ABSTRACT

An earthquake swarm occurred at Sofu Seamount near Izu-Torishima in the Izu-Ogasawara (Bonin) Arc, Japan, on 8 October 2023, followed by the arrival of unexpectedly-large tsunamis over a wide area of the Pacific coast of southwest Japan. On 20 October, aerial observation identified floating pumice rafts extending for ~80 km in the area near the seamount, and subsequently the pumice clasts that may have constituted the raft were sampled and found to comprise white-colored rhyolitic pumice containing dark-colored patches. Subsequently, in the summer and autumn of 2024, white pumice, which can be clearly distinguished from the pre-existing gray and black pumice from recent eruptions of oceanic volcanoes, was stranded on the coasts of the Nansei and Ogasawara Islands. This study presented petrographic and geochemical characteristics of the white drift pumice to investigate the dispersal of the small-scale pumice rafting.White pumice clasts were collected at Okinawa (Nansei Islands) and Hahajima (Ogasawara Islands), and found to have the same petrographic and geochemical characteristics. The clasts consist of microlite-free white pumice with fine vesicles and gray-colored frothy patches, similar to the clasts collected immediately after the earthquake swarm at Sofu Seamount in October 2023. In addition, the clasts contain black enclaves containing Mg-rich olivine and clinopyroxene, and Ca-rich plagioclase. Although the white pumice clasts collected at similar times included those with different characteristics, the finding of the same pumice type over a distance of >1000 km suggests that the pumice clasts drifted over a wide area despite the small size of the pumice rafts.