Volcanic lateral collapses represent major hazards through associated eruptive activity, landslide inundation and, in island or coastal settings, the generation of tsunamis. Forecasting the timing, precursory indicators, and magnitude of lateral collapses still remains a significant gap in volcanic risk management, a challenge exemplified by the 2018 lateral collapse of Anak Krakatau, Indonesia. The volcano’s southwestern flank collapsed without recognised warning, resulting in a devastating tsunami which inundated > 300 km of regional shorelines. Since then, the edifice has rapidly regrown, prompting a necessary assessment of its future stability. Here we analyse Anak Krakatau’s uniquely detailed archival growth records, in combination with satellite and drone datasets, to reconstruct historical edifice development from 1919 to 2023, using this to then project and contextualise its future growth trajectories and assess potential instability conditions. Our time series of 3D models reveals that post-collapse regrowth has mimicked historical development trends, but on a considerably accelerated timescale. If future growth follows the long-term pre-collapse (1960–2018) average, then a subaerial edifice morphology equivalent to 2018, but with larger overall dimensions, would be met by around the year 2100. If future development maintains the much higher 2019–2023 growth trends, then a morphology comparable to 2018 could instead be reached by the 2030s. We consider the continuation of such high growth rates unlikely, but this highlights the importance of monitoring edifice growth and flank instability signals over the coming decades, following the methodology provided here. Additionally, the submarine SW flank now has a considerably reduced gradient relative to 2018, which is likely to promote stability further into the future. To comprehensively assess future stability, the role of alteration, hydrothermal activity and the structure of the submarine flank should also be evaluated.

Volcanic edifices are known to be unstable and their collapses have been observed several times in history. A collapse of island or coastal volcanoes is especially hazardous for the population living on regional shorelines due to the generation of destructive tsunami waves. A recent example of this is the tsunami‐generating collapse of Anak Krakatau's (Sunda Strait, Indonesia) southwestern flank in December 2018. At Anak Krakatau, deformation through sliding of the SW‐flank, preceding the collapse, is evident from published InSAR data. However, little is known about the lithology and internal structure of the pre‐collapse edifice. Here, we combine direct shear experiments and finite‐element models to understand edifice deformation and gravitational edifice instability at Anak Krakatau. Our model results suggest that Anak Krakatau deformed through a proto shear plane within the edifice prior to the collapse. To cause a gravitationally unstable edifice, our models indicate that the proto shear plane must dip >12°, or the rock mass or proto shear plane must be weakened to μ < 0.3. These threshold values however seem unlikely in nature and thus, an external factor is needed to trigger catastrophic collapse. Our direct shear experiments indicate that samples of powdered gouge from ash and scoria are strong, exhibiting μ > 0.6. Furthermore, the results of the direct shear experiments support catastrophic collapse along a proto shear plane due to velocity‐weakening frictional behavior. Hence, assessment of flank collapse potential requires knowledge of material properties and the presence, strength, and geometry of weak planes within the edifice.

Within the insurance and reinsurance sectors, volcanoes and their secondary impacts are often an overlooked risk due to the long return periods associated with large explosive eruptions, and relatively low economic and insured losses from eruption events compared to other natural hazards such as large magnitude earthquakes. However, with continued population growth, globalisation and climate change increasing exposure to volcanoes, and more sophisticated monitoring and modelling methods revealing the true extent of both primary volcanic hazards and secondary effects, this is a peril that should be more thoroughly considered. This study reviews exposure to active Holocene volcanoes, compares economic and insured losses of significant eruptions, and explores how analogues of historic events could affect the modern world. We show that the past 40 years have mostly seen eruptions of Volcanic Explosivity Index (VEI) 3–4; significant but not “super-catastrophes.” Should a larger VEI 6+ event occur near a densely populated area or in a country with high insurance penetration, losses could be far higher. Countries with the highest exposed populations to volcanoes include Indonesia, the Philippines and Guatemala. However, this differs from countries at greatest risk of insurable losses, such as China, Japan and the US, and lower insurance penetration in more exposed countries identifies a significant protection gap. Eruptions in smaller nations show particular financial vulnerability, with recent eruptions in Tonga and La Palma leading to large losses in proportion to their GDP; as much as 1/3 of their economy (30-37%). The economic losses of accumulated volcanic activity have totalled $152.6 billion over 20 years (an average of $7.6 billion a year). Recent estimates that a large, long return period, global climate-affecting eruption might lead to losses in the multi-trillions, and would impact pre-existing reinsurance markets in a similar manner to tropical cyclones, highlights the need for greater attention, preparedness and resilience measures. With this in mind, we discuss current research and industry initiatives to collate volcanic data, to aid disaster risk reduction strategies. It is hoped that by working with such organisations, and helping facilitate global standardisation of data and risk communication, the insurance industry can be better prepared for future volcanic eruption scenarios.

AbstractCompositions of plagioclase‐melt pairs are commonly used to constrain temperatures (T), dissolved water contents (H2O) and pressures (P) of pre‐eruptive magma storage and transport. However, previous plagioclase‐based thermometers, hygrometers, and barometers can have significant errors, leading to imprecise reconstructions of conditions during plagioclase growth. Here, we explore whether we can refine existing plagioclase‐based hygrothermobarometers with either plagioclase‐melt or melt‐only chemistry (±T/H2O), calibrated using random forest machine learning on experimental petrology data (n = 1,152). We find that both the plagioclase‐melt and melt‐only models return similar cross‐validation root‐mean‐square errors (RMSEs), as the melt holds most of the P‐T‐H2O information rather than the plagioclase. T/H2O‐dependent melt models have test set RMSEs of 25°C, 0.70 wt.% and 76 MPa for temperature, H2O content and pressure, respectively, while T/H2O‐independent models have RMSEs of 38°C, 0.97 wt.% and 91 MPa. The melt thermometer and hygrometer are applicable to a wide range of plagioclase‐bearing melts at temperatures between 664 and 1355°C, and with H2O concentrations up to 11.2 wt.%, while the melt barometer is suitable for pressures of ≤500 MPa. An updated plagioclase‐melt equilibrium model has also been calibrated, allowing the equilibrium anorthite content to be predicted with an error of 5.8 mol%. The new P‐T‐H2O‐An models were applied to matrix glasses and melt inclusions from the 1980 Mount St Helens (USA) and 2014–2015 Holuhraun (Iceland) eruptions, corroborating previous independent estimates and observations. Models are available at https://github.com/kyra‐cutler/Plag‐saturated‐melt‐P‐T‐H2O‐An, enabling assessment of plagioclase‐melt equilibrium and characterization of last‐equilibrated P‐T‐H2O conditions of plagioclase‐saturated magmas.

Island nations may have potential long-term survival value for humanity in global catastrophes such as sun-blocking catastrophes from nuclear winter and large magnitude volcanic eruptions. One way to explore this issue further is to understand the impact on islands after the largest historically observed volcanic eruption: that of Mt Tambora in 1815. For each of the 31 large, populated islands selected, we conducted literature searches for relevant historical and palaeoclimate studies. We also analysed results from a reconstruction (EKF400v2), which uses atmospheric-only general circulation model simulations with assimilated observational and proxy data. From the literature review, there was widespread evidence for weather/climate anomalies in 1815–1817 for these islands (29/29 for those with data). But missing data was an issue for other dimensions such as impaired food production (seen in 8 islands out of only 12 with data). Based on the EKF400v2 reconstruction for temperature anomalies (compared to the relatively “non-volcanic” reference period of 1779 to 1808), the islands had lower temperature anomalies in the 1815–1818 period than latitudinally equivalent continental sites (at 100 km and 1000 km inland). This was statistically significant for the great majority of the comparisons for group analyses by hemisphere, oceans, and temperate/tropical zone. When considering just the islands, all but four showed statistically anomalous temperature reductions in the 1816–1817 period (for most p < 0.00001). In the peak impact year of 1816, the lowest anomalies were seen for islands in the Southern Hemisphere (p < 0.0001), the Indian Ocean (p < 0.0001), and in the tropics and subtropics of the Southern Hemisphere (p = 0.0057). In conclusion, the findings of both the literature review and reconstruction simulations suggest climatic impacts of the Tambora eruption for nearly all these 31 large islands, albeit less than for continental sites. Islands with the smallest temperature anomalies were in the Southern Hemisphere, in particular the Indian Ocean and the tropics and subtropics of the Southern Hemisphere.

Volcano geoengineering is the practice of altering the state of volcanic systems and/or volcanic eruptions to exploit them or mitigate their risk. Although many in the field insist there is little that can be done to mitigate the hazard, past examples of both intentional and inadvertent volcano interventions demonstrate that it is technically feasible to reach volcano plumbing systems or alter atmospheric processes following eruptions. Furthermore, we suggest that economical, political, and environmental pressures may make such interventions more common in the future. If volcano geoengineering ever becomes a discipline, it will need to overcome many safety and ethical concerns, including dealing with uncertainty, deciding on philosophical approaches such as a consequentialism or precautionary principle, justice and inequality, military uses, cultural values, and communication. We highlight that while volcano geoengineering has significant potential benefits, the risks and uncertainties are too great to justify its use in the short term. Despite this, because of the potential large benefits to society, we believe there is a strong ethical case to support research into the efficacy and safety of volcano geoengineering for its potential future use. We propose that rigorous governance and regulation of any volcano geoengineering is required to protect against potential risks, to enable potentially valuable and publicly available research (e.g., quantification of efficacy and safety), to ensure that any future policy must be co-created through community engagement, and that volcano geoengineering should only be considered as part of larger mitigation practices.

More must be done to forecast and try to manage globally disruptive volcanic eruptions. The risks are greater than people think. More must be done to forecast and try to manage globally disruptive volcanic eruptions. The risks are greater than people think.

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 after the collapse and rapid edifice regrowth. Not only does rapid regrowth potentially obscure evidence of past collapses, but it also emphasises the finely balanced relationship between edifice loading and crustal magma storage.

Abstract The conditions under which halogens partition in favor of an exsolved fluid relative to the coexisting melt are key for understanding many magmatic processes, including volcanic degassing, evolution of crustal melt bodies, and ore formation. We report new F, Cl, and Br fluid/melt partition coefficients for intermediate to silicic melts, for which F and Br data are particularly lacking; and for varying CO2- H2O contents to assess the efects of changing fluid composition (XH2O) on Br fluid/melt partitioning for the first time. The experiments were conducted at pressures 50–120 MPa, temperatures 800–1100 °C, and volatile compositions [molar XH2O = H2O/(H2O +CO2)] of 0.55 to 1, with redox conditions around the Nickel-Nickel Oxygen bufer (ƒO2 ≈ NNO). Experiments were not doped with Cl, Br, or F and were conducted on natural crystal-bearing volcanic products at conditions close to their respective pre-eruptive state. The experiments therefore provide realistic constraints on halogen partitioning at naturally occurring, brine-undersaturated conditions. Measurements of Br, Cl, and F were made by Secondary Ion Mass Spectrometry (SIMS) on 13 experimental glass products spanning andesite to rhyolitic compositions, together with their natural starting materials from Kelud volcano, Indonesia, and Quizapu volcano, Chile. Fluid compositions were constrained by mass balance. Average bulk halogen fluid/melt partition coefficients and standard deviations are: DCl fluid/melt = 3.4 (±3.7 1 s.d.), DFfluid/melt = 1.7 (±1.7), and DBrfluid/melt = 7.1 (±6.4) for the Kelud starting material (bulk basaltic andesite), and DCl fluid/melt = 11.1 (±3.5), DFfluid/melt = 0.8 (±0.8), and DBr fluid/melt = 31.3 (±20.9) for Quizapu starting material (bulk dacite). The large range in average partition coeficients is a product of changing XH2O, pressure and temperature. In agreement with studies on synthetic melts, our data show an exponential increase of halogen Dfluid/melt with increasing ionic radius, with partitioning behavior controlled by melt composition according to the nature of the complexes forming in the melt (e.g., SiF4, NaCl, KBr). The fundamental chemistry of the diferent halogens (differing ionic size and electronegativities) controls the way in which partitioning responds to changes in melt composition and other variables. Experimental results confirm that more Cl partitions into the fluid at higher bulk Cl contents, higher melt Na, higher fluid XH2O ratios, and lower temperatures. Bromine shows similar behavior, though it seems to be more sensitive to temperature and less sensitive to Na content and XH2O. In contrast, F partitioning into the fluid increases as the melt silica content decreases (from 72 to 56 wt% SiO2), which we attribute to the lower abundance of Si available to form F complexes in the melt. These new data provide more insights into the conditions and processes that control halogen degassing from magmas and may help to inform the collection and interpretation of melt inclusions and volcano gas data.

The caldera-forming Samalas eruption of the Samalas-Rinjani volcanic complex on Lombok, Indonesia, in 1257 CE ranks as one of the most explosive and sulphur-rich eruptions of the past thousand years. Along with other significant events (e.g. Tambora 1815, El Chichon 1982), Samalas 1257 forms a class of powerful eruptions of magmas of intermediate alkaline composition, which remain relatively understudied. This study aims to better understand the pre-eruptive magma reservoir conditions and volatile storage capacities of intermediate alkaline systems, using the 1257 Samalas eruption as a case study. Systematic partial-equilibrium experiments were run to constrain potential magma reservoir conditions (25-200 MPa and 850-1000 degrees C) by examining changes in the chemistry and textures of experimental glass and mineral products. Natural trachydacite pumice from the 1257 Samalas eruption was used as the experimental charge for a series of water-saturated experiments run in cold-seal pressure vessels, buffered at an oxygen fugacity integral O-2 of NNO/NNO + 1 log units. Assessing the match between natural and experimental glass compositions and mineral phases and textures reveals that pre-eruptive magma reservoir conditions were between 875 degrees C and 930 degrees C and ca. 100-150 MPa (4.5 +/- 1km depth). Breakdown of experimental amphibole at pressures below 75 MPa, and plagioclase instability at 950 degrees C/100 MPa, provide the strongest constraints on phase stabilities that are consistent with the magma storage region. In the observed temperature and pressure range, the natural compositions of plagioclase, amphibole and orthopyroxene are replicated in experimentally precipitated rims. Clinopyroxene and high anorthite plagioclase (An(>64)) in the natural mineral assemblage could not be replicated in the experiments, implying that these compositions most likely did not grow in the shallow pre-eruptive reservoir of Mt Samalas. The pre-eruptive storage depths of trachydacite magma are significantly shallower than the postulated magma storage at the currently active basaltic-andesite magmatic system at Rinjani, and thus monitoring magma storage depth may be a useful indicator of changing magma composition in the future. Our findings show that highly explosive VEI 7 eruptions of intermediate alkaline magma can be fed from a relatively limited range of storage pressures (100-150 MPa), suggesting accumulation of magma in one place, rather than the rapid extraction of magma from a vertically extensive, transcrustal magma system.

Explosive, caldera-forming eruptions are exceptional and hazardous volcanic phenomena. The 1883 eruption of Krakatau is the largest such event for which there are detailed contemporary written accounts, allowing information on the eruptive progression to be integrated with the stratigraphy and geochemistry of its products. Freshly exposed sequences of the 1883 eruptive deposits of Krakatau, stripped of vegetation by a tsunami generated by the flank collapse of Anak Krakatau in 2018, shed new light on the eruptive sequence. Matrix glass from the base of the stratigraphy is chemically distinct and more evolved than the overlying sequence indicating the presence of a shallow, silicic, melt-rich region that was evacuated during the early eruptive activity from May 1883 onwards. Disruption of the shallow, silicic magma may have led to the coalescence and mixing of chemically similar melts representative of a range of magmatic conditions, as evidenced by complex and varied plagioclase phenocryst zoning profiles. This mixing, over a period of two to three months, culminated in the onset of the climactic phase of the eruption on 26th August 1883. Pyroclastic density currents (PDCs) emplaced during this phase of the eruption show a change in transport direction from north east to south west, coinciding with the deposition of a lithic lag breccia unit. This may be attributed to partial collapse of an elevated portion of the island, resulting in the removal of a topographic barrier. Edifice destruction potentially further reduced the overburden on the underlying magmatic system, leading to the most explosive and energetic phase of the eruption in the morning of 27th August 1883. This phase of the eruption culminated in a final period of caldera collapse, which is recorded in the stratigraphy as a second lithic lag breccia. The massive PDC deposits emplaced during this final phase contain glassy blocks up to 8 m in size, observed for the first time in 2019, which are chemically similar to the pyroclastic sequence. These blocks are interpreted as representing stagnant, shallow portions of the magma reservoir disrupted during the final stages of caldera formation. This study provides new evidence for the role that precursory eruptions and amalgamation of shallow crustal magma bodies potentially play in the months leading up to caldera-forming eruptions. (c) 2021 Elsevier B.V. All rights reserved.

The flank failure and collapse of Anak Krakatau on December 22nd, 2018 triggered a destructive tsunami. Whether the prior activity of the volcano led to this collapse, or it was triggered by another means, remains a challenge to understand. This study seeks to investigate the recent volcano submarine mass-landslide deposit and emplacement processes, including the seafloor morphology of the flank collapse and the landslide deposit extent. Bathymetry and sparker seismic data were used during this study. Bathymetry data collected in August, 2019 shows the run-out area and the seafloor landslide deposit morphology. Bathymetry data acquired in May, 2017, is used as the base limit of the collapse to estimate the volume of the flank collapse. Comparisons between seismic data acquired in 2017 and 2019 provide an insight into the landslide emplacement processes, the deposit sequence, and structure below the seafloor. From these results we highlight two areas of the submarine-mass landslide deposit, one proximal to Anak Krakatau island (∼1.6 km) and one distal (∼1.4 km). The resulting analysis suggests that the submarine-mass landslide deposit might be produced by a frontally compressional, faulted, landslide, triggered by the critical stability slope, and due to the recent volcanic activity. Blocky seabed features clearly lie to the southwest of Anak Krakatau, and may represent the collapse blocks of the landslide. The seismic analysis of the data acquired in August, 2019 reveals that the blocky facies extends to ∼1.62 km in the width around Anak Krakatau, and the block thicknesses vary up to 70.4 m. The marine data provides a new insight into the landslide run out and extent, together with the landslide deposit morphology and structure that are not available from satellite imagery or subaerial surveys. We conclude that the landslide run out area southwest of the recent collapse, is ∼7.02 ± 0.21 km2.

As demonstrated at Anak Krakatau on December 22 nd , 2018, tsunamis generated by volcanic flank collapse are incompletely understood and can be devastating. Here, we present the first high-resolution characterisation of both subaerial and submarine components of the collapse. Combined Synthetic Aperture Radar data and aerial photographs reveal an extensive subaerial failure that bounds pre-event deformation and volcanic products. To the southwest of the volcano, bathymetric and seismic reflection data reveal a blocky landslide deposit (0.214 ± 0.036 km 3 ) emplaced over 1.5 km into the adjacent basin. Our findings are consistent with en-masse lateral collapse with a volume ≥0.175 km 3 , resolving several ambiguities in previous reconstructions. Post-collapse eruptions produced an additional ~0.3 km 3 of tephra, burying the scar and landslide deposit. The event provides a model for lateral collapse scenarios at other arc-volcanic islands showing that rapid island growth can lead to large-scale failure and that even faster rebuilding can obscure pre-existing collapse.

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 theart 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 km(3), 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 km(3). 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 km(3). 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 similar to 30 min. For both modeled rheologies, the 0.224 km(3) 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.

IODP Expedition 340 successfully drilled, for the first time, large and likely tsunamigenic volcanic island arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition, origin, and mode of transport of those deposits. Sites in the medial to distal parts of the landslide deposits offshore Montserrat and Martinique recovered seafloor sediment, comprising turbidites and hemipelagic deposits, and lacked the coarse and chaotic subaerial volcanic debris avalanche material. This supports the concepts that (i) the volcanic debris avalanche component of these landslides is restricted to proximal areas and tends to stop at the slope break and (ii) emplacement of volcanic debris avalanches in marine settings can trigger widespread and voluminous failures of preexisting low-gradient seafloor sediment. The most likely mechanism for generating these large-scale seafloor sediment failures appears to be the propagation of a decollement, from proximal areas that are loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation by volcanic island landslides. Volcanic island landslides composed of mainly seafloor sediment may form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water.

We use satellite imagery to investigate the shoreline changes associated with volcanic activity in 2018–2019 at Anak Krakatau, Indonesia, spanning a major lateral collapse and period of regrowth through explosive activity. The shoreline changes have been analyzed and validated through the adaptation of an existing methodology based on Sentinel-2 multispectral imagery and developed on Google Earth Engine. This work tests the results of this method in a highly dynamic volcanic environment and validates them with manually digitized shorelines. The analysis shows that the size of the Anak Krakatau Island increased from 2.84 km2 to 3.19 km2 during 15 May 2018–1 November 2019 despite the loss of area in the 22 December 2018 lateral collapse. The lateral collapse reduced the island area to ~1.5 km2 but this was followed by a rapid increase in area in the first two months of 2019, reaching up to 3.27 km2. This was followed by a period of little change as volcanic activity declined and then by a net decrease from May 2019 to 1 November 2019 that resulted from erosion on the SW side of the island. This history of post-collapse eruptive regrowth and coastal erosion derived from the shoreline changes illuminates the potential for satellite-based automated shoreline mapping to provide databases for monitoring remote island volcanoes.

Explosive eruptions that occur with little or no precursory unrest (less than a month) pose the greatest hazards from volcanoes to nearby populations. Here we focus on the preeruptive conditions for these explosive events, their triggers and how these eruptions evolve. We concentrate on Kelud volcano, where we have conducted a set of petrological experiments to understand preeruptive storage conditions for several recent eruptions. For the 2014 explosive eruption, we combine this with an analysis of interferometric synthetic aperture radar measured deformation. Our data suggest that both explosive and effusive eruptions at Kelud are sourced from a magma storage system at 2–4 km. However, explosive eruptions are fed by magma stored under relatively cool (~1000 °C) and water‐saturated conditions, whereas effusive eruptions are fed by slightly hotter (~1050 °C), water‐undersaturated magmas. We propose that the initial phase of the 2014 eruption was triggered by volatile overpressure, which then fostered top‐down decompression tapping discrete magma bodies. By compiling a global data set of monitoring signatures of explosive eruptions, we show that the onset of unrest rarely points to the shallow ascent of magma to the surface, as ascent mostly occurs in a matter of hours or minutes. We relate the timescale of preeruptive unrest to eruption triggering mechanisms, with yearly/decadal periods of unrest relating to magma injection events (which may or may not precede a magmatic eruption), whereas internal triggering (e.g., second boiling) of an already present, cooling magma body can lead to explosive eruptions with little warning.