Long-term monitoring on a closed-conduit volcano: A 25 year long time-series of temperatures recorded at La Fossa cone (Vulcano Island, Italy), ranging from 250 °C to 520 °C

Abstract The longest record of temperature data from an active volcano in southern Italy is presented. The dataset comes from continuous monitoring of fumarole temperatures from the La Fossa cone of Vulcano (Aeolian Islands) running from 1991 to 2016. The discussion includes an empirical approach, based on a large number of direct measurements. At Vulcano Island, geochemical monitoring of the uprising fluids allows detection of the surface effects of perturbation in the state variables of the buried hydrothermal and magmatic systems. The presented datasets show that fumaroles' changing temperatures, which are related to surface heat flow, are useful indicators. Over the past 25 years, the combined effects of runoff and chemo-physical alterations were negligible on the output temperature of the earliest monitored fumaroles. The maximum recorded variation was 298 °C (measured in the ground very close to the steaming vents, at a depth of 0.5 m). Repetition of output temperature values occurred after 19 years in the same position; the time variations suggest a cyclic characteristic, although more years are needed to register the complete cyclic modulation. A combination of minor cyclical variations has also been registered in the fumarole output. The minor cycles appeared in this long series of data after 1995, and they can be interpreted as one of the surface effects of temporary departures from a stationary state assumed for the system feeding the La Fossa area. In this sector of the Mediterranean area, the steady state pressure field, as well as the steady state temperature gradients, can be perturbed either by magmatism or by seismo-tectonic processes related to regional dynamics. This long-term monitoring allowed comparisons of many temperature subsets with other validated geochemical and geophysical data series and highlighted common source mechanisms accounting for endogenous processes. Changes in the magma source and/or seismo-tectonic activity have been the primary causes of the time variations. The collected data show the effectiveness of the geochemical approach for following the heat flow changes that originated from a deep source in real time, even though an estimation of magmatic and/or hydrothermal energy release cannot be retrieved by surface temperature monitoring alone.