箱根火山大涌谷テフラ群から見つかった2種類の火山ガラス

—その給源火山と年代への制約—

抄録

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.