Janoo Volcano (550-750 m in basal diameter;150 m in height) is located in the northwestern part of the post-caldera central cones of Aso Volcano, central Kyushu, southwestern Japan. The volcano had been thought to be a cinder cone composed entirely of scoria-fall deposit and older than 7.3 ka. Fieldwork in and around the volcano has re-examined the detailed tephra stratigraphy and eruption age of Janoo Volcano. A black humic paleosol divides an upper pumice-fall deposit from a lower scoria-fall deposit. The upper pumice-fall deposit shows only two pure pumice bed sections with pumice clasts scattered in a brown massive ash elsewhere in the deposit. The deposit is composed mainly of light gray well-vesiculated dacitic (SiO₂=65.4-67.7 wt.%) pumiceous clasts containing biotite phenocrysts, and abundant banded pumices, suggesting a mixture of silicic and mafic magmas. Based on the phenocryst assemblage and age, the pumice-fall deposit is correlated to the Aso central cone pumice 1 (ACP1;4.1 ka), which is the only pumice-fall deposit erupted from Aso Volcano during Holocene time. The lower scoria-fall deposit is more than 30 m thick and constitutes most of the Janoo cinder cone. It includes brownish black to brown well-vesiculated basaltic andesite (SiO₂=54.7-55.5 wt.%) scoriaceous clasts and cauliflower bombs with radially arranged cooling joints. The Akamizu lava (SiO₂=57-59 wt.%) distributed west of the Janoo cinder cone, whose source was previously unknown, is attributed to Janoo Volcano based on the lava’s petrographic characteristics. A ¹⁴C age of 3830±30 years BP, which corresponds to 4.2-4.1 ka, was obtained from the humic paleosol interbedded between the ACP1 and Janoo scoria. The stratigraphy and characteristics of the tephra deposits suggest the following eruption sequence. The initial eruption at Janoo Volcano occurred at 4.9-4.3 ka and was strombolian in style forming the Janoo cinder cone. After lying in repose for a few hundred years, Janoo Volcano erupted again, and produced the ACP1 tephra containing abundant banded pumices and Akamizu lava at 4.1 ka. The southern half of the Janoo cinder cone was destroyed probably by the effusion of Akamizu lava. Volcanic activity forming Kishimadake, Ojodake, Komezuka and Kamikomezuka volcanoes in the northwestern part of post-caldera central cones at 4-3.3 ka was derived from basaltic to basaltic andesite magmas, whereas the eruption products of Janoo Volcano have a wide range in chemistry from basaltic andesite to dacite. Activity of Janoo Volcano is characterized by the presence of a dormant period (a few hundred years), allowing a paleosol to develop on the scoria-fall deposit, before ejection of both mafic and silicic magmas in the late eruption.

We propose a simplified method for the estimation of water content in volcanic glass using the quantitative analysis of oxygen concentration with an energy dispersive X-ray spectrometry (EDS). The difference between the bulk concentration of oxygen measured by EDS and the stoichiometric amount of oxygen to form all the elements in the sample oxide is assumed as the oxygen which consists of water. For precise quantitation of oxygen, the evaluation of the thickness of carbon coating film on the sample is crucial. The thickness of carbon coating on the sample was estimated using the stoichiometric ratio among the elements in the standard mineral put in the next of the unknown samples. The results of this methods show good correlation to the measurement using FT-IRand SIMS methods.

We determined origins of glass shards contained in Owakidani Tephra Group (Hk-Ow). Hk-Ow is the latest (approximately<3 ka) eruption products of Hakone volcano and is composed of five units;Hk-Ow1 to 5 in ascending order. They were formed by phreatic eruptions and are mainly composed of ash fall, debris flow and surge deposits. In the previous study, the surge deposit was reported to contain fresh brown glass shards. To find evidence of magmatic involvement in the eruptions, we conducted systematic analysis on glass shards contained in Hk-Ow deposits and interbedded paleosols. We found that two types of glass shards, brown and colorless, are contained in Hk-Ow3 to 5 but not in Hk-Ow2 (approximately 2 ka). These types of glass shards were also found in interbedded paleosols upper than Hk-Ow2. Shapes, refractive indices and chemical compositions of the two types of glass shards indicate that the sources of brown and colorless glass shards are Fuji and probably Kozushima (Iz-Kt tephra;AD838) or Niijima (Iz-Nm tephra;AD886) volcanoes, respectively. Since the inferred eruption center of the base surge deposit is different from that of the known Hk-Ow eruptions, it is suggested that the base surge is not identical to Hk-Ow3 to 5. Based on these results, we concluded that the base surge deposit formed in the 9th century or later. Absence of juvenile material in the Hk-Ow suggests that the Hk-Ow events were purely phreatic, and magmatic involvement to the eruptions was unlikely.

Free CO₂ gas of deep (mantle+subducting slab) origin has been discharging accompanied by cold spring water of Hyoshimizu Hot Spring located at the eastern coast of Himeshima island, which is a Pleistocene volcanic island located northeast of Kyushu, Japan. On the western side of this island, it can be also observed free gas upwelling off the coast of Nishiura village. Therefore, we sampled upwelling bubble gas for the first time and report chemical and isotopic compositions of the sample gas. The major chemical component of the bubble gas from the Nishiura submarine gas discharge (Nishiura SGD) is CO₂ as in the gas associated with the Hyoshimizu Hot Spring. The He-Ar-N₂ relative composition and the ³He/⁴He-⁴He/²⁰Ne relation of the Nishiura SGD gas are common with those of the free gas of Hyoshimizu Hot Spring and indicate that the Nishiura SGD gas will be of deep origin. However, the Nishiura SGD gas is rich in CH₄ while its concentration in the free gas in Hyoshimizu Hot Spring is negligibly small. The carbon isotope composition (δ¹³C) of CH₄ in the Nishiura SGD gas suggests that it should be of abiogenic origin. Since we found gases released on the eastern and western coasts of Himeshima island are both derived from deep, it is expected that such deep-originated gas seeps out to the atmosphere from ground inland of Himeshima island.