16:00 〜 16:15
[SVC30-09] The history of 54 ka eruption of Kuju volcano
キーワード：九重火山、噴火史、層序
The 54 ka eruption of Kuju volcano released various pyroclastic deposits including ash, surge, large-scale pyroclastic flow deposits and plinian pumice fall deposits, which is the largest scale in the eruption history of Kuju volcano. However, there is no consensus on the eruption history. Then, in this study, to examine the history of Kuju volcano 54 ka eruption, geological investigation of the pyroclastic deposits, especially focusing on Kj-D which is the key tephra to elucidating the stratigraphy.
In this study, we use the classification of Matsumoto (1983) and Ohta (1991) on the pfls: the pfl that has formed Handa highland at the northern to northwestern foot is Hnd pfl (Matsumoto, 1983), and the one that has formed Kuju highland at the southern foot is K pfl (Kpf of Ohta, 1991). At the bottom of the whole ejecta, Kj-D tephra is overlain by Hnd pfl deposits. There are pumice flow deposits including bi-cpx bearing opx-hbl dacite pumice. Large-scale secondary pyroclastic flows were deposited after the deposition of both pfls, and Kj-D and Kj-P1 overly them. Kj-P1 is constituted with Kj-P1 lower and upper layer, and a small pyroclastic flow of K2 pfl that overlies K pfl at Kuju highland.
Kj-D is overlain by Kj-P1 at the eastern foot. Kj-D is composed of lower lapilli layers and upper volcanic ash layers. The upper layer of Kj-D was classified into five unites; Kj-D-u1, 3 and 5: lowermost, middle and uppermost bluish gray ash layers, and Kj-D-u2 and 4: lower and upper light brown volcanic ash layers. The blue-gray volcanic ash layer in the upper layer is a stack of dozen of thin layers mainly composed of fine-grained sand-sized volcanic fragments. Kj-D-u2 and 4 are both fine-grained sand-sized volcanic ash, mainly composed of pumice-type glass, and contains mafic minerals of hbl, opx, cpx and bi as decreasing order that is similar mineral composition of that of K pfl. The lower part of Kj-D-u4 contains a pumice layer with a maximum grain size of ~5 cm in diameter. The pumice is opx-hbl dacite. Based on the Kj-D-u2 and 4 isopach maps, the source of each is found in the area of the Hnd pfl and K plf. The source of the Kj-D-u4 pfa is found in the central or eastern part of Kuju volcano based on the isopach map.
The history of the eruption is considered as follows: The eruption began with two lapilli layers (Kj-D l1 and 2). After that, relatively small-scale explosive eruptions occurred repeatedly, and deposited Kj-D-u blue-gray ash layers. During this repeated adh deposit, three large-scale pyroclastic flow eruption occurred in the central part of the volcano. In the first two times, Hnd pfl flowed down to the northerm foot and released Kj-D-u2 as co-ignimbrite ash. A plinian eruption occurred to deposit the Kj-D-u4 pumice layer in the late Kj-D upper ash deposition, and the K pfl, an intra-plinian pfl, was deposited at the southern foot. Along with this, Kj-D-u4 was deposited as a co-ignimbrite ash. The eruption of Kj-D-u5 blue-gray ash continued for a while, and two huge plinian eruptions occurred at the end of the eruption, depositing the lower and upper layers of Kj-P1. The last eruption deposited K2 pfl, an intra-plinian pfl at the southern foot.
In this study, we use the classification of Matsumoto (1983) and Ohta (1991) on the pfls: the pfl that has formed Handa highland at the northern to northwestern foot is Hnd pfl (Matsumoto, 1983), and the one that has formed Kuju highland at the southern foot is K pfl (Kpf of Ohta, 1991). At the bottom of the whole ejecta, Kj-D tephra is overlain by Hnd pfl deposits. There are pumice flow deposits including bi-cpx bearing opx-hbl dacite pumice. Large-scale secondary pyroclastic flows were deposited after the deposition of both pfls, and Kj-D and Kj-P1 overly them. Kj-P1 is constituted with Kj-P1 lower and upper layer, and a small pyroclastic flow of K2 pfl that overlies K pfl at Kuju highland.
Kj-D is overlain by Kj-P1 at the eastern foot. Kj-D is composed of lower lapilli layers and upper volcanic ash layers. The upper layer of Kj-D was classified into five unites; Kj-D-u1, 3 and 5: lowermost, middle and uppermost bluish gray ash layers, and Kj-D-u2 and 4: lower and upper light brown volcanic ash layers. The blue-gray volcanic ash layer in the upper layer is a stack of dozen of thin layers mainly composed of fine-grained sand-sized volcanic fragments. Kj-D-u2 and 4 are both fine-grained sand-sized volcanic ash, mainly composed of pumice-type glass, and contains mafic minerals of hbl, opx, cpx and bi as decreasing order that is similar mineral composition of that of K pfl. The lower part of Kj-D-u4 contains a pumice layer with a maximum grain size of ~5 cm in diameter. The pumice is opx-hbl dacite. Based on the Kj-D-u2 and 4 isopach maps, the source of each is found in the area of the Hnd pfl and K plf. The source of the Kj-D-u4 pfa is found in the central or eastern part of Kuju volcano based on the isopach map.
The history of the eruption is considered as follows: The eruption began with two lapilli layers (Kj-D l1 and 2). After that, relatively small-scale explosive eruptions occurred repeatedly, and deposited Kj-D-u blue-gray ash layers. During this repeated adh deposit, three large-scale pyroclastic flow eruption occurred in the central part of the volcano. In the first two times, Hnd pfl flowed down to the northerm foot and released Kj-D-u2 as co-ignimbrite ash. A plinian eruption occurred to deposit the Kj-D-u4 pumice layer in the late Kj-D upper ash deposition, and the K pfl, an intra-plinian pfl, was deposited at the southern foot. Along with this, Kj-D-u4 was deposited as a co-ignimbrite ash. The eruption of Kj-D-u5 blue-gray ash continued for a while, and two huge plinian eruptions occurred at the end of the eruption, depositing the lower and upper layers of Kj-P1. The last eruption deposited K2 pfl, an intra-plinian pfl at the southern foot.