The Eruptive Products from Jigoku-numa Hot Pool in Kita-Hakkoda Volcano Group, Northeast Japan : Eruption Style, Magnitude and Age

Abstract

We discovered the latest eruptive products from Jigoku-numa hot pool (about 80 m in diameter) on the southwestern foot of Odake volcano of Kita-Hakkoda volcano group, northeast Japan. We identified three tephra layers (Hk-J1, Hk-J2 and Hk-J3 in descending order) produced by phreatic eruptions from Jigoku-numa hot pool. The distributions of these tephras are limited within 200 m from the vent. The magnitude of each eruption is the order of 10² m³ (Hk-J1 and Hk-J3) and 10⁴ m³ (Hk-J2) in bulk volume. On the basis of radiocarbon dates, their ages are estimated to be 15-17th century for Hk-J1 and Hk-J2, and 13-14th century for Hk-J3. However, the historical document that recorded these eruptions has not been discovered. Lithology of lithic fragments from Hk-J1, Hk-J2 and Hk-J3 suggest that these explosions occurred at shallower than the depth of 50 m beneath Jigoku-numa hot pool. These eruptions probably resulted from the explosion of a confined pocket of steam in the active hydrothermal system with no direct involvement of magma. The hydrothermal activity around Jigoku-numa hot pool is still active. Therefore, it should be noted that similar phreatic eruptions could occur in the future.