Furnas is located in the easternmost of three active trachytic volcanoes on the island of São Miguel, in the historically active Volcanic Complex of Furnas. With a complicated 100,000 year history, the central caldera of the Furnas Volcano is a naturally explosive trachyte structure composed of two main calderas, that through formation, collapse and explosion have marked the natural history of the massif. The oldest caldera has a diameter of approximately 7*5.5 km whose escarpment cuts the lava structures of the neighbouring Volcanic Complex of Povoação to the north and northeast and southeast to valley of Ribeira Quente. The second caldera (4.5*3.5 km diameter) dates back 35000 years (the youngest caldera) and is geomorphologically identifiable by the 200 metre wall to the north and northwest. Within the caldera are numerous pumice cones and maars resulting from Plinian and Phreto-plinian eruptions from 5000 years.