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.

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The sequence and eruption styles of the 1813 eruption of Suwanose-jima Volcano, SW Japan, are reconstructed on the basis of detailed stratigraphical and petrographic features of juvenile materials. The mode of generation of the proximal deposits is discussed in the light of their distribution, surface morphology and also inner structure. It is suggested that a large pyroclastic cone was generated by successive fountaining from fissure and some craters, and that lava flows are generated in two ways; by effusion directly from vents and by secondary flowage of the pyroclastic cone. The juvenile materials of the 1813 eruption are divided into three types in terms of water content and apparent density; low porosity scoria (LPS), high porosity scoria (HPS) and dense spatters and lava flows (DSL). Direct measurements of water content for fresh samples show that the water content of LPS is systematically higher (0.2-4 wt%) than HPS and DSL (0.1-0.2 wt%), whereas mineralogical data of phenocrysts indicate that all these types had similar water content (-3 wt%) just before the eruption. It is suggested that a part of magma quenched and stopped exsolving gas due to interaction with ground water at the depth of〜100 m to form LPS, while the rest continued exsolving gas up to the surface to form HPS and DSL. The difference in apparent density between HPS and DSL is accounted for by different degrees of bubble separation at the surface.

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