Sete Cidades is an active central volcano with an approximately circular summit caldera and is located on the western part of S. Miguel Island. Inside the caldera pumice cones, maars and domes can be observed while the dominant structures on the outer slopes are scoria cones and domes.
The main fracture systems identified at Sete Cidades volcano reflect the regional tectonic regime. Actually, the Mosteiros graben, a NW-SE tectonic structure on the NW volcano flank, is interpreted as a sub-aerial segment of the so-called Terceira Rift. This structure crosses the caldera and proceeds to SE controlling the location of several scoria cones. In addition, it is admitted that the E-W alignment of domes in the W side of the volcano is the superficial expression of a deep oceanic fracture. As pointed out for other Azorean volcanoes, the intersection of NW-SE and E-W trends probably determined the location of Sete Cidades polygenetic centre.
The stratigraphy of Sete Cidades Volcanic Complex includes two main groups. The Inferior Group covers the thick assemblage of lava flows and volcaniclastic deposits that built the sub-aerial basis of the central volcano with more than 200000 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 36000 years including pumice and scoria fall deposits, pyroclastic flows and surge deposits and minor lava flows.
Rocks 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.
The actual model for the caldera genesis, based on morphologic, structural, geochronological and volcanological arguments, rejects prior hypotheses that explain the formation of such structure as the result of a single instantaneous event or incremental subsidence episodes. It is sustained that at least three important phases of collapse occurred.
The first caldera-forming event took place approximately 36000 years ago following the paroxysmal eruption that originated the deposits of Risco Formation. A second phase of collapse, represented by the deposits of Bretanha Formation, occurred 29000 years ago and enlarged the caldera to NW. Finally, at about 16000 years ago happened the chaotic collapse of the caldera northern sector producing the deposits of Santa Barbara Formation.
Around 5000 years ago the intracaldera volcanic activity changed from a magmatic to a hydromagmatic dominant character. Since then at least 17 intracaldera eruptions occurred, the latest of which took place at Caldeira Seca some 700 years ago. The recent eruptive history of Sete Cidades makes it the most active central volcano known in the region.