Furnas contains at least two calderas, a younger one that is 6-km wide and a larger older one that is less topographically distinct. The eastern wall of the 500-m-deep caldera of Furnas overlaps the western wall of the Povoaçao caldera of Nordeste volcano. Volcanic activity at Furnas dates back about 100,000 years. At least 11 trachytic pumice layers, all erupted during the past 5000 years, post-date the caldera. The most prominent post-caldera feature is the Pico do Gaspar lava dome, east of the Lagoa das Furnas caldera lake. A zone of late-Pleistocene to Holocene basaltic-to-trachytic cinder cones and lava domes is located between Furnas and Agua de Pau volcanoes. Two historical eruptions have occurred at Furnas, one sometime between 1439 and 1443 and the other in 1630. The latter was one of the largest Holocene explosive eruptions in the Azores and caused significant damage and fatalities.
