Abstract
Several features of the of the 15 January 2022 Hunga volcano eruption (Tonga, South West Pacific) were differed from existing models of explosive volcanism. The eruption involved exceptionally powerful explosions and major atmospheric pressure waves and tsunami. Did the interplay between magmatic, volcano-structural and water depth create a new class of eruption? The top of the Hunga plume reached 58 km high, pumping water into the mesosphere. Its rapid rise was rivalled by its rapid lateral spread, indicating little density contrast between the ambient atmosphere and the plume. The SO (sub 2) emission was only approximately 2% of that expected from the magma volume and composition, so that much of it was scrubbed into seawater, or converted to SO (sub 4) . Unprecedented rates of lightning in the plumes showed large proportions of ice present. All eyewitness images and videos of the plume show it was dominated by steam. This corroborates with thin fall deposits on land and sea, accounting for only approximately 10% of the magma volume erupted. Most of the magma erupted was ejected over an hour to form >11 km (super 3) of submarine density currents. Pyroclast textures, shapes and grain-size distributions from land and sea attest to violent fragmentation of a hot, crystal poor, low-viscosity basaltic-andesite magma by contact with water. The scale of this eruption was not driven by a Plinian process (steady eruption of an expanding magma foam from a narrow conduit). Instead, early activation of caldera ring-faults via mass/pressure loss, led to rapid penetration of sea-water to meet rising magma within the submarine Hunga edifice. This led to exceptionally large contact areas between pressurized water and magma at >1100 degrees C, driving extreme explosions and a cycle of fault opening and seawater ingress. Alongside this magma erupted from ring-faults during progressive caldera collapse. The central eruption "column" was akin to an atmospheric "thermal", driven by flashing of seawater to steam and carrying little ash. Jetting and low-fountaining of dense, low-viscosity mafic magma from ring faults fed concentric high-velocity submarine density currents that raced >90 km from source. The Hungan eruption style combines an interplay between large volumes of eruptable gas-poor, low-viscosity (mafic) magma, pre-existing caldera structures, and a water depth not great enough to subdue explosivity.
Oral presentation.