LAUSR

Abstract Surtseyan eruptions have long been studied from their edifice deposits, emplaced subaqueously or subaerially. Here we characterize a Surtseyan eruption from both edifice and ash-sheet deposits using a combination of techniques. The volcano studied is Black Point, which erupted into Lake Russell about 13,000 years ago from a water depth of 105 m, producing a total of ~0.8 km3 of tephra. The techniques, applied to samples acquired by targeted sampling based on geological observations in the field, include geochemical characterization of the eruption products, grain-size analysis of the ash-size fraction at proximal and medial sites, three-dimensional characterization of vesicles in lapilli, and particle-shape analysis for ash grains using the recently introduced freeware PARTISAN©. The aim of this combined approach is to determine how the eruption evolved through its course, from before magma fragmentation to the mechanisms of formation, transport and deposition of the tephra. Field observations are consistent with the ash-sheet deposits forming from eruption-fed density currents, and/or fallout through the water column from a subaerial plume +/− umbrella cloud. Geochemical correlation links the mound deposits, as well as the tuff rings, with a stratigraphic sequence at about 2 km west from the edifice. Both show non-linear but broadly correlative geochemical variations in deposits from the subaqueous stage of the eruption through the subaerial one. Pyroclast vesicle textures indicate that volatile expansion played only a minor role in magma fragmentation, and that there was a high level of bubble coalescence and hence outgassing during the eruption. We infer that fragmentation was mostly phreatomagmatic, resulting from explosive interactions between rising magma and water-saturated sediments. Two-dimensional particle-shape analysis of ash grains shows a large variability in the shape parameters of convexity and solidity; this tells us little about fragmentation style but serves as a proxy for magma vesicularity. The most interesting aspect of this eruption is that none of the shape parameters, or qualitative observations, show any sort of systematic change during the course of the eruption, despite the eruption having begun in 100 m of water and ended up subaerial.