São Jorge is a Portuguese island in the central Azorean archipelago of Portugal. It is separated from its nearest neighbors (Pico and Faial islands) by a 15 km strait (consequently, the three islands are sometimes referred to colloquially as the "Triangulo" (Triangle) group or just "The Triangle"). São Jorge is a relatively long thin island with tall cliffs, and where the population (9500 inhabitants.) is concentrated on various deltas along the north and south coasts (its east to west length is 53 km and its north to south width is 8 km and its area is 237.59 km² 95 sq. miles).
Unique among the islands of Azores, São Jorge is uncharacteristically long and slender and susceptible to ocean erosion. The island was built on fissural
volcanism associated with the plate tectonics of the mid-Atlantic Ridge and a transform fault that extends from the Ridge to the island of São Miguel (referred to as the São Jorge Fault).
Through successive fissural eruptions the island was built-up:
- the only remnants of these forces are the line of volcanic cones that extend along the central ridge (approximately 700 metres in altitude). The island was built-up from successive morphological structures of progressively younger materials, these include:
- Topo Volcanic Complex - predominantly built of basalt, hawaiite, and mugearite lavas and pyroclastic deposits and Strombolian cinder cones. Materials in this formation were produced are about 600,000 years in age.
- Rosais Volcanic Complex - consisting of the northeastern portion of the island, with a similar composition to those materials in Topo.
- Manadas Volcanic Complex - most recent formation composed of Strombolian cones and two Surtseyan cones (Morro de Lemos and Morro Velho), as well as craters and tuff rings resulting from Phreatomagmatic activity.