Past eruptions of a newly discovered active, shallow, silicic submarine volcano near Tokyo Bay, Japan

Abstract

Oomurodashi is a newly discovered active, shallow, silicic submarine volcano only 60 km from Tokyo Bay. We reveal its past eruptive activity, and potential future hazards, by examining volatile contents of its subaerial and submarine pumice and lava deposits. These novel data for shallow silicic submarine eruption products were obtained using new Fourier transform infrared spectroscopy (FTIR) analytical techniques for vesicular and hydrated glasses. All matrix glasses have H2O species data consistent with low-temperature hydration following eruption. We therefore used unaltered OH data to investigate past eruptions. Geochemistry confirmed that Oomurodashi was the source of a ca. 13.5 ka subaerial tephra deposit on nearby inhabited islands. We infer from pumice OH contents and tephra characteristics that this deposit was formed by explosive submarine phreatomagmatic activity that produced the shallow crater in the submarine edifice. OH contents of in-place submarine lavas are lower than expected for their current water depth; comparison with past sea level implies that these lavas erupted at ca. 7–10 ka and ca. 14 ka when sea level was lower. Oomurodashi has also erupted submarine pumice with different densities, quench depths, and dispersal histories; however, any pumice sufficiently buoyant to produce floating pumice rafts will have been lost from the local geological record, so pumice rafts remain a potential future hazard.