著者

小林 哲夫

出版者

日本地質学会

雑誌

地質学論集 (ISSN:03858545)

巻号頁・発行日

no.24, pp.93-107, 1984-11-30

被引用文献数

6

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Yufu-Tsurumi volcanoes are situated at the northeastern part of Kyushu island, Japan. Lavas and pyroclastic rocks ranging from the late Pliocene to the late Quaternary in age form the basemment of this area and are cut by many active faults bringing about the blocks tilted variously. Judging from the tephrochronology of the wide-spread tephra, it has been confirmed that both Yufu and Tsurumi volcanoes started their activities more than 35,000 years ago. Yufu-dake is a stratovolcano with several parasitic lava domes and lava flows. The latest eruption occurred ca. 2,000-1,500 years ago issued the Tsukahara pyrodastic flow, the Ikeshlro lava, the Yufu-dake summit lava, and the Yufu-dake ash. The Tsukahara pyroclastic flow deposit is oxidized under the high temperature condition and has secondary fumarolic pipes. The Tsukahara pyroclastic flow is judged to have been formed by the collapse of an ascending lava dome. Stratigraphic relation between the pyroclastic flow and the volcanic ash shows that the collapse of the lava dome happened repeatedly at least two times. The last ascending of lava formed the Ikeshiro lava dome accompanied with lava flow. Shortly after that, the Yufu dake summit lava was issued and formed a summit lava dome. All the activities mentioned above represent a single cycle of eruption ceased within a short period of time. Based on the documentary record, however, it is convinced that the fumarolic activity was continued until the time ca. 1,100 years ago. Tsurumi volcano is composed of highly dissected volcanic edifice and the younger parasitic volcanoes. Based on the tephrochronology of wide-spread tephra, the younger parasitic volcanoes other than the Tsurumi-dake summit lava are judged to have been formed during the period of time ranging from 22,000 to 6,300 years before present. Garan-dake, Uchi-yama, and Nanpeidai are all lava domes and Oninomi-yama alone is a scoria cone with basaltic lava flow. The Tsurumi-dake summit lava which filled up the summit crater and formed a summit lava dome flowed down southward and eastward. One of the lobes of lava flow dammed up the river resulting in the developemment of a volcanic fan formed from the tip of a lava flow. Judging from the distrbution pattern of historic sites on the fan and the descriptions in the ancient manuscripts, it is reasonably concluded that the lava was issued about 1,500-1,200 years ago. Most of the rocks in this area is hornblende andesite ordacite, but a complicated assemblage of phenocryst, namely the olivine-clinopyroxene-orthopyroxene-hornblende-biotite-quartz-plagioclase-Fe-Ti oxides is frequently found in the rocks of of these volcanoes.