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[SVC31-19] Characteristics of drifting pumice from the Fukutoku-Oka-no-Ba 2021 eruption and their relationship to eruption sequence.
Keywords:Fukutoku-Oka-no-Ba, drifting pumice, satellite images, eruption sequence, submarine volcano
Fukutoku-Oka-no-Ba (FOB) 2021 eruption produced a large amounts of pumice rafts that arrived at Japan from the Nansei Islands to the Izu Islands. Drifting pumice has social impacts such as interfering ship navigation and fishing activities, and understanding a phenomenon is important for disaster countermeasures. In addition, the actual state of eruptive activity of submarine volcanoes is not clear enough due to the difficulty of geological survey. Therefore, drifting pumice is a valuable phenomenon that enables us to study submarine volcanoes by material science.
In this study, with the accompanying study of Takeuchi et al. (2022), we collected information on drifting pumice from media and SNS, conducted field surveys and laboratory experiments, and analyzed satellite images in order to clarify the characteristics of drifting pumice and eruptive phenomena of FOB 2021 eruption.
We conducted field surveys in the southern and central area of the main island of Okinawa on October 19-20 and November 1-3, and in Izu-Oshima Island on December 22-24. The drifting pumice was collected at sea by developing a tool that can collect pumice in a fixed area. The thickness of the drifting pumice layer, the drifting concentration and the drifting pumice filling rate were determined by a flotation simulation experiment using a fish tank, and grain size distribution (GSD) analysis was conducted using dried drifting pumice. We also conducted GSD analysis and compositional analysis of the drifted pumice. In the compositional analysis, pumice was classified into gray pumice, black pumice, black-gray striped pumice, pale gray pumice, brown pumice, and dark brown pumice based on color and bubble characteristics. More than 300 particles (8-11.2 mm) of the drifted samples were appraised per site.
From these geological investigations and laboratory experiments, drifting pumice was classified into three types (Type A to C) based on the difference in composition and scale of drifting. Type A is characterized by the fact that most of its component are gray pumice, and is observed in pumice that drifted to the southern part of the main island of Okinawa in late October. Type B is characterized by the fact that most of its component are gray pumice, but a few other types are also produced, as observed in the pumice that drifted to Izu-Oshima Island in mid-November. Type C is characterized by the universal content of pumice other than gray pumice, which corresponds to the pumice that drifted on a small scale from northern to southern Okinawa Island in mid-October. Type C is characterized by the frequent inclusion of a variety of pumice and is observed in small drifted pumice from northern to southern Okinawa Island in mid-October.
We also observed the eruption sequence using satellite data from Himawari-8 satellites. The FOB eruption was broadly classified into three phases (Phases 1-3), and drifting pumice (drifting pumice groups 1-3) were observed in each phase. Phase 1 is eruptions with relatively large umbrella cloud formation from 5:57 to 20:00 on August 13 (JST), Phase 2 is intermittent eruptions with relatively short-term umbrella cloud formation from 20:00 on August 13 to 12:00 on August 14, and Phase 3 is intermittent eruptions without umbrella cloud formation from 12:00 on August 14 to August 15.
In addition, we tracked the drift paths of each drifting pumice group using ESA satellite data and revealed that drifting pumice groups were separated from each other early in drifting, and furthermore, a switch in the westward order of groups occurred midway through the drifting. After that, we found that each group drifted to the Nansei Islands in a certain degree of grouping. First, small-scale Group 3 reached Minami-Daito Island on October 4 and Okinawa Island around October 13. In addition, Group 2 reached the Amami Islands on October 10, and some of them moved northward to reach the Izu Islands in mid-November. Group 1 had the largest drifting area and reached Okinawa Island after late October.
Based on the results of satellite image analysis and geological survey, the difference in the composition of drifted pumice according to the time and place of drift is considered to reflect the difference in drifting pumice groups corresponding to the eruption phase. In other words, drifting pumice group 1 has type A, group 2 has type B, and group 3 has type C components. This suggests that a large amount of homogeneous gray pumice is erupted in Phase 1, and black pumice and striated pumice increase in Phases 2 and 3. Such systematic changes in the ejecta corresponding to the transition of the eruption sequence may reflect the transition of eruption dynamics.
In this study, with the accompanying study of Takeuchi et al. (2022), we collected information on drifting pumice from media and SNS, conducted field surveys and laboratory experiments, and analyzed satellite images in order to clarify the characteristics of drifting pumice and eruptive phenomena of FOB 2021 eruption.
We conducted field surveys in the southern and central area of the main island of Okinawa on October 19-20 and November 1-3, and in Izu-Oshima Island on December 22-24. The drifting pumice was collected at sea by developing a tool that can collect pumice in a fixed area. The thickness of the drifting pumice layer, the drifting concentration and the drifting pumice filling rate were determined by a flotation simulation experiment using a fish tank, and grain size distribution (GSD) analysis was conducted using dried drifting pumice. We also conducted GSD analysis and compositional analysis of the drifted pumice. In the compositional analysis, pumice was classified into gray pumice, black pumice, black-gray striped pumice, pale gray pumice, brown pumice, and dark brown pumice based on color and bubble characteristics. More than 300 particles (8-11.2 mm) of the drifted samples were appraised per site.
From these geological investigations and laboratory experiments, drifting pumice was classified into three types (Type A to C) based on the difference in composition and scale of drifting. Type A is characterized by the fact that most of its component are gray pumice, and is observed in pumice that drifted to the southern part of the main island of Okinawa in late October. Type B is characterized by the fact that most of its component are gray pumice, but a few other types are also produced, as observed in the pumice that drifted to Izu-Oshima Island in mid-November. Type C is characterized by the universal content of pumice other than gray pumice, which corresponds to the pumice that drifted on a small scale from northern to southern Okinawa Island in mid-October. Type C is characterized by the frequent inclusion of a variety of pumice and is observed in small drifted pumice from northern to southern Okinawa Island in mid-October.
We also observed the eruption sequence using satellite data from Himawari-8 satellites. The FOB eruption was broadly classified into three phases (Phases 1-3), and drifting pumice (drifting pumice groups 1-3) were observed in each phase. Phase 1 is eruptions with relatively large umbrella cloud formation from 5:57 to 20:00 on August 13 (JST), Phase 2 is intermittent eruptions with relatively short-term umbrella cloud formation from 20:00 on August 13 to 12:00 on August 14, and Phase 3 is intermittent eruptions without umbrella cloud formation from 12:00 on August 14 to August 15.
In addition, we tracked the drift paths of each drifting pumice group using ESA satellite data and revealed that drifting pumice groups were separated from each other early in drifting, and furthermore, a switch in the westward order of groups occurred midway through the drifting. After that, we found that each group drifted to the Nansei Islands in a certain degree of grouping. First, small-scale Group 3 reached Minami-Daito Island on October 4 and Okinawa Island around October 13. In addition, Group 2 reached the Amami Islands on October 10, and some of them moved northward to reach the Izu Islands in mid-November. Group 1 had the largest drifting area and reached Okinawa Island after late October.
Based on the results of satellite image analysis and geological survey, the difference in the composition of drifted pumice according to the time and place of drift is considered to reflect the difference in drifting pumice groups corresponding to the eruption phase. In other words, drifting pumice group 1 has type A, group 2 has type B, and group 3 has type C components. This suggests that a large amount of homogeneous gray pumice is erupted in Phase 1, and black pumice and striated pumice increase in Phases 2 and 3. Such systematic changes in the ejecta corresponding to the transition of the eruption sequence may reflect the transition of eruption dynamics.