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La Soufriere is part of a recent volcanic complex on the southern Basse-Terre island of Guadeloupe. It is bounded on the south by the Monts Caraibes volcanoes and on the north by the Sans Toucher and Pitons de Bouillante massifs. The complex consists of two juxtaposed edifices: firstly, the composite volcano of la Grande Decouverte (C.V.G.D.) of which la Soufriere, sensu stricto, represents the most recent activity; and secondly, la Madeleine eruptive ensemble, which lies south of C.V.G.D., and is of lesser extent. The construction of C.V.G.D. has been subdivided into three main stages: 1. The Grande Decouverte stage (ca. 0.2 to 0.1 MA): growth, on the southern flank of Sans Toucher, of the main edifice, with dimensions of 20 × 15 km, by a sequence of effusive and pyroclastic events. This stage ended in a major Plinian eruption (several km3), giving rise to the Pintade pumice flows and falls, and to the formation of la Grande Decouverte caldera, only the northern rampart of which is still visible; 2. The Carmichael stage (ca. 0.1 MA to ca. 3000 B.P.4): buildup of the Carmichael volcano inside la Grande Decouverte caldera. The end of this stage is marked by two catastrophic sector collapses, dated at 11 500 and 3100 B.P., each of which was accompanied by large-scale explosions. The two events are characterized by the formation of km-size amphitheater craters (Carmichael and Amic craters), extensive debris flows, and blast deposits (to the west and south of C.V.G.D., respectively) . The older event has no magmatic component and is likened to the 1888 Bandai-san eruption, whereas the younger event might have been similar to the 18 May 1980 Mount St. Helens eruption; 3. La Soufriere stage (ca. 3000 B.P. to the present): formation of la Soufriere volcano, sensu stricto, within the Amic crater. The present dome was emplaced at the end of the last magmatic activity, a mild pelean eruption, in cal AD 1440. Two major eruptions, in 1797 and 1976, and several minor ones have occurred in recorded history (since 1635); all were of the phreatic type. During most of its evolution, C.V.G.D. lavas have remained of a rather constant andesitic composition. However, contrasted basaltic and dacitic products characterize single events, both in the older part of the history of the volcano, and the recent period. Trace element geochemistry indicates that C.V.G.D. had a magma supply system independent of other volcanoes elsewhere in southern Basse-Terre. |
