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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.

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.

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.

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.

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.

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.

We conducted microlite textural analyses for the 1991–1995 eruptions at Unzen, Japan, in order to clarify the relationship between the discharge rate (exit velocity) and the kinetics of the microlite crystallization processes. The temporal variations in the plagioclase microlite crystallinity and the average size are negatively correlated with the variation in the magma discharge rate. On the other hand, the variation in the microlite number density (MND) exhibits a positive correlation with the discharge rate without a significant time lag. Groundmass microlites contain calcic plagioclase microlite (An 45–65 ) and pargasite, suggesting that the microlite crystallization occurred at a depth (70–160 MPa) in the region of stability of the pargasite and plagioclase. The MND is determined at a depth (nucleation depth) with a certain effective undercooling rate (d T /d t ) that is proportional to the exsolution rate of H 2 O from magma (d C H2O /d t ). According to the MND water-exsolution rate meter, d C H2O /d t is calculated as 3.5 to 34.6 × 10 − 6 wt.%/s at the final stage of the nucleation depth ( z n : 70–100 MPa). By assuming that H 2 O exsolution occurs in equilibrium, the temporal variation in the decompression rate can be associated with the ascent velocity, provided the conduit flow is steady. The calculated ascent velocity of magma at z n (70–100 MPa; 0.8–7.6 cm/s) is higher than the exit velocity of magma (0.1 MPa; 0.2–3.9 mm/s). Using reasonable bulk densities, the difference in the velocities at the nucleation depth and surface results in the cross-section of the conduit (ellipse elongated dykes 2:5) at the nucleation depth (from 3 to 21 m) to be smaller than that at the surface (20 × 50–100 m) according to the mass conservation law along the conduit. The estimated temporal variation in the ascent velocity at z n (70–100 MPa) is positively correlated with the observed discharge rate of magma, except at the final stage of eruptive activity. Furthermore, the variation in the conduit dimension at z n is correlated with the variation in the exit velocity, thereby implying that the observed discharge rate of magma is controlled by the ascent velocity at z n (70–100 MPa) and the conduit dimension. With regard to the final stage of eruptive activity, a decrease in the conduit dimension from 19 to 3 m indicates that the conduit is finally closed due to the termination of the magma supply.

The latest eruption of Haruna volcano at Futatsudake took place in the middle of the sixth century, starting with a Plinian fall, followed by pyroclastic flows, and ending with lava dome formation. Gray pumices found in the first Plinian phase (lower fall) and the dome lavas are the products of mixing between felsic (andesitic) magma having 50 vol. % phenocrysts and mafic magma.The mafic magma was aphyric in the initial phase, whereas it was relatively phyric during the final phase.The aphyric magma is chemically equivalent to the melt part of the phyric mafic magma and probably resulted from the separation of phenocrysts at their storage depth of � 15 km. The major part of the felsic magma erupted as white pumice, without mixing and heating prior to the eruption, after the mixed magma (gray pumice) and heated felsic magma (white pumice) of the lower fall deposit. Although the mafic magma was injected into the felsic magma reservoir (at � 7 km depth), part of the product (lower fall ejecta) preceded eruption of the felsic reservoir magma, as a consequence of upward dragging by the convecting reservoir of felsic magma. The mafic magma injection made the nearly rigid felsic magma erupt, letting low-viscosity mixed and heated magmas open the conduit and vent. Indeed the lower fall white pumices preserve a record of syneruptive slow ascent of magma to 2 km depth, probably associated with conduit formation.

Oki-Dōzen (Japan) is a Late Miocene (7–5 Ma) intraplate alkalic volcano composed of a central trachytic pyroclastic complex surrounded by a ring-shaped succession of basaltic to trachybasaltic lavas and pyroclastic rocks and dispersed trachytic bodies. The central trachytic complex is in contact with a syenite that was intruded into the basement early Miocene volcano-sedimentary succession. In the centre of the system there are no alkalic basaltic rocks that are correlative of the outer flank ring. We present whole-rock major and trace element chemistry, Sr–Nd–Pb isotopic compositions and petrological data from the central trachytic volcanic complex and the intrusive syenite body, as well as from the outer ring basaltic succession. We also present and discuss a new set of zircon U/Pb ages collected from the central trachyte and syenite bodies. All the eruptive products of Oki-Dōzen, as well as the syenite, plot on a single liquid line of descent initiated from a mantle-derived alkalic basaltic parent. A younger (2.8 Ma) basaltic eruption (Uzuka basalt) has isotopic compositions that distinguish it from the rest of the system. Geochemical modelling indicates that magmatic differentiation through crystal fractionation and minor crustal assimilation occurred in crustal and shallow sub-volcanic magma reservoirs. In the central part of the system, a number of vertically spaced reservoirs acted as a filter, capturing basaltic dykes and hindering their ascent. In the outer region, dykes either reached the surface unhindered and erupted to form the basaltic/trachybasaltic succession or stalled at crustal levels and differentiated to trachyte before forming dispersed domes/flows. The central plumbing system “filter” resulted in a nest-shaped volcano, with a trachytic core surrounded by basaltic products, and stopped direct injection of basaltic magmas into the shallow syenitic magma reservoir, likely preventing its destabilization and explosive eruption.

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.

During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (

Crystallization experiments were conducted on dry glasses from the Unzen 1992 dacite at 100–300MPa, 775–875 C, various water activities, and fO2 buffered by the Ni–NiO buffer. The compositions of the experimental products and natural phases are used to constrain the temperature and water contents of the lowtemperature and high-temperature magmas prior to the magma mixing event leading to the 1991–1995 eruption. A temperature of 1050 75 C is determined for the high-temperature magma based on two-pyroxene thermometry. The investigation of glass inclusions suggests that the water content of the rhyolitic lowtemperature magma could be as high as 8 wt % H2O. The phase relations at 300MPa and in the temperature range 870–900 C, which are conditions assumed to be representative of the main magma chamber after mixing, show that the main phenocrysts (orthopyroxene, plagioclase, hornblende) coexist only at reduced water activity; the water content of the post-mixing dacitic melt is estimated to be 6 1 wt % H2O. Quartz and biotite, also present as phenocrysts in the dacite, are observed only at low temperature (below 800– 775 C). It is concluded that the erupted dacitic magma resulted from the mixing of c. 35 wt % of an almost aphyric pyroxenebearing andesitic magma (1050 75 C; 4 1 wt % H2O in the melt) with 65 wt % of a phenocryst-rich low-temperature magma (760–780 C) in which the melt phase was rhyolitic, containing up to 8 1 wt % H2O. The proportions of rhyolitic melt and phenocrysts in the low-temperature magma are estimated to be 65% and 35%, respectively. It is emphasized that the strong variations of phenocryst compositions, especially plagioclase, can be explained only if there were variations of temperature and/or water activity (in time and/or space) in the low-temperature magma.

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.

Effusive eruption of dacite magma (2.1×108 m3) during 1991–1995 formed a lava dome at the summit of Unzen Volcano, Japan. The effusion rate was highest at the beginning, 4.0×105 m3/day (4.6 m3/s), and decreased roughly with time, to almost zero before this pattern was repeated with a second pulse of magma supply. The whole-rock chemistry of lavas shows significant variation attributable to variations in phenocryst abundance; the more mafic, the more abundant the phenocrysts. The pattern of chemical variation with time shows some difference from that of the effusion rate. All phenocrysts in dacite (plagioclase, hornblende, biotite, quartz and magnetite) show evidence of disequilibrium with melt. Although a glomerophyric aggregation of phenocrysts suggests coexistence with each other, phenocrysts are isotopically heterogeneous from species to species. The calculated initial melt composition was rhyodacite, and was nearly constant throughout the activity. In contrast, the bulk phenocryst population is andesite. A model explaining the textures and the isotopic heterogeneity is the capture of diorite fragments (or xenocrysts) by parental rhyodacite magma. It is suggested that, when effusion rate was high, less viscous crystal-poor magma exited from the reservoir. Groundmass glass and plagioclase microlite rims show temporal chemical variations correlating with the effusion rate; the higher the effusion rate, the more evolved the compositions. Groundmass crystallinity increased with decreasing effusion rate; from 33% to 50%. Textures in dome lavas suggest that groundmass crystallization had been mostly completed when magma reached the conduit top. The Fe–Ti oxide temperature (880–780°C) was low when the crystallinity was high. Micropumice erupted before dome growth provided a sample recording magmatic foam in the conduit. Porosity of dome lavas was lower at lower effusion rates. Collapse of foam magma and simultaneous escape of volatiles through the conduit top were probably responsible for the accompanying low-frequency earthquakes. Phenocrysts were broken and the breakdown rims on hornblende phenocrysts were torn off during collapse and successive compaction. When effusion waned, degassing and the resultant crystallization proceeded more completely, so that the magma became too viscous to flow in the conduit top and behaved as a plug, resulting in a temporary halt of effusion. In turn, groundmass crystallization in magma below the plug increased excess pressure in the upper parts of conduit due to slow cooling. The plug was scavenged when rising excess pressure overcame its effective strength. Then, the second pulse of magma supply began. Strong endogenous growth and extrusion of a lava spine in the later stage probably occurred for the same reason.

Water contents and hydrogen isotopic ratios were determined for blocks from pyroclastic flow deposits, and bread-crust bombs and blocks from the 1991 Vulcanian eruptions of Unzen volcano, Japan. Groundmass water contents and δD values of samples were calculated by subtracting the contribution of major hydrous minerals (hornblende and biotite) from the bulk rock analyses, and range from 0.1 to 0.5 wt.% and −83 to −49‰, respectively. The samples do not show a systematic H2O–δD relationship, although the block samples tend to have lower δD values than the bomb samples. The non-systematic H2O–δD relationship is likely a result of near surface, kinetically-controlled gas loss. High viscosity of this magma would hinder attainment of hydrogen isotopic equilibrium between exsolved vapor and melt in the final degassing stage. The near surface degassing, however, was accompanied by kinetic fractionation resulting in enrichment of deuterium in the final products as exemplified by bread-crust bombs with high H2O–low δD margins and low H2O–high δD cores. Relatively high δD values of the blocks and bombs as well as high temperature volcanic gas (−30 to −35‰) suggest a closed system degassing of an initial water-rich magma (H2O=6 wt.%) until its water content was reduced to 0.5 wt.%. The pre-eruptive δD value (−46‰) was estimated from the volcanic gas data and D/H analysis of hornblende phenocrysts coupled with assumed isotopic equilibration in the initial hydrous magma.

Pargasite commonly occurs in the dacitic groundmass of the 1991–1995 eruption products of Unzen volcano. We described the occurrence and chemical compositions of amphibole in the dacite, and also carried out melting experiments to determine the low-pressure stability limit of amphibole in the dacite. The 1991–1995 ejecta of the Unzen volcano show petrographic evidence of magma mixing, such as reverse compositional zoning of plagioclase and amphibole phenocrysts, and we used a groundmass separate as a starting material for the experiments. Reversed experiments show that the maximum temperature for the crystallization of amphibole is 930°C at 196 MPa, 900°C at 98 MPa, and 820°C at 49 MPa. Compared with the experimental results on the Mount St. Helens dacite, present experiments on the Unzen dacitic groundmass show that amphibole is stable to pressures ca. 50 MPa lower at 850°C. Available Fe–Ti oxide thermometry indicates the crystallization temperature of the groundmass of the Unzen dacite to be 880±30°C, suggesting that the groundmass pargasite crystallized at >70 MPa, corresponding to a depth of more than 3 km in the conduit. The chlorine content of the groundmass pargasite is much lower than that of phenocrystic magnesiohornblende in the 1991–1995 dacite of Unzen volcano, indicating that vesiculation/degassing of magma took place before the crystallization of the groundmass pargasite. The present study shows that the magma was water oversaturated and that the degassing of magma along with magma mixing caused crystallization of the groundmass amphibole at depths of more than 3 km in the conduit.

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.

SILICA-rich lavas, erupted at island-arc or continental volcanoes, are often produced by a complex process involving the assimilation of crust into a crystallizing, mantle-derived basaltic magma1. The different strontium, neodymium and oxygen isotopic compositions of mantle-derived magmas and continental crust provide a powerful method for tracing the different contributions to continental silicic magmas, and for understanding the parameters controlling the composition and volume of erupted magma1–4. In the large rhyolite eruptive centres of the western United States, the largest-volume, explosive rhyolite eruptions have more mantle-like Nd isotope ratios than other silicic lavas from the same centre2–4, a relationship that has been interpreted as reflecting increased influx of mantle-derived basaltic magma to a crustal magma chamber before large-volume eruptions1. Here we report isotope data for lavas from Unzen volcano, which suggest a similar relationship: the Nd isotope composition is more mantle-like in three larger-volume dacite eruptions (>0.1 km3) than in one small-volume (0.02 km3) eruption. We accordingly suggest that, in small-volume systems like Unzen, where the timescales for magma-chamber evolution are of the order of decades, isotope data such as those presented here might be used in volcanic hazard evaluation.

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.