霧島火山群高千穂複合火山の噴火活動史

Abstract

Takachiho composite volcano, located in south Ky ?? sh ??, Japan, is one of major members of the Kirishima volcano group. The composite volcano consists of four volcanic edifices which are partly overlapping and aligned in E-W direction. On the basis of field geology and tephrochronology, its growth-history can be divided into four stages. During the first stage, a conical stratovolcano, Futagoishi, was formed in the eastern end of the composite volcano. It is com-posed of an alternation of pyroclastic beds and lava flows. The activity of Futagoishi ended before the eruption of Ito pyroclastic flow deposit (22, 000 yBP) from the Aira caldera. The second stage activity started about 10, 000 years ago, after a repose of more than 12, 000 years. It was characterized by vulcanian eruption which continued semi-persistently through a long period of time, probably more than a thousand years. During this period a stratovolcano, Old-Takachiho, was built up on the western flank of Futagoishi, and a large amount of volcanic ash accumulated concentrically around the volcano. The third stage activity was characterized by the eruption of several thin lava flows and scoria from the vent close to Old-Takachiho Volcano. This activity resulted in formation of a small conical stratovolcano, Takachiho-no-mine, on the western slope of Old-Takachiho. At the end of this stage, enormous lava flows were successively effused from the top crater and flowed downward widely. Later lava flows, however, did not reached the foot of the slope due to decrease in effusion rate and piled up around the crater, resulting in the construction of an exogeneous lava dome on the summit. After several hundred years of quiescence, the present stage activity began about 2, 500 years ago. This activity was characterized by sub-plinian eruptions which occurred intermittently and produced thick scoria and ash beds on the eastern foot of the composite vlocano. By repeated eruptions of a great volume of scoria and lava flows, a new vlocano, Ohachi, was born on the western flank of Takachiho-no-mine volcano, and has been active since then. The growth-history of Takachiho composite volcano revealed the following two features: 1) the volcanic activity may have been controlled by the underlying E-W fracture, as suggested by the shifting of the vent position from east to west, and 2) the styles of explosive activity of the composite volcano have gradually changed with time from vulcanian eruption to sub-plinian one.