13:45 〜 15:15
[SVC36-P11] The growth History before and after the 54 ka Handa eruption at Kuju volcano
キーワード：九重火山、飯田噴火、噴出源
[Introduction]
Kuju volcano is an active volcano group sitting on the volcanic front on Kyushu Island. It consists of lava domes, lava flows, and small stratovolcanoes. The volcano group has the eruption history as long as about 200,000 years. In this history, the three large-scale pyroclastic flow eruptions occurred, and the 54 ka Handa eruption was the largest one of those pyroclastic flow eruptions. The eruption released Handa pyroclastic flow deposit (Kj-Hd pfd), Kj-D ash and Kj-P1 pumice fall deposit. Those ejecta can be traced from the near to the far side of the volcano group, and previous studies discussed the stratigraphic relationship between the Handa eruption and lava ejections of Kuju volcano. However, they have a different suggestion. Furthermore, Tsuji et al. (in preparation) showed the possibility of several eruption sources within the Handa eruption. Studying in more detail of the eruption history before and after the Handa eruption and Handa eruption sources can clarify the activity pattern of the entire volcano group. In this study, we reconsider the Handa eruption souces, and the growth history of the volcanoes pre- and post-Handa eruption in western and central regions. It’s based on topographic interpretation, geological survey, and petrographic description.
[The source of Handa eruption: Kutsukake-yama volcano]
Kutsukake-yama volcano is located between the western to central regions, and has crator-like landform at the summit with a diameter of 1.3 km. In the vicinity of the crator-like landform, we observed gray-white pyroclastic flow deposit and yellowish-white pumice block. The grain size of yellowish-white pumice is at least bolder. Based on mineral assemblage, the pyroclastic flow deposit and pumice block are presumed to be Kj-Hd1 pfd and Kj-P1 upper unit, respectively. Therefore, we considered that Kj-Hd1 and Kj-P1 upper unit might erupted from the Kutsukake-yama crator. On the other hand, Tsuji et al. (in preparation) followed Kamata and Mimura (1983) and considered that the eruption center for Kj-Hd3 was around Mimata-yama, Hossho-zan, and Kuju-san, these mountains are located in the central region. In addition, Tsuji et al. (in preparation) considered that Kj-P1 upper unit was around Kuju- san. Thus, it is thought that Kj-Hd and Kj-P1 had several eruption centers.
[The post-Handa eruption history of the Kuju volcano]
In this study, the growth history of Nakadake and Mimata-yama volcanoes in the central region since the Handa eruption was studied in more detail. Nakadake volcano has plural peaks such as Mt. Nakadake, Mt. Kuju-san, Mt. Inabosi-yama and Mt. Shirakuti-dake. This volcano mainly consists of lava flows and lava domes. However, At the top of Mt. Inabosi-yama, the distribution of the stratigraphic structure of pyroclastic flow deposits is newly clarified (Inabosi-yama pyroclastic cone deposits; InPc, Newly defined). InPc was not found on the lava domes of Mt. Kuju-san and Mt. Nakadake, suggesting that InPc erupted at an older age than those lava domes.
Mimata-yama volcano has lava domes and lava flows such as Mimata-Gairinzan lava (MmG), Mimatayama-lava (Mm), Yubiyama-lava (Yb), and Yuzawayama-lava (Yz). We identified newly the hornblende-orthopyroxene-bearing olivine-clinopyroxene basaltic andesite pyroclastic rock (Mimata-yama pyroclastic rock; MmP, newly defined) on the MmG. This petrographic character is different from those of lava domes and lava flows that are characterised two pyroxene-hornblende andesite. Previous studies didn’t identify basaltic andesite rock into the Matsunodai debris avalanche deposit from the collapse of the MmG, suggesting that MmP is yonger than the collapse.
Kuju volcano is an active volcano group sitting on the volcanic front on Kyushu Island. It consists of lava domes, lava flows, and small stratovolcanoes. The volcano group has the eruption history as long as about 200,000 years. In this history, the three large-scale pyroclastic flow eruptions occurred, and the 54 ka Handa eruption was the largest one of those pyroclastic flow eruptions. The eruption released Handa pyroclastic flow deposit (Kj-Hd pfd), Kj-D ash and Kj-P1 pumice fall deposit. Those ejecta can be traced from the near to the far side of the volcano group, and previous studies discussed the stratigraphic relationship between the Handa eruption and lava ejections of Kuju volcano. However, they have a different suggestion. Furthermore, Tsuji et al. (in preparation) showed the possibility of several eruption sources within the Handa eruption. Studying in more detail of the eruption history before and after the Handa eruption and Handa eruption sources can clarify the activity pattern of the entire volcano group. In this study, we reconsider the Handa eruption souces, and the growth history of the volcanoes pre- and post-Handa eruption in western and central regions. It’s based on topographic interpretation, geological survey, and petrographic description.
[The source of Handa eruption: Kutsukake-yama volcano]
Kutsukake-yama volcano is located between the western to central regions, and has crator-like landform at the summit with a diameter of 1.3 km. In the vicinity of the crator-like landform, we observed gray-white pyroclastic flow deposit and yellowish-white pumice block. The grain size of yellowish-white pumice is at least bolder. Based on mineral assemblage, the pyroclastic flow deposit and pumice block are presumed to be Kj-Hd1 pfd and Kj-P1 upper unit, respectively. Therefore, we considered that Kj-Hd1 and Kj-P1 upper unit might erupted from the Kutsukake-yama crator. On the other hand, Tsuji et al. (in preparation) followed Kamata and Mimura (1983) and considered that the eruption center for Kj-Hd3 was around Mimata-yama, Hossho-zan, and Kuju-san, these mountains are located in the central region. In addition, Tsuji et al. (in preparation) considered that Kj-P1 upper unit was around Kuju- san. Thus, it is thought that Kj-Hd and Kj-P1 had several eruption centers.
[The post-Handa eruption history of the Kuju volcano]
In this study, the growth history of Nakadake and Mimata-yama volcanoes in the central region since the Handa eruption was studied in more detail. Nakadake volcano has plural peaks such as Mt. Nakadake, Mt. Kuju-san, Mt. Inabosi-yama and Mt. Shirakuti-dake. This volcano mainly consists of lava flows and lava domes. However, At the top of Mt. Inabosi-yama, the distribution of the stratigraphic structure of pyroclastic flow deposits is newly clarified (Inabosi-yama pyroclastic cone deposits; InPc, Newly defined). InPc was not found on the lava domes of Mt. Kuju-san and Mt. Nakadake, suggesting that InPc erupted at an older age than those lava domes.
Mimata-yama volcano has lava domes and lava flows such as Mimata-Gairinzan lava (MmG), Mimatayama-lava (Mm), Yubiyama-lava (Yb), and Yuzawayama-lava (Yz). We identified newly the hornblende-orthopyroxene-bearing olivine-clinopyroxene basaltic andesite pyroclastic rock (Mimata-yama pyroclastic rock; MmP, newly defined) on the MmG. This petrographic character is different from those of lava domes and lava flows that are characterised two pyroxene-hornblende andesite. Previous studies didn’t identify basaltic andesite rock into the Matsunodai debris avalanche deposit from the collapse of the MmG, suggesting that MmP is yonger than the collapse.