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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.

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...

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.

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.

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.

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 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.

Borehole USDP-4 was drilled into the Unzen volcanic conduit 9 years after its last eruption. The sub-surface mechanism of eruption at Unzen Volcano was investigated by taking cores and by geophysical logging. The drilling operations were carried out in 2003–04, as a joint research program sponsored by the Japanese Government and the International Continental Scientific Drilling Program (ICDP). Borehole USDP-4 was a directional 6-1/4 in. (158.8 mm) borehole drilled from a site located on the northern slope of the volcano at 840 m above sea level, and was designed to penetrate into the conduit at sea level with a final deviation angle of over 70° from vertical and a borehole depth of 1800 m. An igneous dyke with a high likelihood of being the Unzen conduit was encountered at 1996 m depth and core samples were taken from that location. Chemical analysis of drill cores confirmed the identification of the conduit. Geophysical logging, including recording of gamma ray, resistivity, self-potential, density, neutron porosity, sonic velocity, and temperature suggested an alternation of dykes and flows in rock penetrated during the drilling, and in-hole pictures confirmed the lithologic identifications. Although borehole collapse and high temperature had been expected in the conduit, the actual drilling and logging into the conduit experienced no gas or fluid kick, and the measured temperatures within the dyke were below 200 °C.

Megacrysts (large crystals of 2–15 mm in length) of plagioclases extracted from the lavas of the last three eruptions of southwestern Japan's Unzen volcano (AD 1663, AD 1792 and AD 1991–1995) have limited 87 Sr/ 86 Sr ratios of 0.70439–0.70454. Results of micro-drilling analyses indicated that three of eight megacrysts showed a gradual decrease in 87 Sr/ 86 Sr ratios from the core to the rim, whereas other megacrysts showed a homogeneous 87 Sr/ 86 Sr ratio. This relative homogeneity contrasts sharply to the wide variation of 87 Sr/ 86 Sr ratios of matrixes of the lavas of the last three eruptions (0.71417, 0.70467, 0.70447–0.70450, respectively). Most megacrysts show isotopic disequilibrium between their outer rims and their matrixes, suggesting that they did not grow in the host magmas. In addition, seven of the eight megacrysts of plagioclase also have similar trace element abundance ratios (La/Nd and Sr/Ba). Their similar chemical and isotopic compositions suggest that they crystallized from the same parent magma, which suggests to us that they were formed prior to or during the eruption in 1663. The isotopic ratios of the 1663 eruption lavas of 0.71417, however, rules out the possibility that it is the parent magma for the plagioclase megacrysts. The lavas erupted 4000 and 5000 years ago have 87 Sr/ 86 Sr ratios of 0.70454 and 0.70442, respectively, [Chen, C.-H., DePaolo, D.J., Nakada, S., Shieh, Y.-N., 1993. Relationship between eruption volume and neodymium isotopic composition at Unzen volcano. Nature 362, 831–834]; they are inferred to be of the parent magma because of their Sr isotope ratios. Consequently, the micro-analytical results seem to suggest that plagioclases with a single origin can be supplied to volcanic products of several different eruptions surviving several eruption events.

Major and trace element abundances, Sr and Nd isotopic ratios, and 238U-230Th radioactive disequilibria have been analyzed for samples from five volcanoes, Oshima, Miyakejima, Niijima, Teishi Knoll and Fuji, located in the northern Izu arc to investigate across-arc and regional variations of chemical compositions of mantle and fluids expelled from a subducting slab. The abundances and abundance ratios of both fluid mobile and immobile trace elements show across-arc variations. All samples but one have radioactive disequilibria, with (238U/230Th) greater than unity (herein, a ratio in parentheses denotes the activity ratio). The observed disequilibrium is similar to those reported for other arc systems, such as Mariana, Tonga, Kermadec, Chile. It originates from higher mobility of uranium relative to thorium during dewatering of the subducting slab. The degree of radioactive disequilibrium of (238U/230Th) decreases with depth to the Wadati-Benioff zone at each volcano. The radioactivity ratios of (230Th/232Th) of the samples from Miyakejima, however, deviate from an across-arc trend formed by Oshima, Niijima, Teishi Knoll and Fuji. The combined data of (230Th/232Th)-(238U/232Th) of the samples from the Izu and the Mariana arc revealed a well defined array in an isochron diagram with the exceptions of the samples from Miyakejima and Alamagan. This finding requires (230Th/232Th) of the sub-arc mantle beneath the arcs of Izu and Mariana except the magma sources of the two islands of Miyakejima and Alamagan was around 1.1 and relatively homogeneous before the final addition of fluid from slab. The homogeneous (230Th/232Th) necessitates input of Th with homogeneous (230Th/232Th) contributed from sediment melt, which dominates the budget of Th in the sub-arc mantle. It also suggests that influence of fluid addition before the final one had not caused a variation in Th isotopic compositions of the sub-arc mantle.

The variation of sulphur isotopic composition during the 2000 eruption of the Miyakejima Volcano was examined in order to monitor the temporal change of the volcanic activity. The δ34S values of water-soluble sulphate leached from volcanic ash effused during intermittent eruptions from July to September 2005 range from +5 to +11‰ with a fluctuation of ca 3‰ within a single eruption. The δ34S value of sulphuric acid mist collected with ‘Cu-metal trap’ placed on the flank of the volcano from December 2000 to January 2001 is +6.2‰. These sulphur isotopic compositions of sulphate, which were isotopically equilibrated in the subvolcanic hydrothermal system, indicate that the temperature of the hydrothermal system beneath the caldera increased after the period of intermittent phreatic and phreatomagmatic eruptions. Then, the δ34S value of sulphuric acid trapped from January to March 2001 was +9.0‰ and the δ34S value of water-soluble sulphate on volcanic ash emitted with minor eruption in May 2001 was +11.0‰, suggesting a decrease in temperature of the subvolcanic hydrothermal system.

Anatahan Island is located at the southern end of the Mariana volcanic chain. On May 10, 2003, the eastern crater of the island erupted for the first time in recorded history. The Plinian eruption column reached an altitude as high as 13 km on May 11, and a thick layer of ash covered the island. The eruptive activity continued to June 2003, but most of the erupted material was expelled during the first week. The volcanic activity declined in the second half of 2003, but resumed in April 2004. In order to determine crustal deformation associated with the eruption, we conducted GPS measurements in July 2003 at a benchmark (ANAT) located approximately 7 km west–northwest of the active crater, where GPS campaign measurements had been repeated four times since 1992. In the period from January to July 2003 during the eruption, significant subsidence – as much as 21 cm – was detected, but horizontal movement was negligible. We began taking continuous GPS measurements at the same site in July 2003 to monitor the transient deformation that was probably associated with magma migration. To assess the spatial extent of the deformation more accurately, we established another permanent GPS site (ANA2) at a site approximately 3 km from the active crater in the northeastern part of the island in January 2004. The coordinates of this time series at ANAT probably show a change in trends at the beginning of 2004. Another subsidence of 2.8 cm and a westward motion of 2.1 cm were estimated to have occurred in the period from July to December 2003. This was followed by an uplift of 5.2 cm and movement in an eastward direction of 1.0 cm in the period from January to June 2004. We developed three preliminarily models of inflation/deflation sources for three different time periods. During the period from January to July 2003, a deformation source was located beneath the ANAT site and acted as a deflation source. Considering the gap in the GPS time series and errors in data (especially after July 2003), we expected that the deformation sources were located beneath the western part of Anatahan Island and not below the active crater.

Anatahan Volcano, Northern Mariana Islands, began erupting in May-June 2003. A series of subplinian explosive eruptions of andesite magma began at the Eastern Crater in the eastern part of the summit caldera on the evening of 10 May. Brown tephra was sent mainly westward by strong winds. Small-scale pyroclastic surges were discharged eastward outside the caldera in late May. An andesite lava dome that had once filled the inner crater was fragmented by phreatomagmatic explosions in the middle of June. The phreatomagmatic explosions probably occurred due to interaction of the magma head with groundwater around the crater, and abundant very fine ash (gray tephra) was discharged within the caldera and over most of the island. The volume of eruption products of the May-June eruption was estimated to be 1.4 x 10 7 m 3 dense-rock-equivalent. Erupted pumices and lava are aphyric andesite and are variously colored depending on their vesicularity. The SiO 2 contents of erupted materials decreased slightly with time. The fine gray ash is depleted in alkalies, probably due to leaching by acid hydrothermal fluids during explosions. Seismic activity resumed in late March 2004, and small strombolian-like explosions were repeated in May and June 2004. About half of the inner crater was filled with new scoria and lava.

Lateral migration of magma away from Miyakejima volcanic island, Japan, generated summit subsidence, associated with summit explosions in the summer of 2000. An earthquake swarm beneath Miyakejima began on the evening of 26 June 2000, followed by a submarine eruption the next morning. Strong seismic activity continued under the sea from beneath the coast of Miyakejima to a few tens of kilometers northwest of the island. Summit eruptive event began with subsidence of the summit on 8 July and both explosions and subsidence continued intermittently through July and August. The most intense eruptive event occurred on 18 August and was vulcanian to subplinian in type. Ash lofted into the stratosphere fell over the entire island, and abundant volcanic bombs were erupted at this time. Another large explosion took place on 29 August. This generated a low-temperature pyroclastic surge, which covered a residential area on the northern coast of the island. The total volume of tephra erupted was 9.3×106 m3 (DRE), much smaller than the volume of the resulting caldera (6×108 m3). Migration of magma away from Miyakejima was associated with crustal extension northwest of Miyakejima and coincident shrinkage of Miyakejima Island itself during July–August 2000. This magma migration probably caused stoping of roof rock into the magma reservoir, generating subsurface cavities filled with hydrothermal fluid and/or magmatic foam and formation of a caldera (Oyama Caldera) at the summit. Interaction of hydrothermal fluid with ascending magma drove a series of phreatic to phreatomagmatic eruptions. It is likely that new magma was supplied to the reservoir from the bottom during waning stage of magma’s migration, resulting in explosive discharge on 18 August. The 18 August event and phreatic explosions on 29 August produced a conduit system that allowed abundant SO2 emission (as high as 460 kg s−1) after the major eruptive events were over. At the time of writing, inhabitants of the island (about 3,000) have been evacuated from Miyakejima for more than 3 years.

During the early part of a seismic swarm preceding eruption and caldera formation at Miyakejima Volcano, discoloured sea surfaces were observed ∼1.5 km off the western coast of Miyakejima on 27 June 2000. A later survey of the area using a multi-beam side scan sonar and a remotely operated small submarine revealed four craters of 20–30 m diameter aligned east-west in a 100×10–30 m area on the seafloor, with hot water at ∼140°C being released from one of the centres. Each crater consists of submarine spatter overlain in part by scoria lapilli. Dredged spatter from the craters was fresh, and there was no evidence of activity of marine organisms on the spatter surface, indicating that the discoloured sea surface resulted from magmatic eruption on the seafloor. This eruption occurred when a westward-propagating seismic swarm, initiated beneath Miyakejima’s summit, passed through the area. Finding new magma on the seafloor demonstrates that this seismic swarm was associated with intruding magma, moving outward from beneath Miyakejima. Submarine spatter shows flattened shapes with a brittle crust formed by cooling in water, and its composition is aphyric andesite of ∼54 wt% SiO2. The spatter is similar in whole rock and mineral composition to spatter erupted in 1983. However, the wide range of Cl in melt inclusions in plagioclase of the 27 June submarine spatter shows that it is not simply a remnant of the 1983 magma, which has only high Cl melt inclusions in plagioclase. The mixed character of melt inclusions suggests involvement of a magma with low Cl melt inclusions. The magma erupted explosively on 18 August from Miyakejima’s summit, considered as the second juvenile magma in this eruption, contains low Cl melt inclusions in plagioclase. Based on these observations and the eruption sequence, we present the following model: (1) A shallow magma chamber was filled with a remnant of 1983 magma that had evolved to a composition of 54–55 wt% SiO2. (2) Injection of the 18 August magma into this chamber generated a mixed magma having a wide range of Cl in melt inclusions contained plagioclase. The magma mixing might have occurred shortly before the submarine eruption and could have been a trigger for the initiation of the removal of magma from the chamber as an extensive dyke, which eventually led to caldera subsidence.

Based on the combination of petrologic characteristics of the deposits and carbon-14 dating of charcoal pieces, we determined the burial age of the ancient Roman villa, believed to belong to the first Roman Emperor Augustus, at the northern foot of Mt. Vesuvius. Volcanic deposits covering the site consist of three geological formations related to the eruptive activity of Mt. Vesuvius. Juvenile material of the lowest formation show the same whole-rock chemical composition as that of the AD472 eruption, and carbon-14 ages of charcoal in and below the deposits coincide with AD472, indicating that the villa was first buried by the AD472 eruption. The villa itself is thought to have been already abandoned, judging from the depositional relation of the Plinian-fall and pyroclastic surge deposits to the damaged building frames.

Fuji Volcano is the largest and closest active volcano to the Tokyo metropolitan area. The 1707 Hoei eruption was the last of this volcano. Fuji Volcano has two extreme risks inherent to its geological and geographical features. The first is a result of its unique tectonic setting; the volcano is located on the collision boundary between the Northeast Japan and Izu-Mariana arcs. Large earthquakes have occurred repeatedly in this zone triggering sector collapses and ensuing debris avalanches, including the Gotemba debris avalanche at 900 BC. The Fujikawa-kako faults cutting the volcano is the most active fault system in Japan, and a forecast indicates that the scale of the next earthquake would be M 8 and that it is likely to occur some time in the near future within several hundred years. The second risk is related to damage of electric networks in the Tokyo metropolitan area by explosive eruptions. It was forecasted that the fallout thickness by the Hoei-type Plinian eruptions would be ten to several centimeters around the Tokyo Bay area. The electricity in this area highly depends on thermal power plants clustered around the Tokyo Bay. Furthermore, half of the thermal power plants have gas turbines. Although there is an air filter system in the thermal power plants, it may be clogged up by ash and partially or totally block the airflows into the gas turbines. To protect the thermal power plants from the ashfall hazard, the electric company must stop the plant, remove the fallout, and replace the air filters or other broken parts. As a result, massive outages may disrupt social and economic activities in the metropolitan area.

Phase 1 of the most recent eruption of Unzen volcano (Japan) occurred between 1991 and 1993 and involved the endogenous and exogenous addition of magma to a growing summit lava dome. The temporal variation between endogenous and exogenous styles of dome growth has an important bearing on the hazard posed by dome collapse and explosive events. However, such data are difficult to acquire and have previously only been obtained in a fully quantitative manner for the Mount St. Helens dome, in that case using detailed photogrammetry. Here we investigate these competing dome growth processes at Unzen via analysis of their thermal emission signals present in shortwave infrared (SWIR) satellite imagery. We find that both the endogenous and exogenous growth styles exhibit identifiable SWIR signatures whose magnitude can be related to their respective magma supply rates. We find that the SWIR radiant area corresponding to endogenous growth is around four times larger than that corresponding to exogenous growth at the same magma discharge rate. Analysis of time series imagery indicates that, like Mount St. Helens, Unzen experienced a temporally decreasing magma discharge rate during phase 1. As time progressed and the discharge rate fell, the growth style of the Unzen dome changed from ‘exogenous only’, through ‘exogenous decreasing and endogenous increasing’ to ‘endogenous only’. Thus relative growth rates at the endogenous and exogenous areas of the Unzen dome appear to be at least partly controlled by the overall magma discharge rate. As with Mount St. Helens, the cause of this relationship was most likely the formation and strengthening of a brittle crust at the surface vent, which impeded exogenous growth more effectively as time elapsed and the discharge rate fell. These results suggest the similarity of the processes operating at the Unzen and Mount St. Helens domes, though important differences remain in that dome growth was basically continuous at Unzen, whilst at Mount St. Helens growth episodes were interspersed by significant periods of repose.

Constraining physical parameters of tephra dispersion and deposition from explosive volcanic eruptions is a significant challenge, because of both the complexity of the relationship between tephra distribution and distance from the vent and the difficulties associated with direct and comprehensive real-time observations. Three andesitic subplinian explosions in January 2011 at Shinmoedake volcano, Japan, are used as a case study to validate selected empirical and theoretical models using observations and field data. Tephra volumes are estimated using relationships between dispersal area and tephra thickness or mass/area. A new cubic B-spline interpolation method is also examined. Magma discharge rate is estimated using theoretical plume models incorporating the effect of wind. Results are consistent with observed plume heights (6.4–7.3 km above the vent) and eruption durations. Estimated tephra volumes were 15–34 × 106 m3 for explosions on the afternoon of 26 January and morning of 27 January, and 5.0–7.6 × 106 m3 for the afternoon of 27 January; magma discharge rates were in the range 1–2 × 106 kg/s for all three explosions. Clast dispersal models estimated plume height at 7.1 ± 1 km above the vent for each explosion. The three subplinian explosions occurred with approximately 12-h reposes and had similar mass discharge rates and plume heights but decreasing erupted magma volumes and durations.

A collapsed caldera, 1.6 km in diameter and 450 m in depth, was formed at the summit of Miyakejima Volcano during the 2000 eruption. The collapsed caldera appeared on 8 July, with a minor phreatic eruption, 12 days after seismic activity and magma intrusion occurred northwest of the volcano. Growth of the caldera took from 8 July to the middle of August, with seismic swarms associated with the continuous intrusion of magma northwest of the volcano. The growth rate of the caldera was about 1.4×107 m3/day, and the final volume of the collapsed caldera was about 6×108 m3. Major phreatomagmatic eruptions produced a total of about 1.6×1010 kg (1.1×107 m3) of volcanic ash after caldera growth. The caldera structure, and the nature of the eruptive materials of the first collapse on 8 July, suggest that the surface subsidence was caused by the upward migration of a steam-filled cavity, with stoping of the roof rock above the magma reservoir. The diameter of the stoping column was estimated to be 600–700 m from circumferential faults that developed in the caldera floor, and the collapse of the caldera wall enlarged the diameter of the caldera to 1.6 km. The total volume of the caldera and the horizontal diameter of the stoping column gave a subsidence of the caldera floor of 1.6–2.1 km.

To understand degassing processes during the 2000 Miyakejima volcanic activity, we applied the following two methods : 1) repeated analyses of adhered water-soluble gas component such as SO4and Cl ions on ashes produced at eruptions from July 8 and mid-September, and 2) SO2 flux measurements by COSPEC since August 26.The repetitive analyses of soluble component show remarkable changrd in Cl/S. Until August 18, the adhered SO4 concentrations are quite high and Cl concentrations are always low with Cl/S of 0.01-0.05, indicating that a certain mass of groundwater existed in aquifers beneath the summit crater and Cl component selectively dissolved in the groundwater. Since August 29, chlorine concentrations became greater and Cl/S ratios were determined as 0.1-0.14 on August 29 and 0.5-1.5 in September, which coincided with the strong volcanic gas emissions which started in mid-August. Groundwater boiling and establishment of gas conduit are likely to occur to prevent HCl from being absorbed in the groundwater.Sulfur dioxide emission rate has been monitored since August 26. The SO2flux increased in midSeptember from thousands to tens of thousands tons a day. The average SO2 flux after midSeptember to the present is 48 ktons/day. The highest flux was observed on December 7 to be 230 ktons/day. The mass rate of the magma degassing is estimated as 20 Mtons/day and the total volume of the degassed magma is calculated to be 1 km3 so far. The continuous magma degassing without eruptions occurs at a shallow environment by the convective transport of magma from a chamber to a magma head through conduits. A huge degassing rate is likely to be due to a large surface at the magma head, which would be made by a piston-cylinder type of collapse of the volcanic body.

Thermal imaging from satellites is one methodology used for the monitoring and scientific investigation of volcanoes, including those characterised by active lava domes. To be most effective, the remote sensing techniques employed must allow the cause of any observed thermal anomaly to be identified, ideally using information contained within the remote sensing data itself, whilst using any ancillary field data to guide analysis and the necessary assumptions. This study investigates a method by which such discrimination maybe accomplished for activity at lava domes. For this purpose we use three Landsat Thematic Mapper (TM) scenes of Unzen Volcano (Japan) to determine the temperature structure of the active lava dome existing during 1991–1993. These data are of particularly high quality since the TM scenes were obtained during night-time overpasses and a large amount of supplementary data are available to parameterise the model used to retrieve the subpixel temperature structure of the dome surface. The results are matched to near-contemporaneous geological sketch maps of the then current situation in order to identify the TM pixel groupings corresponding to the different styles of dome activity, namely fumarolic degassing, exogenous and endogenous dome growth, and collapse deposits resulting from block and ash flows, small scale pyroclastic flows and rockfalls. The spatial and statistical characteristics of the resultant TM-derived temperature distributions are then investigated to determine rule-based criteria that may be used to differentiate the activity styles of lava domes based on their thermal structure within the remote sensing data. Results indicate that fumarolically active locations, regions of active dome growth, and areas of collapse deposition on the Unzen dome can be differentiated using only the statistical distribution of the hotspot temperatures and fractional areas identified via analysis of TM imagery. The statistics derived for hotspots at the fumarolically active areas show them to be, in general, significantly hotter (and smaller) than those found at the areas of active dome growth, whereas hotspots retrieved on the areas of collapse deposition are cooler (and larger). We find that the frequency distributions of hotspot temperatures extracted for the areas of endogenous and exogenous dome growth exhibit statistically inseparable means, but that discrimination between these two dome growth styles is possible using analysis of the spatial arrangement of the retrieved hotspots. Specifically the exogenous lobes are characterised by structures interpreted to be the effusing vent of new magma and the collapsing lobe front. These features are absent at locations showing only endogenous growth. It is hoped that the criteria developed here will prove useful during future quantitative analysis of the extended TM time-series available for Unzen, and for identification of thermal anomalies of uncertain origin at other active domes observed via satellite remote sensing.

Large-scale collapse of a dacite dome in the late afternoon of 15 September 1991 generated a series of pyroclastic-flow events at Unzen Volcano. Pyroclastic flows with a volume of 1×106 m3 (as DRE) descended the northeastern slope of the volcano, changing their courses to the southeast due to topographic control. After they exited a narrow gorge, an ash-cloud surge rushed straight ahead, detaching the main body of the flow that turned and followed the topographic lows to the east. The surge swept the Kita-Kamikoba area, which had been devastated by the previous pyroclastic-flow events, and transported a car as far as 120 m. Following detachment, the surge lost its force after it moved several hundred meters, but maintained a high temperature. The deposits consist of a bottom layer of better-sorted ash (unit 1), a thick layer of block and ash (unit 2), and a thin top layer of fall-out ash (unit 3). Unit 2 overlies unit 1 with an erosional contact. The upper part of unit 2 grades into better-sorted ash. At distal block-and-ash flow deposits, the bottom part of unit 2 also consists of better-sorted ash, and the contact with the unit 1 deposits becomes ambiguous. Video footage of cascading pyroclastic flows during the 1991–1995 eruption, traveling over surfaces without any topographic barriers, revealed that lobes of ash cloud protruded intermittently from the moving head and sides, and that these lobes surged ahead on the ground surface. This fact, together with the inspection by helicopter shortly after the events, suggests that the protruded lobes consisted of better-sorted ash, and resulted in the deposits of unit 1. The highest ash-cloud plume at the Oshigadani valley exit, and the thickest deposition of fall-out ash over Kita-Kamikoba and Ohnokoba, indicate that abundant ash was also produced when the flow passed through a narrow gorge. In the model presented here, the ash clouds from the pyroclastic flows were composed of a basal turbulent current of high concentration (main body), an overriding and intermediate fluidization zone, and an overlying dilute cloud. Release of pressurized gas in lava block pores, due to collisions among blocks and the resulting upward current, caused a zone of fluidization just above the main body. The mixture of gas and ash sorted in the fluidization zone moved ahead and to the side of the main body as a gravitational current, where the ash was deposited as surge deposits. The main body, which had high internal friction and shear near its base, then overran the surge deposits, partially eroding them. When the upward current of gas (fluidization) waned, better-sorted ash suspended in the fluidization zone was deposited on block-and-ash deposits. In the distal places of block-and-ash deposits, unit 2 probably was deposited in non-turbulent fashion without any erosion of the underlying layer (unit 1).

Juvenile materials were found among products of phreatomagmatic eruptions that preceded dacite dome growth at Unzen Volcano in 1991. They give evidence showing that the hydrous magma started degassing with the resultant crystallization around 100 MPa, and was quenched soon thereafter. Ascending at a rate as low as 13 m/d while degassing, however, the still-molten part inside reacted with water. Phreatomagmatic eruptions started when the magma reached about 1.2 km in depth, and strong ones started at about 0.6 km. Volcanic tremors had occurred at these depths, where the sources of vulcanian explosions and the ground deformation were also located, implying the existence of a possible gas pocket.

The central Kyushu is a north-south spreading region characterized by east-west trending grabens and abundant active volcanoes. Available data indicate that the width of graben is 11-12 km apart from eruption centers of Unzen volcano, whereas it narrows down to about 7 km dose to the eruption centers. Focal mechanisms of earthquakes suggest that north-south tensional axis is not altered by the presence of volcanic conduit. Width of graben in general is pos tively correlated with thickness of lithosphere, and the width of graben is narrowed locally in accordance with the thinning of lithosphere beneath active volcanoes by the presence of magma chamber in middle crust. Evidences supporting this idea include shallow cutoff depth of seismicity and mid-crust low velocity area beneath the active volcano, shallow Curie temperature distribution, and high oxygen isotopic composition of the volcanic rocks.

RECENT modelling of volcanic eruptions has shown that the efficiency of subsurface degassing of magmas determines whether magma erupts explosively or effuses quietly1,2. Slow uprise of magma is often accompanied by effective degassing, leading to the extrusion of lava flows and domes. Although lava dome extrusion is one of the less explosive modes of eruption, it is often accompanied by explosive pyroclastic activities3–5. The 1991 eruption of Unzen volcano provided an opportunity to observe at close range several types of small-scale pyroclastic flow (glowing avalanches) originating from lava domes. Most of the pyroclastic flows are of Merapi type, caused by blocks falling from a collapsing dome; others are of Pelean type originating in an explosion from the side of a dome. The lavas apparently show variable degrees of self-explosivity. We suggest that variable degrees of degassing of the magma produced a wide range of excess pore pressures in the extruded lava domes, resulting in both Merapi-type and Pelean-type pyroclastic flows from the domes.

In November 1989, a swarm of earthquakes deep beneath Tachibana Bay, Kyushu Island, Japan, heralded the inexorable rise of magma toward the summit of Unzen Volcano, some 15 km upward and 15 km eastward, on the Shimabara Peninsula. When the “magma head” emerged in Jigokuato Crater on May 20, 1991, a beautiful but tragic drama began. It started peacefully as a budding flower unfolding lava petals (Figure 1). But by the time lava stopped flowing in February 1995, it had cost the city of Shimabara and the surrounding towns over $2 billion in damage and 44 human lives. At its height, the crisis required the prolonged evacuation of 11,000 residents. Amid this tragedy, however, volcanologists were able to make unprecedented visual and geophysical observations of processes of magma ascent, dome growth, and dome-fed pyroclastic flows.

A dacite dome at Unzen volcano grew mainly exogenously when it was small and the effusion rate was high, but endogenously when the dome became large and the effusion rate declined. The endogenous dome that has grown since late 1993 shows a shape classified as “Pelean,” whereas the earlier stage had a “low lava dome” shape. The carapace of the endogenous dome moved like the crust of a basaltic lava pillow, although the dome is several hundred times larger than such pillows. The surface carapace was carried from the inside of the dome where it had been produced, and it thickened as it cooled. The crater floor was strongly deformed by the advancing endogenous dome. The movement and crater floor deformation can be compared to that of a tractor tread moving on unconsolidated ground.

After a precursory phreatic stage (2008 to 2010), the 2011 Shinmoe-dake eruption entered a phreatomagmatic stage on January 19, a sub-Plinian and lava accumulation stage at the end of January, a vulcanian stage in February–April, and a second phreatomagmatic stage in June–August. Component ratio, bulk composition, and particle size of the samples helped us define the eruptive stages. The juvenile particles were first found in the January 19 sample as pumice (8 vol%) and were consistently present as scoria and pumice particles thereafter (generally ~50 vol%, decreasing in weaker events). The January 19 pumice has water-quench texture. After the lava accumulation, particles of that lava origin came to account for 30~70 vol% of the ash. The second phreatomagmatic stage is proposed because of fine ash and long eruption period. The SiO2 contents of bulk ash are lower in post-January 19, 2011 eruptions, reflecting lower average SiO2 contents in 2011 ejecta than in past ejecta. The free-crystal assemblages were two pyroxenes + plagioclase + Fe-Ti oxides until 2010; olivine joined the assemblage in 2011, when juvenile ash was erupted. This change is consistent with the absence or smaller sizes of olivine phenocrysts in past ejecta.