When a hydrothermal system is shaken by the magmatic fluids: the thermal area of Levante Bay during the 2021-22 unrest of La Fossa volcano (Vulcano Island, Aeolian Archipelago)

The INGV monitoring system operating in the volcano Island since three decades, recorded a new phase of unrest at the La Fossa volcano since September 2021. The main set of the crisis was the central hydrothermal system, deeply affected by the input of heat and chemicals from the magmatic source. The Levante Bay, located northwest of the La Fossa edifice, is a thermal area, where the vapor, coming from a local hydrothermal aquifer, is emitted from several low temperature (100°C) fumaroles along the beach and in the near off shore. The composition of the gas is typical of hydrothermal systems, and indicates equilibrium at temperature close to 200°C. By the onset of the crisis, in September 2021, the composition of the gas emitted from these fumaroles showed a smooth trend of increasing contribution of the magmatic gas. In May 2022, a sudden release of gas occurred in the Levante Bay, which was testified by the whitening of the seawater in the bay, due the formation of sulfur flakes, and by the appearance of typical pockmark structures on the seafloor. The drastic increase of the gas flux from the underwater gas vents, coupled to the presence of the pockmarks, suggested that an explosive emission of gas occurred in May 2022. The chemical and isotopic composition (He and C isotopes) of the gas emitted from the fumaroles revealed the prevailing presence of the magmatic component, closely approaching the composition of the gas emitted from crater fumaroles. This episode drove the attention of the scientific community to this area, currently affected by a significant input of the magmatic vapor, because of the risk related to the huge gas emission and the eventual overpressurization of the local hydrothermal aquifer.