75-25 Comparison of the 2021 and 1971-72 Lava Domes of La Soufriere, St. Vincent and Potential for Transition to Explosive Behavior

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

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Abstract:

The island of St. Vincent in the Lesser Antilles volcanic arc formed from the subduction of the North American plate beneath the Caribbean plate. The northern part of the island is dominated by La Soufriere, a stratovolcano which has had seven major eruptions in the historical record, including a Plinian eruption in April 2021 that was preceded by a lava dome extrusion. The effusive-explosive transition in 2021 was the fourth eruption in the last 125 years, making St. Vincent amongst the most volcanic hazard-prone regions in the world. In 2021, 16,000 residents were successfully evacuated <24 hours before explosive activity began, highlighting the need to understand why volcanoes may change eruptive styles during active periods. 

In contrast to the 2021 eruption, from October 1971 to March 1972, a lava dome emerged from the crater floor of La Soufriere, but never transitioned to an explosive phase. In order to understand this contrasting behavior, we are characterizing the petrology of these lava domes to ascertain potential differences in magma source, storage conditions and/or ascent. Both the 1971-72 and 2021 lava dome are basaltic-andesite, with a phase assemblage of 30% plagioclase, 10% orthopyroxene, 5% clinopyroxene, 5% Fe-Ti oxides, and ~1% olivine. The plagioclase in both samples are mostly normally zoned, with compositions of An_85-98 (cores) and An_55-73 (rims) in the 1971-72 dome and An_83-98 (cores) and An_55-68 (rims) in the 2021 dome. Plagioclase microlites are more sodic, An_55-72, and similar to the rim compositions. Reverse zoning is present in some plagioclase microlites and within glomerocrysts. Cpx compositions are unzoned and broadly similar between the samples, but the 2021 cpx are slightly more Mg-rich, Wo_41-45En_35-43Fs_13-21 (1971-72) versus Wo_41-45En_40-43Fs_13-17 (2021). Opx compositions are nearly indistinguishable, with Wo_3-4En_64-68Fs_29-33 (1971-72) versus Wo_3-4En_63-67Fs_30-33 (2021). In contrast, olivine compositions and textures are distinctive. Olivine in the 1971-72 dome has a fairly restricted composition of Fo_61-66 and is unzoned, whereas the 2021 dome has some normally zoned olivines, with Fo_73-74 cores and Fo53-54 rims, in addition to an unzoned population (Fo_59-62) and one primitive crystal (Fo₉₅). Olivine in the modern dome has a symplectite exsolution texture, typically associated with more oxidizing conditions and higher temperatures. Further analysis of olivine, as well as titanomagnetite, may yield additional insights into different pre-eruptive conditions of the lava domes.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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