Terceira Island contains multiple stratovolcanoes constructed along a prominent ESE-WNW fissure zone that cuts across the island. Historically active Santa Barbara volcano at the western end of the island is truncated by two calderas, the youngest of which formed about 15,000 years ago. Comenditic lava domes fill and surround the caldera. Pico Alto lies north of the fissure zone in the north-central part of the island and contains a Pleistocene caldera largely filled by lava domes and lava flows. Guilherme Moniz caldera lies along the fissure zone immediately to the south, and 7-km-wide Cinquio Picos caldera is at the SE end of the island. Historical eruptions have occurred from Pico Alto, the fissure zone between Pico Alto and Santa Barbara, and from submarine vents west of Santa Barbara. Most Holocene eruptions have produced basaltic-to-rhyolitic lava flows from the fissure zone.