A series of volcanic ash layers preserved in peat soil at an isolated bog (Oda bog) on the flank of Chokai volcano was examined to determine frequency and eruption types during the last 4,500 years. A total of 54 ash layers overlies the peat soil of which calibrated age is ca. 4500 cal yrs BP (the AMS age is ca. 4,000 years), implying that the frequency of explosive eruption is higher than once every 83 years. The layer of pale-yellow fine ash derived from afar was compared in terms of glass composition and age with Holocene widespread tephras around middle to south Tohoku, and To-b (Towada-b tephra) is consequently the most plausible candidate for correlation with the ash. Hydrothermally-altered lithic fragments and blocky- and irregular-shaped juvenile fragments coexist in most ash layers, implying that phreatomagmatic eruption is dominant at Chokai. Wide variety of proportion of juvenile to altered ash grains demonstrates the wide spectrum of eruption types from magma-dominant to hydrothermal-dominant types. Juvenile fragments in individual ash layers show a wide compositional range from basaltic andesite to rhyolite (SiO₂= 55-75 %). These data suggest that batches of compositionally heterogeneous magma repeatedly uprise and interact with subvolcanic hydrothermal system in various degrees, producing a wide variety of eruption styles.

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