Unsinkable, long-drifting, millimeter-sized pumice of the 2021 eruption of Fukutoku-Oka-no-Ba submarine volcano

Understanding the relationship between pumice formation and long-term floatability in seawater is becoming increasingly important in terms of eruption dynamics, material cycles, biological and environmental effects, and ocean hazards. Pumice rafts were produced during the 2021 eruption of the Fukutoku-Oka-no-Ba submarine volcano in the Pacific Ocean, far from the Japanese archipelago. The pumice rafts reached the Amami-Ōshima and Okinawa Islands approximately two months after the eruption and continued to cover the sea surface at several bays and ports, providing a great opportunity to study the characteristics of raft pumices. Sieve analysis of the floating pumice indicated that the pumice rafts characteristically contained several millimeter-sized particles with a peak at 2–4 mm. This evidence raises an important question why the millimeter-sized, Fukutoku-Oka-no-Ba 2021 pumice particles were able to float for over two months, which exceeds the floatation time shown in previous studies. To answer this question, a porosity measurement technique for millimeter-to-centimeter-sized pumice particles was established and applied to drifting pumice that erupted during the Fukutoku-Oka-no-Ba 2021 eruption. The total, connected, and isolated porosities (including errors) were acquired for floating and sunken pumice particles. As studied for the floating pumice from past submarine eruptions, most floating pumice particles contain a high amount of isolated porosity (> 30 vol%) and are thus unsinkable even when the pumice size is in the millimeter scale. This study emphasizes that skeletal density is a useful measure for the determination of pumice floatability (sinkable or unsinkable). As the particle size decreases, crystals are lost from the floating pumice particles, suggesting that the particle size of the floating pumice is affected by its petrological properties (crystal content and size). A comparison with natural pumices from subaerial eruptions and experimental pumices from magma decompression experiments suggests that the Fukutoku-Oka-no-Ba 2021 pumices contain abundant isolated pores due to the suppression of expansion after fragmentation by quenching in seawater, and that the relatively low to moderate crystal content in the magma (< 17 vol%) may contribute to favorable conditions to produce abundant, millimeter-sized, unsinkable pumice.

Most floating pumices have lower saturation density than seawater due to abundant isolated porosity (> 30 vol%) and are thus unsinkable even when the pumice size is in the millimeter scale.