Abstract
Hunga Volcano (Tonga) is a ~6 km-diameter caldera up to 140-150 m below sea level. Surtseyan eruptions in the last 40 years occurred along caldera ring faults. The latest eruption sequence began on 20 Dec 2021, near the 2014-15 vent. On 14 January 2022 a >20 km-high plume formed and was followed by 18 hours of harmonic sea-level disturbance. The climactic event on 15 Jan had a peak eruption rate of ~109 kg/s, producing global air-pressure waves, tsunami and a >55 km-high eruption column. The bulk pumice composition was similar to past events: 56-57 wt% SiO2 andesite. Mingled, phenocryst- and microlite-poor glass spans 50-66 wt% SiO2 over scales of microns to millimeters. Up to ~10% volcanic/hydrothermal xenoliths are present. The roughly concentric fall deposit 65-100 km on land and sea is poorly sorted and fine-grained (4-7 wt% >1 mm). Juvenile clasts are dense (>2.7 g/cm3), with isolated and collapsed vesicles. A sparse basal layer of scoria lapilli (up to 50 mm) fell first (densities of 0.8-2.0 g/cm3). Over 70% of fine particles show hackle lines, stepped fractures, and conchoidal fractures. Geochemical and isotopic data suggests that the eruption was triggered by immiscible magma mixing between two andesitic melts. The mixing likely drove intense gas pressurization of the magmatic and hydrothermal system. As the hydrothermal seal catastrophically failed, rapid decompression and fracturing led to runaway magma rise and interaction with seawater infiltrating the edifice. Intense magma-water and magmatic-gas driven explosions generated tsunami within 35 min of the climactic eruption, but sudden caldera collapse possibly triggered another tsunami, ~1 20 min later, with a final tsunami and eruptive pulse ~4 hours later. The total caldera collapse during involved at least 6.5 km3 of volume change.