Three main fumarolic fileds are located at Fogo Volcano (S. Miguel Island). Two of them (Caldeira Velha and Caldeiras da Ribeira Grande) are regularly sampled and show hydrothermal composition (Ferreira et al., 2005; Caliro et al., 2015; Viveiros et al., 2020). The gases released in the third fumarolic field, named Pico Vermelho, are atmospheric contaminated.
Geothermometers applied to the gases released in the fumaroles at Caldeiras da Ribeira Grande area suggested feeding temperatures ranging between 188 and 278ºC (Caliro et al., 2015). Recent studies suggested lower feeding temperatures for the Caldeira Velha fumaroles (Viveiros et al., 2020). All the hydrothermal manifestations found out at Fogo Volcano are located in the north flank of the volcano, inside the so-called Ribeira Grande Graben. In this area, a geothermal power plant is also running since the early eighties with eight wells currently in production. The wells are fed by a two-phase liquid dominated reservoir with temperatures around 245 ºC (Rangel et al., 2015).
Despite the visible gas manifestations, in the north flank of the volcano, several diffuse degassing areas were also identified (Marcos, 2006; Viveiros et al., 2015; Oliveira et al., 2018). Detailed soil CO2 flux measurements were carried out in the area surrounding the Caldeiras da Ribeira Grande, where a value around 70 t/d was estimated and a thermal energy of 8 MW was calculated (Viveiros et al., 2020). This degassing area expanded after the drilling of a geothermal well in 2010 and not only new fumarolic areas appeared and expanded in the area, but also soil CO2 and temperature anomalies developed in the area, controlled by tectonic structures. The degassing phenomena also extended towards the inhabited area of Caldeiras da Ribeira Grande and caused not only high indoor CO2 values, but also affected the vegetation and several small animals were found death in depressions and low ventilated areas.
There are other sites, such as the Ribeira Seca village, where in 1997 some persons were also dislodged due to high indoor CO2. This site was sampled in what concerns the soil CO2 concentrations, but the soil CO2 fluxes were not measured. In addition, the origin of the carbon is still under debate (Viveiros et al., 2015).