Abstract This paper provides an overview of the eruption of La Soufrière Volcano on the island of St Vincent which occurred between 27 December 2020 and 22 April 2022. It sets the stage for the 17 papers included in this Special Publication that showcase the initial scientific findings arising from analysis of the crisis. Here we present a chronology of the eruption and discuss the key findings from these papers while underscoring the areas for which further research is needed. The detailed account of the eruption offers several lessons for volcanic crisis management and provides insights into the most effective communication process through this type of crisis. It highlights the need for and benefits of planning and preparedness activities prior to an eruption as well as of long-term engagement with disaster management officials and at-risk communities. The value of partnerships both within the island and with external collaborators was shown to be critical as was the use of a multiparametric dataset to assess the course of the eruption. We contend that the papers contained in this publication provide key insights into the mechanisms by which volcanic eruptions can impact populations at risk. The suite of analyses and data have generated a canonical dataset that can provide the framework for new advances in understanding the causes and consequences of varying eruptions worldwide.

Abstract The 2020–21 eruption of La Soufrière, St Vincent consisted of two distinct phases. The initial, effusive phase was characterized by the extrusion of a small lava dome inside the 1.5 km wide Summit Crater, and was followed by a violent explosive phase. Growth and evolution of the lava dome was monitored by visual observations and photogrammetry using imagery acquired from fixed-wing aircraft, helicopters and consumer-grade unmanned aerial vehicles. Additional monitoring was provided by satellite multispectral and radar imagery. Following the emergence of lava at the surface on 27 December 2020, volume and growth rate calculations showed the extrusion occurred at a steady rate of c . 1.8 m 3 s −1 and c . 16–19 × 10 6 m 3 of lava was extruded. The initial near-perfect hemispherical dome transitioned into a dome with a more elliptical footprint, eventually, evolving into a coulée-like feature with two lobes. The dome was destroyed by the onset of explosive activity on 9 April 2021.

Abstract Measurements of surface deformation provide valuable insight into sub-volcanic processes operating before, during and after eruptions. Here, we investigate the drivers behind the 2020–21 effusive–explosive episode at La Soufrière volcano in St Vincent using Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data between 2018 and 2021, and geodetic modelling. We observe inflation up to six months before the start to the effusive phase, which continued as the dome extruded. Once the eruption transitioned to the explosive phase, the volcano rapidly deflated, the bulk happening within the first three days of explosions. Our analytical modelling distinguishes three pressure source depth ranges contributing to this eruptive episode: 16–20, c. 6 and <1 km. Deformation data are therefore in line with a vertically extensive magmatic system being tapped pre- and syn-eruption with interaction between deep and shallow reservoirs by ascending magma batches. The combined use of GPS and InSAR proved to be instrumental for constraining the deformation field active during this eruptive episode. The direction of future geodetic monitoring at La Soufrière should therefore utilize both techniques with a view towards maximizing coverage while making up for shortfalls in station upkeep and variations in satellite overpass regularity.

Abstract Real-time Seismic Amplitude Measurement signals and eruption cloud height measurements were used to estimate peak intensities of 40 explosive events during the 8–22 April 2021 activity of La Soufrière volcano. We estimated magma supply rates and erupted volumes in each explosion, characterized uncertainty by stochastic modelling and identified four eruptive stages. Stage 1 included an intense period of 9.5 hours with 11 explosive events with peak eruption intensity between 2000 and 4000 m 3 s −1 and magma supply rate reaching 828 m 3 s −1 . Twelve high-intensity explosions ( c. 4000 m 3 s −1 ) occurred in Stage 2 with average magma supply rate of 251 m 3 s −1 . Stage 3 involved both declining intensity and magma supply rate and lengthening repose periods between explosions. Stage 4 involved three much weaker explosions. The total erupted volume of magma is estimated at 38.5 × 10 6 m 3 (90% credible interval: [22.0 .. 61.9] × 10 6 m 3 ) consistent with independent estimates from analysis of tephra deposits and volcano subsidence sourced at c. 6 km depth. The 150-fold increase in magma supply rate, from the preceding effusive phase to Stage 1 of the explosive phase, is attributed to replacement of very high-viscosity degassed magma occupying the shallow conduit system with new, lower-viscosity, volatile-rich magma from the magma chamber.

Abstract This paper forensically reconstructs the timings, impacts and processes that drove the sequence of explosive eruptions of La Soufrière, St Vincent in April 2021 using a combination of field-based stratigraphy and textural dissection of the deposit character together with contemporary visual observations. Explosive activity on 9 April and early on 10 April involved destruction of almost all of the 2020/2021 lava dome, c. 60% of the 1979 dome and formation of a 600 m diameter crater by 2pm UTC on 10 April. Following the initial explosion, plumes rose to altitudes of c. 15 km and pyroclastic density currents (PDCs), formed by column collapse, first occurred on 10 April, only after >24 h of explosive activity. Dense PDCs reached the sea only in the Larikai and Roseau valleys, and dilute PDCs were restricted to within 2.5 km of the Summit Crater rim. The tephra fallout deposits are stratified, composed of numerous layers of both lapilli-rich and ash-rich layers, which we have grouped into at least 7 units, based on their common characteristics (Units 1 to 7). Volume estimates, using a range of techniques to constrain uncertainties, indicate that the bulk volume of tephra (fallout and PDC) is 1.19 × 10 8 m 3 ± 20% making this a VEI 4 eruption.

Abstract Eruption source parameters (ESPs) used to characterize explosive eruptions are estimated from tephra deposit data using different models (statistical or numerical) and inversion approaches. The ESPs thus derived are subject to substantial uncertainties when the bulk of the tephra deposit, including information about its full spatial extent and spatial variation in grain-size distribution is missing due to geographical and environmental conditions. We use an advection–diffusion model coupled with a Bayesian inversion and uncertainty quantification algorithm to investigate how ESPs can be robustly estimated given such conditions. The 2021 eruption of La Soufrière volcano (St Vincent and the Grenadines) is our case study. An inversion is conducted for the first two explosive phases of this eruption (U1 and U2). We estimate: an erupted mass of 3.3 × 10 10 ± 1 × 10 10 kg for U1 and 3.1 × 10 10 ± 1.9 × 10 9 kg for U2, with an average particles release height of c. 13.5 km a.s.l. ±0.5 km for both phases. Given the efficiency of the proposed approach and the plausibility of the stochastic inversion results, we recommend this procedure for estimating ESPs for explosive eruptions for which the bulk of the deposit is missing or is inaccessible.

Abstract Petrological analysis of the 2020–21 La Soufrière lava dome reveals ubiquitous oxidation textures. Comparison of the natural dome rock to subsequent explosive scoria phases highlights the lack of any oxidation features in the latter, indicating that oxidation processes affected only the dome-forming magma, either during pre-eruptive storage or upon emplacement. To investigate the causes of oxidation we present a series of one-atmosphere experiments, using fresh natural basaltic andesite scoria as a starting material. Experiments were performed at 900 and 1020°C and at oxygen fugacities between NNO − 2 and air. Experimental results show that iron oxide nanolites nucleate on the rims of pyroxene microlites and phenocrysts under all experimental conditions except at NNO − 2. Orthopyroxene phenocrysts become unstable at 1020°C, at and above NNO + 2. Olivine symplectites form in all experiments at and above NNO. Titanomagnetite co-exsolves titanohematite and an Mg–Fe–Al spinel (pleonaste–magnesioferrite) at and above NNO + 2. Well-developed Mg–Fe–Al spinel trellis exsolution lamellae in titanomagnetite phenocrysts, as seen in the dome, only form in the presence of air at 900°C. The combination of textures and compositions observed in the natural dome indicates that oxidation of the dome magma occurred during emplacement at Earth's surface, with air percolating through the dome at temperatures ≤900°C.

Abstract We apply dynamical models to estimate the rheological properties of magma and lava during the 2021–22 eruption of La Soufrière Volcano, St Vincent. Analysis of the emplacement of a lava coulée gives viscosities in the range 0.94 × 10 10 Pa s to 5.97 × 10 10 Pa s. A static Bingham model gives a yield strength of 4.1 × 10 5 Pa. A dynamical model of conduit flow during the explosive phase of the eruption gives a viscosity range from 1.2 × 10 7 to 2.3 × 10 9 Pa s. A petrological model of magma viscosity in the source region falls in the range 10 2 to 10 3 Pa s. The very high viscosity of the lava is attributed to the presence of a remnant degassed and partially crystallized magma from previous eruptions that occupied the shallow conduit–vent system prior to onset of the 2021 explosive eruptions. Gas-rich magma pushed this degassed remnant magma out over a three-month period at a steady rate of about 1.2 m 3 s −1 and the explosive phase began when volatile-rich and much lower viscosity magma reached the surface.

Abstract The 2020–21 eruption of La Soufrière, St Vincent began with extrusion of a viscous lava dome, which was destroyed upon transition to a major explosive phase. Here we present petrological data to reconstruct the processes leading up to these events. Bulk-rock SiO 2 contents range from 52.8 to 55.4 wt%, classifying the lava and the subsequent scoria as basaltic andesite, the latter being slightly more mafic. Macrocrystal chemistry and modes (plag–cpx–opx–tmt–ol) and crystallinity (45–50 vol%) are largely identical for both phases of the eruption. Pyroxenes are homogenous and precipitated mostly from andesitic melts. Conversely, plagioclase shows strong normal zonation resulting from magma ascent and stalling at multiple crustal levels. Clinopyroxene thermobarometry reveals that crystallization predominantly took place between 8 and 13 km depth at temperatures of 997 − 35 + 18 ∘ C . A lack of evidence for mafic recharge and changes in volatile content and the omnipresence of xenoliths, suggests pre-eruptive destabilization of an andesitic–dacitic melt pocket that disrupted and entrained antecedent mush. Olivine diffusion profiles show that this interaction preceded the onset of eruption. Low dissolved sulfur contents (≤270 ppm S) place constraints on the total SO 2 gas release. Melt–mush disruption appears to be a dominant driver of eruptions at La Soufrière. Supplementary material: Supplementary figures and tables, as well as electronic data tables, are available at https://doi.org/10.6084/m9.figshare.c.6484877

Abstract After more than three months of lava dome extrusion, La Soufrière (St Vincent) transitioned to a series of explosive eruptions in April 2021. Here we present a time-series petrologic analysis of the phenocryst and microlite populations during the first c. 48 h of explosivity to constrain ascent conditions and processes that drove changes in behaviour. Primary eruptive products were crystal-rich (45–50 vol%) basaltic andesites with similar phenocryst phase assemblages and compositions. The change in eruptive style is consistent with overpressurization as a consequence of second boiling from anhydrous microlite crystallization. The microlites display variation between the explosive phases, with two populations: (1) ‘inherited’ − normally zoned high-An plagioclase (>An 70 ) + olivine (Fo 62–79 ) + clinopyroxene + titanomagnetite, inferred to have crystallized at depths >15 km and high water pressures; (2) ‘juvenile’ − unzoned plagioclase (An 45–65 ) + clinopyroxene + orthopyroxene + intermediate pyroxene (Wo 12–38 ) + titanomagnetite, inferred to have crystallized upon ascent due to decompression and degassing. Scoria from the first explosions featured extensive groundmass crystallization and a significant ‘inherited’ microlite population. Later explosions had a more abundant ‘juvenile’ microlite population and lower crystallinity, consistent with more rapid ascent from depth, initiated by decompression following initial blasts and destruction of the lava dome.

Abstract Determining SO 2 emission time-series from explosive eruptions can provide important insights into the driving magmatic processes, however accurate measurements are difficult to collect. Satellite-based platforms provide SO 2 imagery, however translating this to the altitude- and time-resolved emission history required to unravel volcanic processes is a major challenge. This means SO 2 emission time-series are rarely quantified for major eruptions, producing a gap in our understanding of explosive volcanism. Here, we combine SO 2 imagery collected by the TROPOspheric Monitoring Instrument (TROPOMI) with PlumeTraj, a back-trajectory analysis toolkit, to reconstruct the SO 2 emission prior to, and during, the explosive eruption of La Soufrière volcano, St Vincent, in April 2021. Precursory SO 2 emissions were quantified the day before the eruption, with emission rates in agreement with ground-based measurements. We estimate initial magma sulfur contents by comparing the measured SO 2 emissions with erupted magma volumes, finding that the initial explosion was sulfur poor (730 ppm S) compared to the main eruption phase (up to 3400 ppm S). This suggests that the initial explosion cleared old, previously degassed magma resident in the shallow plumbing system, followed by the eruption of the main, mostly un-degassed magma source.

Abstract Lahars are energetic flows of loosely consolidated volcanic debris and water, which have occurred frequently after rainfall events on St Vincent in the Eastern Caribbean since the April 2021 explosive phase of the 2020–21 eruption of La Soufrière volcano. Using scientific observations and information from social media, we have constructed a detailed timeline of the 25 lahar events that occurred during 2021, and summarized lahar impacts and losses. 20 mm daily rainfall on a river catchment is (and remains) sufficient to result in a lahar. We used this threshold, with field estimates of lahar volumes, to conduct both an island-wide assessment of potentially impacted locations using the Laharz model, and a detailed reconstruction of one lahar event, using the dynamic model LaharFlow. A simplified catchment hydrology approach with runoff ratios typical for the Caribbean showed good agreement with observations of flow properties near the coast. Lahars will continue to be an important hazard in St Vincent into the future, and our modelling approach can assess future lahar impacts and provide early warnings. Social media provided key information about lahars and impacts, and allowed communities to alert each other. Future hazard mitigation should strengthen links between communities and with national risk management.

Abstract Following a month of unrest in June–July 2019 and 3 months of effusive activity from late December 2020 to March 2021, La Soufrière volcano, St Vincent, transitioned to a 2-week period of explosions on 9 April 2021. During initial unrest, there was one working seismograph station on the volcano, providing the only information for tracking fluctuations in seismicity. Subsequently, full capability for locating volcanic earthquakes was achieved on 27 January 2021. Because events prior to this did not have reliable locations, unconventional data tracking approaches were adopted for assessing unrest evolution. Holistic inferences, suggested by combining these novel analyses with decades of knowledge of Lesser Antilles volcanoes, provided punctual evidential support for assessing the possibility that unrest could culminate in explosive activity. However, this case history is not offered as a paradigm for minimalist seismic monitoring at active volcanoes, notwithstanding that information gains were possible. This said, evaluations of seismic moment release, using event duration magnitudes as proxies for moment magnitudes, allowed the magma volume intruded or activated in the volcano to be estimated: 68 ± 14 × 10 6 m 3 dense rock equivalent (DRE) magma; this is a value remarkably similar to a geological estimate of total erupted volume (71 ± 14 × 10 6 m 3 DRE), with matching uncertainties.

Abstract The April 2021 La Soufrière of St Vincent eruption generated several pyroclastic density currents (PDCs) during the 2 weeks of the crisis, from 9 to 22 April. To support the hazard assessment team during this eruption, numerical simulations were performed in real time and generated rapid scenario-based PDC invasion maps with the two-phase version of the code VolcFlow, which was able to simulate both the concentrated and dilute regime of PDCs. To generate the maps, only the source properties (shape and location) and the initial volume used to generate the PDCs were varied, all other input parameters were kept constant and estimated from previous simulations. New simulations were then performed based on the field-based deposit map to assess the code's ability to simulate such PDCs. Results show that the syn-crisis invasion maps satisfactorily mimic the observed valley-confined PDCs, while unconfined dilute PDCs were overestimated. The results also highlight that simulation results are greatly improved with additional field-based data, which help constrain the PDC sequence. Numerous lessons were learned, including (1) how to choose the most critical input parameters, (2) the importance of syn-eruptive radar imagery and (3) the potential of this two-phase model for rapid hazard assessment purposes.

Abstract La Soufrière, St Vincent, began an extrusive eruption on 27 December 2020. The lava dome was destroyed, along with much of the pre-existing 1979 dome, in explosive eruptions from 9 to 22 April 2021. Lava domes generate crystalline silica – inhalation of which can cause silicosis in occupational settings – which can become hazardous when dome material is incorporated into volcanic ash. La Soufrière ash (17 samples) was analysed, according to IVHHN protocols, to rapidly quantify crystalline silica and test for other health-relevant properties. The basaltic andesitic ash contained <5 wt% crystalline silica, which agrees with previous analyses of ash of similar compositions and mirrors the low quantities measured in dome samples (2 area %). It contained substantial inhalable material (7–21 vol% <10 µm). Few fibre-like particles were observed, reducing concern about particle shape. Leaching assays found low concentrations of potentially toxic elements, which indicates low potential to impact health, contaminate drinking-water sources or harm grazing animals through ingestion. Collectively, these data indicate that the primary health concern from this eruption was the potential for fine-grained ash to increase ambient particulate matter, an environmental risk factor for respiratory and cardiovascular morbidity and mortality. Precautionary measures were advised to minimize exposure.

Abstract Social media has become an effective bidirectional communication tool used by risk management agencies during crises. Its interactive format can provide near real-time insight into public attitudes toward hazard and risk. While there is a growing body of research on risk communication and risk perception in developing countries, there is limited understanding of real-time public response to low-frequency, high-impact phenomenon like volcanic eruptions. Facebook, the social media platform, was one of the primary mass communication tools used by The University of the West Indies Seismic Research Centre during the 2020–21 eruption of La Soufrière volcano in St Vincent. This study analyses public response to hazard and risk information shared via Facebook. A thematic analysis of user comments and iconographic reactions during the eruption revealed a complex social response to the event and information on the event shared by scientists. Direct access to scientific information was widely appreciated, as was the scientific team, with whom the public developed a familiar relationship. The interplay of faith and science remains a key feature of risk perception in Caribbean society. Findings also highlight the utility of combining old and new broadcast technologies to engage diverse audiences.

Abstract During the 2020–21 eruption of La Soufrière, St Vincent, the University of the West Indies, Seismic Research Centre played a major role in supporting communication of hazard and risk information to publics and stakeholders across St Vincent. Due to COVID-19 restrictions on in-person education and outreach activities, the communications campaign was heavily reliant on social media platforms, and TV and radio broadcasts. Although the communications approach sought to be inclusive of all members of the affected communities, we consider that more vulnerable residents, such as the elderly, children, and those with low literacy levels and limited digital access were likely excluded from the communication efforts. In order to establish effectiveness of the crisis communications campaign at engaging communities and stakeholders with relevant information, and to identify areas for improvement, a large-scale evaluation campaign was conducted in St Vincent in August 2021. The results demonstrate that radio broadcasts are the most important communication tool for broad community reach, but that person-to-person information sharing was more important in the most exposed communities. Agencies such as the Red Cross and grassroots community disaster preparedness groups were instrumental in amplifying the reach of information to vulnerable members of at-risk communities and for evacuation co-ordination.

Abstract La Soufrière, St Vincent is the northernmost stratovolcano on mainland St Vincent. Over 25 000 residents were issued orders to evacuate their homes in the high-risk zones close to the La Soufrière Volcano in St Vincent and the Grenadines because of the imminent explosive volcanic eruption. The aim of this paper is to explore the experiences of residents who evacuated and sought shelter as a result of the 2021 eruption of La Soufrière Volcano. The researchers conducted surveys of affected residents of the red and orange zones (evacuees and non-evacuees) as well as shelter volunteers. The results indicate that residents are willing to evacuate if they are provided with information to guide their decision-making and if they feel that they have a suitable place to stay and that they are likely to receive support, especially food supplies. Despite their willingness to evacuate, people are generally concerned about the safety and security of their house, crops and livestock. In relation to future eruptions, people would be more prepared if they received more information, were evacuated earlier and took more supplies with them when they were evacuated.