近畿地方へ東から飛んできた縄文時代後・晩期火山灰層の発見

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In late Holocene swamp deposits near and around Lake Biwa, central Japan, a characteristic volcanic ash layer which lies above the Akahoya volcanic ash layer (K-Ah; ca. 6, 300yrs BP) was reported by several previous studies. In order to confirm their correlation, we collected three volcanic ash layers in Sonenuma, Kojorogaike, and Torihama Archaeological Site by coring, and analyzed them for the eight major elements and refractive indices of their volcanic glass shards. As a result of these analyses, these volcanic ash layers are safely correlatable. On the other hand, chemical composition and refractive index were measured in the volcanic glass shards of Kawagotaira pyroclastic flow deposit derived from Kawagotaira Volcano, Izu Peninsula, central Japan. On the basis of the similarities of the chemical and optical properties of their glass shards, Kawagotaira Volcano is identified as the source of the above described volcanic ash layers.In the mid-latitude area of the northern hemisphere, westerlies prevail throughout the year. However, the Kawagotaira volcanic ash layer was found on the western side of the source volcano, indicating that an easterly wind must have prevailed at the time of the Kawagotaira eruption. Three types of pressure distribution result in a prevailing easterly wind: the Baiu frontal type in early summer, the Akisame frontal type in autumn, and the type associated with a huge typhoon. One of them was probably the pressure distribution condition at the time of the Kawagotaira eruption.

近畿地方へ東から飛んできた縄文時代後・晩期火山灰層の発見

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