Complex sentence: Geologically, these islands were formed by volcanic and seismic events, although the last terrestrial volcanic eruption occurred in 1957--58 (Capelinhos) and minor earthquakes occur sporadically, usually of low intensity.
Split into simpler sentences: Geologically, these islands were formed by volcanic and seismic events. The last terrestrial volcanic eruption occurred in 1957--58 (Capelinhos) and minor earthquakes occur sporadically, usually of low intensity.