Stratigraphy and volcanic activities of Hijiori volcano, Northeastern Japan arc

Abstract

The volcanic activity of Hijiori volcano (N38°36’35”, E140°9’20”; WGS84) is reported in detail as a case study to understand how a new felsic volcano commences the activity. Hijiori volcano, a small caldera with approximately 2km in diameter, is an active volcano of Japan, which started a series of eruptions 12,000 years ago (in Calendar age), where no volcanic body had existed before. The eruptive history of Hijiori volcano was divided into four major stages (Stage 1, 2, 3, 4) and the subdivisions (Stage 1, Stage 2-1, 2-2, Stage 3-1, 3-2, 3-3 [a,b], Stage 4-1 [a,b,c,d,e], 4-2, 4-3). Time span of each hiatus among the four major stages is shorter enough to produce no black soils, and no significant difference in the ^<14>C age have been recognized between the stage 1 and stage 3. The major eruptive product of Hijiori volcano turns out to be valley filling non-welded (Stage1, 3-3 [a,b]) and partly welded (Stage 2-2) pumice flow deposits with total maximum thickness of about 150m and with a range 5km to the southward and 9km to the northward. The accompanying pumice fall (Stage 1, 2-2, 3-3 [a,b]) and ash fall (Stage 4-2) are dispersed 60km to the eastward. All the essential pumice from the eruptions were hypersthene-hornblende-quartz dacite. There are phreatic (or phreatomagmatic) eruptions at the beginning of Stage 2, 3, and 4 which deposited lapilli falls and flows (Stage 2-1, 3-1, 4-1 [a-e]) in the proximity. The volume of the valley filling pyroclastic flow deposit and the air fall deposit are 1.4 and 0.6km³, respectively, and the caldera filling deposit is estimated to be 0.3km³. Therefore, total eruptive volume of Hijiori volcano is estimated to be about 2.3km³.