Ongoing Activity at Hunga Submarine Volcano, Tonga: The Case for Better Monitoring of Submarine Volcanoes Worldwide

The powerful eruption of Hunga volcano (15-January-2022) excavated ∼6.3 km³ of pre-existing material, leaving behind an 855 m deep crater. The scientific and humanitarian response to this event was challenging due to the remote location, safety concerns, and COVID-19 pandemic restrictions. To investigate the status of ongoing eruptive/hydrothermal activity, this study used, for the first time, an un-crewed surface vessel operated remotely from >16,000 km away to make direct water column measurements within the crater and map its structure in detail. Intense turbidity and oxidation-reduction potential (ORP) anomalies located ongoing activity at sites on the steep inside crater slopes near both remaining islands. Mid-water acoustic reflectors indicated ongoing degassing, and positive ORP anomalies suggested gas composition was dominated by CO₂. At least 75% of the crater rim is shallower than 100 m, so any exchange with the surrounding ocean is limited by the depths of breaches in the rim (185 m between the islands and 290 m on the ENE side). This post-eruption bathymetry results in accumulation of emission products within the deep crater. There were no indications of the ongoing activity visible at the ocean surface, which highlights the limitations and inherent biases associated with relying on discolored surface water and/or atmospheric disturbances to determine eruption start/end dates at submarine volcanoes. This study demonstrates the value and need to add repeat hydrothermal plume and bathymetric surveys to our toolbox for monitoring submarine volcanoes, and the potential for un-crewed, remotely operated vessels to contribute significantly to these efforts.