La Soufrière Geological Background, the 1976-1977 Crisis and Evacuation
Guadeloupe is located in the central Lesser Antilles and consists of nine individual islands, the biggest being Basse-Terre (848 square km) and Grande-Terre (590 square km). These are separated by a natural marine channel, Rivière Salèe. The Grand-Découverte Soufrière (GDS) volcanic complex, which showed activity since at least the last 200'000 years, is on Basse-Terre. Together with Les Saintes, it is the only entirely volcanic island of the Guadeloupe archipelago.
Basse-Terre consists of many distinct eruptive centres grouped into 7 main eruptive fields up to at last 3 Million years old. They create a continuous, 55 km-long volcanic chain trending NNW, up to 25 km wide. The highest point (1467m.a.s.l) is the Soufrière dome, formed during the last magmatic eruption, ca. 1440 AD. The islands youngest complex is another dome, Madeleine Trois-Rivière, and is probably younger than 150'000 years. However, the GDS complex is the only one considered still active.
Guadeloupe, called "butterfly island" for its shape, as seen from space. Image courtesy NASA.
The GDS complex is a large massif with a diameter of about 8 km, composed by medium-K calc-alkaline basaltic andesites and andesites. Its products, erupted in three main phases (Grande Découverte, Carmichael and Soufrière) include andesite to dacite lava flows, domes and associated pyroclastic products. Three successive edifices have partially collapsed during sector-collapse eruptions. Chlorine-rich degassing has caused the only areas free from trees in a heavily forested area. After the dome construction of 1440, other more recent deposits were emplaced between 1550 and 1600. Since then, six phreatic-only explosive phases have been observed (1690, 1797-98, 1812, 1836-37, 1956 and 1976-77).
An unnecessary evacuation?
The last phreatic phase was particularly violent, lasting from July 1976 to March 1977. It was preceded by at least one year of increased seismic activity. The eruption caused much anxiety in the local population. Amongst scientists, a debate developed about the presence of juvenile magmatic components in the erupted products, which would have had considerable risk implications. Finally, more than 60'000 people from southern Basse-Terre were evacuated between 15 August and 15 December 1976. Indeed, between August and November many strong explosions were observed, but the eruption did not transform from phreatic to magmatic. Finally the activity slowly decreased from November 1976 to June 1977, when the crisis was declared over. The crisis lead to the publication of several scientific papers with sometimes strongly opposing viewpoints, but also had the positive effect of increased funding for volcano monitoring and research.
Although the population was strongly affected by the last eruption (earthquakes, water and air contamination, etc.) the month long evacuation, in hindsight many people considered it unnecessary or at least largely exaggerated. After all the eruption never entered a magmatic phase. On the contrary, the current view of scientists at Guadeloupe is that the evacuation of 1976 was fully justified on the base of the data then available. Moreover, a similar crisis - in the view of the same scientists - would be difficult to manage today, due to the considerable growth of the town located in the endangered zone and the future developments plan for Southern Basse-Terre.