COMPARISON OF THE 1902 ERUPTION TO THE OTHER HISTORIC ERUPTIONS OF SOUFRIERE, ST. VINCENT, LESSER ANTILLES

Soufriere volcano on St. Vincent, Lesser Antilles erupted in 1718, 1812, 1902, 1971, and 1979. The most famous of these began on May 6, 1902 with a series of phreatic explosions, entered into its climactic phase on May 7, and continued with sporadic explosions until April 1903. Stratigraphic studies on the west coast indicate that the initial deposit, which rests directly on the pre-1902 paleosol, are roof tiles, these are overlain by a series of surge deposits, often rich in accretionary lapilli that pass upwards into a succession of scoria and ash flow and surge deposits. At the base of the flow-surge sequence is a layer rich in carbonized and non-carbonized wood. This flow-surge sequence is itself overlain by numerous thin ash and lapilli fall beds. Both fall and flow deposits are composed of juvenile clasts, lithic fragments, crystals and accretionary lapilli. Although the presence of accretionary lapilli indicates a significant hydrologic component throughout the eruption, the variations in their amounts between different beds suggests that the water-magma interaction fluctuated significantly during the eruption. The pre-1902 eruptions were both explosive, with that of 1812 producing a new crater. In contrast, the 1971 eruption was entirely non-explosive resulting in the extrusion of a dome into the summit crater lake. The final historic eruption in 1979, began with an explosive phase that destroyed the 1971 dome, and terminated with the extrusion of a new dome. Clasts lithologically similar to the 1971 dome in both the pre-1902 and 1902 deposits lends support to the idea of the extrusion of domes into the crater lake prior to these eruptions. The deposits from the pre-1902, 1971 and 1979 eruptions as well as the initial deposits from the 1902 eruption all show a similar restricted range in major and trace element composition, in contrast the later 1902 deposits are considerably more basic. This difference suggests that the pre-1902, 1971 and 1979 eruptions only tapped magma that was present in the upper levels of the magma chamber, some of this magma may have also seeped out as the domes noted above. In contrast the larger compositional range in the 1902 deposits suggest either the eruption tapped deeper levels of the magma chamber as it progressed or the eruption involved the influx of new basic magma into the chamber.