The 6–8 Aug 2019 eruption of ‘Volcano F’ in the Tofua Arc, Tonga

Abstract In August 2019 a large raft of pumice appeared in the territorial waters of Tonga. As in many other cases, this pumice raft was the only surface expression of a major submarine volcanic eruption. Discolored water and reconstruction of the drift path of the pumice raft using satellite imagery points towards ‘Volcano F’ in the Tofua Arc NW of the island of Vava’u as the most likely volcanic source. Here we present imagery from ESA’s Sentinel-2 satellite that captured the start of the submarine eruption on 6 August 2019 and the waning of the eruption on 8 August, followed by observations of the drifting pumice raft until 14 August. This start time is consistent with T-phase records at the seismic stations on Niue Island and Rarotonga and the signal delay time of 733 s between the two stations is consistent with an origin at or at least near Volcano F. On 8 August, a >136.7 km2 large raft of pumice appears at the sea surface. The modelled minimum raft volume is 8.2–41.0*106 m3, which is equivalent to 2.5–12.3*106 m3 dense rock. The eruption thus corresponds to a volcanic explosivity index (VEI) 2–3 eruption in the submarine environment. Prior to the volcanic eruption, a series of earthquakes close to Volcano F was recorded. The series started on 5 August with a Mb 4.7 event, followed by at least six shallow earthquakes (Mb >3.9) on 6 August. In December 2018 and January 2019, we surveyed the seafloor around Volcano F with multibeam sonar. Combining our data with pre-existing information, we present the first comprehensive bathymetric map of the volcanic edifice and its geologic setting. We show that Volcano F represents a major arc volcanic complex that is situated in an extensional setting. The basal diameter of the volcanic apron is >50 km with a large central, 8.7 x 6 km caldera with a floor at ∼700 m water depth. The top of the post-caldera constructional cone complex had a summit depth of 35 m below sea level in 2004. The volcano shows geochemical differences to the adjacent arc volcanoes on Fonualei and Late islands. The volcano’s pristine volcanic morphology and two documented eruptions (2001 and 2019) indicate a highly active volcanic system that warrants further scientific attention.