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Abstract:
The April 2021 explosive eruption of La Soufriere St Vincent generated a bulk volume of approximately 1.2 x 108 m3 of tephra, indicating that this was a low VEI 4 event. The explosive phase lasted two weeks and, after an initial explosion on the morning of 9th April, became semi continuous for ~48 hours, comprising numerous closely spaced explosions with eruption columns ~ 15km high. Later explosions became more Vulcanian-like with lengthening time gaps between events. Evidence indicates that the first Pyroclastic density currents (PDCs) to travel out of the crater did not occur until ~30 hrs after the onset of explosive activity, associated with Unit 3, late on 10th April and we propose this is related to vent widening, following destruction of lava domes during the initial explosive activity (Unit 1). PDC deposits extended to the sea in two valleys; the Lariaki and Roseau valleys draining the lowest part of the crater rim, 3.2 and 4.2km from the Summit crater rim respectively. To the south and southeast, dense PDCs reached 2.8 km and 1.2 km respectively. Dilute and dense deposits comprise several units interbedded with fallout layers (at least Unit 3, 4 and 5), indicating PDCs were formed by column collapse associated with several explosions. Despite only occurring in fallout layers, ash aggregates formed during the explosive activity appear to be associated with PDC generation. Although several types of aggregate were formed, the proportions of types change with time through the sequence, indicating style of PDC generation controlled aggregate formation.
