担当部執筆（解説・紹介）

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 15-34x10(6) m(3) for explosions on the afternoon of 26 January and morning of 27 January, and 5.0-7.6x10(6) m(3) for the afternoon of 27 January; magma discharge rates were in the range 1-2x10(6) 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.

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 (SiO2 = 57-58 wt.%; 960-980 degrees 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 SiO2 = 62-63 wt.% and a temperature of 870 degrees C, and contains 43 vol.% phenocrysts of pyroxene, plagioclase, and Fe-Ti oxide. The BA magma had SiO2 = 55 wt.% and a temperature of 1030 degrees 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 H2O 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 <5 km is consistent with the proposed magma system model, because olivine crystallization ceased after magma mixing. Our petrological model is consistent with a geophysical model that explains whole crustal deformation as being due to a single source located 7-8 km northwest of the Shinmoe-dake summit. However, even the shallowest estimated source of this deformation (7.5-6.2 km) is deeper than the SA reservoir, which thus requires a contribution of deeper BA magmas to the observed deformation.
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 degrees 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. (C) 2013 Elsevier B.V. All rights reserved.

The climactic phase of the 2011 eruption at Shinmoe-dake was a mixture of subplinian and vulcanian eruptive events, successive lava accumulation (lava dome) within the crater, and repetition of vulcanian events after the dome growth. It was preceded by inflation and elevated seismicity for about one year and by phreatomagmatic explosions of one week before. Small pyroclastic flows and ash-cloud surges formed during the subplinian events, when the eruption column reached the highest level and the vent was widened. A lava dome, which was extruded close to the vent of subplinian events, grew by swelling upward and filling the crater. After the vent was covered by the lava, an intense vulcanian event occurred from the base of the dome and the swelled dome became deflated. After that, vulcanian events were repeated for three months. Simultaneous eruption styles in the crater (vulcanian events, continuous ash emission and dome growth) and some phreatomagmatic events in the vulcanian stage probably are due to a complex upper-conduit system developed in water-saturated country rock.

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 similar to 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 similar to 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.

To constrain the timing and conditions of syneruptive magma ascent that are responsible for shifting eruption intensity we have investigated a basaltic sub-Plinian eruption that produced Yufune-2 scoria in Fuji volcano 2200 years ago We deduced magmatic decompression conditions from groundmass microlite textures including decompression path (i e evolution in decompression rate) and approximate decompression rate in order to relate them to eruption intensity The microlites revealed decompression conditions after water saturation at 700-1100 m depth
The temporal change in scoria size indicates that the magma discharge rate and resultant eruption intensity Increased from unit a to unit b and then declined toward ending units d and e The overall decompression rate in each eruptive unit has a positive correlation with eruption intensity The variation in decompression rate was enlarged in the final units where the maximum remained the same as the peak through the eruption (0 13-022 MPa/s for units b and c) while the minimum was 0 025 MPa/s The large variation here is due to 1) variation in flow velocity across conduit and 2) part of the erupted magma in unit d experienced remarkably slow decompression (0 002-0 003 MPa/s) resulting from decreased overpressure in the reservoir following the major eruption of unit b Furthermore crystal size distribution (CSD) of microlites implied that the earliest erupted magma (unit a) had once been decompressed slowly (0 005-0 012 MPa/s) having been arrested by material in the conduit-vent system which was followed by an increase in decompression rate due to removal of the material at the initiation of the eruption In addition the magma that had been ascending slowly before the unit-d eruption may record the increase in decompression rate This increased rate resulted from being pushed up by the successive magma at the start of that eruption
Two factors had a major impact on eruption intensity First magma decompression rate determined the degree of gas phase separation from ascending magma Judging from CSD different decompression rates had been generated at least at the start of microlite crystallization The second factor is the conduit radius that in combination with magma ascent rate controlled the magma discharge rate Before the major eruption of unit b the conduit radius likely Increased as evidenced by xenoliths of basaltic lava and lithic fragments with the same petrography as the xenoliths in unit a. In unit e the conduit radius decreased through inward development of high-density magma from the conduit margin (c) 2010 Elsevier B V All rights reserved

Asama volcano erupted in the midnight of 2 February 2009 with the ejection of ash and ballistics. The ash was dispersed toward the southeast, and observed in areas up to the southeast of the Kanto plain in the next morning. The ash fall deposits at the southeastern foot of the volcano were surveyed in order to determine a dispersal axis and detail isopleth contours. Isopleth contours of the ash fall deposit stretch out long from northwest to southeast, and they are denser in the western side of the dispersal axis than in the east. In the summit crater area, the ash is not recognized in the northern side. These indicate that the distribution of ash fall was strongly affected by a wind from the northwest. Based on the isopleth contour map, the total weight of ash fall is estimated to be 27,000-31,000ton, using a log area (m²)-log weight (g/m²) plot. The weight is approximately three fifth of the eruption on 1 September 2004 and the same order as ones on 13 November 2004 and 26 April 1982. Major components of the ash sampled at about 8km southeast from the source are non-altered and altered lava, individual crystals, and ceramisite, but minor glass particles (less than 1wt%) are also included in fine grains. The glass particles can be identified as juveniles and divided into two groups based on their shape and glass composition. One is 'dense-type' with rhyolitic composition which is the same as juveniles in the 2004 eruption, and the other is 'vesicular-type' with dacitic composition which is different from any juveniles in the recent eruptions including the 1783 Tenmei eruption. These chemical characteristics of juvenile particles indicate that two-types of magma have recently coexisted beneath Asama volcano and were erupted on 2 February 2009.

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 similar to 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 similar to 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.

We experimentally studied the dacitic magma ejected during the first event in the Usu 2000 eruption to investigate the conditions of syneruptive magmatic ascent. Geophysical data revealed that the magma reached under West Nishiyama, the location of the event's craters, after rising beneath the summit. Prior study of bubble-size distributions of ejecta shows two stages (stage 1 and stage 2) with different magma ascent rates, as the magma accelerated beneath West Nishiyama with the start of the second stage. To simulate ascent of stage 1 from the main reservoir, which was located at a depth of 4-6 km (125 MPa) to 2 km (50 MPa) beneath West Nishiyama, decompression experiments were conducted isothermally at 900 degrees C following two paths. Single step decompression (SSD) samples were decompressed rapidly (0.67 MPa/s) to their final pressure and held for 12 to 144 hours. Multiple step decompression (MSD) samples were decompressed stepwise to their final pressure and quenched instantly. In MSD, the average decompression rates and total experimental durations varied between 0.01389 to 0.00015 MPa/s and 1.5 to 144 hours, respectively. Syneruptive crystallization was confined to stage 1, and the conditions of ascent were determined by documenting similarities in decompression-induced crystallization between ejecta and experiments. Core compositions, number densities, and shapes of experimental microlites indicate that ascent to 2 km depth occurred in less than 1.5 h. Volumes and number densities of experimental microlites from the SSD experiments that best replicate the decompression rate to 2 km indicate that the magma remained at 2 km for approximately 24 h before the eruption. Stagnation at a depth of 2 km corresponds with horizontal transport through a dike from beneath the summit to West Nishiyama, according to geodetic results. The total magma transport timescale including stage 2 is tens of hours and is shorter than the timescale of precursory seismicity (3.5 days), indicating that the erupted magma did not move out of the reservoir for the first 2 days. This is consistent with the temporal change in numbers of earthquakes, which reached a peak after 2 days.

This paper reviews the principles and the methods used to investigate the ascent process of water-saturated magmas based on crystal texture and composition, which are especially relevant to recent progress made in decompression experiments. The primary cause of syneruptive crystallization is an increase in the liquidus temperature and resultant undercooling due to a decrease in dissolved H_2O in the melt. Analyses of ejecta crystal texture provide time-resolvable information. Firstly, recent decompression experiments have improved knowledge regarding crystallization kinetics and have confir...

Study on products of phreatomagmatic eruption on 31 March 2000 at Usu volcano gives insight into timing of bubble nucleation and gas escape from system during magma ascent from the reservoir. The essential fragrnents (pumice and micropumice) vary in vesicularity which is negatively correlated to water content in the rhyolitic groundmass glass. This implies that variable vesicularity resulted from different degree of water exsolution from melt to bubbles, rather than different degree of gas escape. The difference in water exsolution probably reflects the different timing of quenching, where ...

A phreatomagmatic phase at Usu volcano, Hokkaido, on 31 March, 2000, issued essential dacite pumice and micropumice in ash. The magma ascent process of the eruption was investigated using these pumiceous samples. Compositions of individual phenocryst (magnetite, plagioclase, orthopyroxene) rims and groundmass (microlite plus glass, 75.0 wt.% SiO2) are uniform among the pumiceous fragments, showing chemically homogeneous melt before eruption. The water content in the melt before eruption is estimated to be around 5 wt.%, based on both indirectly estimated water content of glass inclusions in...

The activity of Higashi-Izu monogenetic volcano group produced basalt, andesite, dacite, and rhyolite, which can be classified into rock types by whole-rock composition and phenocryst assemblage. It has been widely accepted that felsic upper crust has an important role in forming compositionally diverse ejecta; andesites are formed through the assimilation of felsic crust by the basaltic magma and the felsic magmas through remelting of the felsic crust. To reinvestigate genesis of felsic magma, I examine the physicochemical conditions of both pre-eruption felsic magmas and the felsic crust....