Sete Cidades is one of the three main active central volcanoes of S. Miguel Island, Azores. Its geological setting is dominated by the NW-SE regional tectonic regime that characterizes the so-called Terceira Rift.
Recent field surveys conducted at Sete Cidades volcano allowed to define its stratigraphy considering two main geological units: the Inferior Group covers the thick assemblage of lava flows and volcaniclastic deposits that built the subaerial basis of the central volcano, with more than 200,000 years; the Superior Group is divided in six geologic formations (Risco, Ajuda, Bretanha, Lombas, Santa Bárbara and Lagoas) and comprises the material erupted in the last 36,000 years, including pumice and scoria fall deposits, pyroclastic flows and surge deposits and minor lava flows.
Volcanic products from Sete Cidades define an alkaline series with a potassic tendency, including alkali basalts, hawaiites, mugearites (and shoshonites), benmoreites and trachytes. The diversity of magmatic liquids in terms of major and trace elements agrees with an evolution dominated by low-pressure fractional crystallisation.
Based on morphologic, structural, geochronological and volcanological data a new model was established to account the evolution of Sete Cidades caldera. It is sustained that important caldera-forming events occurred approximately 36,000, 29,000 and 16,000 years ago following massive paroxysmal eruptions that originated pyroclastic flows, surges and pumice fall deposits.