Mila Huebsch

The 2022 Hunga volcano eruption generated one of the largest and most destructive volcanic tsunamis ever recorded. We interpret the timing, nature and relative hazard of volcanic-tsunamigenic processes by combining geophysical, oceanographic, atmospheric, and eye-witness observations of this event. Hydro-acoustic signal ( T waves) detected on seismic stations up to 2600 km away provided a key to discerning tsunami triggers from explosions, volcanic mass flows, and caldera subsidence. The largest tsunami was generated by the first of at least two sudden caldera-subsidence events. This occurred 1 hour after globally detected explosions that produced smaller tsunami. Our results provide rare insights into the rates of caldera subsidence and highlight the complex multiple sources of tsunamis resulting from eruptions of oceanic volcanoes.

Submarine volcanic eruptions can form subaerial plumes that frequently reach the troposphere or even the stratosphere. Despite this, the impact of submarine eruptions on ash transport and related hazards remains unclear due to a lack of clear geological record. Here, we review the impact of submarine volcanoes on ash generation and transport in the Earth system by combining thermal, textural and chemical analysis of volcanic ash from the 15 January 2022 eruption of Hunga volcano (Tonga). We used flash differential scanning calorimetry to perform enthalpy relaxation geospeedometry, which allowed us to determine the natural cooling rates of individual ash grains formed during magma-seawater interaction. Synchrotron-based nano-tomography and subsequent 3D image analysis were used to link initial magma texture, thermal crack propagation and resulting ash characteristics (density and morphology). Chemical analysis included quantification of leachate concentration and isotopic d34S and d37Cl signatures of the Hunga ash. Thermal and 3D textural analysis revealed that high cooling rates (hundreds of K.s-1) during magma-seawater interaction led to high levels of thermal stress, fracturing and pervasive fine ash generation. Ash morphology, density and porosity following thermal granulation were strongly influenced by the starting vesicle size distribution. Heat transfer and magma cooling were accompanied by intense evaporation of seawater and subsequent sea-salt (dominantly halite and Ca-sulphate) formation, with a limited role of gas scavenging on salt precipitation and volatile budget during this eruption. Sea salt formation promoted fine ash aggregation, thereby reducing the residence time of volcanic ash within the troposphere and stratosphere. Together, these processes may explain the ash-poor and sulphate-poor nature of volcanic clouds formed during submarine eruptions and the lack of clear geological record, despite evidence for repeated intrusions of submarine plumes in the stratosphere in historical times.

The stratovolcano Mt. Pelée, Martinique, exhibits eruptive styles ranging from dome formation to sustained, highly violent explosive activity. Historical eruptions have produced lava domes and pyroclastic density currents, collectively termed Peléan activity. In pre-colonial times, several Plinian eruptions took place. Here, we explore physical controls on the proportions of fine particles produced—i.e., the fragmentation efficiency—during primary fragmentation. Samples were collected from ignimbrites from the 1929–1932 and 1902–1905 Peléan eruptions and the P1 (1300 CE), P2 (280 CE), and P3 (79 CE) Plinian eruptions. All samples are andesitic in bulk composition and contain a rhyolitic groundmass glass. The Peléan materials are more crystalline and less porous than their Plinian counterparts, a consequence of more extensive outgassing during dome formation. Representative blocks were cored and experimentally fragmented following rapid decompression (> 1 GPa·s⁻¹ from initial pressure between 5 and 20 MPa). Dry sieving allowed for determining grain size distributions, from which the fractal dimensions, D_f, were calculated as a quantification of fragmentation efficiency. Our results indicate different behaviors for Peléan and Plinian samples. While fragmentation efficiency is positively correlated with applied potential energy for Peléan samples, this relationship is not observed for the Plinian samples, possibly due to syn-fragmentation gas escape above a certain porosity. The rapid decompression experiments were designed to minimize secondary fragmentation by shear along the walls or impact while preserving the entirety of produced materials. Thus, our experimental grainsize data are physically linked to sample textures and overpressure. By comparison with natural pyroclastic products—commonly incompletely preserved—we can approach quantitatively constraining the energetic conditions underlying individual eruptions.