Furnas is a stratovolcano with a large summit caldera about 4 miles (6 km) in diameter and 1,000-1,200 feet (300-400 m) deep. There is a beautiful lake on the west side of the caldera. There is a solfatara on the east side of the caldera.
The only historic eruption of Furnas was in 1630. It lasted about a week and was on the south rim of the caldera, at the Pico de Areia vent. This large (VEI=4) violent, pumice-forming explosive eruption was probably Vulcanian in type. The eruption formed a lava dome and produced pyroclastic flows. Ejecta fell over most of the island. More than 200 fatalities were caused by mudflows. The Povoacao caldera is extinct.
The volcanoes of San Miguel Island and nearby waters based on van Padang and others (1967).
Sources of Information:
Booth, B., Croasdale, R., and Walker, G.P.L., 1978, A quantitative study of five thousand years of volcanism on Sao Miguel, Azores: Phil. Transactions of the Royal Society of London, Series A, v. 288, p. 271-319.
Martin, J.A., 1982, Excursion guide for field trip V1, Islands of Sao Miguel: Proceedings International Symposium Activity Oceanic Volcanoes, Archipelago University Azores, 3: p. 315-328.
Moore, R.B., 1983, Preliminary geologic map of Furnas volcano, Sao Miguel, Azores: U.S. Geological Survey Open-File Report, 83-395.
Moore, R.B., 1990, Volcanic geology and eruption frequency, Sao Miguel, Azores: Bulletin of Volcanology, v. 52, p. 602-614.
Neumann van Padang, M., Richards, A.F., Machado, F., Bravo, T., Baker, E., Le Maitre, W., 1967, Part XXI, Atlantic Ocean: Catalogue of the active volcanoes of the world, International Association of Volcanology, Rome, Italy, 128 p.
Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, Tucson, Arizona, 349 p.