Quaternary volcanic products erupted from five volcanoes located in the central part of the Hida Mountain Range. These are the Kaminorouka Volcanic Rocks, the Warimodake Andesite, and eruptives from Iwagokekotani, Kumonotaira, and Washibaike volcanoes. We have determined the K-Ar ages for five lava samples collected from above volcanoes except for Washibaike Volcano. The K-Ar ages are measured for separated groundmass from the lavas. Based on the newly obtained ages, previously reported seven radiometric ages, and volcanostrati-graphic data, we identify activity periods of each centers. These are : ca. 0.9 Ma for Iwagokekotani Volcano, 0.4-0.2 Ma for the Kaminorouka Volcanic Rocks, 0.3-0.1 Ma for Kumonotaira Volcano, ca. 0.3 Ma for Warimodake andesite, and 0.12-0.01 Ma for Washibaike Volcano, most of which are in the late early Pleistocene to the late Pleistocene.

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It is well known that cosmic-ray muons are illuminating every substance on the earth. Muons arriving vertically from the sky have an intensity of one muon every square centimeter every minute with a mean energy of a few GeV. It is also well known that muons with a smaller average intensity, but with a higher intensity at energies higher than a few hundred GeV, are arriving horizontally along the earth. For the purpose of probing the internal structure of a volcano, these horizontally arriving muons can more easily be used, provided that the muon flux is reasonably high and that the size of the detection system is realistic. An improved cosmic-ray muon detection system comprising two detectors which are vertically segmented into 10 channels and horizontally segmented into 10 channels, with multiplicity cut of the soft-component background of cosmic ray was developed. The multiplicity cut method eliminates soft components by using cascade shower generation through an intermediate iron absorber. By applying to the measurement in internal structure of the volcano Mt. Asama, we clearly detected the hidden crater in the vicinity of the top. There, we proved that the volume occupancy in the region of a crater is less than 30% by comparing to the detailed Monte Carlo simulation. The future prospects of the present method is also discussed.

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K-Ar dating of volcanic rocks from northern vicinity of Matsukawa geothermal field was carried out. Ages of 0.85±0.04 Ma and 0.58±0.10 Ma for Shimokura-Nakakura volcano and 0.72±0.03 Ma for 1470 m peak lava were obtained. The ages of Shimokura-Nakakura volcano are correlated with the early stage of advanced argillic alteration around Matsukawa geothermal field. The contemporaneity of hydrothermal and magma activities in the area implies the volcanic-hydrothermal activity in the core of Shimokura-Nakakura stratocone. The new dating of this study, together with the previously reported K-Ar ages and the geological studies, suggests distinct temporal change in alignments of volcanic centers in Sengan geothermal field at 0.6 Ma. The change was caused by outspread of volcanic activity from upheaval zone, which is bounded by reverse faults, to the outward zone. The volcanic alignment older than 0.6 Ma is located on the upheaval zone and alignment direction is transverse with regional maximum horizontal stress direction. On the other hand, younger volcanic alignments are located outside the upheaval zone and parallel to the regional stress direction, except for Akita Komagatake which is younger than 0.6 Ma but located on the upheaval zone.

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