3:45 PM - 4:00 PM
[SVC31-18] Date, amount, and size distribution of washed-up pumice clasts erupted from Fukutoku-Oka-no-Ba in 2021
Fukutoku-Oka-no-Ba (FOB) volcano, located about 5 km northeast of Minami-Ioto Island, is one of the central cones of the Kita-Fukutoku Caldera, southeast of the Kita-Fukutoku-Tai. At around 6:00 am on 13 August 2021, FOB volcano erupted and continued until around 4:00 pm on August (Oikawa et al., 2021). The total volume of the erupted pumices was estimated to be 0.11–0.26 km3(DRE), and the volcanic explosivity index was 4 (Oikawa et al., 2021). The chemical composition of the pumice clasts shows trachytic compositions and is like that of the previous eruption at FOB volcano (Yoshida et al., 2022). When submarine volcanoes or inland volcanoes near the coast ejected large amounts of pumice, pumice clasts sometimes start to drift in the ocean. In the vicinity of Japan, pumice drift in the ocean was recorded in the 1924 submarine volcano NNE of Iriomotejima eruption, the 1929 Hokkaido-Komagatake eruption, and the 1986 FOB eruption (e.g., Seki, 1927, 1930; Kato, 1988). In this study, we report the date, amount, and size distribution of washed-up pumice clasts by the 2021 eruption.
We referred to Twitter and news reports and contacted personal, local governments, and NPOs directly to find out when and where pumice drifted, and to ask for samples of pumice clasts and photographs. We also did field surveys in Nansei Islands in October, and in the Kanto area after November. Then, in order to check the pumice clasts we collected was from FOB, we measured the refractive indices and the major element composition of volcanic glass shards using 62–120μm fraction. The refractive indices of volcanic glass shards were measured using the RIMS2000 (Kyoto Fission Track Co., Ltd) in Tokyo Metropolitan University. The major element composition of volcanic glass shards was measured using an electron probe micro analyzer (JEOL JXA-8200) in Center for Advanced Marine Core Research, Kochi University.
The 2021 FOB pumice erupted on 13 August 2021 drifted to Minami-daito Island, Kita-daito Island, and Kikai Island in late September to early October, and then drifted to Amami Oshima Island and Okinawa Island. From mid-November to early December, pumice clasts washed up on Sakishima Islands, Taiwan, and Philippines. In the Kanto area, it began washing up to Izu Islands around mid-November and drifted to Tokyo Bay and Boso Peninsula. In addition, it drifted to Kii Peninsula around the same time and drifted to Satsuma Peninsula and Muroto Peninsula in early January 2022.
The mean major axis of pumice clasts was 1.1–12.8 cm, the weight per square meter was 2 g to 21 kg, and the bulk density was 0.3–0.8 g/cm3 by using the glass beads method (Sasaki and Katsui, 1981). The refractive indices of volcanic glass shards were n = 1.508–1.511 (mode: 1.510) for gray pumice, and some other types of pumice showed higher refractive indices. The major element composition of volcanic glass shards of the pumice collected in each area was SiO2 = 63.5–66.1 wt.% and Na2O3 + K2O = 10.2–11.3 wt.%. It is similar to that of the pumice clasts collected by the RV Keifu-Maru of the Japan Meteorological Agency on 22 August.
The mean major axis and the weight per square meter at each coast decreased with time after the eruption. This suggests the possibilities below: (1) pumice is crushed and abraded during drifting, (2) pumice sinks to the seafloor, and (3) pumice spreads on the sea. It is noted that pumice is common flotsam on the coast. So it is important to estimate the source of the pumice by considering not only its appearance but also its chemical compositions.
We are grateful to the many people who provided us information on FOB pumice and provided us with precious pumice samples, including the Japan Meteorological Agency. This work was supported by the Sasakawa Scientific Research Grant from The Japan Science Society. This study was partially performed under the cooperative research program of Center for Advanced Marine Core Research (CMCR), Kochi University (Accept No. 20A033, 20B030).
We referred to Twitter and news reports and contacted personal, local governments, and NPOs directly to find out when and where pumice drifted, and to ask for samples of pumice clasts and photographs. We also did field surveys in Nansei Islands in October, and in the Kanto area after November. Then, in order to check the pumice clasts we collected was from FOB, we measured the refractive indices and the major element composition of volcanic glass shards using 62–120μm fraction. The refractive indices of volcanic glass shards were measured using the RIMS2000 (Kyoto Fission Track Co., Ltd) in Tokyo Metropolitan University. The major element composition of volcanic glass shards was measured using an electron probe micro analyzer (JEOL JXA-8200) in Center for Advanced Marine Core Research, Kochi University.
The 2021 FOB pumice erupted on 13 August 2021 drifted to Minami-daito Island, Kita-daito Island, and Kikai Island in late September to early October, and then drifted to Amami Oshima Island and Okinawa Island. From mid-November to early December, pumice clasts washed up on Sakishima Islands, Taiwan, and Philippines. In the Kanto area, it began washing up to Izu Islands around mid-November and drifted to Tokyo Bay and Boso Peninsula. In addition, it drifted to Kii Peninsula around the same time and drifted to Satsuma Peninsula and Muroto Peninsula in early January 2022.
The mean major axis of pumice clasts was 1.1–12.8 cm, the weight per square meter was 2 g to 21 kg, and the bulk density was 0.3–0.8 g/cm3 by using the glass beads method (Sasaki and Katsui, 1981). The refractive indices of volcanic glass shards were n = 1.508–1.511 (mode: 1.510) for gray pumice, and some other types of pumice showed higher refractive indices. The major element composition of volcanic glass shards of the pumice collected in each area was SiO2 = 63.5–66.1 wt.% and Na2O3 + K2O = 10.2–11.3 wt.%. It is similar to that of the pumice clasts collected by the RV Keifu-Maru of the Japan Meteorological Agency on 22 August.
The mean major axis and the weight per square meter at each coast decreased with time after the eruption. This suggests the possibilities below: (1) pumice is crushed and abraded during drifting, (2) pumice sinks to the seafloor, and (3) pumice spreads on the sea. It is noted that pumice is common flotsam on the coast. So it is important to estimate the source of the pumice by considering not only its appearance but also its chemical compositions.
We are grateful to the many people who provided us information on FOB pumice and provided us with precious pumice samples, including the Japan Meteorological Agency. This work was supported by the Sasakawa Scientific Research Grant from The Japan Science Society. This study was partially performed under the cooperative research program of Center for Advanced Marine Core Research (CMCR), Kochi University (Accept No. 20A033, 20B030).