[S-VC30] Volcanic and igneous activities, and these long-term forecasting

[SVC30-06] Volcanic activities of Mt. Fuji around the end of the Hoshiyama Stage at 19 ka

*Tatsuji Nishizawa¹, Mitsuhiro Yoshimoto¹, Nobuko Kametani¹, ATSUSHI YASUDA² (1.Volcanic Disaster Research Center, Mount Fuji Research Institute, Yamanashi Prefectural Government, 2.Earthquake Research Institute, The University of Tokyo)

Keywords:Mt.Fuji, Hoshiyama stage, explosive eruption, tephra, eruption history

The present volcanic edifice of Mt. Fuji, known as Younger Fuji, is covered by volcanic deposits ejected during the Fujinomiya stage (from 17 ka to 8 ka) and the Subashiri stage (from 8 ka to the present). On the other hand, the outcrops of the Hoshiyama stage (from 100 ka to 17 ka), are limited and the eruption history of the Older Fuji have not been fully understood (e.g., Yamamoto et al., 2021). In order to reveal the long-term activity of Mt. Fuji, we are working on the eruption history based on geological surveys. Relatively continuous outcrops were exposed at a highway construction site about 18 km east of the summit of Mt. Fuji. In the area, there are several tephra layers, soil layers, and debris avalanche deposits (D.A.D.), but no lava flows. The deposits of the Subashiri stage are, from the top, pumice and scoria layers deposited during the AD1707 Hoei eruption, the Omika D.A.D. at 1.1 ka (Yamamoto et al., 2020), six scoria layers, the Gotenba D.A.D. at 2.9 ka, five scoria layers, and the Fuji Black Soil that deposited from 8 ka to 5.6 ka. The thickness of each the Younger Fuji tephra layer ranges from 10 cm to 40 cm. The grain sizes range from 0.5 cm to 2 cm and the maximum grain sizes less than 4 cm. The grain shapes are different in each layer. Beneath them are dozens of tephra layers deposited before the Subashiri stage, and the Mabusegawa D.A.D. at 19 ka (Nishizawa et al., 2023). In addition, beneath the D.A.D. layer, there are several tephra layers. The interface between the D.A.D. layer and the top of the tephra layers is sharp and almost horizontal.
Eight scoria layers were identified beneath the Mabusegawa D.A.D. The thickness of each layer is 5 cm to 40 cm and these layers are composed of slightly angular scoria grains with a size of 1 cm to 4 cm. In addition, the fourth and eighth layers from the top contain several large scoria grains and/or volcanic bombs with grain sizes of 8 cm to 15 cm, and their interiors are red oxidized. Similar stratigraphy and the large scoria grains and/or volcanic bombs are also observed in a large outcrop at a road construction site about 2 km to the east of this outcrop.
The time of deposition of the Mabusegawa D.A.D. is estimated to be around the end of the Hoshiyama stage, so the tephra layers beneath the D.A.D. are associated with the explosive eruptions of the Older Fuji. Compared to the Younger Fuji tephra layers observed in this outcrop, the grain sizes of the Older Fuji tephra layers are relatively large. Two possible factors have resulted in the Older Fuji tephra layers containing such large grains. One possibility is that the Older Fuji had more explosive eruptions than that of the Younger Fuji. In this case, even if the previous crater was located in the same position as the present summit crater, it would have been possible to transport large scoria grains and/or volcanic bombs over long distances. Another possibility is that the previous crater was located further east than the present summit crater. In this case, even if the eruptions were as explosive as the Younger Fuji, the grain sizes of the Older Fuji tephra layers at the eastern foot would be larger than that of the Younger Fuji. The Older Fuji tephra layers seems to have formed by either or both factors.