Università di Firenze, Dipartimento di Scienze della Terra, via G. La Pira, 4, I-50121, Firenze, Italy.

Stromboli volcano (Italy) is characterized by a permanent mild explosive activity disrupted by major and paroxysmal eruptions. These strong eruptions could be triggered by phreatomagmatic processes. With the aim of obtaining a better understanding of ground water flow in the vicinity of the active vents, we carried out a set of geophysical measurements along two profiles crossing the Fossa area (through the Pizzo, the Large and the Small Fossa craters). These measurements include electrical resistivity, induced polarization, self-potential, temperature and CO2 ground concentration. These methods are used in order to delineate the crater boundaries, which act as preferential fluid flow pathways for the upflow of hydrothermal fluids. The absence of fumarolic activity in the Fossa area and the ground temperature close to 100 °C at a depth of 30 cm indicate that the hydrothermal fluids condense close to the ground surface. Part of this condensed water forms a shallow drainage network (<20 m) in which groundwater flows downslope toward a perched aquifer. The piezometric surface of this aquifer is located ∼20 m below the topographic low of the Small Fossa crater and is close (<100 m) to the active vents. Electrical resistivity tomography, temperature and CO2 measurements show that this shallow aquifer separates the underlying hydrothermal body from the ground surface. Further studies are needed to ascertain the size of this aquifer and to check its possible implications for the major and paroxysmal events observed at the Stromboli volcano.