All of the nine islands and the few islets that make up the archipelago are of volcanic origin. The volcanic activity that created these islands ranges from effusive eruptions, characterized by a steady outpouring of lava, to explosive events, with violent fragmentation of lava. The oldest volcanic rocks are dated at 8.12±0.85Ma and can be observed in the Santa Maria Island (Abdel-Monen et al., 1975). Since the settlement of the archipelago, which took place in the 15th century, about 30 eruptions occurred both inland and at sea. The last eruption was a submarine event offshore between 1998 and 2000 close to Terceira Island (Gaspar et al., 2003).