Basse-Terre is a volcanic island. The Lesser Antilles are at the outer edge of the Caribbean Plate, and Guadeloupe is part of the outer arc of the Lesser Antilles Volcanic Arc. Many of the islands were formed as a result of the subduction of oceanic crust of the Atlantic Plate under the Caribbean Plate in the Lesser Antilles subduction zone. This process is ongoing and is responsible for volcanic and earthquake activity in the region. Guadeloupe was formed from multiple volcanoes, of which only La Grande Soufrière is not extinct. Its last eruption was in 1976, and led to the evacuation of the southern part of Basse-Terre. 73,600 people were displaced over a course of three and a half months following the eruption.