14:30 〜 14:45
[SVC30-04] Discovery of new block and ash flow deposit from Azuma-kofuji volcano, Fukushima, Japan
キーワード：吾妻火山、火山灰火山岩塊流、福島、溶岩
Azuma volcano is located on the west side of Fukushima-city. The volcanic complex consists of three volcanoes, called Nishi-Azuma, Naka-Azuma and Higashi-Azuma (from west to east). The Higashi-Azuma volcano has a large (2 km in diameter) summit crater, Jododaira, with an opening to the east side. It formed by sector collapse/Plinian eruption and produced debris avalanche deposits on eastern foot of the volcano at around 140 ka. Azuma-kofuji is one of the active volcanoes in the Azuma volcanic complex generated on the Jododaira crater during 6 ky to 5 ky ago. Azuma-kofuji is composed of a scoria cone, and widely distributed lava flows that covers the debris avalanche deposits at the eastern base of Higashi-Azuma. In this study, we discovered a new block and ash flow deposit (BAF) from Azuma-kofuji that has not been recognized and has not been considered for volcanic hazard assessment of Fukushima-city before. Here, we present the geological (lithofacies, stratigraphy, distribution) and petrological (petrography, bulk-rock chemistry) characteristics of the deposit (Azuma-kofuji block and ash flow deposit: Ak-BAF) and discuss its generation/emplacement processes.
Ak-BAF is distributed at the valley between the eroded cliffs of the debris avalanche deposit and the edge of the most extensive lava flow (Shiratugawa lava flow) 10 km east from Azuma-kofuji cone. On the basis of the estimated distribution area and the average thickness (10~20 m) of the deposit, the volume of Ak-BAF can be estimated to be 1 x 10-3 cubic km. Ak-BAF is a gray colored, coarse ash matrix, supported and lava block rich pyroclastic flow deposit covered by the Nm-NK tephra that was derived from Numazawa volcano around the 5 ka. Large mega-blocks (> 2~3 m in diameter) of lava flow are contained at the most upstream outcrop (loc. A at attached figure). Smaller (< 1 m) vesiculated gray blocks are commonly included and can be determined as juvenile materials in Ak-BAF. Petrologic features of the smaller gray blocks are homogeneous, whereas those of mega-blocks are relatively various. Bulk-rock compositions of these blocks are basaltic andesite to andesite (SiO2=59-62%) and are almost as same as those of lava flows of Auzma-kofuji except for some mega-blocks.
Ak-BAF is not recognized at upper-stream area from the edge (tip) of the Shiratugawa lava flow inferring the Ak-BAF was not derived from Azuma-kofuji cone. There exist at least two multiple flow units at the end of Shiratsugawa lava flow. One flow unit flowed down a longer distance with smoother surface/lava levees indicative of a more fluidal nature (= lower aspect ratio lava: LARL). Another unit (adjacent to the north side of LARL) shows a characteristic morphology, it is more rugged (blocky) and has a spreading distribution (= higher aspect ratio lava: HARL). The distribution of Ak-BAF starts at the southern edge (lobe) of the HARL (loc. A at attached figure) at the steep cliff of the debris avalanche deposit. We suggest that the HARL unit occurred and grew slowly after the emplacement of LARL unit. Then, the southern edge, which might have hardened thick crust, finally reached and collapsed at the steep cliff to generate Ak-BAF. We are investigating a more detailed distribution on the downstream side for volcanic hazard evaluation of Fukushima city.
Ak-BAF is distributed at the valley between the eroded cliffs of the debris avalanche deposit and the edge of the most extensive lava flow (Shiratugawa lava flow) 10 km east from Azuma-kofuji cone. On the basis of the estimated distribution area and the average thickness (10~20 m) of the deposit, the volume of Ak-BAF can be estimated to be 1 x 10-3 cubic km. Ak-BAF is a gray colored, coarse ash matrix, supported and lava block rich pyroclastic flow deposit covered by the Nm-NK tephra that was derived from Numazawa volcano around the 5 ka. Large mega-blocks (> 2~3 m in diameter) of lava flow are contained at the most upstream outcrop (loc. A at attached figure). Smaller (< 1 m) vesiculated gray blocks are commonly included and can be determined as juvenile materials in Ak-BAF. Petrologic features of the smaller gray blocks are homogeneous, whereas those of mega-blocks are relatively various. Bulk-rock compositions of these blocks are basaltic andesite to andesite (SiO2=59-62%) and are almost as same as those of lava flows of Auzma-kofuji except for some mega-blocks.
Ak-BAF is not recognized at upper-stream area from the edge (tip) of the Shiratugawa lava flow inferring the Ak-BAF was not derived from Azuma-kofuji cone. There exist at least two multiple flow units at the end of Shiratsugawa lava flow. One flow unit flowed down a longer distance with smoother surface/lava levees indicative of a more fluidal nature (= lower aspect ratio lava: LARL). Another unit (adjacent to the north side of LARL) shows a characteristic morphology, it is more rugged (blocky) and has a spreading distribution (= higher aspect ratio lava: HARL). The distribution of Ak-BAF starts at the southern edge (lobe) of the HARL (loc. A at attached figure) at the steep cliff of the debris avalanche deposit. We suggest that the HARL unit occurred and grew slowly after the emplacement of LARL unit. Then, the southern edge, which might have hardened thick crust, finally reached and collapsed at the steep cliff to generate Ak-BAF. We are investigating a more detailed distribution on the downstream side for volcanic hazard evaluation of Fukushima city.