Tracking Post-unrest Instability in Long-Lived Hydrothermal Systems: A Multiparametric Analysis of the August 2025 reactivation of La Fossa Cone (Vulcano, Italy).

In closed-conduit volcanoes dominated by long-lived hydrothermal activity, both the escalation toward unrest and return to background conditions are non-linear processes. Post-unrest recovery phases are often characterised by intermittent fluctuations and reversals, complicating the definition of a stable baseline and the likely return to the pre-unrest state. Due to the low intensity of this kind of volcanic activity with respect to those involving magma bodies directly, anomalous patterns can often be hidden by external factors (e.g. meteorological and solar heating effects). For this reason, a multiparametric monitoring is essential to discriminate between background variability and actual changes leading to phases of potential concern. After the major hydrothermal unrest of 2021-2022 at La Fossa Cone (Vulcano Island, Italy), the system gradually evolved into a recovery phase, although intermittent periods of instability marked by notable change in the geophysical and geochemical parameters occurred. This study analyses the transient episodes occurring in Summer 2025 by integrating geochemical, seismic, and thermal datasets. In late July, the bulk crater SO₂ flux increased, indicating a further contribution of magmatic volatiles from the shallow feeding system to the hydrothermal one. In early August, this new input of hot magmatic fluids was followed by a sharp peak in the volcano-seismic signals, reflecting a pressurization of the hydrothermal system. Ground temperatures, measured continuously by four permanent stations located at the summit and inner crater area, recorded a significant increase in temperature. Initially this increase occurred in the most permeable areas, near the fumarole field, and then rapidly expanded laterally, affecting a wide area of the crater. Although the episode was short-lived (late July to late September 2025), it was characterised by a significant release of heat and fluids. Remarkably, despite the rapid temporal evolution, soil temperatures reached peak values comparable to those observed during the main unrest phases of 2021–2022, highlighting the system's capacity to quickly restore critical conditions potentially suitable for phreatic/phreatomagmatic explosions. Interpreted as a late-stage fluctuation of the 2021-2022 crisis, the August 2025 episode underscores the need to continuously monitor the La Fossa Cone, in order to define robust baselines to correctly assess volcanic hazard.