Abstract
Kita-Hakkoda volcanic group, located in the Northeast Japan arc, comprises eleven small-scale stratovolcanoes. The volcanic activity iniciated around 0.40 Ma and continued until present. Each volcano had been active in a specific period. The position of the eruptive center has converged to the central part through time. Each volcano consists predominantly of lava flows and domes associated with small-scale pyroclastic fall and flow deposits by vulcanian, strombolian and phreatic eruptions. Lithology of the eruptive rocks is basalt to dacite in 0.4-0.2 Ma, and andesite to dacite in 0.2 Ma-present. There are several tephra layers probably derived from Kita-Hakkoda volcanic group that are distributed to Kamikita Plain. They indicate several large-scale pyroclastic eruptions of 0.1 km3 to several km3 in volume occurred during 0.27-0.17 Ma in the volcanic group. In total 15 km3 (DRE) of magmas has been erupted during 0.4 million years. The eruption rate for the entire activity is estimated to be 0.04 km3/1000 yr. Eruption rates are relatively high (0.054-0.040 km3/ky) in 0.4-0.1 Ma, and low (0.016 km3/ky) in 0.1 Ma-present. The temporal changes of the eruption rate and the eruption style show that the culmination of the volcanic activity was in 0.4-0.1 Ma. The intensity of the volcanic activity has declined at least after 0.1 Ma. Temporal change of eruption rate and the convergence of the vent position can be explained by the process of cooling of a mantle diapir. Based on the mantle diapir model, it seems that the current activity of this volcanic group is the state that going toward an end in the long term.
