Time-series insights of the geomorphologic changes of the Hunga submarine caldera

Caldera volcanoes produce the most impactful eruptions worldwide, yet caldera formation is rarely observed, especially in the submarine environment. We report repeat high-resolution seafloor mapping and quantify the geomorphological changes after the 15 January 2022 eruption of Hunga volcano (Tonga). While the ~1800 m-high stratovolcano retained its overall form, islands, shoals and the shallow caldera floor (~150-200 m water depth) collapsed, forming a ~3.5 km diameter basin, ~850 m deep, with its structural floor ~100-150 m below. The total displaced volume is 8.9 ± 0.1 km³. Of this, 6.85 ± 0.05 km³ is attributed to caldera collapse, reflecting loss of magma from the ~2–6 km deep reservoir; and ~2 km3 was removed by erosion from eruption-fed density currents and undercutting of the southern edifice flanks. Caldera morphology suggests multiple episodes of trapdoor and piston subsidence, while inner-caldera landslide deposits intercalated with pyroclastic infill indicates caldera collapse was syn-eruptive. Observed fluid vents in May 2022 highlighted two vent zones, below the collapsed 2014-2015 tephra cone in the NW, and at the SE edge of the caldera basin. These results provide new insights into the mechanisms and geomorphic evolution of highly hazardous mafic caldera eruptions.