1:45 PM - 2:15 PM
[SVC32-01] Reconstruction of the 10,000-year high-resolution history of Asama-Maekake volcano and evolution of eruption size and style
★Invited Papers
Keywords:Asama-Maekake volcano, eruption history, eruption style
Asama-Maekake volcano, which has been active since 10,000 years ago, can be divided into two types of eruptions: large-scale eruptions such as the Tenmei eruption (1783CE), which left strata, and small- to medium-scale eruptions such as the Vulcanian eruption and ash emissions, which left no clear strata (e.g., the 2004 eruption). History of the large-scale eruptions were reconstructed with high resolution first and the long-term evolution of the eruption scale and style were evaluated. In order to understand old buried deposits, trenching in as wide an area as possible near the volcanic edifice is effective. In addition, a lot of dating should be carried out to obtain information on the eruption period as a material for correlation between sites. Isopach maps of pyroclastic fall deposits should be made as much as possible. The history was reconstructed based on the distribution of the deposits and the overall stratigraphy.
Trench excavations at 21 locations at the foot of Asama-Maeagake volcano were used to examine the deposits buried below the 12th century fallout pyroclastic deposit As-B. Radiocarbon dating of soils directly under the same pumice layer shows a width of several hundred years, but As-C, Group E, and As-F are thought to be generated around 2,000, 6,300, and 8,300 years ago, respectively. In the soil between As-C and Group E, many pumice fall layers with similar occurrence were sandwiched between them, and defined them as Group D. The age was around 3,000 to 6,000 years ago. As a result of comparing the scale using the area surrounded by the 16 cm and 64 cm isopach lines, the scale of subplinian eruptions leaving pumice layers as geological units was diverse. They were divided from Class 1, which is recognized in large scale up to more than 50 km from the crater, to Class 4, in which pumice particles are scattered in the soil near the volcanic edifice. Class 1: As-A, As-B', As-B, As-C, Group E (E-ADF), Class 2: an example of Group D (D-SSW), Group E (E-My and E-UB) and As-F, Class 3: As-A', part of Group D Class 3: As-A', part of D-Group and E-Group, and E-F, and below As-F; Class 4: part of Group D and Group E, and E-F, and subordinate deposits of As-F. This indicates that most of the deposits older than As-C are of a scale that rarely reaches distant areas. Since 2000 years ago, large-scale eruptions such as As-A, B, and C, which formed ash layers farther than 50 km, have occurred with low frequency. The average frequency is about once every 700 years. Before then, just thin pumice fall layers have been formed within 20 km from the summit crater.
As for small- and medium-scale eruptions, since the 20th century when most of the records were kept, continuous ash emissions and Vulcanian eruptions were active until around 1959, but after that, single eruptions such as the 2004 eruption occurred every 10 years or so. In small and medium-scale eruptions, individual plumes do not leave clear strata, but traces of the eruption can be recognized as volcanic ash particles of lava fragments in the soil. In other words, we can evaluate the activity of small- and medium-scale eruptions from vertical changes in the particle size of volcanic ash particles in soil at the same location. For example, the diameters of the volcanic ash in the borehole core samples at the southeast foot of the volcano are coarse between the pumice fall layers of Group D. It is noteworthy that the volcanic ash in the area between As-B and As-C, and in the area below As-F, the ash particle content is low and black soils have developed, indicating that the volcanic activity was inactive or almost dormant. In Group D, there were intermittent periods of active small- and medium-sized eruptions during a long period of intermittent small sub-Plinian eruptions of Class 3 and 4, suggesting a relationship between the two eruption styles. The style and frequency of eruptions changed after 3,000 years ago, and large-scale sub-plinian eruptions with pyroclastic flows and lava flows occurred infrequently after 2,000 years ago until the Tenmei eruption.
Trench excavations at 21 locations at the foot of Asama-Maeagake volcano were used to examine the deposits buried below the 12th century fallout pyroclastic deposit As-B. Radiocarbon dating of soils directly under the same pumice layer shows a width of several hundred years, but As-C, Group E, and As-F are thought to be generated around 2,000, 6,300, and 8,300 years ago, respectively. In the soil between As-C and Group E, many pumice fall layers with similar occurrence were sandwiched between them, and defined them as Group D. The age was around 3,000 to 6,000 years ago. As a result of comparing the scale using the area surrounded by the 16 cm and 64 cm isopach lines, the scale of subplinian eruptions leaving pumice layers as geological units was diverse. They were divided from Class 1, which is recognized in large scale up to more than 50 km from the crater, to Class 4, in which pumice particles are scattered in the soil near the volcanic edifice. Class 1: As-A, As-B', As-B, As-C, Group E (E-ADF), Class 2: an example of Group D (D-SSW), Group E (E-My and E-UB) and As-F, Class 3: As-A', part of Group D Class 3: As-A', part of D-Group and E-Group, and E-F, and below As-F; Class 4: part of Group D and Group E, and E-F, and subordinate deposits of As-F. This indicates that most of the deposits older than As-C are of a scale that rarely reaches distant areas. Since 2000 years ago, large-scale eruptions such as As-A, B, and C, which formed ash layers farther than 50 km, have occurred with low frequency. The average frequency is about once every 700 years. Before then, just thin pumice fall layers have been formed within 20 km from the summit crater.
As for small- and medium-scale eruptions, since the 20th century when most of the records were kept, continuous ash emissions and Vulcanian eruptions were active until around 1959, but after that, single eruptions such as the 2004 eruption occurred every 10 years or so. In small and medium-scale eruptions, individual plumes do not leave clear strata, but traces of the eruption can be recognized as volcanic ash particles of lava fragments in the soil. In other words, we can evaluate the activity of small- and medium-scale eruptions from vertical changes in the particle size of volcanic ash particles in soil at the same location. For example, the diameters of the volcanic ash in the borehole core samples at the southeast foot of the volcano are coarse between the pumice fall layers of Group D. It is noteworthy that the volcanic ash in the area between As-B and As-C, and in the area below As-F, the ash particle content is low and black soils have developed, indicating that the volcanic activity was inactive or almost dormant. In Group D, there were intermittent periods of active small- and medium-sized eruptions during a long period of intermittent small sub-Plinian eruptions of Class 3 and 4, suggesting a relationship between the two eruption styles. The style and frequency of eruptions changed after 3,000 years ago, and large-scale sub-plinian eruptions with pyroclastic flows and lava flows occurred infrequently after 2,000 years ago until the Tenmei eruption.