テフラから見た最近45万年間の恐山火山の噴火活動史

抄録

Ohatagawa tephra (Oh), Tanabu A, B, C and D tephras (Tn-A, Tn-B, Tn-C and Tn-D) erupted in descending order from Osore-zan volcano, Northeast Japan, are recognized in and around the Tanabu lowland on the downwind side of this volcano. These tephras are thought individually to represent single eruption cycles of Osore-zan. Tn-D contains the Otoshinosawa pumice flow deposit (Tn-D-Ot). Tn-C, Tn-B and Tn-A contain ash fall, pumice fall, ash and lapilli fall, pumice flow, and pyroclastic surge deposits. During the eruption cycles of Tn-C, Tn-B and Tn-A, pumice fall layers covered the downwind side area, and voluminous pumice flow deposits (Tn-C-Sh, Tn-A-Dt and Tn-A-Sk) were emplaced on the northeastern foot. Usori caldera at the summit of Osore-zan is covered also with the Tn-C-Sh, Tn-A-Dt and Tn-A-Sk pumice flow deposits. Ash fall deposits are commonly parallel-laminated and contain accretionary lapilli. They are distributed mainly on the east-northeastern or eastern downwind side. They are interpreted to have been produced by phreatomagmatic eruptions in direct contact of magma with the lake-water of Usori caldera. Eruption volumes of Tn-C, Tn-B and Tn-A were estimated to be about 1.9, 0.3 and 1.5 km³ (DRE), respectively. The Osore-zan's last tephra, Oh, is a thin fall deposit of ash and lapilli which is ＜0.1 km³ (DRE) in volume.