Explosive activity of the last 1000 years at La Soufrière, St Vincent, Lesser Antilles

Abstract

The products of explosive activity of La Soufrière volcano on the island of St Vincent over the last 1000 years are described. Dates for the different eruptions were determined using information from contemporary accounts, fieldwork and radiocarbon dating. Scoria-flow type pyroclastic density currents (PDCs) dominate the products of both the historical eruptions (1979, 1902-03, 1812 CE) and prehistoric eruptions (~1580 and 1440 CE) with subordinate fallout components associated with several eruptions. Radiocarbon dating shows that these six eruptions define a crude cyclicity with repose periods ranging between 77 and ~140 years and systematically decreasing in more recent times. Two prehistoric eruptions, in ~1440 and 1580 CE respectively, both produced magmatic lapilli fallout and PDCs, and were fed by slightly more evolved magmas than the historical eruptions. The eruptions in 1902 and 1812 CE had ash-rich, possible phreatomagmatic activity at their onset. The iconic 1902-03 CE eruption generated radial distributed PDCs, which were responsible for the deaths of ~1500 people. However, only small remnants of these deposits remain and the original distribution cannot be determined from the preserved geology, which has important implications for hazard studies. Petrochemical work has shown that magmas involved in the explosive eruptions were quite narrow in compositional range, mainly comprising basaltic andesites. The 1902-03 eruption involved a late stage basaltic component in March 1903. However, activity in the last 1000 years generated notably more homogeneous magmas with a narrower range than the older eruptive periods previously reported in the literature, suggesting a significant variation in the magmatic reservoir feeding system with time.

Petrological insights on the last 1000 years of explosive activity at La Soufrière volcano, St. Vincent (Lesser Antilles)

“…When whole-rock data are taken into account, these mostly display a very coherent behaviour with increasing differentiation. As already reported by Cole et al (2019), crystal fractionation of mainly ol, cpx and pl (from basalt to basaltic andesite), followed by pl, cpx, opx and opaques (to more evolved basaltic andesites), in line with petrography, mineral chemistry and literature data, satisfactorily explains the observed geochemical trends. Simple major-and trace elements mass balance calculations can efficiently reproduce the evolutionary processes of La Soufrière magmas (see Electronic Appendix 1).…”

“…According to Heath et al (1998a), the products from the oldest Pre-Somma Lavas and Yellow Tuff formations span a wide compositional range, including the MgO-richest concentrations (i.e., from ~2 up to 10-13.5 wt.%), whereas the rocks of the youngest Crater Lavas and Pyroclastic formations are more homogeneous and evolved (e.g., MgO ~2-6 wt.%, only occasionally up to ~8 wt.%). Time-related geochemical gradients have also been reported within single eruptions, like that of 1902-03, for which the existence of a zoned magma chamber was suggested (Roobol and Smith, 1975;Cole et al, 2019). The products of the historical eruptions of 1902-03 and 1979 bear also evidence for mixing phenomena (i.e., banded and mingled juvenile clasts) between basaltic andesite and dacite magmas (Carey and Sigurdsson, 1978;Graham and Thirlwall, 1981).…”

“…The samples from the 1718-1812 and 1902-03 historical eruptions include i) a more common scoria type dominated by large pl phenocrysts with generally smaller cpx, opx, few ol microphenocrysts and accessory opaque microphenocrysts/microcrysts, and ii) a less common "mafic-rich" scoria type present only in 1902-03 products, characterised by approximately similar abundances of pl and colourless/light green cpx, plus ol phenocrysts. Anhedral pl (only in the main scoria type, particularly abundant and large in sample SVG14; Cole et al, 2019) and ol grains (larger than the euhedral/subhedral microphenocrysts, in both scoria types) are also occasionally observed.…”

“…La Soufrière has been the site of at least five main historical eruptions (dated 1718, 1812, 1902-03, 1971 and 1979) during which basaltic andesite domes (in the crater) and pyroclastic density current deposits were emplaced, along with subordinate fallout components (Aspinall et al, 1973;Shepherd et al, 1979;Graham and Thirlwall, 1981;Pyle et al, 2018;Cole et al, 2019). Recently, two major prehistoric eruptions predating the 1718 event were identified by Cole et al (2019), who reported 14 C ages pointing to 1440 and 1580 CE. On this basis, the authors proposed for the last ~600 years of activity at La Soufrière a crude periodicity with repose periods ranging between 77 and ~140 years and systematically decreasing in more recent times.…”

“…The products of the last ~1000 years of activity of the La Soufrière volcano of St. Vincent (referring to the historical eruptions of 1902-03 and 1718-1812, and the prehistoric eruptions of 1580 CE and 1440 CE) have been recently investigated by Cole et al (2019). Here we provide a more detailed petrological characterization and also present for the first time new data including: i) wholerock major-and trace element composition for juvenile clasts from older undated eruptions (i.e., pre-1440); ii) whole-rock trace element concentrations (including Rare Earth Elements) for all the juvenile clasts; iii) mineral and glass microanalyses for juvenile products (tripling the previous set of available data); iv) mineral microanalyses for xenoliths embedded within the investigated juvenile clasts.…”