Broken crystals are often contained in the eruptive products of explosive and nonexplosive eruptions. Although little is known of the actual mechanism of crystal fragmentation, the broken crystal itself is useful for understanding phenomena in the course from magma ascent to eruption. In order to describe the nature of broken crystals, the degrees of fragmentation of plagioclase (referred to as “b/a value”, hereafter) were measured in this study. The b/a values were obtained by dividing the length of a broken surface (b) by the circumference of the crystal (a) on the thin section. Plagioclase contained in the following lava flows and pumice fall deposits of Sakurajima Volcano and Asama Volcano were measured. In the case of the 1783 Asama eruption, the activity culminated to its climactic, pyroclastic eruption that generated Plinian pumice falls and clastogenic lava flows and formed a pyroclastic cone after the intermittent Vulcanian eruptions for about three months. The 1914-1915 Sakurajima eruption progressed from the initial vigorous pyroclastic eruption (Stage 1) via lava flowage with intermittent Vulcanian eruptions (Stage 2) to the long-lasting, gentle outflow of lava (Stage 3). The eruptive style of Stage 1 is similar to that of the climactic stage of the Asama 1783 eruption. The eruptive style of the 1946 Sakurajima eruption was similar to that of Stage 2. Concerning the pumice fall deposits, crystals in a single pumice clast and free crystals of various grain sizes were measured. Multiple timings and fields of crystal fragmentation are indicated from the following results. In the case of Stage 3 of the Sakurajima 1914-1915 eruption, a small amount of plagioclase shows smaller b/a values suggesting that a small amount of poorly fragmented crystals was produced prior to the eruption. In this case, crystal breakage related to melt inclusions foaming during the decompression of ascending magma and to shear-induced fragmentation of ascending magma near the conduit wall are instanced as the possible mechanism. In the cases of Stage 2 and the 1946 eruption, b/a values are higher than that in Stage 3. Crystal fragmentation within the conduit could occur, probably due to shock by repetitive Vulcanian explosions. In the cases of Stage 1 of the Sakurajima 1914-1915 eruption and the climactic stage of the Asama 1783 eruption, average b/a values increased in the order of crystals in pumice clast, free crystals, and crystals in lava. Free crystals consisting of broken surfaces without glass coating and crystal faces covered by vesicular glass are dominant. It is considered that the abundant broken free crystals are produced by magma fragmentation and then by quenching of erupting pyroclastic materials in the eruptive column. Additional fragmentation during flowage can be considered within the clastogenic lava flows.

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Katsuma-Yama volcano is located on the Okushiri Island 15 km west of Oshima Peninsula, southwest Hokkaido. Effused from the Katsuma-Yama crater of the volcano at about 20 ka or a little bit older time, Katsuma-Yama rhyolite lava entered the Horonai-Gawa caldera lake and intruded into the lake deposits. The rhyolite lava is almost entirely glassy but hydrated to form perlitic rocks with a water content up to 2～3 wt.%. Relatively fresh, dark and dense part of the lava remains in the inner part of the source area and is replaced with a light grey glassy rock mainly along the flow layers or flow-parallel minor fractures. Dark dense glass locally fills fractures of a light grey glassy part, and curviplanar cracks are developed normal to the columnar joints and further normal to the resulting cracks. In addition to these thermal contraction cracks, more curved and more closely spaced cracks are developed in light grey rocks, likely produced by volumetric change with glass hydration. Thermal contraction cracks were presumably developed in the relatively fresh part immediately below the glass transition temperature with a rapid volume change. Hydration likely proceeded with water-permeation through the cooling cracks and cracking proceeded further with volume expansion of hydrated domains. The glass transition temperature is estimated empirically to be about 700℃ with a minimum water-content, 0.3 wt.% in a relatively fresh dark glassy rock. Hydration is likely to have almost ceased at about 400℃ as the rate of water diffusion becomes too small to across crack-to-crack distance before the lava entirely cooled below 400℃.

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The 30 ka caldera-forming eruption of Ohachidaira started with plinian pumice fall and pyroclastic flows. The deposits contain pumice (SiO₂=64.9-68.4wt.%), scoria (SiO₂=56.6-59.0wt.%), and banded pumice. This study examined the evolution processes of silicic magma chamber through mineralogical and petrological analyses of the eruption products. Three types of plagioclase phenocrysts such as An-rich (type A: An_70-90), An-poor (type B: A_36-56), and intermediate (type C: An_56-70) were observed. Type-A plagioclase phenocrysts were further classified into two sub-types on the basis of MgO content in the cores; type A1 (MgO＞0.05wt.%) and type A2 (MgO＜0.05wt.%). Type-A1 and type-A2 plagioclase phenocrysts were derived from mafic magma, type-B plagioclase phenocrysts were derived from silicic magma, and type-C plagioclase phenocrysts were derived from hybrid magma formed by the mixing of mafic and silicic magmas. The pumice mainly contains type-B plagioclase phenocrysts with rare type-A2 and type-C plagioclase phenocrysts. The scoria contains type-A1, type-A2, and type-B plagioclase phenocrysts with rare type-C plagioclase phenocrysts. These assemblages in the products can be explained by the mixing of magmas. Initially, mafic magma including the type-A1 plagioclase phenocrysts was injected into the bottom of the silicic magma chamber, and a density-stratified magma chamber was formed. The first mixing occurred at the interface of mafic and silicic magmas, and a hybrid magma was formed at the interface of the two magmas. During the period from the mixing to the eruption, type-A2 plagioclase phenocrysts were formed due to the diffusion of MgO in type-A1 plagioclase phenocrysts. Whereas, type-C plagioclase phenocrysts were derived from hybrid magmas. During the eruptions, the lower-layer magmas (hybrid and mafic magmas) were sucked into the conduit due to the viscous force of the upper-layer silicic magma. Outer part of the conduit, silicic and hybrid magmas mixed. The mixed magma contained type-B, type-A2, and type-C plagioclase phenocrysts. Whereas, in the center of the conduit, the mixing of the three magmas (mafic, hybrid, and silicic magmas) occurred, and the mixed magma containing the type-A1, type-A2, type-B, and type-C plagioclase phenocrysts was formed.

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Shinmoedake (Kyushu, Japan), which is one of the Kirishima Volcanoes, experienced several small eruptions in 2010, finally culminating in a sub-plinian eruption on January 26-27, 2011. After this sub-plinian phase, the eruption style shifts to the phase of vulcanian eruption or ash emission. This volcanic activity is still occurring. We here summarize the eruption history of Shinmoedake during the Edo period on the basis of historical records. The eruptions of Shinmoedake during the Edo period occurred in AD 1716-1717 (Kyoho eruption) and AD 1822 (the 4th year of Bunsei eruption). The Kyoho eruption, which was a large-scale (total amount of tephra: 2×10¹¹ kg) eruption, is divided into the following seven stages. Stage 1 (Apr. 10, 1716 to May 7, 1716): small eruptions occurred over two months; Stage 2 (Sep. 26, 1716): falling ash first observed at the foot of Shinmoedake; Stage 3 (Nov. 9 to 10, 1716): the first large eruption was observed, with pumice falling over a wide area; Stage 4 (Dec. 4 to 6, 1716): small eruptions; Stage 5 (Feb. 9 to 20, 1717): the second pumice fall eruption, with an intermittent ash fall eruption thereafter; Stage 6 (Mar. 3, Mar. 8, Mar 13, Apr. 8, 1717): ash fall eruptions; Stage 7 (Sep. 9, 1717): the last ash fall eruption. These eruptions, which continued intermittently over 17 months, were characterized by multiple repetitions of a large eruption. Based on the results of a comparison between the Kyoho eruption and the 2011 eruption, the eruptions from March 30, 2010 to January 26, 2011, were similar to Stages 1 to 3 of the Kyoho eruption; the eruptions after January 26, 2011, were similar to Stages 5 to 6 of the Kyoho eruption. In addition, the relatively large eruption events of Stages 3 and 5 of the Kyoho eruption and the January 26-27, 2011, eruption began without any noticeable precursors. The eruption in the 4th year of Bunsei (AD 1822) was a small eruption that lasted less than a day. The recent eruption sequences, which were also similar to the Edo period eruptions, are divided into a small-scale eruption (the 1959 eruption) and a large-scale eruption (the 2011 eruption). The eruption duration time of the small-scale (total amount of tephra: ＜ 10¹⁰ kg) eruption was less than a day. The eruption duration time of the large-scale (total amount of tephra: ＞ 10¹⁰ kg) eruption could be a few months or years. Both eruption sequences began with a small eruption. A large-scale eruption can occur a few months after the start of the eruption sequence. This is an important turning point in the eruption sequence of Shinmoedake.

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We produced an SO₂ imager of a low-cost version with the use of digital cameras of consumer use. General configuration and characteristics of the instrument are presented. Calibration with SO₂ cells of known column concentration confirmed a comparable absorption coefficient to the device which is previously invented by Mori and Burton (2006). Although simultaneous shooting with two cameras is necessary to overcome the temporal change of an object because of somewhat long exposure time (5 to 10 sec) required, this device operates without external power or control PCs, and thus, is suitable for mobile use. We performed a field test of the instrument at Sakurajima volcano and confirmed its validity as an SO₂ imager. However, further improvement of the optical system or a special care with UV scattering in front of a plume is necessary for quantitative applications in a field operation.

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We applied the tomography method to the P- and S-wave arrival times of 829 local earthquakes observed at 101 stations in central and southern Kyushu, and revealed the detailed three-dimensional seismic velocity structure of the crust, especially the region beneath the Aira caldera. The structure obtained beneath the Aira caldera was characterized by a compacted high Poisson’s ratio zone at about 20 km depth, suggesting fluid saturation such as partial melts relating to volcanism. We also found that the low frequency earthquakes occurred so as to infill the lower crust between the high Poisson’s ratio zone and the moho discontinuity beneath the Aira caldera. It was also obvious that earthquakes clearly concentrated in regions with low Vp/Vs (low Poisson’s ratio) in the upper crust of the whole of southern Kyushu.

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