Furnas

Geography & Climate

Furnas is located in Portugal, within the Azores-Terceira Rift Volcanic Province of the broader Atlantic Ocean Volcanic Regions. Situated at 37.77° N, 25.32° W in the Northern Hemisphere, the volcano lies within a temperate climate zone. With a summit elevation of 805 meters above sea level, Furnas is a moderately sized peak that remains accessible to hikers and researchers for much of the year. The volcanic landform is characterized as a composite, which describes the physical shape and structure of the volcanic edifice as observed from the surface.

Geological Context

Furnas is situated along a rift zone, where tectonic plates are pulling apart. As the crust thins and fractures, magma rises to fill the gap, creating volcanic activity. Rift volcanism typically produces more fluid lava flows rather than explosive blasts, which means communities near Furnas in Portugal generally face slower-moving hazards like lava flows and ground fissures rather than sudden pyroclastic events. However, rift volcanoes can still generate significant ash emissions and gas releases that affect local air quality. The dominant rock type is trachyte / trachydacite, an intermediate to silica-rich volcanic rock. This composition typically produces more viscous magma, leading to more explosive eruptions with ash columns and pyroclastic flows. Andesitic and dacitic magmas build pressure before erupting, which is why volcanoes with this rock chemistry are often associated with Plinian-style eruptions that can send ash tens of kilometers into the atmosphere and disrupt aviation across entire regions.

Eruption History Summary

Furnas has 9 recorded eruptions in the geological database, spanning from 4570 BCE to 1630 CE. The most powerful recorded event was a paroxysmal eruption with devastating regional consequences in 1630 CE, reaching VEI 5 on the Volcanic Explosivity Index. Notable eruptions include 1630 CE (VEI 5), 1441 CE (VEI 4), 1430 CE (VEI 3). This eruption frequency indicates a moderately active volcanic system with periodic reawakenings.

Significance

With a maximum recorded VEI of 5, Furnas has demonstrated the capacity for paroxysmal eruptions comparable to the 1980 eruption of Mount St. Helens. Eruptions of this scale can devastate areas within 20-30 kilometers and produce ash fall that disrupts aviation and agriculture across hundreds of kilometers. With 9 recorded eruptions on file, Furnas has enough documented activity to help scientists identify patterns in its eruptive behavior.

GVP Reference Summary

Furnas volcano lies at the eastern end of Sao Miguel Island, immediately west of the older Nordeste shield volcano and its Povoaçao caldera. There are 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 Furnas caldera overlaps the western wall of the Nordeste Povoaçao caldera. Volcanic activity 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, 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.

— Smithsonian Institution, Global Volcanism Program