Thermal surveys of the Vulcano Fossa fumarole field 1994–1999: evidence for fumarole migration and sealing

Abstract

Between 1994 and 1999 we carried out five thermal surveys of the Vulcano Fossa fumarole field. Each consisted of 485–1213 vent temperature measurements distributed across the field. These show that the field has been cooling and expanding, where mean and maximum vent temperatures decreased from 216 to180°C and 541 to 428°C, respectively. Between 1994 and 1997, however, expansion of the field resulted in increased heat loss from 45 to 67MW. We suggest that cooling and expansion has been caused by increased gas flux and/or permeability, where increased flow of fluids of magmatic origin during 1996 is implied by chemical and isotopic data (Capasso, G. Favara, R., Francofonte, S., Inguaggiato, S., 1999. Chemical and isotopic variations in fumarolic discharge and thermal waters at Vulcano Island (Aeolian Islands, Italy) during 1996: evidence of resumed volcanic activity. J. Volcanol. Geotherm. Res. 88 (1999) 167–175.). We propose that the size of the conduit has increased, causing the fumarole field to expand and vent temperatures to generally decline as gas and heat losses become distributed over a wider area. The increase in exhaling area has outweighed the modulating effect of cooling on total heat losses, so that heat loss has increased due to an increased input from the magmatic source and/or an increase in system permeability.Within these field-wide trends, the rim and then the middle zones have been cooling. Conversely, the lower zone showed evidence of warming during 1994–98, before cooling in 1999. Throughout the measurement period a SE–S thermal migration has been apparent, and a new hot zone has developed on the field's southern edge. We suggest that sealing of fumaroles by complete boiling of marine fluids on the northern edge of the system and higher permeability on the southern edge may have influenced this expansion direction. If vent temperatures drop below 100°C then shallow sulfur sealing will quickly diminish fumarole life expectancy and contribute to the abandonment of vents in cooling zones. In our model, increased gas flux and/or permeability (e.g. due to increased magmatic degassing and/or fracturing) is the primary mechanism driving observed changes in the field temperature, size and location. Deep and shallow conduit sealing are secondary mechanisms influencing the location of new zones once increased gas and heat fluxes have been triggered.