Yunotani spa is located to the west part of the Central cones of Aso Volcano. Since the 14th Century, many people have been visiting this hot spring. A few explosions were referred in an old document “Nagano-ke-nikki; Diary of Nagano family”. However, the details of these events are not yet clear. We have found a set of historical records on the explosion called "Yunotani Catastrophe" that took place at the solfatara of Yunotani spa in July 6, 1816. These records, two drawings and five reports on the explosion, describe the events and damages in detail. A lot of “hot-mud” with many blocks were ejected from two vents, and destroyed 12 bathrooms and injured one person in the area about 100 m from the vents. Judging from the descriptions and occurrences of the hydrothermally altered ejecta around the vents, it can be said that this event was caused by steam explosions from the solfatara. By the revelation of these records, it became clear that an explosion occurred not only at Naka-dake but also in the area outside of Naka-dake in the historic age. This fact is important for the prevention of volcanic disaster in Aso Volcano.

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Melt inclusions in phenocrysts of ejecta of Miyakejima volcano have been analyzed with electron microprobe focused on sulfur abundance and its speciation. Samples were collected from three different volcanic events to characterize their source magma; (1) submarine eruption on June 27, 2000, (2) summit eruption on August 18, 2000, (3) lateral fissure eruption in 1983. Major element concentrations of melt inclusions derived from submarine eruption and 1983 eruption suggest their common origin. Melt inclusions within ejecta of summit eruption show less evolved feature. Although sulfur concentrations in the melt inclusions vary from 0.32 to 0.05 wt% SO₂ possibly reflecting pre- and post-entrapment degassing, no significant difference in S concentration was observed among three magmas. Measured S⁶⁺/S_total ratios in melt inclusions with SO₂ higher than 0.1 wt% yield pre-eruptive redox conditions of ΔNNO=+0.2～+0.5 log unit, and show no correlation with SO₂ concentration. On the other hand, melt inclusions with SO₂ less than 0.1 wt% indicate redox conditions of ΔNNO=0～-0.5 log unit, implying rapid degassing of SO₂ during eruption causes decrease of oxygen fugacity in the melts.

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The Myojin-sho volcano is one of the active submarine volcanos in the northern part of the Izu-Ogasawara arc about 400 km south of Tokyo. This volcano is a somma edifice of the Myojin-sho caldera, 6.5 km×8 km in diameter and 1000 m deep. The topography, seismic profiler, magnetic and gravity surveys around the Myojin-sho caldera were conducted by the Hydrographic Department, Japan (JHD) in 1998 and 1999. The geophysical structures of the caldera were derived, and the possible cause of the caldera formation is discussed. The residual gravity anomalies were calculated from the observed free-air anomalies by subtracting the gravity effect of 2-layer subbottom model structure, which amounts to 10 m Gals in a localized zone from the caldera to the northern somma. Bouguer gravity anomalies with the assumed density of 2.0 and 2.4 g/cm³ also show the positive anomaly over the same zone, which is accompanied by the acoustic and magnetic basement depression. Moreover, it seems that the sediment volume nearby Myojin-sho caldera cannot compensate the volume loss of caldera (20 to 41 km³). These features insist that the Myojin-sho caldera is caused by the collapse of the pre-caldera edifice rather than the explosion. The origin of the high gravity caldera may be ascribed to the magma pocket causing the depression, instead of the high density erupted material filling the caldera floor. The magnetization intensity of 4.8-5.3 A/m at the Myojin-sho volcano is derived from the magnetic anomaly, which may claim that the Myojin-sho volcano consists of andesitic to basaltic rock rather than dacitic rock. On the other hand, magnetization of the central cone of Takane-sho volcano is estimated to be 1.1-1.9 A/m, which is consistent with the fact that dacite pumices were sampled.

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The ca. 2.2 ka eruption of Yufu volcano, Kyushu, Japan successively produced two lavas and several pyroclastic flows. Tsukahara debris avalanche deposit lies tephrostratigraphically in the same horizon with the neighboring pyroclastic flow deposits. However, its exact relation to the 2.2 ka eruption deposit is not clear in any outcrop. We thus considered this problem by using the progressive thermal demagnetization method that estimates emplacement temperatures of the pyroclastic deposits. The results indicate that the Tsukahara debris avalanche deposit includes hot accessory blocks and that the emplacement temperatures of pyroclastic flow deposits varied for each fiow unit. The following is the outline of 2.2 ka eruption estimated by geological and temperature data. Before the main eruption, the inner part of the volcano was heated by rising magma. The northern slope of Yufu volcano subsequently collapsed to form Tsukahara debris avalanche which has some accessory high-temperature-blocks (300-500℃). No blast was produced during this collapse. Shortly after the collapse occurred, Ikeshiro lava dome erupted and occupied completely the collapsed concave. Block and ash flows (Yufu-north pyroclastic flow) were generated in association with the lava dome growth. Yufu-dake summit lava erupted near the summit of the volcano, and block and ash flows (Yufu-south pyroclastic flow) accumulated as a result of collapsing materials from the lava.

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噴火中の火山で, どのような地形変化が起きているかを知ることは, 活動状況を把握するうえで, 最も重要な点の一つである. しかし, 我が国のような湿潤な気候帯では, しばしば, 山頂部が雲に覆われ観測できない事態が起きる. このような場合, 合成開ロレーダー(SAR)は, 昼夜, 天候, 噴煙等に左右されないため, 火山の地形変化の監視に有効であると考えられる. しかし, SAR 画像は, 独自の歪みをもっため,地形図との対応関係を知るのが容易でないなどの問題がある.本論は,RADARSATSAR 画像をGround Control Points (GCP) を用いたマッチングにより地形図上に重ね合わせ, 2000年7月に噴火を開始した三宅島陥没カルデラ縁の拡大状況の推定を行った. この際フォアショートニングによる歪みを抑えるため, カルデラ縁に近い場所にGCPを設定した. 得られた結果を空中写真等からの推定と比較すると,両者のずれは大きい部分でも1OOm程度であり, 比較的良く一致していることがわかった. このことから, RADARSATSAR 画像は, 緊急時にカルデラ縁の位置を推定するといった用途には十分利用できると考えられる.

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