Ash generation and transport during explosive submarine eruptions

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.