浅間前掛火山12世紀以前の噴火履歴復元の手がかり

—御代田軽石を中心とした新知見—

抄録

The Asama-Maekake volcano has been active for about 10,000 years. Although the many pyroclastic fall deposits, which mainly consist of pumice fall layers, are indicative of past sub-Plinian eruptions, the stratigraphy of the deposits over the entire history of the Asama-Maekake Volcano has not yet been revealed. This is because the pyroclastic fall deposits are distributed in the various directions from the crater. Moreover, the deposits predating the 12^th century have a similar occurrence and petrography, making it difficult to correlate deposits among different localities. Therefore, little information is available for the older eruptions. The pyroclastic fall deposit in the direction not leeward of the dominant wind is focused in this study. A pyroclastic fall deposit called Miyota pumice (referred to as As-My hereafter), which is characteristically distributed in the southern direction from the crater, was targeted. The distribution of As-My has barely been mapped owing to the difficulty resulting from its poor exposure. The ¹⁴C ages of the samples of black humus soil immediately beneath As-My at two localities were dated to 5720±30yBP and 5530±30yBP. These ages are almost the same as those of the pyroclastic fall deposits As-UB (Ubagahara) and As-Kn (Kuni) distributed on the northern flank. Since the stratigraphic relation among these deposits is difficult to determine, it is not clear whether As-My, As-UB, and As-Kn are products from a single eruption or different eruptive activities. At least, it can be considered that multiple sub-Plinian eruptions occurred around 6,000 years ago. In addition, black humus soil and intercalating four pumice fall deposits including As-My at a major outcrop on the SSE flank slope (Locality M72) provided information on the activity over the last 8,000 years, back to the earliest stage of the volcano. The ¹⁴C ages of samples of black humus soil taken from immediately beneath the pumice fall deposits were 3,830±30yBP, 4,710±30yBP, 5,530±30yBP (just beneath As-My), and 7,470±30yBP. This is the first time that such sequential data concerning eruptive ages on the flank slope has been obtained. Black soil contains angular lithic fragments, which are similar to the ash grains from Vulcanian eruptions (e.g., the 2004 eruption). This suggests that Vulcanian eruptions have occurred between sub-Plinian eruptions since the early stage of the Asama-Maekake volcano. Such information from outcrops on the flank slopes with various directions is required to reconstruct the detailed eruptive history.