The chemical and isotopic composition of hot spring waters and fumarolic gases from the Kirishima volcanic area were investigated in order to delineate the geochemical structure of the hydrothermal systems. There are active volcanoes, Iwoyama, Shinmoe and Ohachi locate in the area, and each of them has characteristic hydrothermal systems. At Iwoyama volcano, chloride-sulfate type waters are discharging from central part, and acid sulfate type waters are located on its north and west flanks. On the south west flank of Shinmoe volcano, CO₂ type waters are located. At a deep part on the west flank of both volcanoes chloride type waters are distributed. This variety of different type of waters can be derived from differentiations of a hypothetical parent fluid. The chloride type water seems to be a liquid phase separated from the mixture of the parental fluid and ground water. The vapor phase separated from the liquid phase heats ground water and produces the sulfate type and the CO₂ type waters. In the hydrothermal systems beneath Iwoyama volcano, a parental fluid is differentiated to a chloride-sulfate type water and a H₂S rich fumarolic gas. The parent fluid in the hydrothermal system of Shinmoe volcano seems to be depleted in sulfur species than the parental fluid for Iwoyama. Ohachi volcano has a faint fumarolic activity at the summit. The fumarolic gas losses most of water vapor prior to the discharge due to a conductive cooling and condensation of the vapor up to the summit. A model calculation evaluates the original composition of the fumarolic gas from Ohachi volcano. The original composition resembles to those of the fumarolic gases from the summit of Shinmoe volcano.

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The Akita-Komagatake, located in the central part of the Ou Back-Bone Range, northeast Japan, is a Quaternary volcano with two calderas. The tephra, younger than 13,500 y.B.P., erupted from this volcano, are stratigraphically divided into thirteen formations, namely AK-13 to AK-1 in ascending order, on the basis of buried soil and loess intercalated. The tephra consists of three types of lithologies: such coarse-grained essential materials as pumice and scoria (type 1), well-sorted black to brown sandy ash (type 2), and fine-grained ash frequently including accretionary lapilli (type 3). These lithologic types are resulted from different styles of volcanic eruption: Plinian to subplinian style eruption (type 1), vulcanian to phreatoplinian style eruption (type 2) and phreatoplinian style eruption (type 3). ¹⁴C measurements on humic soil or woods just below or above the tephra reveal the tendency that the explosive eruptions at the Akita-Komagatake volcano occured during three stages; 13,500 to 11,600 y.B,P., 10,000 to 7,100 y.B.P. and 4,000 to 1,000 y.B.P. In the first stage, AK-13, main part of which is called the Koiwai Pumice, and AK-12, the Yanagisawa Pumice, were erupted. During the eruption of the Koiwai Pumice, the Obonai Pyroclasitic Flow Deposits was produced. The volumes of erupted materials for the tephra including the pyroclastic flow deposit are more than 0.6 km³, and two calderas are thought to have been formed in this stage. In the second stage, AK-11 to AK-6 were erupted. AK-9, the Arasawa Pumice (newly named in this paper), and AK-8, the Horikiri Pumice, are dominated by pumice and scoria (lithologic type 1), and their volumes of erupted materials are more than 0.1 km³. The volumes for other tephra in this stage is more than 0.046km³. Through the second stage to the last stage, the volumes of erupted materials tend to become smaller than 0.046km³.

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This paper describes the distribution and trends of dikes in the Yatsugatake volcanic chain, central Japan. Total number of dikes is 213. Among them 121 are in the northern area and 92 are in the southern area. They are composed of andesite and basalt. In the northernmost part of the northern area, the predominant strike of 84 dikes is NW-SE, in agreement with the topographic elongation of some Younger lava cones. In the southern area, dikes show radial patterns, although N-S trending dikes are conspicuous in the central part of the area, and volcanic necks are arranged in the N-S direction. The whole dike trend is consistent with the arrangement of eruptive centers in the volcanic chain. Dike trends, arrangement of conduits and topographic elongation of some Younger lava cones suggest that the maximum horizontal compressional stress axis is NW-SE in the northern area, and N-S in the southern area.

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The eruption of Unzen volcano began on November 17, 1990 and extrusion of dacite lava lasted from May 20, 1991, until February 1995. Pyroclastic flows occurred frequently from partial collapses of the growing lava dome and had caused severe damage to the local residents. The total volume of the erupted lava is estimated to be about 2×10⁸ m³. Although it was very difficult to predict the beginning day of the eruption, some precursory phenomena of each eruption stage could be recognized. From November 1989 on wards, active seismic area migrated from the western side of the volcano with a depth of about 15 km, where a magma reservoir had been proposed, to the summit area. Anomalous attenuation of seismic waves passing through the area beneath the summit was also observed. From July 1990, about four months before the first steam eruption, volcanic trernor began to be observed. Just before the initiation of the lava extrusion, bulging of the summit area was detected by tilt and EDM (electro-optical distance measuring insturument) observations. Also large demagnetizations were observed by proton magnetometers. Daily magma supply rate had been monitored based on the amplitude of long-period oscillations recorded by tiltmeter, which was very useful for evaluating the ongoing eruptive activity. Variation of SO₂ emission rate measured using COSPEC and the deformation of volcanic edifice measured using leveling, GPS and EDM were rather useful for the relatively long-term prediction of the activity. Twenty-four hours watching was carried out by GSDF (Ground Self Defence Forces: Japan Army) and SEVO (Shimabara Earthquake and Volcano Observatory). The information was immediately released to the local society. This system was practical for supporting the risk management conducted by the local governments and mitigating the volcanic disaster.

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