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<jats:title>Abstract</jats:title> <jats:p> 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 <jats:sup>10</jats:sup> ± 1 × 10 <jats:sup>10</jats:sup> kg for U1 and 3.1 × 10 <jats:sup>10</jats:sup> ± 1.9 × 10 <jats:sup>9</jats:sup> kg for U2, with an average particles release height of <jats:italic>c.</jats:italic> 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. </jats:p>

<jats:title>Abstract</jats:title> <jats:p>This paper provides an overview of the eruption of La Soufrière Volcano on the island of St. Vincent which occurred between 27 December 2020 to 22 April 2022. It sets the stage for the 19 articles 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.</jats:p> <jats:p>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.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>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.</jats:p> <jats:p>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 &lt;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% &lt;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.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>During the 2020–2021 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.</jats:p> <jats:p>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 coordination.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>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.</jats:p> <jats:p> Explosive activity on 9 April and early on 10 April involved destruction of almost all of the 2020/2021 lava dome, <jats:italic>c.</jats:italic> 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 <jats:italic>c.</jats:italic> 15 km and pyroclastic density currents (PDCs), formed by column collapse, first occurred on 10 April, only after &gt;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. </jats:p> <jats:p>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).</jats:p> <jats:p> Volume estimates, using a range of techniques to constrain uncertainties, indicate that the bulk volume of tephra (fallout and PDC) is 1.19 × 10 <jats:sup>8</jats:sup> m <jats:sup>3</jats:sup> ± 20% making this a VEI 4 eruption. </jats:p>

<jats:title>Abstract</jats:title> <jats:p> 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 <jats:sub>2</jats:sub> 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 <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msubsup> <mml:mn>997</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>35</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>18</mml:mn> </mml:mrow> </mml:msubsup> <mml:msup> <mml:mrow> <mml:mspace width="thickmathspace" /> </mml:mrow> <mml:mo>∘</mml:mo> </mml:msup> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> </mml:math> </jats:inline-formula> . 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 <jats:sub>2</jats:sub> gas release. Melt–mush disruption appears to be a dominant driver of eruptions at La Soufrière. </jats:p> <jats:p content-type="supplementary-material"> <jats:bold>Supplementary material:</jats:bold> Supplementary figures and tables, as well as electronic data tables, are available at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6484877">https://doi.org/10.6084/m9.figshare.c.6484877</jats:ext-link> </jats:p>

<jats:title>Abstract</jats:title> <jats:p> Determining SO <jats:sub>2</jats:sub> 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 <jats:sub>2</jats:sub> imagery, however translating this to the altitude- and time-resolved emission history required to unravel volcanic processes is a major challenge. This means SO <jats:sub>2</jats:sub> emission time-series are rarely quantified for major eruptions, producing a gap in our understanding of explosive volcanism. </jats:p> <jats:p> Here, we combine SO <jats:sub>2</jats:sub> imagery collected by the TROPOspheric Monitoring Instrument (TROPOMI) with PlumeTraj, a back-trajectory analysis toolkit, to reconstruct the SO <jats:sub>2</jats:sub> emission prior to, and during, the explosive eruption of La Soufrière volcano, St Vincent, in April 2021. Precursory SO <jats:sub>2</jats:sub> 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 <jats:sub>2</jats:sub> 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. </jats:p>

<jats:title>Abstract</jats:title> <jats:p> 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, <jats:italic>c.</jats:italic> 6 and &lt;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. </jats:p>

<jats:title>Abstract</jats:title> <jats:p>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.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Scroll bars are curvilinear ridges of sediment deposited as meandering channels migrate laterally. It is unclear exactly which variables control the spacing, height, and curvature of scroll ridges, and in turn how these properties record channel migration histories. This study tests the relationship between channel and scroll bar geometries in river systems with laterally migrating chute channels that deposit their own scroll packages. Lidar- and SRTM-derived DEMs are used to calculate the spacing, relief, wavelength, and curvature of scroll bars in eight different chute-channel-dominated river systems with a range of channel dimensions, slopes, and water and sediment discharge regimes. Seven of the eight rivers have statistically indistinguishable chute channel (CC) and main channel (MC) scroll spacings, even though the chute channels have smaller channel dimensions. Additionally, there is no correlation between scroll spacing and relief for main or chute channels. For lidar-derived DEMs, the ratio between median scroll spacing and median channel width is low and falls within a narrow range for MC scrolls (from 0.13–0.22; mean: 0.16), but is higher with a wider range for CC scrolls (from 0.48–1.33; mean: 0.8). CC scrolls are straighter overall than MC scrolls, but there is no correlation between curvature and scroll spacing. Overall, these results show that 1) chute channel scrolls are often indistinguishable from main channel scrolls based on spacing and relief, and 2) MC and CC scrolls have different relationships with formative channel widths, which suggests that other variables, such as lateral migration rate and flood inundation depth, are key controls on scroll bar geometries.</jats:p> <jats:p content-type="supplementary-material"> Supplementary material at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6606605">https://doi.org/10.6084/m9.figshare.c.6606605</jats:ext-link> </jats:p>