Sete Cidades

Geography & Climate

Sete Cidades is located in Portugal, within the Azores-Terceira Rift Volcanic Province of the broader Atlantic Ocean Volcanic Regions. Situated at 37.87° N, 25.79° W in the Northern Hemisphere, the volcano lies within a temperate climate zone. With a summit elevation of 842 meters above sea level, Sete Cidades 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

Sete Cidades 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 Sete Cidades 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

Sete Cidades has 16 recorded eruptions in the geological database, spanning from 3050 BCE to 1880 CE. The most powerful recorded event was a cataclysmic eruption with ash columns reaching the stratosphere in 1444 CE, reaching VEI 4 on the Volcanic Explosivity Index. Notable eruptions include 1811 CE (VEI 3), 1444 CE (VEI 4), 670 CE (VEI 3). This level of eruptive frequency indicates a persistently active volcanic system that warrants ongoing monitoring.

Significance

With a maximum recorded VEI of 4, Sete Cidades has demonstrated the capacity for significant explosive eruptions that can impact regional aviation, agriculture, and communities tens of kilometers from the summit. VEI 4 eruptions produce ash columns reaching 15-25 kilometers into the atmosphere. With 16 recorded eruptions, Sete Cidades has a substantial eruptive history that provides valuable data for understanding volcanic behavior and assessing future hazards in the Atlantic Ocean Volcanic Regions region.

GVP Reference Summary

Sete Cidades volcano at the western end of Sao Miguel Island contains a 5-km-wide summit caldera occupied by two caldera lakes. The steep-walled, 500-m-deep caldera was formed about 22,000 years ago, and at least 22 post-caldera eruptions have occurred. A large group of Pleistocene post-caldera trachytic lava domes, lava flows, and pyroclastic-flow deposits is found on the western-to-northern flanks. A nearly circular ring of six Holocene pyroclastic cones on the caldera floor have been the source of a dozen trachytic pumice-fall deposits erupted during the past 5,000 years. Recorded eruptions date back to the 15th century and have occurred both within the caldera and from submarine vents off the west coast.

— Smithsonian Institution, Global Volcanism Program