著者

早川 由紀夫

出版者

東京大学地震研究所

雑誌

東京大学地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.60, no.4, pp.507-592, 1986-03-31

---

The eruptive activity of Towada Volcano is documented by the tephrostratigraphic study. More than 20 eruptive episodes are described in detail with isopach maps and isograde maps of maximum pumice size, maximum lithic size, and median diameter for the individual fallout deposits. The activity of the volcano started about 200,000 years ago along the NE-SW trending line crossing the present lake Towadako and relatively small-scale volcanoes were formed. A caldera 11km×11km wide was formed as a result of several eruptive episodes during the period about 55,000 to 13,000 years ago, three of which included voluminous pyroclastic flow eruptions. The post-caldera activity occured at a rate of one eruptive episode every 1,000 to 2,000 years and a stratovolcano and two lava domes were formed in the caldera. The latest eruptive episode was dated 1,250 y B. P. by the radiocarbon method. Rocks of Towada Volcano cover a wide range from basaltic andesite to rhyodacite (SiO2 : 51-70wt.%) with phenocrysts of plagioclase, augite, hypersthene, and magnetite with occasional olivine. Horn-blende is characteristically found in the pyroclastic deposits of 13,000 years old and in some earlier deposits. Volumes, V, of two plinian deposits are determined by the crystal method : 6.7km3 for the Chuseri deposit and 2.2km3 for the Nambu deposit. Then an empirical formula, V=12.2 TS, is obtained for the practical volume estimation, where T is the thickness of an isopach and S is the area enclosed by the isopach. Application of the formula to the fallout deposits of Towada Volcano suggests that the total magma erupted during the past 55,000 years amounts to 1.5×1017g. This corresponds to the discharge rate of dense rock equivalent to 1.1km3 per thousand years. The cross-wind range, Rc, of the pyroclasts of a given size may be a good indicator of the maximum height reached by the pyroclasts in the eruption column. It is found that the Rc is relatively large for those deposits whose erupted masses are relatively large. The dispersal of a fallout deposit is also seriously affected by winds. A plausible solution of the eruption condition for the Nambu deposit is that 4mm size lithic fragments reached the maximum height of 15km in the eruption column, then they were detached from the column and displaced by winds having an average velocity of 30m/s. After a 20 min flight, they fell upon the ground 48km east of the source. Whole-deposit grain size populations are determined for the Chuseri and Nambu plinian deposits. The Chuseri population is similar to the New Zealand examples. However, the Nambu population is distinctly coarser than the others.