Volcanic Geology of Terceira Island, Azores: A Field Guide to Geodiversity

Terceira in the Azores archipelago is one of the most geologically diverse islands of the Atlantic Ocean and its geodiversity is due to the close relationship between tectonics and magmatism. This strong interaction is translated by the presence of large central volcanoes with calderas and a WNW-ESE-trending fissure zone that crosses them. The active tectonics of Terceira is well expressed by the Lajes Graben, which was the source of some of the more destructive earthquakes in the Azores. Moreover, the bimodal geochemistry of Terceira is recorded, on the one hand, by the numerous basaltic eruptions that punctuate the island and, on the other hand, by the less frequent but large trachytic (peralkaline) explosive eruptions of the central volcanoes. Among the latter, ignimbrite-forming eruptions are the most hazardous phenomena, and in case of such a future eruption the island would suffer catastrophic consequences. Historical eruptions have occurred on land (1761) and at sea offshore Terceira (1867 and 1998–2001), but with no record of casualties. This chapter highlights the diversity of geological structures found on Terceira Island, including tectonic faults, volcanoes and their products, volcanic caves, and geothermal areas, with attention to the associated volcanic hazards.