A technique based on micro reflectance spectroscopy using a vacuum Furier Transform infrared (FT-IR) spectrometer has developed for the quantitative measurement of dissolved water in melt inclusions in phenocrysts. The technique is superior in sample preparation to the conventional transmission method, which requires doubly polished samples with known thickness. Four synthesized glass samples with H₂O contents ranging 1.17 to 4.05wt% were examined in this study. A linear correlation between the logarithm of reflectance intensities around 3650cm^-1by H-bearing species and the water content of the glasses is obtained. At the lateral resolution of 30μm, the precision of the measurement caused by random error is 0.23wt% (1σ), and the value is low enough to apply the technique to the measurement of dissolved water in melt inclusions. A method which corrects possible contamination of inclusion spectra by the olivine host is also proposed.

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We carried out the geological and petrological investigations around the summit area of Yotei Volcano. Four eruptive crater groups are recognized: Summit, Hinangoya, Niseko and Kitayama in ascending order. They erupted six pyroclastic deposits (from S-6 to S-1 in ascending order) and five lava flows. Lava effusion occurred from the Hinangoya, Niseko and Kitayama crater groups. These activities were mainly Strombolian. Whole-rock chemistry of juvenile materials is distinct among four crater groups, indicating distinct magma system has been active beneath different craters. The radiocarbon age from unit S-2 fallout deposit from the Kitayama crater group was obtained as 4010±30cal. yBP. On the basis of estimated accumulation rate of soil layers, the latest (S-1) and S-4 eruptions from Kitayama group are considered to have occurred in ～2,500yBP and >5,000yBP, respectively. Eruptive deposits of Kitayama crater group do not show the evidence of interval more than several thousand years between unit S-4 and S-5. Thus, it could be concluded that the Kitayama group started its activity from mid of Holocene. Eruptive volumes of each eruptive group except for the Summit crater group range from 0.1 to 0.18km³. After the last magmatic eruption in 2.5ka, there are no evidences of eruptions from the Kitayama group, indicating that activity of the group has finished. However, considering newly revealed eruption history of the summit area, it should be noted that next eruptive group with distinct magma system might start its activity from another crater.

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The 1783 activity of Asama volcano was reviewed from May to the end of the year based on 166 old documents, including those recorded at a distance. 1. Prior to the 1783 Asama eruption, the level of magma head ascended at Kama-yama crater-pit. Moderately explosive eruptions commenced on May 9, and repeatedly blew off the plugged magma. 2. Depending on the wind direction, ash fell N, NNE, and NE of the crater including Sado Island, Tohoku and Kanto districts. From August 3 to 5, climactic plinian eruption dispersed pyroclastic materials. Distributions of 8 tephra- fall units were presented. 3. The timing of rumbling and quakes at distant places farther than 100km from the crater well correspond with explosive events witnessed by neighbors of the volcano. 4. Duration of a single eruptive event rarely exceed 6 hours. It was true even during the culminating plinian stage from Aug. 3 to 5, 1783. The eruption was so violent in this stage that huge blocks larger than 10m were thrown from Kama-yama crater. 5. Documents concerning with Kambara pyroclastic flow and subsequent debris avalanche occurred on August 5, suggested that an explosion on the northern flank triggered collapse of northern sector. The event occurred at about 08:00 to 08:30am, which is 90 to 120 minutes earlier than estimations appear in previous work. 6. Small and less frequent eruptions continued until January 15, 1784.

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Old historical documents on 1779 AD An’ei eruption of Sakurajima, southwest Japan were collected from distal places as well as those from neighboring area of the volcano. These records revealed that the ash-fall front traveled to the northeast at about 50-100km/h, reached as far as Tohoku district 1200km from Sakurajima, and covered area of ca. 2.33×10⁵km². Investigation of old documents helped to improve understanding of behavior of the volcano and environmental effects at the time of infrequent and great eruption. The wide distribution of ash-fall in 1779 Sakurajima eruption suggests that there is a high potential that ash discharged by future eruption of Sakurajima may cover down through the mainland of Japan. We should keep in mind both physical and economical effects of ash-fall in assessing the activity and making the scenario of an eruption.

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