5:15 PM - 7:15 PM
[SVC32-P01] Re-investigation of the Latest Eruption deposits from Asahidake volcano, Taisetsu Volcano Group, Hokkaido, Japan
Introduction
Asahidake is an active volcano formed by the most recent activity (approximately 15,000 years ago to the Present) of the Taisetsu Volcano Group in central Hokkaido. Clarifying details of recent eruptive activity is crucial for volcanic disaster prevention because the volcano is a popular destination for tourists and climbers.
Wada et al. (2003) and Sato et al. (2015) recognized an ashfall deposit derived from Asahidake above the Tarumai-a tephra (Ta-a: 1739 AD) at the northwestern foot of the volcano, and concluded that the latest eruption of Asahidake occurred after 1739 AD. The ashfall does not contain fresh glass shards so these studies inferred that it was produced by a phreatic eruption. Ishige et al. (2018) describe lahar deposits at the same stratigraphic level as the ashfall.
We conducted a geological survey at Asahidake to re-investigate the stratigraphy and distribution of the ashfall and lahar deposits. Here, we refer to both deposits as Ash-A1 (Sato et al., 2015) and report their characteristics.
Stratigraphy, Age, Distribution, and Component
The ashfall lies above Ta-a, and is separated by a soil layer. The lahar deposits directly cover the ashfall. Radiocarbon dating of the soil samples sandwiched between Ta-a and Ash-A1 yielded ages ranging from 661 to 32 cal BP, which is not inconsistent with the age of Ta-a (1739 AD).
The ashfall is distributed over a radius of approximately 500 m from the northwest craters (Ishige et al., 2018) located on the northwestern foot of Asahidake. The distribution of the lahar deposits is more limited than that of the ashfall.
The ashfall layer is brown or pinkish-gray in color, and consists of fine to very fine volcanic ash. Major components of the ash include altered lithic fragments (reddish-brown to pink, white, yellow, and black) and crystals (mainly plagioclase, orthopyroxene, and clinopyroxene). Its maximum thickness is 2-5 cm, becoming thinner and pinching out laterally at the northern, eastern, and southern limits of the distribution area. The lahar deposits are composed of volcanic sand and subangular to subrounded blocks with poor sorting. The thickness is approximately 10-20 cm.
Discussion
We confirmed the stratigraphy of Ash-A1 overlying Ta-a at several locations on the northwestern foot of Asahidake as described by previous studies. We also confirmed that Ash-A1 originated from a phreatic eruption due to the lack of juvenile material. Based on the distribution and change in lateral thickness, the source of Ash-A1 can be estimated to be in the northwest craters. The lahar deposits directly overlie the ashfall, suggesting that a lahar occurred immediately after the eruption. The areas covered by the ashfall and lahar deposits are estimated to be at least about 1×106 m2 and 2×104 m2, respectively. The volume of the ashfall calculated by the method of Legros (2000) is ca 4×103 m3, while the volume of lahar deposits calculated by multiplying the area by average thickness is ca 3×104 m3. Lahar generation just after a phreatic eruption has also been observed in deposits at Tokachi volcano, central Hokkaido. Understanding the generation process of this type of lahar is important for disaster prevention.
Acknowledgments
We are grateful to the Asahikawa Construction Management Department for providing the data for this study. We are also grateful to Dr. Satoshi Okamura of the Hokkaido Soil Research Cooperative Association for the cooperation with SEM-EDS analysis.
[References]
Ishige, K., Nakagawa, M., & Ishizuka, Y. (2018). Jour. Geol Soc Japan, 124, 297-310.
Sato, E., Wada, K., Okuno, M., & Nakamura, M. (2015). 2015 Fall Meet. Volcanol. Soc. Japan, Abstr, 106.
Wada, K., Nakamura, M., Okuno, M., & Sato, E. (2003). 2003 Fall Meet. Volcanol. Soc. Japan, Abstr. 158.
Asahidake is an active volcano formed by the most recent activity (approximately 15,000 years ago to the Present) of the Taisetsu Volcano Group in central Hokkaido. Clarifying details of recent eruptive activity is crucial for volcanic disaster prevention because the volcano is a popular destination for tourists and climbers.
Wada et al. (2003) and Sato et al. (2015) recognized an ashfall deposit derived from Asahidake above the Tarumai-a tephra (Ta-a: 1739 AD) at the northwestern foot of the volcano, and concluded that the latest eruption of Asahidake occurred after 1739 AD. The ashfall does not contain fresh glass shards so these studies inferred that it was produced by a phreatic eruption. Ishige et al. (2018) describe lahar deposits at the same stratigraphic level as the ashfall.
We conducted a geological survey at Asahidake to re-investigate the stratigraphy and distribution of the ashfall and lahar deposits. Here, we refer to both deposits as Ash-A1 (Sato et al., 2015) and report their characteristics.
Stratigraphy, Age, Distribution, and Component
The ashfall lies above Ta-a, and is separated by a soil layer. The lahar deposits directly cover the ashfall. Radiocarbon dating of the soil samples sandwiched between Ta-a and Ash-A1 yielded ages ranging from 661 to 32 cal BP, which is not inconsistent with the age of Ta-a (1739 AD).
The ashfall is distributed over a radius of approximately 500 m from the northwest craters (Ishige et al., 2018) located on the northwestern foot of Asahidake. The distribution of the lahar deposits is more limited than that of the ashfall.
The ashfall layer is brown or pinkish-gray in color, and consists of fine to very fine volcanic ash. Major components of the ash include altered lithic fragments (reddish-brown to pink, white, yellow, and black) and crystals (mainly plagioclase, orthopyroxene, and clinopyroxene). Its maximum thickness is 2-5 cm, becoming thinner and pinching out laterally at the northern, eastern, and southern limits of the distribution area. The lahar deposits are composed of volcanic sand and subangular to subrounded blocks with poor sorting. The thickness is approximately 10-20 cm.
Discussion
We confirmed the stratigraphy of Ash-A1 overlying Ta-a at several locations on the northwestern foot of Asahidake as described by previous studies. We also confirmed that Ash-A1 originated from a phreatic eruption due to the lack of juvenile material. Based on the distribution and change in lateral thickness, the source of Ash-A1 can be estimated to be in the northwest craters. The lahar deposits directly overlie the ashfall, suggesting that a lahar occurred immediately after the eruption. The areas covered by the ashfall and lahar deposits are estimated to be at least about 1×106 m2 and 2×104 m2, respectively. The volume of the ashfall calculated by the method of Legros (2000) is ca 4×103 m3, while the volume of lahar deposits calculated by multiplying the area by average thickness is ca 3×104 m3. Lahar generation just after a phreatic eruption has also been observed in deposits at Tokachi volcano, central Hokkaido. Understanding the generation process of this type of lahar is important for disaster prevention.
Acknowledgments
We are grateful to the Asahikawa Construction Management Department for providing the data for this study. We are also grateful to Dr. Satoshi Okamura of the Hokkaido Soil Research Cooperative Association for the cooperation with SEM-EDS analysis.
[References]
Ishige, K., Nakagawa, M., & Ishizuka, Y. (2018). Jour. Geol Soc Japan, 124, 297-310.
Sato, E., Wada, K., Okuno, M., & Nakamura, M. (2015). 2015 Fall Meet. Volcanol. Soc. Japan, Abstr, 106.
Wada, K., Nakamura, M., Okuno, M., & Sato, E. (2003). 2003 Fall Meet. Volcanol. Soc. Japan, Abstr. 158.