11:00 AM - 1:00 PM
[SVC30-P03] Floating pumice from Fukutoku-Oka-no-Ba, Features of drifting pumice particles on Izu-Oshima
Keywords:pumice raft, fukutokuokanoba, pumice grain
1. Introduction
Floating pumice stones from the August 2021 eruption at Fukutoku-Oka-no-Ba were washed ashore in various parts of Okinawa and became a big problem. Although the frequency of occurrence is low, it is necessary to deepen the understanding as a cause of volcanic disasters. As a floating pumice disaster, there is a case where villagers who fled by boat were stranded at sea due to floating pumice that filled the sea surface during the Taisho eruption of Sakurajima. On the other hand, in the case of geological deposits for which there is no documentary record of floating pumice, it would be useful to have information on particle characteristics, etc. to identify floating pumice.
2. Drifting pumice stones and lepas anatifera
Therefore, we obtained drifting pumice stones from Okinawa and Miyakejima, and conducted a field survey on the coast of Izu-Oshima to observe the drifting situation and pumice stone particles. Whole-rock major element chemistry of collected pumice show similar trend to that of Yoshida et al.(2022).The ones in the northern part of the main island of Okinawa were obtained from Mercari in October. The pumice stones on Miyakejima were collected near Tsubota in November and were provided by Mr. Noda and Mrs. Kikuchi of Kamitsuki. Lepas anatifera with a straight major axis of about 2 mm was attached to the sample in Okinawa. On the other hand, some of Miyakejima's drifting pumice stones reached 1 cm. It probably grew while being carried on the Kuroshio Current.
Among them, there are dozens of lepas anatifera attached to the surface of pumice stones with a diameter of 10 cm, and when they are submerged in water to restore the floating condition, they float stably without capsizing like a ship. There was a nature. It was confirmed that the adhesion of Lepas anatifera was limited to the underwater side and none on the air side. Therefore, when we confirmed the particles of pumice stones during a field survey on Izu-Oshima, there was a large difference in the degree of weathering of the pumice stone surface between the underwater part and the middle part of the air, and the air had a smooth rounded shape. On the other hand, the underwater side was almost unweathered and rich in unevenness. Although the particles with such characteristics were well preserved, there were some particles with smooth top surfaces, masks, and smooth particles, which reached nearly half, but no adhesion of Lepas anatifera was observed (Fig. 1).
3. Hypothesis of origin
Pumice stones originating from the field of Fukutoku-oka-no-ba have uneven parts in density, such as crystal clots called chocolate chips and black bands. Even if the particles crack randomly, the positions of the center of gravity and the center of gravity of the particles do not match, and it is considered that there are many cases where the same direction always comes up when floating. In such a case, it is possible that the weathering of the portion exposed to the air selectively progressed during the floating for 3 months or more, and the weathering progress of the underwater part was slow. Adhesion and growth of Lepas anatifera may further enhance such anisotropy. In particular, some of them have similar cross-sectional and planar shapes to those of ships, and I think that they may have been advantageous for coastal drifting. While moving in a pumice raft, they collide with each other and change their shape, and I think it may have happened.
4. Future issues and plans from now on.
Examine whether there are any pumice stones other than the Fukutoku-oka-no-Ba field that show similar characteristics. We will search for particles with the same shape characteristics from the sediments near the coast in the past and verify whether they are floating pumice. While observing the behavior of pumice stones floating on the surface of the water, engineering studies such as the center of gravity of the pumice stones and the effect of buoyancy will be conducted.
5. reference
Oikawa et al. (2021) Eruption in August 2021 at Fukutoku-Okanoba, Ogasawara Islands. Proceedings of the Volcanological Society of Japan, 120-120.
Yoshida et al. (2022) Variety of the drift pumice clasts from the 2021 Fukutoku-Oka-no-Ba eruption, Japan,https://doi.org/10.1111/iar.12441.
Floating pumice stones from the August 2021 eruption at Fukutoku-Oka-no-Ba were washed ashore in various parts of Okinawa and became a big problem. Although the frequency of occurrence is low, it is necessary to deepen the understanding as a cause of volcanic disasters. As a floating pumice disaster, there is a case where villagers who fled by boat were stranded at sea due to floating pumice that filled the sea surface during the Taisho eruption of Sakurajima. On the other hand, in the case of geological deposits for which there is no documentary record of floating pumice, it would be useful to have information on particle characteristics, etc. to identify floating pumice.
2. Drifting pumice stones and lepas anatifera
Therefore, we obtained drifting pumice stones from Okinawa and Miyakejima, and conducted a field survey on the coast of Izu-Oshima to observe the drifting situation and pumice stone particles. Whole-rock major element chemistry of collected pumice show similar trend to that of Yoshida et al.(2022).The ones in the northern part of the main island of Okinawa were obtained from Mercari in October. The pumice stones on Miyakejima were collected near Tsubota in November and were provided by Mr. Noda and Mrs. Kikuchi of Kamitsuki. Lepas anatifera with a straight major axis of about 2 mm was attached to the sample in Okinawa. On the other hand, some of Miyakejima's drifting pumice stones reached 1 cm. It probably grew while being carried on the Kuroshio Current.
Among them, there are dozens of lepas anatifera attached to the surface of pumice stones with a diameter of 10 cm, and when they are submerged in water to restore the floating condition, they float stably without capsizing like a ship. There was a nature. It was confirmed that the adhesion of Lepas anatifera was limited to the underwater side and none on the air side. Therefore, when we confirmed the particles of pumice stones during a field survey on Izu-Oshima, there was a large difference in the degree of weathering of the pumice stone surface between the underwater part and the middle part of the air, and the air had a smooth rounded shape. On the other hand, the underwater side was almost unweathered and rich in unevenness. Although the particles with such characteristics were well preserved, there were some particles with smooth top surfaces, masks, and smooth particles, which reached nearly half, but no adhesion of Lepas anatifera was observed (Fig. 1).
3. Hypothesis of origin
Pumice stones originating from the field of Fukutoku-oka-no-ba have uneven parts in density, such as crystal clots called chocolate chips and black bands. Even if the particles crack randomly, the positions of the center of gravity and the center of gravity of the particles do not match, and it is considered that there are many cases where the same direction always comes up when floating. In such a case, it is possible that the weathering of the portion exposed to the air selectively progressed during the floating for 3 months or more, and the weathering progress of the underwater part was slow. Adhesion and growth of Lepas anatifera may further enhance such anisotropy. In particular, some of them have similar cross-sectional and planar shapes to those of ships, and I think that they may have been advantageous for coastal drifting. While moving in a pumice raft, they collide with each other and change their shape, and I think it may have happened.
4. Future issues and plans from now on.
Examine whether there are any pumice stones other than the Fukutoku-oka-no-Ba field that show similar characteristics. We will search for particles with the same shape characteristics from the sediments near the coast in the past and verify whether they are floating pumice. While observing the behavior of pumice stones floating on the surface of the water, engineering studies such as the center of gravity of the pumice stones and the effect of buoyancy will be conducted.
5. reference
Oikawa et al. (2021) Eruption in August 2021 at Fukutoku-Okanoba, Ogasawara Islands. Proceedings of the Volcanological Society of Japan, 120-120.
Yoshida et al. (2022) Variety of the drift pumice clasts from the 2021 Fukutoku-Oka-no-Ba eruption, Japan,https://doi.org/10.1111/iar.12441.