The islands are located a top an active triple junction between three large tectonic plates: the North American Plate, the Eurasian Plate and the African Plate.
Volcanism associated with the formation of the islands arises from the fact that the Azores Triple Junction involves rifting, a process whereby the crust is spreading along three ridge legs radiating out from the triple junction. The volcanism is also related to the Azores hotspot. The islands began forming during the Tertiary period. Pico, a volcano that stands 2,351 metres (7,713 ft) high on the island of the same name, has the highest altitude in the Azores and all of Portugal.
The last volcano to erupt was the Capelinhos Volcano (Vulcão dos Capelinhos) in 1957, in the western part of Faial Island, increasing the size of the island by 2.4 km, but it's estimated that the great part of it will be washed away in next 20 years. Santa Maria Island is the oldest Azorean island, formed 4.8 million years ago, and is the only island in the Azores with an abundance of sediments where marine fossils have been discovered.
The Gruta das Torres, Algar do Carvão, Gruta do Natal, Gruta das Cinco Ribeiras are some of the many caves of the Azores.