Abstract

Soufrière is a stratovolcano just higher than 1,000 m which dominates the northern part of the island of St. Vincent in the Lesser Antilles. A succession of pyroclastic fall deposits produced by this volcano covers about 55% of the island. Maximum thicknesses away from the crater area range from 50 m in the northeastern area of the island to 2 m in the Kingstown area at the southern tip. These deposits, believed to be of late Pleistocene age, were produced during a unique phase of extremely violent activity which may not have lasted for more than a few hundred years; during this time, the volcano was in nearly constant activity. The eruptions took place from an open summit crater similar to the present one, which may have been periodically blocked by lava domes during short quiescent spells. The deposits were produced by plinian, vulcanian, and strombolian type eruptions, some of which may have been directed blasts.

The composition of the pyroclastic falls is distinctly more mafic than that of the average Soufrière lava from previous eruptions. Chemical variation in vertical sections shows that tephra with the highest silica content is ejected at the start of an eruption, usually as pumice, and is followed by more mafic material. High-level fractionation of basaltic magma under the volcano is inferred from this pattern of behavior.

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