Variety of the drift pumice clasts from the 2021 Fukutoku‐Oka‐no‐Ba eruption, Japan

Abstract

Pumice rafts that arrived at the Nansei Islands, Japan, provided a unique opportunity to investigate the Fukutoku‐Oka‐no‐Ba (FOB) eruption of August 2021. Despite drifting for 2 months for ~1300 km, the drift pumice raft had a large volume and contained a variety of pumice clasts, some of which were deposited during a high tide in a typhoon, while others were washed up on a sandy beach. Most of the drift pumice clasts are gray in color, vesicular, and have a groundmass containing black enclaves. Rare black pumice and the main gray pumice components have similar trachytic compositions, with SiO2 = 61–62 mass% and total alkalis = 8.6–10 mass% (on an anhydrous basis). Both pumice types contain clinopyroxene, plagioclase, and rare olivine phenocrysts. Thin‐section observations show that the gray pumice has more elongated vesicles as compared with the black pumice that has spherical vesicles, even where the two types of pumice are in the same clast. The glass in the black pumice is transparent and brown in color, while that in the gray pumice is colorless. No micro or nano‐crystals were observed during electron and optical microscopy. Raman spectra of the brown‐colored glass exhibit a clear magnetite peak, suggesting magnetite nanolites cause the brown color. High‐Mg olivine in the black pumice has an equilibrium temperature of c. 1200 °C and a rim diffusion profile indicative of re‐equilibration with the surrounding melt over a period of hours to days. The textural relationships between the gray and black pumice suggest that the black pumice had become black and viscous before the two types of pumice mixed. Therefore, crystallization of magnetite nanolites and a corresponding increase in melt viscosity were important in the eruption preparation process, which then resulted in a large‐scale Plinian eruption.

Coastal ecological impacts from pumice rafts

“…According to a very recent report, the pumice clasts were drifting ashore in Thailand (traveling 4000 km-long distance) across the South China Sea within half a year of this eruption 29 . Most pumice stones were gray, but some pumice was banded, and others were black reflecting some compositional variation 25 , 29 (Fig. 2 d,e).…”

“…2 d,e). The Kuroshio Current is faster than the Kuroshio countercurrent 27 , so some pumice clasts have already reached the main island of Japan 25 . Tracking the dispersal of the pumice will allow a better forecasting model based on observed raft trajectories by considering exact wind effects in the Philippine Sea 30 .…”

“…The Japan Coast Guard reported that a large submarine eruption occurred within the Ogasawara Archipelago (Fukutoku-Oka-no-Ba, Tokyo, Japan: 24.285°N, 141.481°E) on 13 August 2021 22 . Eruption details have been summarized by the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology 23 , Smithsonian Institution Global Volcanism Network 24 , and Yoshida et al 25 . Satellite observations were able to capture the formation and then dispersal of the 300-km 2 pumice raft 26 .…”

“…Pumice is a highly vesiculated glassy volcanic rock fragment where the vesicularity typically ranges from 50–80% making the pumice have a bulk density less than water. It is commonly light colored, reflecting a high silica content 25 . The large amount of pumice stones produced by this eruption was carried by surface winds and ocean currents for approximately 2 months before reaching the Ryukyu Islands, including the main island of Okinawa, which is located approximately 1300 km away from the volcano (Fig.…”

Pumice Raft Detection Using Machine-Learning on Multispectral Satellite Imagery

“…and marine organisms across thousands of kilometers, potentially dispersing nutrients and increasing microbial biomass and biodiversity in areas geographically distant from the site of the eruption (DeVantier, 1992;Risso et al, 2002;Bryan et al, 2012). Pumice rafts can also be a significant hazard for human maritime activities, especially by clogging harbors and affecting near shore sea life and fisheries, disrupting local economies (Bryan et al, 2012;Jutzeler et al, 2020), as observed in the recent raft dispersal from the 2021 eruption of Fukutoku-Okanoba volcano, a submarine volcano in the Bonin Arc (24.285 °N, 141.481 °E) (Fauria et al, 2022;Yoshida et al, 2022).…”