São Jorge, with about 1.3 million years, resulted from basaltic fissure volcanism, causing its elongated shape with numerous volcanic cones of strombolian type aligned toward WNW-ESE (west-northwest – east-southeast).
In morphological terms this island is essentially plateau, with about two-thirds of its surface at altitudes varying between 300 and 800 metres, predominantly the existence of very accentuated and declivitous slopes particularly in the coastal sea cliffs, some of which with around 600 metres high. It is often at the bottom of the sea cliffs, the existence of small areas flattened at sea level, the famous fajãs, a singularity of this island.
The fajãs (coastal platforms) are the most typical geological phenomenon of São Jorge. There are about 74 across the island and formed by two processes: the lava spills, when lava flows advanced into the sea; and mass movements strand (detrital), this is, when by instability of the sea-cliffs and the erosive action, moving debris accumulate at the base of these. In the eastern part of the island (the oldest) have formed the most peculiar detrital fajãs: of Cubres; of Caldeira de Santo Cristo; of Vimes; of Bodes and of São João.
São Jorge was the scene of many historical eruptions since its settlement. The last volcanic phenomena occurring on the island were subaerial eruptions in 1580 and 1808 and submarine eruption of 1964, southwest of Rosais.