5:15 PM - 6:30 PM
[HGM03-P04] Pumice transportation process from inland to sea through rivers: A case study of the Typhoon Hagibis of 2019 along the Tone River, central Japan
Keywords:drift pumice, Tone River, Typhoon Hagibis
Pumice may drift in the sea by own buoyancy, and be transported to a distal place by sea currents. "Drift pumice" is pumice clasts floating on the sea, and is found at the present coasts frequently and also in strata occasionally. Drift pumice found in sediments has been used as an age marker because they are interpreted to be deposited immediately after the eruption (e.g., Toyokura et al., 1991; Shiraishi et al., 1992; Sawada et al., 1997; Aoki and Arai, 2000). However, we could find drift pumice at the present beaches in Japan despite no eruptions and no pumice floating events near Japan. Additionally, almost source of drift pumice at the present Pacific coasts in Japan are pyroclastic flow deposits from Aira caldera in the southern part of the Kyusyu region and Towada caldera in the northern part of the Tohoku region (Hiramine et al., 2020). Pyroclastic flow deposit of Aira-Tn (AT) tephra (Machida and Arai, 2003) is widely distributed as the pyroclastic plateau in the southern part of the Kyusyu region. Therefore, we interpreted that the primary source supplying pumices to the sea in the present condition, in which there is no eruption, is pyroclastic flow deposits on land. However, the source of drift pumice of about 40 % is still unknown in Hiramine et al. (2020). These unknown drift pumices are a possibility that pyroclastic flow deposits and pumice fall deposits distributed inland flow into the sea through rivers. However, the pumice transportation process from inland to sea is not yet revealed. Therefore we focused on the drift pumices along the Tone River just after the flood caused by Typhoon Hagibis and conducted tephra analysis to estimate their source volcanoes. There are quaternary volcanoes in upstream of the Tone River, which have many tephras including pumices: Asama, Haruna and Akagi volcanoes.
In this study, we sampled pumices at Shibukawa Osaki, Shibukawa city, Gunma Pref. (Loc. 1), Tsutsumi, Hanyu City, Saitama Pref. (Loc. 2), Sanno, Goka-machi, Ibaraki Pref. (Loc. 3), Oki, Moriya City, Ibaraki Pref. (Loc. 4) and Niijuku, Tonosho-machi, Chiba Pref. (Loc. 5) toward downstream. Some of them were washed and crushed. Then we measured refractive indices and the major element composition of volcanic glass shards using 62–120μm fraction. Refractive indices of volcanic glass shards were measured using a refractive index measuring system (RIMS 2000: Kyoto Fission Track Co., Ltd), and the major element composition of volcanic glass shards were measured using an energy dispersive X-ray spectrometer (EDAX GENESIS APEX2 and JEOL JSM-6930).
As a result, we collected 51 pumices at Loc. 1, 22 pumices at Loc. 2, 17 pumices at Loc. 3, 17 pumices at Loc. 4, 3 pumices at Loc. 5. Furthermore, they were classified into seven groups (Group A to G) based on outward appearance, mineral composition, and refractive indices of volcanic glass shards. Then we measured the major element composition of volcanic glass shards at least one sample per group. From glass compositions, Group A, D, F and G were identified to tephras from Asama volcano. The data of Integrated Disaster Information Mapping System indicates that some landslides and debris flow occurred in the northern part of Asama volcano, where pyroclastic flow deposits and pumice fall deposits were distributed tickly, at the time when Typhoon Hagibis hit. From this information, we estimated that pumices of pyroclastic flow deposits and pumice fall deposits distributed in upstream area entered to the river because of landslides and/or debris flows, and then pumices were transported to the sea through the Tone River. Besides, the Tone River is one of the longest rivers in Japan, and its river-bed slope is low in the downstream area. Therefore, pumices, we discovered along the Tone River, suggest that pumice of pyroclastic flow deposits and pumice fall deposits distributed on land in Japan could be transported to the sea through the river by one flood event.
In this study, we confirmed the pumice transportation process from inland to sea. Therefore, it is suggested that some source of drift pumice at the present beaches in Japan is pyroclastic flow deposits and pumice fall deposits distributed inland. This transport process of pumices via rivers is speculated one of the mechanisms of continuous supply of pumice to the sea.
In this study, we sampled pumices at Shibukawa Osaki, Shibukawa city, Gunma Pref. (Loc. 1), Tsutsumi, Hanyu City, Saitama Pref. (Loc. 2), Sanno, Goka-machi, Ibaraki Pref. (Loc. 3), Oki, Moriya City, Ibaraki Pref. (Loc. 4) and Niijuku, Tonosho-machi, Chiba Pref. (Loc. 5) toward downstream. Some of them were washed and crushed. Then we measured refractive indices and the major element composition of volcanic glass shards using 62–120μm fraction. Refractive indices of volcanic glass shards were measured using a refractive index measuring system (RIMS 2000: Kyoto Fission Track Co., Ltd), and the major element composition of volcanic glass shards were measured using an energy dispersive X-ray spectrometer (EDAX GENESIS APEX2 and JEOL JSM-6930).
As a result, we collected 51 pumices at Loc. 1, 22 pumices at Loc. 2, 17 pumices at Loc. 3, 17 pumices at Loc. 4, 3 pumices at Loc. 5. Furthermore, they were classified into seven groups (Group A to G) based on outward appearance, mineral composition, and refractive indices of volcanic glass shards. Then we measured the major element composition of volcanic glass shards at least one sample per group. From glass compositions, Group A, D, F and G were identified to tephras from Asama volcano. The data of Integrated Disaster Information Mapping System indicates that some landslides and debris flow occurred in the northern part of Asama volcano, where pyroclastic flow deposits and pumice fall deposits were distributed tickly, at the time when Typhoon Hagibis hit. From this information, we estimated that pumices of pyroclastic flow deposits and pumice fall deposits distributed in upstream area entered to the river because of landslides and/or debris flows, and then pumices were transported to the sea through the Tone River. Besides, the Tone River is one of the longest rivers in Japan, and its river-bed slope is low in the downstream area. Therefore, pumices, we discovered along the Tone River, suggest that pumice of pyroclastic flow deposits and pumice fall deposits distributed on land in Japan could be transported to the sea through the river by one flood event.
In this study, we confirmed the pumice transportation process from inland to sea. Therefore, it is suggested that some source of drift pumice at the present beaches in Japan is pyroclastic flow deposits and pumice fall deposits distributed inland. This transport process of pumices via rivers is speculated one of the mechanisms of continuous supply of pumice to the sea.