[M-IS16] Geophysical fluid dynamics-Transfield approach to geoscience

[MIS16-02] Physical Conditions under which a Volcanic Plume from a Submarine Eruption Reach the Sea Surface

*Yasuhiro Ishimine¹ (1.Mount Fuji Research Institute, Yamanashi Prefectural Government)

Keywords:Submarine Eruption, Volcanic Plume, Hunga Tonga-Hunga Ha'apai, Fukutoku-Oka-no-Ba

I will talk about preliminary results of an investigation on the dynamic and thermodynamic conditions under which a volcanic plume generated from a submarine volcano reaches the sea surface, with the aim of elucidating the characteristics of the peculiar phenomena observed during the 2022 eruption of the Hunga Tonga-Hunga Ha'apai volcano. The eruption of Hunga Tonga-Hunga Ha'apai volcano in the Kingdom of Tonga in January 2022 generated gigantic eruption clouds accompanying global atmospheric pressure fluctuations. Specific tidal changes were also observed across the Pacific Ocean and alerts on tsunami were issued in Japan. However, the physical process of the series of the phenomenon remains to be understood. I examine the characteristics of the eruptions that may have occurred at the Hunga Tonga-Hunga Ha'apai volcano, focusing on the differences from the submarine eruption of the Fukutoku-Oka-no-Ba volcano in August 2021. Although volcanic plumes rise high into the atmosphere during many submarine eruptions, including the 2021 eruption of Fukutoku-Oka-no-Ba volcano, they do not contain much volcanic ash. Therefore, those plumes are supposed to consist of seawater vaporized at the sea surface due to the heat from magmatic materials, and the volcanic plumes themselves generated on the seafloor are cooled and condensed by mixing with seawater, and the plumes in water are regarded as behaving as a liquid suspending solid particles. As a result, pumice generated from magmatic materials float to the sea surface due to buoyancy caused by the density difference between pumice and water. They cannot maintain enough momentum to continue to rise in the atmosphere and drift on the sea surface as a "pumice raft". On the contrary, significant ashfall were reported during the 2022 eruption of Hunga Tonga-Hunga Ha'apai volcano and the plume reach is too high to rise only by water vapor evaporated from the sea surface. Thus, I think the volcanic plume generated on the seafloor possibly have maintained the state of a mixture of a hot gas fluid and suspended solid particles, reaching the sea surface to rise into the atmosphere in a similar manner with subaerial volcanic plumes. To examine the hypothesis, I carried out some simple calculations using a steady-state 1D plume model to search for the dynamic and thermodynamic conditions that realize a hot gas plume in water. The results suggest that the temperature in the plume can be maintained above the boiling point of water if the volcanic vent is about 150 m in depth below the sea surface and the its radius is more than 1 km with the condition that magma temperature is higher than 1000 K. The large amount of seawater may be entrained into the volcanic plume and vaporize to cause rapid volume expansion, which could be the source of the sound waves that caused global atmospheric pressure fluctuations. This implies that tidal changes that simultaneously observed with atmospheric pressure fluctuations may also have been caused by a unique mechanism that can only occur in submarine volcanic eruptions.