Petrographic and geochemical characteristics of Sofu Seamount, a recently-erupted deep submarine volcano in the Izu arc

In early October 2023, earthquake swarms occurred near Izu-Torishima in the southern Izu arc. On October 9, tsunamis reaching up to 60 cm were observed at Hachijojima and along the Pacific coast of the Japanese archipelago where the height of the tsunamis were disproportionate to the earthquake magnitudes (Sandanbata et al., 2023). T-wave analysis identified the epicenter near Sofu Seamount, bathymetric high west of Sofugan volcano, and subsequent bathymetric surveys by JAMSTEC and Japan Coast Guard independently revealed significant topographical changes, including a concave depression near the western summit of Sofu Seamount (Fujiwara et al., 2024; Minami & Tani, 2024). This significant bathymetric change suggests an intense deep-sea eruption even though the water depth of the pre-eruption summit was ~800 m.

We conducted seafloor geological surveys using ROV and dredges during cruises KM25-02Leg2 (March 2025) and KM25-09 (September 2025) by the R/V Kaimei. Sofu Seamount is an east-west trending bathymetric, which was not previously recognized as an active volcano. Its western edifice features a caldera-like structure approximately 5 km in diameter, with a central cone in the northern section now containing a 1.5 km wide crater formed during the 2023 activity. The crater floor lies at a depth of ~1200 m, with the rim at ~900 m. Recent shallowing within the outer caldera was detected, suggesting significant tephra deposition in the proximal area. Rock samples were collected from the inside of the central cone, inside of the outer caldera, and northern slope of the seamount. Around the central cone crater, dark-colored volcanic rocks were collected, some of which featured brown-colored coatings identified as iron oxyhydroxides. Bacterial mats were observed on both sides of the northeastern crater rim, suggesting an active hydrothermal venting. White pumice (clasts to boulders) was found deposited on the upper most surface of the caldera interior, likely originating from the October 2023 event. Large dark-colored volcanic rocks were observed outside the caldera, notably lacking iron oxide coatings.

The collected samples ranged from basaltic andesite to rhyolitic pumice, with SiO2 = 52 – 73 %. Mafic rocks contain olivine (Fo70-75), clinopyroxene, Ca-rich plagioclase, and rare orthopyroxene. Some dredged samples contained xenoliths with less-differentiated olivine (~Fo85) and well-preserved basaltic volcanic glass. Olivine-melt composition suggests P-T conditions of ~1050 C and ~160 MPa. In contrast, pumice samples contain clinopyroxene, orthopyroxene, and Na-rich plagioclase, with the estimated conditions of ~900 C and 135-165 MPa. Thermodynamic modelling indicates the compositional trend cannot be explained by fractional crystallization. As such, the mixing of high-temperature mafic magma and low-temperature felsic magma within a pressure range of 134-165 MPa beneath Sofu Seamount triggered an explosive eruption in October 2023.