Aso central cones located within Aso caldera, central Kyushu, southwestern Japan, initiated their activity soon after the formation of the caldera (ca. 90 ka). The cones have produced voluminous airfall tephra layers and lava flows. Most of the tephra layers distributed in and around Aso caldera are andesite to basaltic-andesite scoria-fall and ash-fall deposits. Their stratigraphy is very complicated because it is difficult to distinguish between scoria-fall layers in the field. However, dacite to rhyolite pumice-fall deposits from some central cones interbedded between the tephra layers are very useful to correlate stratigraphic units at separated localities. Therefore, we used the pumice-fall deposits in order to construct the tephrostratigraphy and eruptive history of Aso central cones during the past 90,000 years. Thirty-six pumice-fall deposits were identified including eleven major key beds. In ascending order they are Nojiri pumice (NjP), Ogashiwa pumice (OgP), Yamasaki pumice 5 (YmP5), Sasakura pumice 2 (SsP2) and 1 (SsP1), Aso central cone pumice 6 to 3 (ACP6-ACP3), Kusasenrigahama pumice (Kpfa) and Aso central cone pumice 1 (ACP1). Phenocrystic minerals of most pumice are plagioclase, ortho- and clino- pyroxene and magnetite, but NjP, ACP5, ACP3 and ACP1 include biotite, and NjP and SsP2 contains hornblende phenocrysts. On the basis of several ^<14>C ages of buried soils just below pumice (above Kpfa) and stratigraphic position eruption ages for the eleven major pumice are estimated as follows: NjP (85 ka), OgP (80 ka), YmP5 (69 ka), SsP2 (57 ka), SsP1 (56 ka), ACP6 (52 ka), ACP5 (45 ka), ACP4 (40 ka), ACP3 (39 ka), Kpfa (31 ka) and ACP1 (4ka). During the past 90,000 years Aso central cones produced pumice-fall deposits at an interval of about 2,500 years. Many of the sources of the pumice appear now buried under the present Aso central cones.

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A partly carbonized wood trunk was discovered below the Omuroyama tephra covered by a lava, and it was dated to be 3,700±100 y. B.P., by the benzene liquid scintillation method for radiocarbon age determination. We have investigated the circumstance of the discovery of the wood sample based on an interview of the person who collected the sample during his operation of building construction. According to him, the wood specimen was found at 1.5～2 m depth beneath the ground level when the operator removed the massive blackish lava block. This specimen was laid down covered by black sandy pebbles. The size of the wood trunk was 105 cm long and 45 cm and 35 cm along wide and short diameter, respectively. We also revealed the burial environment of the wood to be related with the Omuroyama tephras and lava flow, based on our volcanostratigraphic observation of the surrounding area. The tephras covering the trunk were identified as the Omuroyama tephra B, C, and D, which were defined by Koyano et al. (1996) around this area, and these tephras were covered by the Omuroyama lava flow II (Koyano et al., 1996). The wood trunk was mostly fresh and only its branch was burned. It can be estimated that the trunk has been preserved surrounded by ground water, since several tons of ground water gushed out when the aquifer was opened for construction. On this context, this trunk was buried at the early stage of the volcanism of Omuroyama volcano covered by the tephras, and later also covered by the Omuroyama lava II. We can not conclude that the burned branch is ascribed to direct burning by the Omuroyama lava flow II but this ¹⁴C dating resultant is also concordant all archaeological evidences reported.

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The morphology of free crystals contained in fifteen pumice fall deposits from eight volcanoes in Japan was observed. Although most of the crystal faces are filmily covered with vesicular glass, broken surfaces are observed in many free crystals. The ratio of length of broken surface (b) to the total circumference of a crystal (a) “b/a value” in the 2-D picture was measured for plagioclase free crystals contained in the deposits. The b/a values vary from sample to sample and each sample has a wide distribution of the value on the frequency diagram. Previously, broken free crystals in ignimbrites have been reported in many cases, whereas enough has not been known for pumice fall deposits. From the results of this study, broken free crystals are common also in pumice fall deposits indicating that abundant broken free crystals are produced through the eruptive process of explosive eruptions like plinian and subplinian style.

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Two volcaniclastic deposits-a debris-avalanche deposit (NgDA) and a lahar deposit (NgL) were discovered along the Nigorikawa River on the western slope of Aso central cones, southwestern Japan. The Nigorikawa debris-avalanche deposit, having a maximum thickness ＞3m, contains numerous plastically deformed debris-avalanche blocks (＜3.7 m) of volcanic ash and soil layers in a poorly-sorted silty to clay matrix. The Nigorikawa lahar deposit contains many subangular to subrounded lithic clasts (＜0.7 m) and shows clast-supported and matrix-rich depositional structures. We obtained a ¹⁴C age of 2,230±70 years BP from a wood fragment in NgDA, which corresponds to 400-100 cal BC (2σ). The date is consistent with the age of cultural remains (the Yayoi period: 300 BC-300 AD) underlying the debris-avalanche deposit. We obtained a ¹⁴C age of 4,100±60 years BP (2880-2480 cal BC) from a wood fragment in NgL. These ¹⁴C ages indicate that major volcaniclastic flows have inundated the Nigorikawa River multiple times in the past 4,000 years. Although the source and cause of the volcaniclastic flows remain unsolved, this discovery provides important information about volcanic hazards in the western part of Aso central cones.

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Torishima is an active insular volcano located on the Izu Arc (30°29'N, 140°18'E, 394m). In the last century, big eruptions occurred in 1902 and 1939, which made the island uninhabitable. In 1998, a small pit crater was formed beside the Io-yama crater. A large-scale smoke rising from the insular volcano was observed on August 8, 2002. The phreatic eruption occurred after a dormancy of 63 years, resulting in the formation of the 2002 crater, and it was followed by magmatic eruptions with eject of scoria on August 10 and 12. Discolored water was observed on August 12 and 13. From August 13, emission of volcanic ash was intermittent and the volcanic activity ceased before August 19 at latest. The eruption in 2002 was relatively small-scale and short-term activity.

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