Abstract
Caldera-forming eruptions are among the most hazardous volcanic processes, particularly submarine ones. Yet, the dynamics of submarine caldera-forming eruptions and, thus, the hazards they pose are not well understood, owing to the relative inaccessibility of the resulting calderas and eruption products. Here we show the changes in geomorphology of Hunga volcano (Tonga) after the 15 January 2022 eruption. From repeat high-resolution seafloor mapping we find that the caldera deepened from ~150 m to ~850 m, with its structural caldera floor ~150 m below infill, and marginally widened from ~4.5 to 4.8 km in diameter. The total displaced volume is 8.9 ± 0.1 km3. Of this, 6.85 ± 0.05 km3 is attributed to caldera collapse, with the remainder due to surface erosion by eruption-fed density currents. Our post-eruption bathymetry also revealed landslide deposits intercalated with pyroclastic infill, implying syn-eruptive caldera collapse, and inward-stepping terraces, which we interpret as surface normal fault splays over piston subsidence. The caldera and magmatic system geometry suggests that ~8 km3 of Dense Rock Equivalent magma erupted to initiate collapse, less than 30% of the magma reservoir. The magnitude of the resulting tsunami was probably enhanced by the speed and style of caldera formation, highlighting the capacity for relatively small but steep calderas collapses at submarine volcanoes to generate hazardous tsunamis.