Search

The permeability of magma in volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991–1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2 m wide and exhibits a 2 m wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a ∼ 1 m wide, highly sheared region with relatively low porosity and permeability, as well as an outer 20 cm wide cataclastic fault zone. The low-porosity, highly sheared rock further exhibits an anisotropic permeability network, with slightly higher permeability along the shear plane (parallel to the conduit margin), and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3 m long by ∼ 9 cm wide fracture ending bluntly and bordered by a 15–40 cm wide damage zone with permeability enhanced by ∼ 3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure. We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e. ductile) shear during ascent up to the point of rupture, which was estimated by Umakoshi et al. (2008) at ∼ 500 m depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into and along the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the core of the spine, leaving a slight permanent displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.

Kirishima volcano consists of more than 20 eruptive centers. Among them, Shinmoe-dake had magmatic eruptions in October 2017 and March 2018. Subsequently, another active cone, Iwo-yama, had phreatic eruptions in April 2018. These events were unique in that the 2018 eruption was the first effusion-dominated eruption of Shinmoe-dake and the first simultaneous activity of two cones of the Kirishima volcanic group ever documented. We report the detailed sequence of the events by combining areal photos, satellite images, and seismo-acoustic data analyses with the other published information. The seismo-acoustic data clarify the eruption onset and the transitions of the behaviors in three stages for each of the 2017 and 2018 eruptions. For both eruptions, we present regularly repeated tremors or 'drumbeat' earthquakes in the second stage, which interpret as gas separation from magma, leading to the ash-poor plume in the 2017 eruption or the effusive eruption in the 2018 event. We also propose that the 2017 and 2018 eruptions of Shinmoe-dake and the 2018 eruption of Iwo-yama are sequential events linked by the degassing of magma beneath Shinmoe-dake. An eruption like the 2017--2018 eruptions of Shinmoe-dake would leave little geological record and could be captured only by modern techniques. Although Shinmoe-dake has been believed to be an example of less-frequent eruptions, effusive eruptions like the 2018 case might have occurred more frequently in the past. The timelines summarized in this study will be useful in future studies of Kirishima volcanoes and world equivalences.

The permeability of magma in shallow volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991–1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2 m wide and exhibits a 2-m wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a ~1-m wide, highly-sheared region with relatively low porosity and permeability, and an outer 20-cm wide cataclastic fault zone. The low porosity, highly-sheared rock further exhibits an anisotropic permeability network with slightly higher permeability along the shear plane (parallel to the conduit margin) and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3-m long by ~9-cm wide fracture ending bluntly and bordered by a 15–40 cm wide damage zone with an increased permeability of ~3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure. We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e., ductile) shear during ascent up to the point of rupture, estimated by Umakoshi et al. (2008), at ~500 m depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into, and along, the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the spine core with slight displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the upward shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.

A plinian-style eruption with a radially spreading umbrella cloud occurred on February 13, 2014, at Kelud volcano, Indonesia. We present the sequence of this plinian event based on a geological survey of the eruption products and analysis of satellite images. The eruptive deposits were divided into four major depositional units (Units A, B, C, and D) and used to determine the sequence of events. The plinian phase was preceded by partial destruction of the existing lava dome and generation of high-energy pyroclastic density currents (PDCs) mainly toward the NE that produced a series of depositional subunits (Units A0–2) and blown-down trees (stage 1). In the main phase of the plinian eruption, tephra fallout (Unit B) was widely distributed over East Java (stage 2). The winds above the volcano significantly affected the tephra dispersal process. In stage 3, the plinian column collapsed, generating dense PDCs that flowed down the volcano valleys, producing pumiceous lobate deposits (Unit C). The declining phase of the eruption produced fine-rich fallout tephra layers (Unit D1–2) from low-level eruption plumes and/or ash lofted from PDCs. The eruption sequence constructed from field observations is supported by geophysical observations. The deposit features and componentry suggest that newly ascended magma triggered the destruction of the lava dome and the generation of high-energy PDCs, and during the subsequent climactic phase the dome was completely destroyed. We estimate a total erupted volume of 0.25–0.50 km3 (bulk deposit volume, 0.14–0.28 km3 in dense rock equivalent) and a mass eruption rate of 6.5 ± 2.8 × 107 kg/s, with a volcanic explosivity index of 4. The eruption sequence and physical parameters of the 2014 eruption will help assess future volcanic activity and hazards at Kelud volcano.

A new program for the Next Generation Volcano Research and Human Resource Development started in 2016, following the government’s analysis of a volcanic disaster at Mount Ontake in 2014. One of its important purposes is the development of a technology that can provide visualized information of imminent volcanic hazards to the stakeholders. The latter include researchers in the Volcano Disaster Prevention Councils. Since the volcanic activity in Japan has been relatively less in the past few hundred years, larger eruptions are certainly expected to occur in the near future. Volcanic risk management has developed in Japan independently of university or institutional research, and by a national law, researchers are not allowed to officially forecast imminent volcanic eruptions. In the case of large eruptions never being observed, a close communication between the Japan Meteorological Agency and researchers becomes very important. Our project goal is issuing effective information on real-time observational and hazard mitigation simulation data to the stakeholders and researchers. Based on our inspection and interviews we develop information tools using which the above data are provided effectively and the dissemination and education of volcanic disasters are performed.

Shearing of magma during ascent can promote strain localisation near the conduit margins. Anymechanical and thermal discontinuities associated with such events may alter the chemical, physicaland rheological stability of the magma and thus its propensity to erupt. Lava spines can record suchprocesses, preserving a range of macroscopic and microscopic deformation textures, attributed toshearing and friction, as magma ascends through the viscous-brittle transition. Here, we use a multi-disciplinary approach combining petrology, microstructures, crystallography, magnetics and experi-mentation to assess the evidence, role and extent of shearing across a marginal shear zone of the1994–1995 lava spine at Unzen volcano, Japan. Our results show that crystals can effectively moni-tor stress conditions during magma ascent, with viscous remobilisation, crystal plasticity and com-minution all systematically increasing towards the spine margin. Accompanying this, we find anincrease in mineral destabilisation in the form of pargasitic amphibole breakdown displaying tex-tural variations across the shear zone, from symplectitic to granular rims towards the spine margin.In addition, the compaction of pores, chemical and textural alteration of interstitial glass and mag-netic variations all change systematically with shear intensity. The strong correlation between thedegree of shearing, crystal deformation and disequilibrium features, together with distinct magneticproperties, implies a localised thermal input due to shear and frictional processes near the conduitmargin during magma ascent. This was accompanied by late-stage or post-emplacement fluid- andgas-induced alteration of the gouge, as well as oxidation and glass devitrification. Understandingand recognising evidence for strain localisation during magma ascent may, therefore, be vital whenassessing factors that regulate the style of volcanic eruptions, which may provide insights into thecryptic shifts from effusive to explosive activity as observed at many active lava domes

Kelud Volcano is among the most active volcanoes in Indonesia, with repeated explosive eruptions throughout its history. Here, we reconstructed the relationship between the repose period and the cumulative volume of erupted material over the past 100 years and estimated the long-term magma discharge rate and future eruptive potential and hazards. Tephra data and eruption sequences described in historical documents were used to estimate the volume and mass discharge rate. The volumes of the 1901, 1919, 1951, 1966, 1990, and 2014 eruptions were estimated as 51–296 × 106m3. The mass discharge rates were estimated to be on the order of 107kg/s for the 1919, 1951, and 2014 eruptions and the order of 106kg/s for the 1966 and 1990 eruptions. Based on a linear relationship between the repose period and cumulative erupted mass, the long-term mass discharge rate was estimated as ∼ 1.5 × 1010kg/year, explaining the features of the larger eruptions (1919, 1951, and 2014) but not those of the smaller eruptions (1966 and 1990). This estimate is relatively high compared to other typical basaltic-andesitic subduction-zone volcanoes. This result provides important insights into the evolution of magmatic systems and prediction of future eruptions at Kelud Volcano.

Eruption scenarios were prepared as possible sequences in event trees for six active volcanoes in Indonesia, that are located near populated areas or have erupted in recent years (Galunggung, Guntur, Kelud, Merapi, Semeru, and Sinabung). The event trees prepared here show sequences of possible eruption phenomena without probabilities on branches and cover sequences experienced in historical and pre-historical eruptions based on archives and field research results. Changing magma discharge rates during eruption sequences were considered for the event tree of Merapi. This conceptual event tree can also be used as a short-term event tree in which forecasting the coming eruption became possible with geophysical and geochemical monitoring data. Eruption event trees prepared for selected time windows cannot illustrate all plausible hazards and risks associated with an eruption. Therefore, hazards and risks generated from an eruption should be considered in different domains from the event tree.

The nature of sub-volcanic alteration is usually only observable after erosion and exhumation at old inactive volcanoes, via geochemical changes in hydrothermal fluids sampled at the surface, via relatively low-resolution geophysical methods or can be inferred from erupted products. These methods are spatially or temporally removed from the real subsurface and thus provide only indirect information. In contrast, the ICDP deep drilling of the Mt Unzen volcano subsurface affords a snapshot into the in situ interaction between the dacitic dykes that fed dome-forming eruptions and the sub-volcanic hydrothermal system, where the most recent lava dome eruption occurred between 1990 and 1995. Here, we analyse drill core samples from hole USDP-4, constraining their degree and type of alteration. We identify and characterize two clay alteration stages: (1) an unusual argillic alteration infill of fractured or partially dissolved plagioclase and hornblende phenocryst domains with kaolinite and Reichweite 1 illite (70)-smectite and (2) propylitic alteration of amphibole and biotite phenocrysts with the fracture-hosted precipitation of chlorite, sulfide and carbonate minerals. These observations imply that the early clay-forming fluid was acidic and probably had a magmatic component, which is indicated for the fluids related to the second chlorite-carbonate stage by our stable carbon and oxygen isotope data. The porosity in the dyke samples is dominantly fracture-hosted, and fracture-filling mineralization is common, suggesting that the dykes were fractured during magma transport, emplacement and cooling, and that subsequent permeable circulation of hydrothermal fluids led to pore clogging and potential partial sealing of the pore network on a timescale of ~ 9 years from cessation of the last eruption. These observations, in concert with evidence that intermediate, crystal-bearing magmas are susceptible to fracturing during ascent and emplacement, lead us to suggest that arc volcanoes enclosed in highly fractured country rock are susceptible to rapid hydrothermal circulation and alteration, with implications for the development of fluid flow, mineralization, stress regime and volcanic edifice structural stability. We explore these possibilities in the context of alteration at other similar volcanoes., info:eu-repo/semantics/published

The climactic phase of the Shinmoe-dake 2011 eruption was characterized by sub-Plinian events (January 26 p.m., 27 a.m., and 27 p.m.) and lava accumulation in the crater (late on January 27 to January 29), all of which were accompanied by Vulcanian events. The magma discharge rate for lava accumulation was close to that of the sub-Plinian events. We discuss evolution of syneruptive magma ascent through the climactic phase on the basis of decompression-induced crystallization of groundmass microlites, in order to reveal how the final eruption style was determined at the explosive–effusive transition. We examined pumice and lava-like pyroclasts from three sub-Plinian events, the January 28 Vulcanian event, and a lava block ballistically ejected during the February 1 Vulcanian event. The samples exhibit variable groundmass textures due to variable syneruptive ascent conditions, not due to pre-eruptive inhomogeneity of the andesitic magma and different degrees of syneruptive volatile exsolution preceding quenching. Eruptive units were first characterized according to bulk density, assuming that slower ascent resulted in higher density. A decrease in the magma ascent rate led to a gradual shift to an effusive eruption style. There was little variation in ascent rate among magmas erupted simultaneously during the first and second sub-Plinian events (bulk density 0.8–2.1 g/cm3), but differences increased in the third sub-Plinian and January 28 Vulcanian eruptions with minor appearance of slow magma (0.9–2.8 g/cm3). Only slow magma occupied the shallow conduit at the completion of lava accumulation (February 1 lava; 2.1 g/cm3). Among the first and second sub-Plinian units, the early stage of the second sub-Plinian event had the greatest variation in ascent rate, with extension to a slightly higher bulk density (1.0–2.1 g/cm3) than the other events (0.8–1.7 g/cm3). Ascent-rate variations in the early stage of the second sub-Plinian event, the third sub-Plinian event, and the January 28 Vulcanian event are all due to the preceding calm phase in the intermittent explosive activity. The moving magma column, which showed a vertical zonation in the degree of degassing, formed in the shallow conduit during the calm phase due to a decrease in the discharge rate, and this resulted in the co-ejection of magmas with variable degassing upon the start of explosive events. The crystal size distribution (CSD) of plagioclase microlites constrains the syneruptive branching (i.e., a point of divergence) in ascent rate and eruption style, which occurred after the start of microlite crystallization. The CSDs of the different samples show differences only in small crystal sizes; slope at these size fractions is smallest for low-density ( 2.0 g/cm3) of the January 28 and February 1 events, and greatest for high-density pyroclasts (>2.0 g/cm3) of the third sub-Plinian event. We propose, based on the similarity in CSD, that the shallow magma-feeding system was common for the two craters active during the January 28 explosive–effusive hybrid activity, where the Vulcanian event and emplacement of lava of the later February 1 event occurred in different parts of the original crater. The samples have variable plagioclase microlite number densities (1.4 × 105–3.5 × 106 mm−3), based on variations in small crystal sizes among different CSDs. This variability resulted not only from the usual increasing trend in number density with increasing degree of undercooling, but also the decreasing trend associated with the strong undercooling during explosive eruptions.

Japan is one of the countries with abundant active volcanoes. In the field of Nuclear Energy, it is necessary to assess safety against volcanic hazards, and voluntary guidelines were published in J...

We investigated the creation of a volcanic islet and emplacement of lava flows in the sea by analyzing data from the island-forming eruption at Nishinoshima, Japan, that has been continuing since November 2013. Aerial observations and satellite images were used to perform a quantitative analysis of the eruption processes. The most intriguing characteristic of the lava flows is the development of lobes and tubes from breakouts and bifurcations of andesitic ‘a’ā-type lava flows. Internal pathways that fed lava to the active flow front were eventually developed by crust solidification and dominated the lava transport. The average discharge was ~2 × 10 5 m 3 /day, and the total volume of erupted material reached ~0.1 km 3 at the end of February 2015. Fractal analysis of the lava-flow margins suggests that the growth pattern is self-similar, with a fractal dimension (D) of ~1.08–1.18, which is within the range of subaerial basaltic lava flows. The morphological evolution of Nishinoshima is controlled primarily by effusion of lava with an apparent viscosity of 10 4 –10 6 Pa∙s, average discharge of ~2.3 m 3 /s, and eruption duration lasting ~2 yr. Our data and analyses suggest that the effect of lava coming in contact with seawater, as well as the variations in the lava discharge rate on local and overall scales, are important factors affecting the development of crust and the lava transport system.

In order to evaluate airborne ash densities, a real-time volcanic ash dispersion model, PUFF, is applied to the February 13, 2014 eruption of Kelud volcano in Indonesia. The emission rate of the ash mass from the vent is estimated based on the empirical formulae tested at Sakurajima volcano using ground deformation and seismic monitoring data.According to the result of the PUFF model simulation, the circular shape of the anvil ash cloud 17 km in height extends during the first two hours over a radius of 200 km from the volcano. The core region within 50 km of the volcano shows an airborne ash density of 1000 mg/m3. Three hours after the initial eruption, the area with 100 mg/m$^bm 3$ extends 300 km to the west, covering Yogyakarta Airport. Due to low-level winds, Surabaya Airport to the northeast also becomes part of the area with 100 mg/m3. The result of the ash plume dispersal 7 hours into the eruption indicates that the entire island of Java is in the danger zone for commercial airliners, as ash exceeds 10 mg/m3. Although satellite images show that the ash plume is located only in the southern half of western Java, the simulation results quantitatively indicate much wider extents of the aircraft danger zone.

It is our great pleasures to present the sixth JDR Award to Prof. Masato Iguchi. Prof. Iguchi, as the top guest editor, published two JDR special issues entitled “Integrated Study on Mitigation of Multimodal disasters caused by Ejection of Volcanic Products.” These special issues, vol.11, no.1 in 2016 and vol.14, no.1 in 2019, were results of a project (2013–2018) of the Science and Technology Research Partnership for Sustainable Development (SATREPS). The second special issue set a record for the highest annual download. In addition, he was the first author or co-author of 11 papers published by JDR in 2019. Professor Iguchi, as one of most recognized volcanologists in the world, has been leading observation research of active volcanoes as the Director of the Sakurajima Volcano Research Center of Kyoto University’s Disaster Prevention Research Institute. He has promoted geophysical research on volcanic activity not only in Japan but also in Indonesia, evaluating volcanic activity by incorporating the results of material science and demonstrating the effectiveness of long-term, multi-item observations in those processes. He has made a significant contribution to the promotion of Indonesian volcano research, taking over the strong will of former Kyoto University professors, and his contributions have culminated in the aforementioned two special issues. On behalf of the JDR editorial board, I wish to thank Prof. Iguchi for his efforts and to congratulate him as the winner of the sixth JDR Award.

Contributions of Volcano Observatories to the Hyogo (UNISDR 2005) and Sendai (UNISDR 2015) Frameworks for Disaster Risk Reduction. (DOCX 19 kb)

We summarize major findings and best-practice recommendations from three Volcano Observatory Best Practices (VOBP) workshops, which were held in 2011, 2013 and 2016. The workshops brought together representatives from the majority of the world’s volcano observatories for the purpose of sharing information on the operation and practice of these institutions and making best practice recommendations. The first workshop focused on eruption forecasting, the second on hazard communication, and the third on long-term hazard assessment. Subsequent VOBP workshops will address additional issues of broad interest to the international volcano observatory community. The objective of VOBP is to develop synergy among volcano hazards programs and their observatories internationally, so as to more rapidly and broadly advance the field of applied volcanology. Each of the workshop summaries presented here include best practice recommendations for consideration by the world’s volcano observatories.

Table listing participants in VOBP workshops. A. Volcano Observatory Best Practices Workshop #1 (Near-Term Eruption Forecasting) Erice, Sicily, 11 - 15 September 2011. B. Volcano Observatory Best Practices Workshop #2 (Volcano Hazard Communication) Erice, Sicily, 2 - 6 November 2013. C. Volcano Observatory Best Practices Workshop #3 (Long-term Hazard Assessments) Vancouver, USA, 15-18 November 2016. (DOCX 42 kb)

Table listing VOBP-participant observatories, partner emergency management organizations and responsibilities for issuance of volcanic alert levels. (DOCX 28 kb)

The phreatic eruption of the Ontake volcano in 2014 reminded us that even moderately active volcanoes, most of which are tourist attractions in Japan, can sometimes exhibit unpredictable and hazardous behaviors, taking away the lives of those who do not fully recognize their threat. With this adding momentum, the Japanese people want volcanology and its applications to be developed to further improve the precision of volcanic eruption alerts. To meet this expectation, a comprehensive program, the “Integrated program for next-generation volcano research and human resource development,” sponsored by the Ministry of Education, Culture, Sports, Science and Technology, was started in November 2016 on a 10-years plan. The most stunning aspect of this program is the integration of (1) a research project and (2) a human resource development program to mitigate volcanic disasters in Japan from long-term point of view. Both of these are collaboratively supported by many researchers from almost all Japanese universities and national institutions related to volcanology. This special issue compiles several topics in this research project to demonstrate its present stage of development and to indicate its anticipated future destination. The target of the research project is to develop, using multi-disciplinary scientific methods, new ways of evaluating volcanic hazards. Specifically, four research groups jointly (A) construct a data archive and exchange system connecting all Japanese volcanologists, (B) develop new geophysical and geochemical observation techniques and methods of analyzing data, (C) evolve methods of predicting volcanic eruptions based on eruption history from precise geological survey and numerical simulations, and (D) propose the provision of technologies for volcanic disasters. We hope that this program will greatly help to mitigate volcanic disasters in Japan, and we will strive to realize this through the research project.

Japan is one of the countries with abundant active volcanoes and has a long history of developing countermeasures to mitigate volcanic disasters. In the field of nuclear energy, it is also necessar...

Intense rainfall related to the typhoon T1326 on October 15–16, 2013 (total 824 mm; maximum hourly rainfall 118.5 mm) triggered numerous landslides and associated lahars at Izu Oshima Volcano, the northernmost part of Izu Mariana volcanic arc, Japan. The landslides were concentrated mainly in a 2-km 2 area located on the western slope of Izu Oshima Volcano. Most of the landslides were shallow soil slips ( 4 m 3 /km 2 , based on debris volumes trapped by sediment retention dams. The characteristics of rainfall-induced landslides and associated lahars at Izu Oshima Volcano in 2013 provide an important lesson about future non-eruption-related landslide and lahar hazards at tephra-rich volcanoes.

This chapter sketches an overall volcanic profile of the Japanese Islands and provides a context for the volcanic geoheritage of Japan. Mechanisms and particularities of subduction zone volcanism are discussed, and the recent volcanic history of the Japanese Islands is compared with the volcanic histories of Chile and Indonesia. It becomes clear that Japan has not experienced very large eruption activities compared to Chile and Indonesia despite having geological similarities. This quiescence possibly implies a statistical likelihood of major eruptions in the near future due to accumulation of magma. It is also shown that the actual scale of volcanic eruptions is a poor determinant of the human casualty; instead, the locations of eruptions (distance from residential areas and access) and level of preparedness or vulnerability of the affected population are important factors. The chapter argues that although it is possible to provide probable eruption scenarios, accurate detailed forecasting remains difficult, as each volcano is a different system and the eruption style is not always identical even at a single volcano. It is also argued that fundamental research on individual volcanoes is indispensable to understand this dynamic earth heritage, and reflecting on experience of geoparks in Japan, the chapter states that such heritage branding could become effective tools for promoting awareness and resilience of local societies.

Mafic enclaves in the 1991–1995 dacite of Unzen volcano show chemical and textural variability, such as bulk SiO2 contents ranging from 52 to 62 wt% and fine- to coarse-grained microlite textures. In this paper, we investigated the mineral chemistry of plagioclase and hornblende microlites and distinguished three enclave types. Type-I mafic enclaves contain high-Mg plagioclase and low-Cl hornblende as microlites, whereas type-III enclaves include low-Mg plagioclase and high-Cl hornblende. Type-II enclaves have an intermediate mineral chemistry. Type-I mafic enclaves tend to show a finer-grained matrix, have slightly higher bulk rock SiO2 contents (56–60 wt%) when compared with the type-III mafic enclaves (SiO2 = 53–59 wt%), but the overall bulk enclave compositions are within the trend of the basalt–dacite eruptive products of Quaternary monogenetic volcanoes around Unzen volcano. The origin of the variation of mineral chemistry in mafic enclaves is interpreted to reflect different degree of diffusion-controlled re-equilibration of minerals in a low-temperature mushy dacitic magma reservoir. Mafic enclaves with a long residence time in the dacitic magma reservoir, whose constituent minerals were annealed at low-temperature to be in equililbrium with the rhyolitic melt, represent type-III enclaves. In contrast, type-I mafic enclaves result from recent mafic injections with a mineral assemblage that still retains the high-temperature mineral chemistry. Taking temperature, Ca/(Ca + Na) ratio of plagioclase, and water activity of the hydrous Unzen magma into account, the Mg contents of plagioclase indicate that plagioclase microlites in type-III enclaves initially crystallized at high temperature and were subsequently re-equilibrated at low-temperature conditions. Compositional profiles of Mg in plagioclase suggest that older mafic enclaves (Type-III) had a residence time of ~100 years at 800 °C in a stagnant magma reservoir before their incorporation into the mixed dacite of the 1991–1995 Unzen eruption. Presence of different types of mafic enclaves suggests that the 1991–1995 dacite of Unzen volcano tapped mushy magma reservoir intermittently replenished by high-temperature mafic magmas.

Japan is one of the countries with abundant active volcanoes and has a long history of developing countermeasures to mitigate volcanic disasters. In the field of nuclear energy, it is also necessar...

The physical condition of the 1 February, 2011, vulcanian explosion at Shinmoedake volcano, Japan, is estimated based on the size of impact craters created by ballistic ejecta, using a ballistic trajectory model and a scaling law for impact crater formation. The initial velocity, impact velocity and mass of ejecta were estimated at 240–290 m/s, 140 ± 20 m/s and 1–3 ton, respectively. The gas mass fraction at the source was calculated to be 0.04–0.1, using the initial velocity and a theoretical model of vulcanian explosion. This gas mass fraction is higher than the petrologically estimated value for pre-eruptive magma. Low-angle jets from the explosion and the estimated depth and size of a pressurized gas region suggest a shallow source inside the lava dome. The observation and results imply that segregation and accumulation of gas in a shallow conduit played a role in an increase of excess pressure immediately below the dome surface, prior to the vulcanian explosion.

The 2011 Shinmoe-dake eruption started with a phreatomagmatic eruption (Jan 19), followed by climax sub-Plinian events and subsequent explosions (Jan 26–28), lava accumulation in the crater (end of January), and vulcanian eruptions (February–April). We have studied a suite of ejecta to investigate the magmatic system beneath the volcano and remobilization processes in the silicic magma mush. Most of the ejecta, including brown and gray colored pumice clasts (Jan 26–28), ballistically ejected dense lava (Feb 1), and juvenile particles in ash from the phreatomagmatic and vulcanian events are magma mixing products (SiO 2 = 57–58 wt.%; 960–980 °C). Mixing occurred between silicic andesite (SA) and basaltic andesite (BA) magmas at a fixed ratio (40%–30% SA and 60%–70% BA). The SA magma had SiO 2 = 62–63 wt.% and a temperature of 870 °C, and contains 43 vol.% phenocrysts of pyroxene, plagioclase, and Fe–Ti oxide. The BA magma had SiO 2 = 55 wt.% and a temperature of 1030 °C, and contains 9 vol.% phenocrysts of olivine and plagioclase. The SA magma partly erupted without mixing as white parts of pumices and juvenile particles. The two magmatic end-members crystallized at different depths, requiring the presence of two separate magma reservoirs; shallower SA reservoir and deeper BA reservoir. An experimental study reveals that the SA magma had been stored at a pressure of 125 MPa, corresponding to a depth of 5 km. The textures and forms of phenocrysts from the BA magma indicate rapid crystallization directly related to the 2011 eruptive activity. The wide range of H 2 O contents of olivine melt inclusions (5.5–1.6 wt.%) indicates that rapid crystallization was induced by decompression, with olivine crystallization first (≤ 250 MPa), followed by plagioclase addition. The limited occurrence of olivine melt inclusions trapped at depths of Remobilization of mush-like SA magma occurred in two stages before the early sub-Plinian event. Firstly, precursor mixing with BA magma and associated heating occurred (925–871 °C; stage-1 of ≥ 350 h), followed by final mixing with BA magma (stage-2). MgO profiles of magnetite phenocrysts define timescales of 0.7–15.2 h from this final mixing to eruption. The mixed and heated magmas, and stagnant mush that existed in the SA reservoir in the precursor stage, were finally erupted together. Magnetite phenocrysts in the Feb 18 ash reveal the occurrence of continuous erosion of the stagnant mush during the course of the 2011 eruptive activity.

The phreatic eruption at Ontake volcano on 27 September 2014, which caused the worst volcanic disaster in the past half-century in Japan, was reconstructed based on observations of the proximal pyroclastic density current (PDC) and fallout deposits. Witness observations were also used to clarify the eruption process. The deposits are divided into three major depositional units (Units A, B, and C) which are characterized by massive, extremely poorly sorted, and multimodal grain-size distribution with 30–50wt% of fine ash (silt–clay component). The depositional condition was initially dry but eventually changed to wet. Unit A originated from gravity-driven turbulent PDCs in the relatively dry, vent-opening phase. Unit B was then produced mainly by fallout from a vigorous moist plume during vent development. Unit C was derived from wet ash fall in the declining stage. Ballistic ejecta continuously occurred during vent opening and development. As observed in the finest population of the grain-size distribution, aggregate particles were formed throughout the eruption, and the effect of water in the plume on the aggregation increased with time and distance. Based on the deposit thickness, duration, and grain-size data, and by applying a scaling analysis using a depth-averaged model of turbulent gravity currents, the particle concentration and initial flow speed of the PDC at the summit area were estimated as 2×10−4–2×10−3 and 24–28m/s, respectively. The tephra thinning trend in the proximal area shows a steeper slope than in similar-sized magmatic eruptions, indicating a large tephra volume deposited over a short distance owing to the wet dispersal conditions. The Ontake eruption provided an opportunity to examine the deposits from a phreatic eruption with a complex eruption sequence that reflects the effect of external water on the eruption dynamics., Article, Earth, Planets and Space. 68: 82(2016)

The sudden eruption of Mount Ontake on September 27, 2014, led to a tragedy that caused more than 60 fatalities including missing persons. In order to mitigate the potential risks posed by similar volcano-related disasters, it is vital to have a clear understanding of the activity status and progression of eruptions. Because the erupted material was largely disturbed while access was strictly prohibited for a month, we analyzed the aerial photographs taken on September 28. The results showed that there were three large vents in the bottom of the Jigokudani valley on September 28. The vent in the center was considered to have been the main vent involved in the eruption, and the vents on either side were considered to have been formed by non-explosive processes. The pyroclastic flows extended approximately 2.5 km along the valley at an average speed of 32 km/h. The absence of burned or fallen trees in this area indicated that the temperatures and destructive forces associated with the pyroclastic flow were both low. The distribution of ballistics was categorized into four zones based on the number of impact craters per unit area, and the furthest impact crater was located 950 m from the vents. Based on ballistic models, the maximum initial velocity of the ejecta was estimated to be 111 m/s. Just after the beginning of the eruption, very few ballistic ejecta had arrived at the summit, even though the eruption plume had risen above the summit, which suggested that a large amount of ballistic ejecta was expelled from the volcano several tens-of-seconds after the beginning of the eruption. This initial period was characterized by the escape of a vapor phase from the vents, which then caused the explosive eruption phase that generated large amounts of ballistic ejecta via sudden decompression of a hydrothermal reservoir.

We report methods, based on geophysical observations and geological surveys, for the prediction of eruptions and the evaluation of the activity of 4 volcanoes in Indonesia. These are Semeru, Guntur, Kelud and Sinabung volcanoes. Minor increases in tilt were detected by borehole tiltmeters prior to eruptions at the Semeru volcano depending on the seismic amplitude of explosion earthquakes. The results show the possibility of prediction of the type and magnitude of eruption and the effectiveness of observation with a high signalto-noise ratio. The establishment of background data is important for evaluating volcanic activity in longterm prediction. Typical distributions of volcanic and local tectonic earthquakes were obtained around the Guntur volcano, where geodetic monitoring by continuous GPS observation is valuable. The cumulative volume of eruptive products is valuable for evaluating the potential for future eruption. The eruptive rate of the Kelud volcano is ca 2×106m3/y (dense rock equivalent), but the volume of the 2007 eruption was only 2×107m3, suggesting a still high potential for eruption. Based on geological surveys and dating, an eruption scenario is proposed for the activity of Mt. Sinabung, where phreatic eruptions occurred in 2010 after a historically long dormancy.

Before its 2010 eruption, Sinabung Volcano was a Btype volcano, in its Indonesian classification. A series of explosions featuring 1-5 km high phreatic-ash columns occurred from August 27, 2010 untill September 7, 2010. SO2 flux measured during the eruptions showed sizeable gas emission and the youngest volcanic product has age of 1200 years BP obtained from 14C dating. At the end of August 2010, four continuous seismic stations were established around the volcano, and 6 additional stations were deployed in October 2010. Deformation monitoring was conducted temporarily till in February 2011, four continuous GPS stations were installed. All were set up through collaboration between Indonesian and Japanese academic and government institutions. Hypocenter calculations using data of 4 seismic stations fromSeptember to October 2010 showed two concentrations for shallow volcanic earthquakes (VTB) 0.5-2 km beneath the crater and for deep volcanic earthquakes (VTA) 2.5-14 km beneath the crater. These epicenters defined a northeast-southwest lineament, near an elongated sinistral fault zone between Sinabung and Sibayak volcanoes. Earthquake sources went deeper northeastward. Results using the data of 10 stations from November 2010 to February 2011 showed that earthquakes were concentrated at depths of 4-6 km beneath Lake Lau Kawar. Tilt and Electro-optic Distance Measurement (EDM) measurements from August to September 2010 showed no significant changes We inferred that since the last explosion in 7 September 2010, stabilization process both in pressure and energy were low.

A buried, old volcanic body (pre-Komitake Volcano) was discovered during drilling into the northeastern flank of Mount Fuji. The pre-Komitake Volcano is characterized by hornblende-bearing andesite and dacite, in contrast to the porphyritic basaltic rocks of Komitake Volcano and to the olivine-bearing basaltic rocks of Fuji Volcano. K-Ar age determinations and geological analysis of drilling cores suggest that the pre-Komitake Volcano began with effusion of basaltic lava flows around 260 ka and ended with explosive eruptions of basaltic andesite and dacite magma around 160 ka. After deposition of a thin soil layer on the pre-Komitake volcanic rocks, successive effusions of lava flows occurred at Komitake Volcano until 100 ka. Explosive eruptions of Fuji Volcano followed shortly after the activity of Komitake. The long-term eruption rate of about 3 km3/ka or more for Fuji Volcano is much higher than that estimated for pre-Komitake and Komitake. The chemical variation within Fuji Volcano, represented by an increase in incompatible elements at nearly constant SiO2, differs from that within pre-Komitake and other volcanoes in the northern Izu-Bonin arc, where incompatible elements increase with increasing SiO2. These changes in the volcanism in Mount Fuji may have occurred due to a change in regional tectonics around 150 ka, although this remains unproven.

Permeability measurement was made on five rock samples from USDP-4 cores. Rock samples were collected from the conduit zone and its country rock. One sample (C14-1-1) is considered as a part of the feeder dyke for the 1991–1995 eruption. The transient pulse method was employed under confining pressure up to 50 MPa. Compressional wave velocity was measured along with permeability. The measured permeability ranges from 10 − 19 to 10 − 17 m 2 at the atmospheric pressure, and is as low as that reported for tight rocks such as granite. The permeability decreases with increasing confining pressure, while the compressional wave velocity increases. Assuming that pores are parallel elliptical tubes, the pressure dependence of permeability requires aspect ratio of 10 − 4 –10 − 2 at the atmospheric pressure. The pore aperture is estimated to be less than 1 μm. The estimated aspect ratio and pore aperture suggest that connectivity of pores is maintained by narrow cracks. The existence of cracks is supported by the pressure dependence of compressional wave velocity. Narrow cracks (