霧島・御鉢火山における2003年12月以降の噴気活動と明治～大正時代の火山活動

抄録

A new fumarolic activity at Ohachi Volcano of Kirishima Volcano Group started on December 13, 2003 after about 80 years of inactivity. Two small fumarolic vents, T8 and T9, were formed on the slope within the Ohachi crater. Ejected materials were mainly fine-grained mud (altered ash), that were distributed within the southwestern sector of the crater. Small accessory lapilli, which were coated by mud, were scattered within 20 meters of T8 vent. No essential material was found. Alunite, kaolinite, 10 Å-halloysite were present as clay minerals. The volume of ejected material was very small, probably less than 10m³. At the time of this writing, the fumarolic activity at Ohachi crater is still going on. To understand the present activity, we investigated old documents of historic eruptions of this volcano, in particular those that occurred during the Meiji and Taisho eras (1880-1923), to determine the mode of eruptions during that time. The first stage of the eruption started with a fumarolic activity, which became more active. Then magmatic activities continued for about 40 years. The early phase of magmatic activities was characterized by vulcanian eruptions generating a large volume of ash, while the latter phase was characterized by sporadic but strong vulcanian explosions accompanied by ejection of volcanic bombs and blocks. At present, it would be difficult to judge whether the fumarolic activity will become more active, and proceed to magmatic stage. However, the results of our investigation of historical eruptions would still be useful to predict the nature of future eruption of this volcano.