hot crater lake, and a mixed plume. We measured SO2, H2S, CO and HCl, ranging in abundance from ~102–104 ppbv. Neither NO2 nor Cl2 were detected. For the fumarolic gases, molar ratios were measured as follows: H2S/SO2 is ~0.15, H2/SO2 ~0.25, CO/SO2 ~0.02, HCl/SO2 ~0.1. The crater lake plume''s H2S/SO2 is ~0.03. The compositions are discussed in terms of degassing equilibria and plume chemistry. Our instrument design represents a cost-effective, low-power and highly portable system that can be readily adapted for operational surveillance of volcanic gases. [Copyright &y& Elsevier]

Phreatic eruptions occurred at the Meakandake volcano in 1988, 1996, 1998, 2006, and 2008. We conducted geochemical surveillance that included measurements of temperature, SO emission rates, and volcanic gas composition from 2003 to 2008 at the Nakamachineshiri (NM), Northwest (NW), and Akanuma (AK) fumarolic areas, and the 96-1 vent, where historical eruptions had occurred. The elemental compositions of the gases discharged from the different areas are similar compared with the large variations observed in volcanic gases discharged from subduction zones. All the gases showed high apparent equilibrium temperatures, suggesting that all these gases originated from a common magmatic gas. The gases discharged from each area also exhibited different characteristics, which are probably the results of differences in the conditions of meteoric water mixing, quenching of chemical reactions, and vapor-liquid separation. The highest apparent equilibrium temperatures (about 500°C) were observed in the case of NW fumarolic gases, despite the low outlet temperature of about 100°C at these fumaroles. Since the NW fumaroles were formed as a result of the 2006 phreatic eruption, the high-temperature gas supply to the NW fumarole suggests that the phreatic eruption was caused by the ascent of high-temperature magmatic gases. The temperatures, compositions, and emission rates of the NM and 96-1 gases did not show any appreciable change after the 2006 eruption, indicating that each fumarolic system had a separate magmatic-hydrothermal system. The temperatures, compositions, and emission rates of the NM fumarolic gases were apparently constant, and these fumaroles are inferred to be formed by the evaporation of a hydrothermal system with a constant temperature of about 300°C. The 96-1 gas compositions showed large changes during continuous temperature decrease from 390° to 190°C occurred from 2003 to 2008, but the sulfur gas emission rates were almost constant at about four tons/day. At the 96-1 vent, the SO/HS ratio decreased, while the H/HO ratio remained almost constant; this was probably caused by the rock-buffer controlled chemical reaction during the temperature decrease. [ABSTRACT FROM AUTHOR]

The chemical and isotopic compositions of volcanic gases at a borehole and a natural fumarole in the Owakudani geothermal area, Hakone volcano, Japan, have been repeatedly measured since 2001, when a seismic swarm occurred in the area. The CO/HO and CO/HS ratios were high in 2001. It increased in 2006 and again in 2008 when seismic swarms occurred beneath the geothermal area. The observed increases suggest the injection of CO- and SO-rich magmatic gas into the underlying hydrothermal reservoir, implying that the magmatic gas was episodically supplied to the hydrothermal system in 2006 and 2008. The earthquake swarms probably resulted from the injection of gas through the shallow crust accompanying the break of the sealing zone. [ABSTRACT FROM AUTHOR]

Abstract: Chemical and isotopic compositions of low to medium temperature fumarolic gasses were measured at various fumaroles of Kuchinoerabujima volcano from 1993 to 2009 by the combination of fumarolic gas sampling and Multi-GAS measurement of volcanic plumes. Repeated culmination of the volcanic activity was observed as contemporaneous occurrence of seismic swarms, summit inflation and demagnetization, almost every two years after 1999. Fumarolic activity also increased parallel with these activities; new fumarolic fields of low-temperature fumaroles at boiling point formed at the southern and western rims of the summit crater in 2003 and intense degassing activity started at medium-temperature fumaroles inside the summit crater in 2008. The low-temperature fumarolic gasses have peculiar composition with high H2/H2O, CO/CO2 and SO2/H2S ratios, typical features of high-temperature volcanic gasses, but also with low HCl and total S contents, typical features of low-temperature gasses. These features indicate that the gasses are formed by low-temperature differentiation of high-temperature gasses. Variation of H and O isotopic ratios of the low-temperature gasses indicates that the gasses are formed by isoenthalpic processes of vapor–liquid separation at 100°C of a high-temperature gas and meteoric water mixture, implying that the high-temperature gas was injected into a shallow aquifer without cooling. Temperatures of the high-temperature gasses were estimated as 550–700°C based on the equilibrium temperature calculation with considering the low-temperature differentiation. The medium-temperature fumarolic gasses from the summit crater have typical compositions of high-temperature volcanic gasses and are estimated as the source gas of the boiling point fumaroles. The H2/H2O and CO2/St ratios of the low-temperature fumaroles increased from 2004 to 2009, which is interpreted to be caused by the evolution of thermal structure of a shallow gas storage region of a few hundred meters deep. Increase of magmatic gas supply caused enlargement of a high-temperature zone of the gas storage region widening the surrounding temperature transit zone with smaller temperature gradient. The enlargement of the high-temperature zone also reduces the overlying low-temperature aquifer, which caused the decrease of CO2/St ratio by reduction of sulfur removal reaction. The enlargement of the high-temperature zone resulted in the leakage of the high-temperature gasses directly to the surface as the medium-temperature gasses from the summit crater. [Copyright &y& Elsevier]

Abstract: The thermal activity of the Iwodake volcano at Satsuma-Iwojima, Japan, is characterized by the predominant volcanic gas discharge at the summit crater and heat discharge from the high ground temperature area, which is distributed widely from the summit crater to the mountainside. In this study, the surface temperature distribution was obtained by an airborne survey using an infrared thermal camera to estimate the heat flux from the ground surface to the atmosphere. The heat discharge rate from the high ground temperature area was estimated to be 22 and 84MW at the summit and the mountainside, respectively. Earlier studies showed that the volcanic gas at the summit crater was of magmatic origin with a discharge rate of 400kg/s and that the degassing depth was very shallow. The degassing activity of the Iwodake volcano is thought to have continued for more than 800years. The continuous active degassing at shallow depth probably results in the development of a hydrothermal system within the volcano because the volcanic gas ascending the degassing vent from the top of the magma to the surface is diffused to the surrounding formation. The diffused volcanic gas produces a wide-ranging surface temperature anomaly. The development of such a hydrothermal system was studied using a numerical simulation. The results confirmed that the overall thermal activity of the Iwodake volcano, i.e., the volcanic gas ejection at the summit crater, widely distributed ground temperature anomaly on the mountainside, and hot springs along the coast, could be formed by the degassing activity. Important factors controlling the wide-ranging hydrothermal system are the permeability of the volcanic edifice and the depth of degassing. Simulations indicated that degassing depth from 300 to 500m below the ground surface, permeability of the mountain edifice above 7×10−14, and vent permeability from 3.8×10−10 to 9.7×10−10 m2 were suitable conditions for the development of the Iwodake thermal activity. [Copyright &y& Elsevier]

Abstract: Vertical variation of the oxidation state as recorded by Fe–Ti oxides of Takanoobane rhyolite lava (TR lava) is revealed on the basis of petrographic and rock magnetic studies. Petrographic observations of the interior of TR lava showed that Fe–Ti oxides are preferentially oxidized along the flow structure that is characterized by the light-colored band composed of aggregates of deformed minute cavities, whereas both the top and bottom parts of the lava are not oxidized. Rock magnetic studies measuring thermal demagnetization of a three-component isothermal remanent magnetization (IRM) and magnetic susceptibility revealed that hematite of a highly oxidized state is formed preferentially along the light-colored band, consistent with the petrographic observations. The observed preferential oxidation of the light-colored band is adequately explained by post-eruptive degassing of hydrogen from the interior of the lava, rather than migration of atmospheric oxygen into the hot interior of the cooling lava against the back-streaming volcanic gas. The magmatic volatiles emitted from the interior can be transported effectively through the light-colored band and microcracks which ubiquitously develop nearly vertical to the flow direction. The degassing mechanism should be limited to highly viscous silicic lavas such as rhyolite because the mechanism requires development of the flow structure and the microcracks as the pathways for gases. These structures are achieved only in highly viscous silicic lavas. [ABSTRACT FROM AUTHOR]

Abstract: Volcanic edifices often have hydrothermal systems in the shallow subsurface, and these hydrothermal systems may discharge volcanic gases. Electrical conductivity (resistivity) surveys have potential for the estimation of the fluxes of volcanic gases, because interstitial water conductivity steeply increases when salinity increases by the dissolution of volcanic gases. However, in the subsurface of volcanoes, there are essentially two factors increasing bulk conductivity: the salinity of hydrothermal fluids and conductive clays by hydrothermal alteration. Therefore, the estimation of the fluxes of volcanic gases by electromagnetic methods requires to separate bulk conductivity simply into pore water conductivity (σ f ) and matrix conductivity (σ s ), which are related to salinity and hydrothermal alteration, respectively. The objective of this study is to separate observed bulk conductivity simply into σ f and σ s by the use of drillcore samples based on Revil''s model and to map the vertical variation of these two conductivity components for examination relation between bulk conductivity and hydrothermal fluids as the first step toward demonstrating the feasibility and practicality of using geophysical methods to constrain volcanic gas fluxes. As a case study, this study was conducted at the USDP-1 borehole site located on Unzen Volcano, SW Japan, and the bulk conductivity was successfully separated. The results consistently explained the vertical profiles of temperature and core permeability and suggested little hydrothermal alteration at this study site. The simple method of separating bulk conductivity may be applicable to vertical or lateral mapping of hydrothermal fluids and alteration at volcanoes, and separated values of σ f and σ s may be of great help for the estimation of the fluxes of volcanic gases and the intensity of hydrothermal alteration. [Copyright &y& Elsevier]

Abstract: Chemical compositions of volcanic gases of several Japanese active volcanoes have been monitored from distant safe places since the beginning of the 1990s using an FT-IR spectral radiometer. For absorption measurements, an infrared light source behind volcanic gas emissions is necessary in a volcanic environment. In the early observations, infrared radiation from hot lava domes (Unzen volcano) and hot ground heated by high-temperature fumaroles (Usu, Aso, and Satsuma-Iwojima volcanoes) were used as infrared light sources. However, these sources were not available in many cases. This remote FT-IR method became more commonly applied to chemical monitoring of volcanic gases emitted from the summit or slopes of active volcanoes using scattered solar infrared light as infrared light sources (Sakurajima, Miyakejima, and Asama volcanoes). To date, eight species have been measured using this method: SO2, HCl, HF, CO, CO2, COS, SiF4, and H2O. The observations indicate that volcanic gases for each volcano have different chemical composition on a SO2–HCl–HF ternary diagram in spite of similar tectonic settings, suggesting that vapor/melt volume ratios during volcanic gas formation differ among volcanoes. During more than 15years of monitoring, chemical changes in volcanic gases attributable to ascent of magma were observed only at Asama, where HCl/SO2 and HF/HCl ratios in the eruptive period were higher than those in non-eruptive period because of scrubbing of more soluble components in surface hydrothermal systems in the non-eruptive stage or solubility-controlled fractionation processes. Results show that these parameters are the most prospective ones among the various parameters measured using the remote FT-IR method to monitor volcanic activities. [Copyright &y& Elsevier]

Abstract: Two distinctive magmatic fluids were recognized in the Tatun volcanic group (TVG), Taiwan. One is a relatively reduced fluid represented by the fumarolic gases at Hsiao-you-ken (HYK) geothermal field. Another is an oxidized fluid containing high concentrations of HCl represented by the fumarolic gases at Da-you-ken (DYK). An intermediate gas was recognized at Gung-tze-ping (GTP) and She-hung-ping (SHP). The fumarolic gases at HYK and GTP possess the features of so-called primary steam generated on mixing of magmatic gas and meteoric groundwater. The fumarolic gases at DYK are a simple mixture between magmatic gas and water vapor of meteoric origin. The CO2/H2O molar ratio of the magmatic component in the fumarolic gases at DYK was estimated to be 0.018, meanwhile it was estimated to be 0.027 for the fumarolic gases at HYK and GTP, suggesting the magma beneath DYK is depleted in volatiles relative to the magma beneath HYK and GTP. The estimated CO2/H2O ratio for the magmatic component is comparable to that of some active volcanoes in Japan, suggesting the enrichment of volatiles in the magmas beneath TVG. [Copyright &y& Elsevier]

Abstract: After the phreatic eruption in 1982–83, volcanic activities at Kusatsu–Shirane volcano had been decreasing and reached a minimum in 1990, had turned to a temporal rise in activity up to 1994 and then decreased again at least up to 1997. During this low-activity period we observed a relatively short (≤1 y) cyclic variation in polythionates (PT) in the Yugama lake water. Spectral power density analysis of the PT time-series by an autoregressive (maximum entropy spectral estimation, MESE) method indicates that the major frequency in the PT variations is 1.0 y−1 and the minor is 2–3 y−1 (1.0 and 0.3–0.5 y in periodicity, respectively). Annual variations in the lake temperature are ruled out for explaining these periodicities. We attribute these cyclic variations to a cyclic magnification-reduction in meteoric-water injection into a hydrothermal regime where volcanic gases from cooling magma bodies at depth and cooler oxidized groundwater come into contact with each other. This interaction may result in a periodical change in the composition and flux of SO2 and H2S gases being discharged into the lake and forming PT. From a phase deviation (2–3 months) in the cycles between the annual precipitation, including snowmelt, and the PT time-series, we estimated the maximal depth of a hydrothermal reservoir beneath the lake assuming a vertical hydraulic conductivity (5×10− 3 cm/s) of the volcanic detritus around the summit hydrothermal system. Chloride in the lake water reached a maximum 1.5 years faster than PT. This is most likely due to a gradual elevation of the potentiometric front of a concentrated sublimnic solution in the hydrothermal reservoir. Variations of dissolved SO2 and H2S in the lake water were not consistent with those of the fumarolic gases on the north flank of the volcano. This fact together with additional observations strongly suggests that these fumaroles may have the same origin but are chemically modified by a subsurface aquifer. The PT monitoring at active crater lakes during a quiescent period can provide insight into the annual expansions and reductions of a volcano-hosted hydrothermal reservoir. A simple analytical method in the field for the semi-quantitative determination of dissolved SO2 and H2S in crater-lake waters is given in Appendix 2. [Copyright &y& Elsevier]

Stable chlorine isotope compositions (δ37Cl, per-mil: ‰, vs. a standard sample of sea water) of Kusatsu-bandaiko hot water samples, taken regularly in the years between 1974 and 1995 in the Kusatsu-Shirane volcanic region, Japan, were measured mass-spectrometrically. The results show that the δ37Cl values of the waters taken before 1984 were at around-0.12‰, whereas those after 1984 were at around+0.18‰. The δ37Cl values are thus distinct across 1984, which is consistent with the classification by the Cl to S molar ratio (Cl/S): the higher the Cl/S ratio, the larger the δ37Cl value. The δ37Cl value increased as much as 0.30‰ during 5 years between 1980 and 1984. This isotopic enrichment is likely correlated with increasing Cl/S ratios, suggesting that the heavier isotope (37Cl) may have preferentially increased in the original Cl source of the hot spring across 1984 when volcanic activity likely increased at Mt Kusatsu-Shirane. [ABSTRACT FROM AUTHOR]

Abstract: We conducted microlite textural analyses for the 1991–1995 eruptions at Unzen, Japan, in order to clarify the relationship between the discharge rate (exit velocity) and the kinetics of the microlite crystallization processes. The temporal variations in the plagioclase microlite crystallinity and the average size are negatively correlated with the variation in the magma discharge rate. On the other hand, the variation in the microlite number density (MND) exhibits a positive correlation with the discharge rate without a significant time lag. Groundmass microlites contain calcic plagioclase microlite (An45–65) and pargasite, suggesting that the microlite crystallization occurred at a depth (70–160 MPa) in the region of stability of the pargasite and plagioclase. The MND is determined at a depth (nucleation depth) with a certain effective undercooling rate (dT/dt) that is proportional to the exsolution rate of H2O from magma (dC H2O/dt). According to the MND water-exsolution rate meter, dC H2O/dt is calculated as 3.5 to 34.6×10−6 wt.%/s at the final stage of the nucleation depth (z n: 70–100 MPa). By assuming that H2O exsolution occurs in equilibrium, the temporal variation in the decompression rate can be associated with the ascent velocity, provided the conduit flow is steady. The calculated ascent velocity of magma at z n (70–100 MPa; 0.8–7.6 cm/s) is higher than the exit velocity of magma (0.1 MPa; 0.2–3.9 mm/s). Using reasonable bulk densities, the difference in the velocities at the nucleation depth and surface results in the cross-section of the conduit (ellipse elongated dykes 2:5) at the nucleation depth (from 3 to 21 m) to be smaller than that at the surface (20×50–100 m) according to the mass conservation law along the conduit. The estimated temporal variation in the ascent velocity at z n (70–100 MPa) is positively correlated with the observed discharge rate of magma, except at the final stage of eruptive activity. Furthermore, the variation in the conduit dimension at z n is correlated with the variation in the exit velocity, thereby implying that the observed discharge rate of magma is controlled by the ascent velocity at z n (70–100 MPa) and the conduit dimension. With regard to the final stage of eruptive activity, a decrease in the conduit dimension from 19 to 3 m indicates that the conduit is finally closed due to the termination of the magma supply. [Copyright &y& Elsevier]

Abstract: Chemical and isotopic compositions were measured for volcanic gases discharged from the recently erupted Heisei-shinzan dome of Unzen volcano. The volcanic gases are largely mixed with the air, whose content ranges from 60 to 90%. Chemical and isotopic compositions of the gases are corrected for the air mixing accounting not only for the major air components but also for contribution of the atmospheric moisture and CO2. The variation of the corrected composition revealed that these gases are derived from various sources including; 1) the air circulating through the lava domes, 2) the local meteoric water at various degrees of evaporation and vapor loss, 3) the magmatic CO2 likely originated from a deep source, and 4) the gases discharged from the cooling lava dome. The gases discharged from the cooling lava dome are characterized by the oxygen and hydrogen isotopic composition typical of the subduction-type volcanoes but with high Cl content and high Cl/S and F/S ratios. In contrast, discharges of CO2 with the magmatic δ 13C values are not correlated with discharge of heat or acid gas species, suggesting the ascent of CO2-rich fluids from a deep source, possibly through the volcanic conduit. [Copyright &y& Elsevier]

Abstract: During the dome-forming eruption in 1991 to 1995 at Mt. Unzen, Japan high temperature volcanic gases were sampled from fumaroles located at the western basement of the dacitic lava dome. Comparing the composition of volcanic gas, the volatile in the pre-eruptive melt of magma and the groundmass of lava, we propose that the volatile in magma was liberated through three different gas separation steps. The first gas separation happened at depth deeper than 3.9 km. The liberated gas traveled along the conduit then discharged as the observed fumarolic gas. The second gas separation took place beneath the lava dome. The liberated gas was discharged on the top of lava dome. The third gas separation occurred near surface in a so called “open system degassing” process. The large variation in the CO2/H2O ratio of fumarolic gas could be explained by the variation of the bubble fraction in the pre-eruptive melt of magma chamber before the gas separations. The estimated bubble fraction in the melt was less than 1.7 vol.%. Such a low fraction of bubbles in the magma chamber of Mt. Unzen would be one of the reasons why the eruption was non-explosive. The fumarolic gas in the later stage of eruption was enriched in HCl and HF due to the effect of open system degassing of magma, although the gas contained CO2 and H2S with high concentrations. The CO2 and H2S would be derived from a vapor phase of hydrothermal system developed around the conduit. [Copyright &y& Elsevier]

Abstract: The volcano on Miyake Island first erupted in July 2000 and continuous emission of volcanic gas from the collapsed caldera has been observed from the middle of August 2000. The large volcanic emission of SO2 had a strong influence on Tokyo metropolitan area, which is located approximately 150 km north of Miyake Island. We measured major ions in precipitation and dry deposition samples which had been collected at five sampling sites (Yokohama, Kashiwa, Fujisawa, Yokosuka, and Hachioji) in the Tokyo metropolitan area for 12 years since 1990. We have evaluated quantitatively the impact of the volcanic SO2 gas emitted from Miyake Island on the Tokyo metropolitan area by comparing depositional ionic constituents in the volcanic degassing period (from September 2000 to August 2001) with those in the normal period of the past 10 years (September 1990 to August 2000). nss-SO4 2− concentrations in precipitation at the sampling sites in the Tokyo metropolitan area were 59.5–77.0 μeq/L during the degassing period, and 33.3–44.1 μeq/L during the normal period, respectively. The difference of nss-SO4 2− concentrations between the two periods was statistically significant. In contrast, no significant differences were observed in the concentrations of the other major ions (NH4 +, nss-Ca2+, Cl−, and NO3 −) between the two periods. The impact of volcanic degassing from Miyake Island on the ionic concentrations in the precipitation of the Tokyo metropolitan area was seen only in the H+ and nss-SO4 2− concentrations. The annual wet deposition amount of volcanic nss-SO4 2− into the Tokyo metropolitan area has been quantitatively estimated. The annual wet deposition amounts were calculated as 701±277 Meq/year (22.4±8.9 kt SO2/year) on the total area of the Tokyo metropolitan area (14,000 km2). The wet deposition amount of nss-SO4 2− corresponds to only 0.15% of the total annual amount of volcanic SO2 (15 Mt/year) emitted from Miyake Island from September 2000 to August 2001. This estimation is consistent with a previous study which reported that 0.3% of the atmospheric discharge of SO2 from the volcano in Mt. Etna in Sicily was deposited by precipitation to the surrounding area (1200 km2). The annual wet deposition amount of volcanic SO2 to the Tokyo metropolitan area was at the same order of the magnitude (37±15%) as the annual anthropogenic SO2 amount (60 kt/year) emitted from the Tokyo metropolitan area. [Copyright &y& Elsevier]

An extremely large amount of volcanic gas has been released since mid-August 2000 from the volcanic island of Miyakejima, Japan, after formation of a summit caldera of 1.6 km diameter. The volcanic gas emission was continuous with very little extrusive magma activity. Variation of the SO2 emission rate was monitored by repeated measurements with an airborne correlation spectrometer. In December 2000, the SO2 emission rate averaged for the month peaked at 54 kt/d, which is twice the global SO2 emission rate from nonerupting volcanoes evaluated before this activity. The SO2 emission rate gradually decreased, almost linearly when plotted on a log scale, to 7 kt/d by the end of 2002, and then remained constant until at least December 2003. The total SO2 emission amounts to 18 Mt, comparable to the emission of a large explosive eruption such as Pinatubo in 1991. A theoretical evaluation, based on the model of magma convection in a conduit, suggests that extremely large volcanic gas emissions can be caused by formation of a magma pathway with a slightly larger diameter than exists in common systems, because the magma-transport rate is proportional to the fourth power of the conduit radius. Because volcanic gas emissions were initiated by formation of a summit collapse caldera of 1.6 km diameter, the creation of a large magma-conduit system through fractures formed during caldera collapse is likely the underlying cause of the extremely large volcanic gas emissions from the volcano. [ABSTRACT FROM AUTHOR]

• Neutral, non-corrosive hydrothermal waters are ideal for geothermal energy projects. • We studied the concept of the shadow effect by which neutral waters can be found. • Results for the Kakkonda geothermal field revealed the presence of a shadow effect. • The shadow effect is caused by buffering within a neutral hydrothermal reservoir. • Further verification of the shadow effect in different fields will be valuable. Field surveys were carried out at the Kakkonda geothermal field, where the location of the neutral hydrothermal reservoir has been identified, to investigate the shadow effect of the pH distribution in the actual geothermal development field. Methods using the electrical conductivity (EC)–pH relation and catchment area were employed, and the validity of the data was evaluated to select the most appropriate data for verifying the shadow effect. Results consistent with the concept of a shadow effect were obtained because acidic surface waters strongly affected by volcanic gases were not distributed above the neutral hydrothermal reservoir. [ABSTRACT FROM AUTHOR]

Examines the flux of volatiles and ore-forming metals from the magmatic-hydrothermal system Satsuma Iwojima volcano. Concentrations and fluxes of major and trace elements; Implications for the formation of hydrothermal ore deposits.

Unmanned aerial vehicles (UAVs), represented by rotor-based drones, are suitable for volcanic observations owing to the advantages of mobility and safety. In this study, vertical profiles of wind and aerosol concentrations at altitudes up to 1000 m around Mt. Sakurajima, one of the most active volcanoes in Japan, were measured in situ using a drone equipped with an ultrasonic anemometer and aerosol sensor. The drone-measured wind profiles were compared with Doppler LiDAR data and analysis values derived from a meteorological model. Drone-measured vertical profiles collected at a vertical speed of 1 m·s−1 (upward and downward) showed strong agreement with the LiDAR observations, as did the averaged values of hovering drone measurements. Obvious vertical wind shear was found by the drone in the vicinity of Mt. Sakurajima. An aerosol sensor was installed on the drone with the capability to measure fine (PM2.5) and coarse particles (PM10) simultaneously; in this manner, volcanic ash and aerosol pollutants around the volcano could be distinguished. Thus, it was proven that drones could be applied to investigate wind conditions and aerosols in situ, even at dangerous locations near active volcanoes. [ABSTRACT FROM AUTHOR]

Volcanic gas compositions are estimated based on volcanic plume measurement with Multi-GAS at a frequently erupting Sakurajima volcano by application of airborne methods and automatic monitoring station on the flank during 2013–2019. The airborne measurements of CO 2 and H 2 O concentrations often suffer from large fluctuation of background likely due to entrainment of the atmosphere derived from different altitude. By removing the background effect, a representative volcanic CO 2 /SO 2 mole ratio is estimated as 0.4 ± 0.1 and we did not observe clear temporal variation. The volcanic H 2 O/SO 2 ratios are estimated as 30–70 with a large uncertainty because of the strong atmospheric perturbation. The SO 2 /H 2 S ratios show quite a large variation ranging from 1 to 800. The large SO 2 /H 2 S ratios larger than 30 are observed during ash eruption and are suggested as a result of oxidation by atmosphere on hot ashes. The small ratios less than 10 are observed when frequency of explosions is low. Variation of the SO 2 /H 2 S ratios of the small values can be caused by variation of degassing pressure, however, the small variation of the CO 2 /SO 2 ratio suggests a limited range of the pressure variation. • Multi-GAS volcanic gas measurements performed at the inaccessible Sakurajima volcano. • Cessna-borne, UAV-borne and automatic station measurements are combined. • Low altitude atmosphere entrainment largely perturbed the results. • Volcanic CO 2 /SO 2 and H 2 O/SO 2 ratios are estimated as 0.4 and 30. • Large SO 2 /H 2 S ratio variations of 1–800 with large values during ash eruptions. [ABSTRACT FROM AUTHOR]

• Long-term monitoring of thermal waters has been continued at Tokachidake volcano. • The thermal waters are influenced by volcanic gas and magmatic fluid. • The thermal waters have changed in relation to fumarolic and seismic activity. • Monitoring of thermal waters is useful to interpret changes in volcanic activity. Continuous geochemical monitoring of thermal waters at Tokachidake volcano has been conducted for more than three decades to detect changes in its volcanic activity. Based on the geochemical data from 1986 to 2013, the origin and temporal changes of the thermal waters were discussed in our previous study. Here, we supplement the previous dataset with the geochemical data from 2014 to 2018 and comprehensively discuss the results of geochemical and geophysical monitoring to reveal the interactions between deep magmatic fluids, volcanic gases, and shallow aquifers. Thermal waters, discharging in the Tokachidake spa area, are formed by the interaction of volcanic gas and a shallow aquifer, and show the Cl change which have a significant correlation with the change in the volcanic plume height. This suggests a parallel volcanic gas input from the magma to the shallow aquifer and fumarole. Both changes are caused by a variation in the volcanic gas release rate from the magma. Around the 1988–1989 magmatic eruption, thermal waters in the Fukiage spa area showed remarkable thermal and chemical changes due to the interaction with the magmatic fluid (NaCl-type thermal water). Based on the chemical and isotopic compositions of the thermal waters, we calculated the Cl concentration and temperature of the NaCl-type thermal water as being approximately 20,000 mg/L and ∼280 °C, respectively. Our calculation of the supply rate of the NaCl-type thermal water indicates that large amounts of the NaCl-type thermal water are in continuous supply when volcanic activity increases prior to a magmatic eruption. In this area, the recent monitoring results reveal that the changes in the Cl concentration and seismic activity show a relatively good correlation, indicating that at least some of the volcanic earthquakes are related to the migration of the NaCl-type thermal water. Thus, when volcanic activity increases before a future magmatic eruption, chemical changes in the thermal waters in the Fukiage spa area should be apparent and the changes are likely to occur together with seismic activity. Our results propose that it is possible to interpret the present state of an active volcano more precisely through a comprehensive discussion of geochemical data from the thermal waters along with the other observations. [ABSTRACT FROM AUTHOR]

The sulfur-bearing volcanic gases SO 2 and H 2 S have a direct impact on ecosystems and can endanger human health. Timely information regarding dispersion patterns of fumarolic gases is crucial for predicting potential impacts. However, real-time fixed-point observations are difficult in alpine areas where snow accumulation is seasonably variable, in addition to being potentially dangerous to on-the-ground observers. Thus, in this study we tested a novel technique using a colorimeter, digital camera, and an unmanned aerial vehicle (UAV) imagery to evaluate the dispersion of sulfur particles from fumarolic gas over a snow-covered location in the Tateyama Mountain Range, central Japan, from 2013 to 2015. Snow samples were collected at the depths ranging from 0 to 0.5 cm and the sulfur particle content was determined by X-ray fluorescence analysis. The dissolved ion concentrations in snow melt solutions were analyzed to clarify the contribution of Asian yellow dust (KOSA) to the snow surface color. RGB color images captured by the UAV were converted to CIE-Lab color space with three axes: X axis changed from red to green (a *), Y axis from yellow to blue (b *), and Z axis from white to black (L *). The color levels of green and yellow increased in the snow surface of fumarolic areas and showed a significant correlation with the densities of sulfur particles on the snow surface (r 2 = 0.853, P < 0.001). These results indicated that aerial images of snow surface color can be used for estimation of sulfur particle dispersion from fumarolic gases. • Relationship between snow-surface color and fumarolic sulfur was investigated. • Snow surface color was more green-yellow closer to fumarolic areas. • Green-yellow intensities had positive correlations with sulfur particle amounts. • UAV imagery detected a spatial correlation between snow colors and sulfur particles. • UAV system is a useful tool allowing for evaluation of snow surface color changes. [ABSTRACT FROM AUTHOR]

At many volcano observatories, measurements of the plume height are frequently applied. On the other hand, the recent development of the satellite measurements enables the monitoring of the SO2 mass emitted by the passive degassing at sufficient temporal resolution. Using these two techniques, this study focuses on the degassing activity before the 8 October 2016 phreatomagmatic eruption of Aso volcano, Japan. Here we show the temporal variations of the plume height, the SO2 mass, and ground-based SO2 flux during 6 months before the eruption. Our result shows similar temporal changes of them, especially for the maxima and the increase, respectively, in about 2 months and 6 days before the eruption. This result indicates that the degassing system had been stable during the whole study period, but the accumulation of volcanic gas in the conduit since August might trigger the phreatomagmatic eruption. These techniques can be sufficient to monitor the degassing activity and to detect its precursory change. [ABSTRACT FROM AUTHOR]

Volcanic gas composition measurement by Multi-GAS was repeated during the eruptive period in 2014-2015 as well as the quiet period preceding the eruption at Nakadake crater, Aso volcano. The eruptive activity is characterized by continuous ash emission with intermittent Strombolian activity and temporal pauses. Volcanic gas composition measured during the eruptive period showed a rapid and large variation. In particular, the CO2/SO2 and SO2/H2S ratios varied in the rages of 1-8 and 3-300 during the ash eruption with a clear negative correlation. The large variation and the negative correlation of the compositions are attributed to two orders of magnitude difference of degassing pressure, such as 20 and 0.2 MPa; the gases with the large CO2/SO2 and the small SO2/H2S ratios are derived from the high pressure. The rapid and large composition variation suggests frequent ascent of bubbles formed at various depth during the eruption. The maximum CO2/SO2 ratio decreased with decreasing eruption intensity that suggests decrease in contribution of the bubbles derived from a large depth. With time, H2O/SO2 ratio of the gases increases from 30 to > 60, suggesting increase in a hydrothermal contribution. [ABSTRACT FROM AUTHOR]

The behavior of mercury (Hg) in water, sediments, air, dwarf bamboo leaves, and soils around the fumarolic area of Mt. Myoko, Japan, was investigated. Although some of the hot spring water samples contained over 1 µg/L total-Hg, overall, the total-Hg concentrations in the water samples decreased rapidly as the water flowed into a river. The total-Hg concentrations decreased not only due to simple dilution, but also to co-precipitation of Hg2+ with Fe(OH)3. The highest atmospheric Hg concentration, 91.7 ng/m3, was detected near the fumarole, and the concentrations decreased with distance from the fumarolic area. This tendency was also confirmed in the total-Hg concentrations of plant leaf samples. Total-Hg, methylmercury (MeHg), and ethylmercury (EtHg) concentrations in the soil surface layer (n = 13) ranged from 0.19 to 21.7 mg/kg, 0.3 to 9.3 µg/kg, and undetected to 7.7 µg/kg, respectively. The total-Hg concentrations in the surface layer soil samples decreased with distance from the fumarolic area, while the vertical distribution of total-Hg concentrations in the core and outcrop samples showed no clear trends. However, the MeHg and EtHg concentrations had no relationship with the total-Hg concentrations and were higher in the upper layer soils. The results suggest that MeHg and EtHg not only migrate with other Hg species from the volcanic gases, but are also generated on site. It was also confirmed that EtHg occurs in soil at the same level as MeHg in the study area. [ABSTRACT FROM AUTHOR]

Reports on the activity of the Mount Oyama volcano on the island of Miyake, Japan. Amount of sulphur dioxide being emitted by the volcano; Evacuation of island residents due to health risk concerns; Volcanologists' plans to study what seem to be anomalies in the volcanic activity.