I-type cosmic spherules are micrometeorites that formed by melting during atmospheric entry and consist mainly of iron oxides and FeNi metal. I-types are important because they can readily be recovered from sedimentary rocks allowing study of solar system events over geological time. We report the results of a study of the mineralogy and petrology of 88 I-type cosmic spherules recovered from Antarctica in order to evaluate how they formed and evolved during atmospheric entry, to constrain the nature of their precursors and to establish rigorous criteria by which they may be conclusively identified within sediments and sedimentary rocks. Two textural types of I-type cosmic spherule are recognised: (1) metal bead-bearing (MET) spherules dominated by Ni-poor (<1.5 w%) wüstite and FeNi metal (10-95 wt% Ni) with minor magnetite, and (2) metal bead-free (OX) spherules dominated by Ni-rich wüstite (0.5-22.5 wt%) and magnetite. Two varieties of OX spherule are distinguished, magnetite-poor dendritic spherules and magnetite-rich coarse spherules. Six OXMET particles having features of both MET and OX spherules were also observed. The wüstite to magnetite ratios and metal contents of the studied particles testify to their formation by melting of extraterrestrial FeNi grains during progressive oxidation in the atmosphere. Precursors are suggested to be mainly kamacite and rare taenite grains. Vesicle formation within metal beads and extrusion of metallic liquid into surrounding wüstite grain boundaries suggests an evaporated iron sulphide or carbide component within at least 23% of particles. The Ni/Co ratios of metal vary from 14 to >100 and suggest that metal from H-group ordinary, CM, CR and iron meteorites may form the majority of particles. Oxidation during entry heating increases in the series MET80 wt% Ni comprising a particle mass fraction of <0.2. Non-equilibrium effects in the exchange of Ni between wüstite and metal, and magnetite and wüstite are suggested as proxies for the rate of oxidation and cooling rate respectively. Variations in magnetite and wüstite crystal sizes are also suggested to relate to cooling rate allowing relative entry angle of particles to be evaluated. The formation of secondary metal in the form of sub-micron Ni-rich or Pt-group nuggets and as symplectite with magnetite was also identified and suggested to occur largely due to the exsolution of metallic alloys during deco...

A critical challenge during volcanic emergencies is responding to rapid changes in eruptive behaviour. Actionable advice, essential in times of rising uncertainty, demands the rapid synthesis and communication of multiple datasets with prognoses. The 2020–2021 eruption of La Soufrière volcano exemplifies these challenges: a series of explosions from 9–22 April 2021 was preceded by three months of effusive activity, which commenced with a remarkably low level of detected unrest. Here we show how the development of an evolving conceptual model, and the expression of uncertainties via both elicitation and scenarios associated with this model, were key to anticipating this transition. This not only required input from multiple monitoring datasets but contextualisation via state-of-the-art hazard assessments, and evidence-based knowledge of critical decision-making timescales and community needs. In addition, we share strategies employed as a consequence of constraints on recognising and responding to eruptive transitions in a resource-constrained setting, which may guide similarly challenged volcano observatories worldwide.

Proximal deposits of small-volume trachytic eruptions are an under-studied record of eruption dynamics despite being common across a range of settings. The 59 ± 4 ka Echo Canyon deposits, Ascension Island, resulted from a small-volume explosive-effusive trachytic eruption. Variations in juvenile clast texture reveal changes in ascent dynamics and transitions in eruption style. Five dominant textural types are identified within the pumice lapilli population. Early Strombolian-Vulcanian eruption phases are typified by macro- and micro-vesicular equant clast types. Sheared clasts are most abundant at the eruption peak, transitioning to dense clasts in later phases due to shear-induced coalescence, outgassing and vesicle collapse. Melt densification and outgassing via tuffisite veins increased plume density, contributing to partial column collapse and the explosive-effusive transition. Bulk vesicularity distributions indicate a shift in dominant fragmentation mechanism during the eruption, from early-stage bubble interference and rupture to late-stage transient fragmentation, with a transient peak of Plinian activity. Dome and lava groundmass crystallinities of up to 70% indicate near-complete degassing during effusive phases, followed by shallow over pressurisation and a final less explosive phase. We provide textural evidence for high-intensity explosive phases and rapid transitions in eruptive style during small-volume trachytic eruptions and consider the impact of trachytic melt compositions on underlying dynamics of these short-lived, explosive events. This analysis demonstrates the value of detailed stratigraphy in understanding critical changes in eruption dynamics and the timescales over which they may occur which is of particular value in anticipating future eruptions of this type.

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 9-22 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 weight % 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 volume % < 10 µm). Few fibre-like particles were observed, eliminating 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 minimise exposure. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6745583

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 th and early on 10 th April involved destruction of almost all of the 2020/2021 lava dome, ∼ 60% of the 1979 dome and formation of a 600 m diameter crater by 2pm UTC on 10 th April. Following the initial explosion, plumes rose to altitudes of ∼15 km and pyroclastic density currents (PDCs) formed by column collapse, first occurred on 10 th April, only after > 24hrs 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 x 10 8 m 3 +/− 20% making this a VEI 4 eruption. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6474317