The Kirishima Volcano Group is composed of many Quaternary volcanoes occupying an area about 600km². Ohachi Volcano is a small, active stratovolcano situated at the southeastern part of the Kirishima Volcano Group. It has a considerably large crater compared to the size of its volcanic edifice, with thick agglutinates that crop out on the wall of the crater. We studied the eruptive history of Ohachi Volcano by tephrochronological method, and recognized 15 tephra deposits around the volcano. Geological and archeological data and documentary records of eruption indicate that the oldest tephra from the Ohachi Volcano is the Araso tephra (AsT) erupted in ca. AD 700. All other tephra deposits were erupted in historic time such as Katazoe tephra (KzT) in AD 788, Miyasugi tephra (MsT) in ca. AD 1000, Takaharu tephra (ThT) in AD 1235, and Takachihogawara tephra 1-11 (TgT-1 to TgT-11) in the period of AD 1250-1700. Thus, the Ohachi is a very young volcano at 1300 years old. We recognized two types of magmatic eruption : sub-plinian and vulcanian. Lava emission took place at least four times in association with sub-plinian eruptions. Among these eruptions, ThT is the most voluminous, and a pyroclastic flow was also generated. The total volume of erupted magma (volcanic edifice included) was calculated to be about 2.5×10⁸m³ (DRE). Eruption rate was not constant throughout the eruptive history of Ohachi volcano, that is, the eruptive history is conveniently divided into three stages by the eruption rate : the early stage from AD 700 to 1235 (ca. 540 years), the middle stage from AD 1236 to 1717 (480 years), and the late stage from AD 1718 up to present (290 years). This indicates that more than 80% of the total amount of magma was produced in the early stage, while the remaining 20% of magma was erupted in the middle stage. Although there are many recorded eruptions during the late stage, it is difficult to find any tephra deposits around the Ohachi Volcano, which suggests that the scale of volcanic eruption is progressively diminishing through time. The volcanic edifice of Ohachi is composed mainly of succession of thick tephra layers, some of them changed to agglutinates. Correlation of the proximal deposits to the distal tephra suggests that the volcanic edifice had grown mainly during the two eruptive stages of KzT and ThT, which are comparatively large-scale sub-plinian eruptions (order of 10⁷m³ to 10⁶m³). Hence, the most important process in generating agglutinates is rapid accumulation of voluminous tephra around the crater which are generated during sub-plinian eruptions.

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A widespread tephra referred to here as Tamagawa-R4 Tephra (Tmg-R4) is newly recognized. Tmg-R4, derived from the Pre-Yakeyama caldera located in the Sengan geothermal area, the Northeast Japan arc, covers the area from Tohoku to Kanto, northeast of Honshu Island. At the type locality in the proximal area, Tmg-R4 comprises a non-welded pyroclastic flow deposit (ignimbrite) and an immediately overlying welded pyroclastic flow deposit (Kurasawayama Welded Tuff). Absence of plinian fall deposits in the area of ca. 25 km south of the source and the fine vitric ash nature of the distal ash-fall deposits of Tmg-R4 suggest that they are co-ignimbrite ash-fall deposits. Tmg-R4 was identified using a combination of refractive indices and chemical compositions of major and rare earth elements of glass shards (n=1.498-1.501, SiO₂: 78.3-78.6 wt%, K₂O: 4.2-4.5 wt%, Ba: 830-911 ppm), mineral content, refractive indices of hornblende (n₂=1.665-1.686). On the basis of these properties, Tmg-R4 was identified in Boso and Oga peninsulas, Choshi area, and in the core drilled on Musashino upland around 500 km south of the source. Calcareous nannofossil biostratigraphic (Calcareous nannofossil datum 13) and magneto-stratigraphic positions in Boso peninsula and Choshi, and paleomagnetic direction and many radiometric ages determined in the proximal area by previous studies indicate that the age of Tmg-R4 is ca. 2.0 Ma, positioned just below the base of the Olduvai Subchron. The distribution of Tmg-R4 showing emplacement of co-ignimbrite ash-fall deposit in the area 530 km south of the source, emphasizes the upwind transport direction relative to the prevailing westerly winds. This distribution shows similarity to those of a few co-ignimbrite ash-fall deposits derived from calderas in the Northeast Japan arc. As a key marker horizon in this age, the widespread occurrence of Tmg-R4 provides a tie line between many different sections over a distance of 530 km. Additionally, Kd44-Nk Tephra above Tmg-R4 is recognized in Boso peninsula, Choshi, Niigata and east Lake Biwa areas. Characteristic properties and stratigraphic positions indicate that Kd44-Nk possibly derived from the Sengan geothermal area occurred at 1.968-1.781 Ma.

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Temporal and spatial variations in Plio-Pleistocene volcanism in the Ryohaku Mountains, central Japan, have been investigated by newly obtained K-Ar ages on 38 groundmass samples separated from volcanic rocks. The obtained groundmass ages show small variations and errors and are relatively younger than K-Ar ages from bulk rocks reported by previous studies indicating lesser effect of excess argon from phenocrysts. Based on the newly obtained ages and volcano stratigraphic data, we identify activity periods of 1.0-0.8 Ma for Kyogatake Volcano, 0.8-0.7 Ma for Hoonji Volcano, 1.0-0.8 Ma for Toritateyama Volcano, 0.8 Ma for Akausagiyama Volcano, 3.1-2.9 Ma and 2.5 Ma for Gankyoji-Sannomine Volcano, 1.5 Ma for Choshigamine Volcano, 0.3 Ma for Bishamon Volcano, and 1.2 to 1.1Ma for Eboshi-Washigatake Volcano. The volcanoes in the Ryohaku Mountains form two volcanic rows of the Kuzuryu and Hakusan Volcanic Chains which have ESE-WNW and N-S alignments, respectively. Early volcanic activity occurred intermittently from 3.6 Ma to 1.5 Ma, and the volcanoes near the junction of the two volcanic chains were active. The volcanic activity in the ESE-WNW trending Kuzuryu Volcanic Chain was restricted in the age range from 1.2 Ma to 0.7 Ma with migration from Eboshi-Washigatake Volcano (ESE end) to Hoonji Volcano (WNW end). The N-S trending Hakusan Volcanic Chain was active from 0.4 Ma to the present.

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The volcanic island, Izu-Torishima, erupted in August, 2002. The National Research Institute for Earth Science and Disaster Prevention observed the crustal deformation and surface temperature distribution of the island after the eruption by using remote-sensing techniques of Interferometric Synthetic Aperture Rader (InSAR) and airborne Multi-Spectral Scanner (MSS). The InSAR image from RADARSAT-1 data acquired about 15 and 40 days after the eruption exhibits no large deformation exceeding several centimeters for the entire island, showing the limit of the analysis without any ground control points (GCPs). However, a small local movement (less than 2.8cm) is observable at the center of the island. The airborne MSS, VAM-90A, obtained the surface temperature distribution from infrared bands and pseudo-color images from visible and near-infrared bands almost one year after the eruption. The high-temperature area (over 70℃) is limited around the central crater, where the ground surface is covered by sublimed sulfur. The highest temperature of 80℃ is detected in this area.

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