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Theoretical source models of underwater explosions are often applied in studying tsunami hazards associated with subaqueous volcanism; however, their use in numerical codes based on the shallow water equations can neglect the significant dispersion of the generated wavefield. A non-hydrostatic multilayer method is validated against a laboratory-scale experiment of wave generation from instantaneous disturbances and at field-scale subaqueous explosions at Mono Lake, California, utilising the relevant theoretical models. The numerical method accurately reproduces the range of observed wave characteristics for positive disturbances and suggests a relationship of extended initial troughs for negative disturbances at low-dispersivity and high-non-linearity parameters. Satisfactory amplitudes and phase velocities within the initial wave group are found using underwater explosion models at Mono Lake. The scheme is then applied to modelling tsunamis generated by volcanic explosions at Lake Taupō, New Zealand, for a magnitude representing an ejecta volume of 0.1 km 3. Waves reach all shores within 15 min with maximum incident crest amplitudes around 0.2 m at shores near the source. This work shows that the multilayer scheme used is computationally efficient and able to capture a wide range of wave characteristics, including dispersive effects, which is necessary when investigating subaqueous explosions. This research therefore provides the foundation for future studies involving a rigorous probabilistic hazard assessment to quantify the risks and relative significance of this tsunami source mechanism. [ABSTRACT FROM AUTHOR]

Theoretical source models of underwater explosions are often applied in studying tsunami hazards associated with submarine volcanism; however, their use in numerical codes based on the shallow water equations can neglect the significant dispersion of the generated wavefield. A non-hydrostatic multilayer method is validated against a laboratory-scale experiment of wave generation from instantaneous disturbances and at field-scale submarine explosions at Mono Lake, California, utilising the relevant theoretical models. The numerical method accurately reproduces the range of observed wave characteristics for positive disturbances and suggests a previously unreported relationship of extended initial troughs for negative disturbances at low dispersivity and high nonlinearity parameters. Satisfactory amplitudes and phase velocities within the initial wave group are found using underwater explosion models at Mono Lake. The scheme is then applied to modelling tsunamis generated by volcanic explosions at Lake Taupō, New Zealand, for a magnitude range representing ejecta volumes between 0.04–0.4 km3. Waves reach all shores within 15 minutes with maximum incident crest amplitudes around 4 m at shores near the source. This work shows that the multilayer scheme used is computationally efficient and able to capture a wide range of wave characteristics, including dispersive effects, which is necessary when investigating submarine explosions. This research therefore provides the foundation for future studies involving a rigorous probabilistic hazard assessment to quantify the risks and relative significance of this tsunami source mechanism. [ABSTRACT FROM AUTHOR]

Subaqueous to emergent explosive basaltic eruptions, named Surtseyan after the Surtsey eruption in 1963, occupy a peculiar position in the classification of explosive eruptions. The presence of external water clearly affects the eruption dynamics, from magma fragmentation to tephra dispersal. For the present work we selected Black Point volcano, California, USA, which erupted in the Pleistocene Lake Russell about 13,000 years ago, from a depth of 105 m. This volcano provides a rare opportunity to study the entire stratigraphy of a Surtseyan eruption from its edifice deposits (mound and tuff rings) and tephra dispersed at proximal and medial sites in the former host Lake Russell, now vanished. To characterize the Black Point eruption, we adopted a series of techniques never used before in combination for such volcanic activity. These techniques, based on targeted sampling and 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 vesicle characterization of the lapilli-size fraction, and particle shape analysis for ash grains through the recently introduced freeware PARTISAN. The aim of this combined approach is to determine how the eruption evolved through its course, and whether any geochemical or petrological changes took place and affected that evolution. Field observations are consistent with the ash-sheet deposits forming from eruption-fed density currents, fallout through the water column from a subaqueous plume, or fallout through the water column from a subaerial plume. Geochemical correlation links the mound deposits with a stratigraphic sequence at about 2 km from the edifice, with each showing non-linear but consistent geochemical variations during the subaqueous stage of the eruption. There is no systematic change in particle vesicularity upsection through the stratigraphy, but there are small differences between two stratigraphically equivalent deposits, which indicate some textural heterogeneity of the magma. Particle-shape analysis of ash grains from Black Point indicates a wide range of particle convexity and solidity, but these characteristics have not proved useful for identifying the fragmentation style. [ABSTRACT FROM AUTHOR]

Havre caldera volcano's deep (~1 km) submarine eruption in July 2012 produced an ~80 km long vapour plume and ~400 km2 pumice raft at the sea surface. The vapour plume indicates focused delivery of heat to the surface and indicates that the eruption formed some kind of narrow thermal plume. In 2015 during an NSF-funded research cruise we observed and sampled the eruption's deposits in situ with the remote operated vehicle Jason. High-resolution bathymetry was obtained with the autonomous underwater vehicle Sentry, providing a LiDAR-quality map with 1-m resolution of the entire caldera including the multiple vent sites of the 2012 eruption. A surprising product of the eruption is extensive fine and very fine ash, forming part of a multi-layered deposit with local contributions from effusive sources. The main ash layers show no clear thinning or fining trends across the whole of the caldera floor and shoulders, implying wide dispersal. Since significant fractions of this ash show a texture different to the pumice it is unlikely that these grains have been produced by abrasion of the raft pumice. Analysis of the ash will help us determine the driving mechanisms for ash production, which have implications for the eruption dynamics, and for rates and mechanisms of heat transfer to the ocean. There are specific, identifiable energy costs to the process of fragmentation, but energy expended in fragmentation greatly enhances rates of heat transfer by forming additional surface area. The effect of the latter is well known for subaerial eruptions, in which ash production strongly controls generation of buoyant plumes. For Havre's deep-sea eruption, we have conducted heat-transfer and fragmentation experiments by remelting products of the 2012 eruption. Both dense rock from lava domes, and seafloor-collected pumice, were remelted. Physical data from experiments has been combined with 2D and 3D shape analyses of both lab-generated volcanic particles and natural volcanic ash to reveal the dominant ash generation mechanisms involved in the 2012 Havre eruption. Experimental setups and applied analysing techniques will be presented. Preliminary results indicate that magma-water interaction, and specifically fuel-coolant interaction, probably induced, was important in generating most of the ash. This result rests primarily on characteristics of fractal dimension of natural and experimental particles, as assessed using high-resolution (sub-micron) micro-CT imaging and statistical T-tests of significant numbers of analysed particles. The result is interesting in the context of other analyses of the Havre 2012 products, which did not address ash formation, and which concluded that it was a non-explosive high-rate effusive eruption. [ABSTRACT FROM AUTHOR]