Tomoki Sato

Nishinoshima is a volcanic island in the Ogasawara Arc that has exhibited intermittent activity since 2013, including four major eruptive episodes: 2013–2015 (Episode 1), 2017 (Episode 2), 2018 (Episode 3), and 2019–2020 (Episode 4). Previous studies reported a change in eruption style—from a Strombolian eruption with lava flows (Episode 1 to the early phase of Episode 4) to violent Strombolian eruptions (mid to late phase of Episode 4)—accompanied by a shift in the bulk chemical composition of volcanic ash from andesite to basaltic andesite. However, the number of tephra samples from Episode 4 remains limited, and additional data are required to clarify the details about changes in chemical composition during this episode. In this study, we present the major and trace element compositions of offshore tephra collected from Episode 4. We analyzed Kairei in December 2020. The bulk chemical compositions indicate that these samples were classified into andesite and basaltic andesite and were deposited during the mid‐ to late phase of Episode 4. Furthermore, their chemical compositions fall between those of the Nishinoshima andesites and the Episode 4 basalts reported previously. This suggests that the magma composition associated with the eruption shifted from andesite to basaltic andesite within a period of only 1.5 months. In addition, the samples show both vertical and spatial compositional variations. The former may reflect gravitational settling of ash particles through the water column and their floating behavior at the sea surface, whereas the latter may be influenced by eruption timing, local wind directions, or ocean currents.

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.

Pumice is a pyroclastic material of low density and is occasionally buoyant in water. When it erupts from submarine volcanoes, it can drift long distances and wash ashore at remote beaches. Pumice from the eruption of Fukutoku‐Oka‐no‐Ba volcano in the Izu–Bonin arc in October 2021 drifted for about 2 months over a distance of > 1300 km onto the Nansei Islands of Japan, disrupting shipping, fishing, and coastal ecosystems. In March 2024, large amounts of dark brown to black pumice washed ashore on these islands, and subsequently in the Kanto area of Japan. Their geochemical and petrographic characteristics were examined, and Ioto volcano, which is one of the most active volcanoes in Japan, was identified as a possible source based on these characteristics. Small amounts of similar pumice clasts drifting ashore in the Nansei Islands during 2022–2023 were also identified as originating from Ioto volcano. Since July 2022, volcanic activity involving magma eruption has occurred off the southeast coast of Ioto, where pumice rafts generated by the eruptions have been observed. Drift simulations indicate that pumice ejected from Ioto could reach the Nansei Islands, where it is widely distributed. The observation of pumice drifting along the coast of these islands thus provides information on the dispersal of pumice to other parts of Japan, with sources effectively identified through their chemical composition. The results contribute to our understanding of the eruption histories of submarine volcanoes in the Izu–Bonin arc.

Intermittent volcanic activity has continued at Nishinoshima volcano in the Izu–Bonin arc in the western Pacific since 2013, characterized mostly by effusive Strombolian eruptions (Episodes 1–3). Subsequently, the eruption style changed suddenly to explosive violent Strombolian in mid‐June 2020 (Episode 4), after which the volcano entered a period of quiescence. In 2021, volcanic activity restarted at Nishinoshima, although the eruptions were small (Episode 5). The change in eruption style between Episodes 1–3 and 4 was controlled mainly by a change in magma composition. However, the details of the redox state with respect to magma composition remain unclear. We aimed to understand the change in magma composition during Episode 4 and the subsequent activity, focusing on the redox state. Fe–K edge X‐ray absorption near edge structure (XANES) analysis and petrographic observations were performed on volcanic glass in seafloor and subaerial samples from Episodes 4 and 5. The results show that the explosive eruption of Episode 4 was characterized by the intrusion of oxidized basaltic magma into a reduced andesitic magma, which is a similar tendency to another large‐scale eruption in the Izu‐Bonin arc (Fukutoku‐Oka‐no‐Ba). Episode 5 shows different redox changes, involving remnants of the basaltic andesite magma from Episode 4, whose f _O2 had been reduced. The groundmass glass in Episode 5 subaerial ejecta has oxidized and differentiated (SiO ₂ ‐rich) composition possibly due to the subaerial cooling process. The observed geochemical and petrographic characteristics suggest that the small Episode 5 eruption occurred due to the continuous feed of the magma similar to Episode 4, but the magma plumbing system in the volcano made the provided high f _O2 magma reduced, which may mitigate the explosivity of the future eruption.

The experimental study of water–rock reactions under high-temperature and -pressure conditions is a useful approach to constrain controlling factors of the fluid composition in a natural hydrothermal system. Previous studies have focused mainly on the mid-ocean ridge fields, and the hydrothermal alteration of intermediate-to-felsic rocks has been less emphasized despite its potential importance in the fluid chemistry in an arc/back-arc basin setting. We examined the alteration processes of fresh rhyolite and andesite rocks collected from the middle and southern Okinawa Trough, respectively, at 325 °C and 300 bar (the estimated condition at the reaction zone in the fields), especially focusing on the behavior of silica between the solid and liquid phases. The experimental fluids are characterized by the high Si concentration up to 30 mM, indicating the substantial dissolution of volcanic glass in the analyzed rocks. The high Si concentration in the fluids was presumably buffered by amorphous silica, precipitated from the fluids as a precursor of hydrothermal quartz, during the experiments. Our results emphasize a previously overlooked role of volcanic glass/amorphous silica in the fluid composition in the Okinawa Trough and are consistent with the previous model of pumice replacement mineralization for the SMS deposit formation in the trough.

Primary andesitic magmas could be an important component of arc magma genesis and might have played a key role in the advent of continents. Recent studies hypothesized that primary andesitic magmas occur in the oceanic arc, where the crust is thin. The Kermadec arc has the thinnest crust among all the studied oceanic arcs (84–86) and clinopyroxene (Mg# = 81–87). Using olivine addition models, the primary magmas were estimated to contain 55–56 wt.% SiO₂ and 10–12 wt.% MgO, similar to the high-Mg andesites observed in other convergent plate margins, indicating the generation of primary andesitic magma beneath the Kibblewhite Volcano. The trace element and isotopic characteristics of the magnesian andesites are typical of volcanic rocks from the Kermadec arc. This indicates that the subduction of a young plate or melting of a pyroxenitic source is not necessary to produce magnesian andesites. Instead, we propose that the magnesian andesites were produced by the direct melting of the uppermost mantle of the Kermadec arc. The thin crust of the Kermadec arc should yield low-pressure conditions in the uppermost mantle, allowing the sub-arc mantle to generate primary andesitic melts. This study supports the hypothesis that primary andesitic magmas generate in the arc where the crust is thin and provides a new insight into the magma genesis of the Kermadec arc.

Fukutoku‐Oka‐no‐Ba is a submarine volcano located at 24°17.1′ N/141°28.9′ E in the Izu–Bonin arc, and is one of the most active volcanoes in Japan. This volcano produced an explosive eruption in August 2021 that generated a large amount of volcaniclastic material, some of which drifted westward to Japan and the coastal area of East Asia as a pumice raft. The pumice clasts that drifted for >1000 km were mostly homogeneous and identical to those produced by past historical eruptions. The clasts have trachytic compositions (SiO₂ = 61–63 mass% and Na₂O + K₂O = 8.6–10.0 mass%) and contain augite, plagioclase, olivine (Mg# ~65), and magnetite, along with a small number of mafic enclaves containing diopside and high‐Mg olivine (Mg# ~ 92). We undertook a research cruise to investigate the proximal volcaniclastic materials by dredging. The proximal materials include pumice, weakly vesiculated lapilli, and volcanic blocks, which have trachytic composition (SiO₂ contents up to 64.5 mass%). The main minerals in the proximal material are similar to those in the drift pumice, although remnants of mafic magma do not occur in the SiO₂‐rich samples. The petrographic and geochemical characteristics of the proximal and drift ejecta from Fukutoku‐Oka‐no‐Ba suggest the magma reservoir was stratified into two parts. The major part experienced magma mixing with a limited volume of mafic magma, whereas the other part was more differentiated. The differentiated high‐SiO₂ magma accumulated in the upper part of the magma reservoir and avoided the mixing with and feed of volatile from the mafic magma, then were pushed out from the volcanic vent without extensive bubbling to sunk in the proximal area.

It is common knowledge that the Moho is the boundary between the crust and the Earth's mantle. Here we show along several seismic profiles through the Pacific Plate that a correlation exists between the strength of Moho reflections, crustal thickness, and water depth. Where the Moho can be detected clearly, the overlying oceanic crust is systematically thicker and the water depths are shallower. We suggest that two end‐members of oceanic crust exist in fast spreading environments: one thick, underlain by a clear Moho; the other thinner, without a Moho; with all intermediate situations. In the Oman ophiolite, the best‐preserved on‐land analogue of fossil oceanic lithosphere created by fast‐spreading, the boundary between the mantle peridotites and the lower crustal gabbros mainly consists of a dunitic transition zone (DTZ) ranging from a few meters to a few hundred meters in thickness. A sudden influx of seawater down to the base of the crust at the mid‐ocean ridge (MOR) results in the hydrous (re‐)melting of mantle peridotites, producing a dunitic residue at the crust–mantle boundary that represents the most reflective Moho. At the same time, the hydrous melting, in addition to the normal decompression melting, beneath the MOR, increases the thickness of the oceanic crust by enhancing magma production. In the absence of hydrous melting, the DTZ is thin or absent at the crust–mantle boundary, and instead the uppermost mantle harzburgite is intruded by gabbros, and/or the overlying crustal gabbro is intruded by numerous wehrlite bodies, which will be seismically gradational.

The 2021 eruption of Fukutoku-Oka-no-Ba (FOB) in the northwest Pacific on 13 August 2021 produced a large volume of pumice that drifted westward for ~1300 km to the Nansei Islands, Japan, and some extent. In February 2022, pumice with similar characteristics to the FOB pumice was deposited along the Gulf of Thailand. The pumice clasts deposited in Songkhla Province, Thailand, were fine-grained with 2800 km south-westward across the South China Sea.