Azores

Sao Miguel - Geological Highlights

Sete Cidades: 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. The E-W alignment of domes in the W side of the volcano is the surface expression of a deep oceanic fracture. The intersection of NW-SE and E-W trends probably determined the location of Sete Cidades poligenetic centre.

The presently accepted model for the caldera genesis suggests that at least three important phases of collapses occurred. The first caldera forming event took place approximately 36'000 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 29'000 years ago and enlarged the caldera to the NW. Finally, at about 16'000 years ago, the chaotic collapse of the caldera's northern sector produced the deposits of the Santa Barbara Formation. Around 5000 years ago the intracaldera volcanic activity changed from a magmatic to a hydromagmatic character. Since then at least 17 intracaldera eruptions occurred, the latest of which took place at Caldeira Seca some 700 years ago.