Chapter 1 Volcanic geology of São Miguel Island (Azores Archipelago): introduction

The Azores is an archipelago in the Atlantic Ocean composed of nine volcanic islands. The islands are dispersed along a general WNW–ESE trend crossing the Mid-Atlantic Ridge in the area where the Eurasian, African and North American lithospheric plates meet. While Corvo and Flores lie to the west of the Mid-Atlantic Ridge and emerge from a present-day relatively stable geological setting, the other islands are located in an important seismic and volcanically active zone corresponding to the boundary between the Eurasian and African tectonic plates. Volcanic activity has occurred in historic times on the islands of Faial, Pico, Sao Jorge, Sao Miguel and Terceira, and there have also been several recorded submarine eruptions (Weston 1964). Sao Miguel is the largest and most volcanically active island of the archipelago. In the last 5 ka several eruptions took place on the three active central volcanoes – Sete Cidades, Fogo and Furnas – and in the basaltic fissure systems of Picos and Congro. Although details of the first settlement of the island in the fifteenth century are uncertain, there is evidence that Furnas was in eruption at the time the first settlers arrived, some time between 1439 and 1443 (Queiroz et al. 1995). The historian Gaspar Frutuoso (1522–1591?) provided the first geological descriptions of the Azores in his work ‘Saudades da Terra’, giving important details about the earliest volcanic and seismic events. In the sixteenth and seventeenth centuries there were two explosive trachytic eruptions of sub-Plinian scale on Sao Miguel: Fogo 1563 and Furnas 1630. The last eruption on land occurred in the Picos Fissural Volcanic System, in 1652, and involved the extrusion of lava domes. The …