霧島火山群新燃岳の最近300年間の噴火活動

抄録

This paper presents results of geologic investigation of the eruptive activity in the last 300 years of Shinmoedake, an active volcano in the Kirishima Volcano Group. The recent activity of this volcano is divided into four eruptive episodes : the 1716-1717, 1771-1772, 1822 and 1959 episodes. The most important activity occurred in 1716-1717. During the 1716-1717 eruption, fallout deposits, pyroclastic flows and mudflows were widely dispersed around the volcano. The products of this episode show that the eruption progressed with time from phreatic to magmatic. These field data are in good agreement with historic records of eruptive activity. According to the historic records, the eruptive activity lasted from 11 March, 1716 to 19 September, 1717. The 1771-1772 and 1822 activities produced base surges, pyroclastic flows, fallout tephra and mudflows that were confined to the slope and eastern base of the volcano, but historic records do not reveal the details of these eruptions. The field evidence shows the same phreatic to magmatic sequence as the 1716-1717 activity. However, the eruptions of both episodes were on a smaller scale than the 1716-1717 eruption. The 1959 activity was well described. This episode produced minor gray silty to sandy lithic fallout tephra indicating that only phreatic activity occurred. The fallout was distributed northeast of the vent. In conclusion, the field evidence and historical records show that each eruptive episode of the current activity of Shinmoedake progressed from phreatic to magmatic. The eruptions are frequently accompanied by pyroclastic flows and mudflows.