The Evolution of La Grande Découverte (La Soufrière) Volcano, Guadeloupe (F.W.I.)

Abstract

The Grande Découverte 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 Découverte caldera, only the northern rampart of which is still visible; The Carmichaël stage (ca. 0.1 MA to ca. 3000 B.P.⁴): buildup of the Carmichaël volcano inside la Grande Découverte 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 (Carmichaël 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 Bandaï-san eruption, whereas the younger event might have been similar to the 18 May 1980 Mount St. Helens eruption; La Soufrière stage (ca. 3000 B.P. to the present): formation of la Soufrière 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. Volcanic hazard assessment has to take into account the past recurrence of collapses of sectors of the volcano. Abnormal seismicity and deformation that accompanies the development of Mt. St. Helens-type eruptions would most probably be recognized as such by the observatory monitoring networks. However, it may be much more difficult to interpret future unrest as premonitory signs of an impending Bandaï-san-type event.