Terceira Island is an island in the Azores, in the middle of the North Atlantic Ocean, with an area of 396.75 km². The island's length is 29 km and the width is 18km; the perimeter is 90 km. Population is 54,996, down from a peak of 59,000. Population density is 140.73/km².
Terceira Island consists of four overlapping stratovolcanoes built above a fissure zone, which rise from a depth of over 1,500 metres (5,000 ft) on the floor of the Atlantic Ocean. The oldest at over 300,000 years is the Cinquo Picos stratovolcano that forms the eastern part of the island, with a 7 km (4 mi) diameter caldera which is one of the largest in the Azores. Next in age at perhaps 100,000 years is the Guilherme Moniz stratovolcano in the south-central part of the island, which also has a caldera with the highest remaining point on the rim reaching 623 metres (2,044 ft). Just to its north is the Pico Alto stratovolcano, probably less than 60,000 years old. It once had a caldera too, but subsequent eruptions filled it with several smaller lava domes and cones which top out at 808 m (2,651 ft).
