抄録
The Chuseri pumice is a widespread Holocene dacitic tephra layer erupted from the Nakano Umi crater in the Towada caldera. The pumice is directly overlain by the Kanegasawa bedded pumice and Utarube ash. The Chuseri pumice. Kanegasawa pumice, and Utarube ash define the Chuseri tephra formation. Volumes are calculated using isopach maps to be 6.5 km^3 for the Chuseri pumice, 1.2 km3 for the Kanegasawa pumice, and 0.9 km3 for the Utarube ash. Plots of the areas enclosed by the isopleths of maximum pumice size and maximum lithic size show the Chuseri pumice to be an average plinian eruption product. The Utarube ash is an ash-fall deposit containing accretionary lapilli and is interbedded with surge deposit near source area. Grainsize characteristics of the Chuseri pumice are described. Like other plinian deposits, the Chuseri pumice does not contain an appreciable amount of fine ash (finer than 1/16mm), at least, as far as 45 km from the source vent. A new radiocarbon date (GaK-9761) on charcoal from within the Chuseri pumice deposit establishes an age for the eruption of 5,390±140 years B.P.
