Abstract
The January 2022 eruption of Hunga Volcano is likely the most explosive mafic eruption yet documented. It exhibited dynamics of ash plume expansion and atmospheric pressure waves unlike anything previously observed by satellites. It erupted crystal-poor andesitic magma (57-63 wt.% silica glass) and produced an eruptive column of at least 55 km high. Deposits were sampled from four islands across the Kingdom of Tonga within 10 days of the eruption. Textural, grainsize and morphological analyses were completed to assist local response authorities. The tephra (500-5600 mu m) comprises, on average, dark pumice (43%), light pumice (21%), blocky glass (25%), banded pumice (4%), lithics (6%) and free-crystals (Pl, Cpx, Opx) (1%). Specific gravity of particles ranges from 0.4-1.0 (few and rare light pumice lapilli) to approximately 2.1-2.8 (ash-sized material). All inhabited islands reported ashfall, except the northern Nuia group, >300 km from the volcano, with measured thicknesses from 4 (at approximately 60 km distance) to less than 0.1 cm. The general characteristics imply that magmatic vesiculation, as well as deformation and collapse, must have occurred prior to magma-water interaction, i.e. a primary fragmentation process. This could be a combination of rapid decompression and explosive magmatic gas release, along with highly-efficient crack-confined phreatomagmatism, accelerated by stress waves and thermal contraction rapidly increasing magma surface area for interaction. The ash is fine-grained and poorly sorted overall, with 8 wt.% finer than 10 mu m and <0.05 wt.% finer than 1 mu m. Variations in the mode and sorting of ash fall at different locations and angles from the vent show that there was potentially complex dispersal of ash from different phases of the 11 hour-long eruption.