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Abstract: Landslides associated with flank collapse are volumetrically the most significant sediment transport process around volcanic islands. Around Montserrat, in the Lesser Antilles, individual landslide deposits have volumes (1 to 20km3) that are up to two orders of magnitude larger than recent volcanic dome collapses (up to 0.2km3). The largest landslide deposits were emplaced in at least two stages, initiated by the emplacement of volcanic debris avalanches which then triggered larger-scale failure of seafloor sediment, with deformation propagating progressively downslope for up to 30km on gradients of <1°. An unusually detailed seismic, side-scan sonar and bathymetric dataset shows that the largest landslide off Montserrat (forming Deposit 8) incorporated ~70m of in-situ sediment stratigraphy, and comprises ~80% seafloor sediment by volume. Well-preserved internal bedding and a lack of shortening at the frontally-confined toe of the landslide, shows that sediment failure involved only limited downslope transport. We discuss a range of models for progressively-driven failure of in-situ bedded seafloor sediment. For Deposit 8 and for comparable deposits elsewhere in the Lesser Antilles, we suggest that failure was driven by an over-running surface load that generated excess pore pressures in a weak and deforming undrained package of underlying stratigraphy. A propagating basal shear rupture may have also enhanced the downslope extent of sediment failure. Extensive seafloor-sediment failure may commonly follow debris avalanche emplacement around volcanic islands if the avalanche is emplaced onto a fine-grained parallel-bedded substrate. The timing of landslides off Montserrat is clustered, and associated with the deposition of thick submarine pyroclastic fans. These episodes of enhanced marine volcaniclastic input are separated by relatively quiescent periods of several 100ka, and correspond to periods of volcanic edifice maturity when destructive processes dominate over constructive processes. [Copyright &y& Elsevier]

Abstract: A major uncertainty regarding the environmental impacts of volcanic Hg is the extent to which Hg is deposited locally or transported globally. An important control on dispersion and deposition is the oxidation state of Hg compounds: Hg(0) is an inert, insoluble gas, while Hg(II) occurs as reactive gases or in particles, which deposit rapidly and proximally, near the volcanic vent. Using a new high temperature thermodynamic model, we show that although Hg in Etna''s magmatic gases is almost entirely Hg(0) (i.e., gaseous elemental mercury), significant quantities of Hg(II) are likely formed at Etna''s vents as gaseous HgCl2, when magmatic gases are cooled and oxidised by atmospheric gases. These results contrast with an earlier model study and allow us to explain recent measurements of Hg speciation at the crater rim of Etna without invoking rapid (<1min) low temperature oxidation processes. We further model Hg speciation for a series of additional magmatic gas compositions. Compared to Etna, Hg(II) production (i.e., Hg(II)/Hgtot) is enhanced in more HCl-rich magmatic gases, but is independent of the Hg, HBr and HI content of the magmatic gases. Hg(II) production is not strongly influenced by the initial oxidation state of magmatic gases above NNO, although production is hindered in more reduced magmatic gases. The model and results are widely applicable to other open-vent volcanoes and may be used to improve the accuracy of chemical kinetic models for low temperature Hg speciation in volcanic plumes. [Copyright &y& Elsevier]

The Dec. 22, 2018 lateral collapse of the Anak Krakatau (AK) volcano in the Sunda Straits of Indonesia discharged volcaniclastic material into the 250 m deep caldera southwest of the volcano and generated a large tsunami, causing runups of up to 85 m in the near-field, and 13.5 m in the far-field, on the nearby coasts of Sumatra and Java. The tsunami caused 437 fatalities, the greatest number from a volcanically-induced tsunami since the catastrophic explosive caldera-forming eruption of Krakatau in 1883 and the sector collapse of Ritter Island in 1888. For the first time in over 100 years, the 2018 AK event provides an opportunity to study a major volcanically-generated tsunami that caused widespread loss of life and significant damage. Here, we present numerical simulations of the collapse and tsunami generation, propagation, and coastal impact, with state-of the-art numerical models, using both a new parametrization of the collapse and a near-field bathymetric dataset based on our 2019 field surveys and satellite images. These subaerial and submarine data sets are used to constrain the geometry and magnitude of the landslide mechanism, which show that the primary landslide scar bisected the AK edifice, cutting behind the central vent and removing 50% of its subaerial volume. The primary landslide volume is estimated to range from 0.175–0.313 km3, based on uncertainties in the shape of the submerged part of the failure plane. This is supported by an independent estimate of the primary landslide deposit volume of 0.214 ± 0.036 km3. Given uncertainties in the failure volume, we define a range of potential failure surfaces that span these values in 4 collapse scenarios of volume ranging from 0.175 to 0.313 km3. These AK collapses are modeled, assuming either a granular or viscous fluid rheology, together with their corresponding tsunami generation and propagation. Observations of a single tsunami, with no subsequent waves, are consistent with our interpretation of landslide failure in a rapid, single phase of movement rather than a more piecemeal process, generating a tsunami which reached nearby coastlines within ~30 min. For both modeled rheologies, the 0.224 km3 collapse (second and preferred scenario) most successfully reproduces the near- and far-field tsunami flow depth and runup observed in all post-event field survey results, tide gauge records, and eyewitness reports to date, suggesting our estimated landslide volume range is appropriate. This event highlights the significant hazard posed by relatively small-scale lateral volcanic collapses, which can occur en-masse , without any precursory signals, and are an efficient and unpredictable tsunami source. Our successful simulations demonstrate that current numerical models can accurately forecast tsunami hazards from these events. In cases such as Anak Krakatau's, the absence of precursory warning signals, together with the short travel time following tsunami initiation present a major challenge for mitigating tsunami coastal impact, stressing the need to develop and install early warning systems for such events. • New geometric models of Anak Krakatau's (AK) 2018 collapse based on a combination of submarine and subaerial data. • New high-resolution modeling of both AK's collapse and tsunami generation and propagation to the far-field, for 2 slide rheologies (dense fluid and granular medium) and 4 different volumes. • Near-field modeling based on new 3D NHWAVE model including vertical accelerations in slide layer, important on AK's steep slopes. • Comparison of modeled near-field tsunami runups on surrounding AK's islands to new drone surveys and other published field survey data. • New comparison of modeled far-field tsunami impact (runups/inundation/tide gauge) on Java and Sumatra's shores to all published field survey data to date. • New results allow constraining the style and parameters of AK's collapse and testing current volcanic landslide-tsunami models, which can be used to predict the behavior of similar events at other volcanic islands [ABSTRACT FROM AUTHOR]