A 13-month flank eruption of Fayal volcano occurred in 1957 and 1958. Intermittent violent explosions (phreatomagmatic) and ejection of ash characterized the eruption during its first 8 months. Following a 2-day intense seismic period, Capelinhos volcano entered an effusive phase which continued during the last 5 months of activity; Strombolian activity occurred in the crater vent, and a number of lava flows issued from the base of the cone. An outer, semicircular cinder cone, an inner spatter cone, and associated tephra deposits added 2.5 km2 of new land to Fayal Island. Total energy dissipated during the eruption was estimated to be 4×1021 ergs. Differential movements or warping of west Fayal occurred during the seismic crisis. Tephra and lava composition is olivine basalt. Lava viscosity was computed to be between 3×104 and 5×106 poises. Eruption sounds and flashing arcs were tape recorded and analyzed by an audio and subaudio spectrograph; Strombolian-type eruptions produced essentially broadland sound, explosions had maximum energy below 50 cps, and the dominant recorded energy of flashing arcs was between 30 and 50 cps.