Fogo is the largest of the three active central volcanoes of S. Miguel Island and dominates its central part. It is located at the intersection of NW-SE, NE-SW and E-W fault systems and shows a complex morphology with a summit caldera formed as result of numerous collapse and explosive events. The edifice of Fogo has been extensively dissected by erosion, presenting deep valleys some of which shows a clear tectonic control.
The products of Fogo belong to a potassic alkaline suite ranging from alkali basalt to trachyte, reflecting withdrawal of melt from a compositionally zoned trachytic magma reservoir. The oldest subaerial products of Fogo volcano date from about 300,000 years ago and were collected from a borehole in the lower northern flank. Older products are poorly exposed, making stratigraphic correlations difficult, particularly in the north flank where considerable subsidence within the NW-SE Ribeira Grande graben occurred. A more complete stratigraphy for the last 40,000 years was established on the southern flank of the volcano. During this period some large trachytic plinian eruptions, including those of the Ribeira Chã and Fogo A were related to major changes on the caldera morphology. The last major intracaldera eruption occurred in 1563 and deposited a considerable thickness of tephra on the eastern part of the island. Four days after this event there was an effusive basaltic eruption on the northwestern lower flank of the volcano.