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The lateral collapse of Anak Krakatau volcano, Indonesia, in December 2018 highlighted the potentially devastating impacts of volcanic edifice instability. Nonetheless, the trigger for the Anak Krakatau collapse remains obscure. The volcano had been erupting for the previous six months, and although failure was followed by intense explosive activity, it is the period immediately prior to collapse that is potentially key in providing identifiable, pre-collapse warning signals. Here, we integrate physical, microtextural and geochemical characterisation of tephra deposits spanning the collapse period. We demonstrate that the first post-collapse eruptive phase (erupting juvenile clasts with a low microlite areal number density and relatively large microlites, reflecting a crystal-growth dominated regime) is best explained by instantaneous unloading of a relatively stagnant upper conduit. This was followed by the second post-collapse phase, on a timescale of hours, which tapped successively hotter and deeper magma batches, reflected in increasing plagioclase anorthite content and more mafic glass compositions, alongside higher calculated ascent velocities and decompression rates. The characteristics of the post-collapse products imply downward propagating destabilisation of the magma storage system as a response to collapse, rather than pre-collapse magma ascent triggering failure. Importantly, this suggests that the collapse was a consequence of longer-term processes linked to edifice growth and instability, and that no indicative changes in the magmatic system could have signalled the potential for incipient failure. Therefore, monitoring efforts may need to focus on integrating short- and long-term edifice growth and deformation patterns to identify increased susceptibility to lateral collapse. The post-collapse eruptive pattern also suggests a magma pressurisation regime that is highly sensitive to surface-driven perturbations, which led to elevated magma fluxes afte

On Dec. 22, 2018, at approximately 20:55–57 local time, Anak Krakatau volcano, located in the Sunda Straits of Indonesia, experienced a major lateral collapse during a period of eruptive activity that began in June. The collapse discharged volcaniclastic material into the 250 m deep caldera southwest of the volcano, which generated a tsunami with runups of up to 13 m on the adjacent coasts of Sumatra and Java. The tsunami caused at least 437 fatalities, the greatest number from a volcanically-induced tsunami since the catastrophic explosive eruption of Krakatau in 1883 and the sector collapse of Ritter Island in 1888. For the first time in over 100 years, the 2018 Anak Krakatau 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 tsunami, with state-of the-art numerical models, based on a combined landslide-source and bathymetric dataset. We constrain the geometry and magnitude of the landslide source through analyses of pre- and post-event satellite images and aerial photography, which demonstrate that the primary landslide scar bisected the Anak Krakatau volcano, cutting behind the central vent and removing 50% of its subaerial extent. Estimated submarine collapse geometries result in a primary landslide volume range of 0.22–0.30 km3, which is used to initialize a tsunami generation and propagation model with two different landslide rheologies (granular and fluid). 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 minutes. Both modelled rheologies successfully reproduce observed tsunami characteristics from post-event field survey results, tide gauge records, and eyewitness reports, suggesting our estimated landslide volume range

Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass-wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (?930 ka to ?900 ka, ?810 ka to ?760 ka, and ?190 ka to ?120 ka) that coincide with periods of increased volcano instability and mass-wasting. The youngest of these periods marks the peak in activity at the Soufrière Hills volcano. The largest flank collapse of this volcano (?130 ka) occurred towards the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufrière Hills edifice, and their timing also coincides with periods of rapid sea-level rise (>5 m/ka). Available age data from other island arc volcanoes suggests a general correlation between the timing of large landslides and periods of rapid sea-level rise, but this is not observed for volcanoes in intra-plate ocean settings. We thus infer that rapid sea-level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean-island settings.

The chalcophile elements are important both in terms of their economic value and as potential tracers of magmatic processes at convergent margins. However, because of analytical difficulties, comprehensive datasets of chalcophile element concentrations for volcanic rocks are rare. Here, we present analyses of a near complete suite of chalcophile elements (S, Cu, Ag, Se, As, Sb, Sn, W, Mo, Pb, Bi, Tl, Zn, Ga, Co) for volcanic rock samples collected from a typical continental arc stratovolcano in southern Chile (Antuco). Enrichment in Pb, Bi, W, Tl, Sb and As relative to Parental-MORB indicates that these elements have been mobilised from the subducting slab into the sub-arc mantle wedge, in contrast to Cu and Ag. Very low Se concentrations suggest that Se, like S, was lost during co-eruptive degassing of the Antuco magmas. Previous studies on oceanic arcs have demonstrated that as higher fO2 subduction-related magmas ascend through the overlying lithosphere, magnetite fractionation may trigger sulfide fractionation during crystallisation. If such a process is extensive and has a sharp onset, this would result in a plummet in the Cu, Se and Ag contents of the residual melt. At Antuco, although a decrease in the Fe2O3(T) and TiO2 concentrations at ∼55 wt.% SiO2 (∼3 wt.% MgO) indicates magnetite fractionation, this is not associated with a corresponding drop in Cu contents. Instead, we observe a general decrease in Cu and a decrease in Cu/Ag with increasing SiO2 and decreasing MgO. Furthermore, Cu/Ag in the most primitive Antuco rocks are lower than the global MORB array, indicating that the melts were sulfide saturated at an early stage in their crustal evolution. Through modelling fractional crystallisation, we show that only a minor volume (0.5–0.6 vol.%) of fractionating sulfide is needed to produce divergent trends in Cu and Ag, as observed in the Antuco samples. Our results show that sulfide

The chalcophile elements are important both in terms of their economic value and as potential tracers of magmatic processes at convergent margins. However, because of analytical difficulties, comprehensive datasets of chalcophile element concentrations for volcanic rocks are rare. Here, we present analyses of a near complete suite of chalcophile elements (S, Cu, Ag, Se, As, Sb, Sn, W, Mo, Pb, Bi, Tl, Zn, Ga, Co) for volcanic rock samples collected from a typical continental arc stratovolcano in southern Chile (Antuco). Enrichment in Pb, Bi, W, Tl, Sb and As relative to Parental-MORB indicates that these elements have been mobilised from the subducting slab into the sub-arc mantle wedge, in contrast to Cu and Ag. Very low Se concentrations suggest that Se, like S, was lost during co-eruptive degassing of the Antuco magmas. Previous studies on oceanic arcs have demonstrated that as higher fO2 subduction-related magmas ascend through the overlying lithosphere, magnetite fractionation may trigger sulfide fractionation during crystallisation. If such a process is extensive and has a sharp onset, this would result in a plummet in the Cu, Se and Ag contents of the residual melt. At Antuco, although a decrease in the Fe2O3(T) and TiO2 concentrations at ∼55 wt.% SiO2 (∼3 wt.% MgO) indicates magnetite fractionation, this is not associated with a corresponding drop in Cu contents. Instead, we observe a general decrease in Cu and a decrease in Cu/Ag with increasing SiO2 and decreasing MgO. Furthermore, Cu/Ag in the most primitive Antuco rocks are lower than the global MORB array, indicating that the melts were sulfide saturated at an early stage in their crustal evolution. Through modelling fractional crystallisation, we show that only a minor volume (0.5–0.6 vol.%) of fractionating sulfide is needed to produce divergent trends in Cu and Ag, as observed in the Antuco samples. Our results show that sulfide