Abstract
We propose a new 40 thousand year-old eruption history of Hokkaido-Komagatake volcano, northern Japan, from new field observations, radiocarbon dating of charcoal woods and whole-rock chemistry of juvenile materials. In previous studies, eight distinct plinian eruption units have been recognized in the last 40 thousand years (Ko-a, 1929 AD; Ko-c1, 1856 AD; Ko-c2, 1694 AD; Ko-d, 1640 AD; Ko-f, 6.3 cal ka; Ko-g, 6.8 cal ka; Ko-h, 20 cal ka; Ko-i, 39 cal ka). Our study has revealed seven new eruption units between Ko-d and Ko-i in the northern flank area of the volcano. Eruption ages of the new units are estimated to be: P1 (6.5−6.3 cal ka); P2 (6.5−6.3 cal ka); P3 (ca. 12.8 cal ka); P4 (ca. 14.8 cal ka); P5 (ca. 17.4 cal ka); P6 (ca. 17.7 cal ka); and P7 (ca. 19 cal ka). The eruption history can be divided into four stages, separated by long periods of dormancy: pre-39 cal ka, 20 cal ka to 12.8 cal ka, 6.8 cal ka to 6.3 cal ka, and post-1640 AD. Each of the four stages is characterized by distinct whole-rock chemistry. The last three stages are characterized by an initial large eruption and subsequent medium to smaller scale eruptions. Our new findings could provide important background information and context for understanding the present state of Hokkaido-Komagatake volcano.
