Investigating hydrothermal unrest conditions of phreatic and phreatomagmatic events at Vulcano Island (Aeolian archipelago, Italy): insights from X-ray microtomography and in-situ experimental data

Periods of volcanic unrest present significant challenges for emergency management due to the inherent uncertainties. Rapid and unpredictable changes in the system conditions can signal either the onset of eruptive activity or a transition back to a quiescent state following non-eruptive unrest. In dormant volcanoes, such unrest is often driven by important variations in the hydrothermal system. Even in the absence of magma ascent or eruption, these variations can lead to hazardous phenomena, including phreatic eruptions caused by the rapid heating and vaporisation of fluids at shallow depths or landslides resulting from the failure of altered pyroclastic units.

Vulcano Island, an open-conduit system during the Middle and Modern Ages, currently represents a closed-conduit volcano with significant volcanic risk due to its potential for renewal of eruptive activity and associated hazards. This risk becomes particularly high during the summer, when tourism is at its peak. In fall 2021, Vulcano experienced one of the most significant episodes of unrest at La Fossa Crater in decades, marking a potential progression towards an eruption. Observations in September showed an increase in monitored parameters such as fumarole temperatures, steam emissions and concentrations of acid gases such as CO₂ and SO₂ as well as seismicity and uplift. In response, the Italian Civil Protection raised the alert level for Vulcano from green to yellow on 1 October. This phase of unrest ended in December 2023.

This escalation has prompted new research to deepen our understanding of the volcano's hydrothermal system and its dynamic behaviour, shedding light on the potential causes of phreatic and phreatomagmatic unrest phases. We present a comprehensive dataset on the microstructural characteristics of Vulcano's rocks, including pore content and size distribution, hydraulic and elastic properties, and mechanical behaviour. Volcanic samples were collected from various outcrops on Vulcano island, assuming they could represent the rock sequence down to approximately 1000 metres, through the correlation with the “horizons” identified along two cores extracted from the geothermal wells drilled around La Fossa cone in the 1970-80s. X-ray microtomography, an advanced imaging technique, is used to produce high-resolution (1 μm) 3D images of the volcanic rocks in a non-destructive manner. These analyses were further complemented by laboratory experiments such as uniaxial compression and tensile tests using 4D time-resolved imaging. The study of rocks microstructure and their geomechanical behaviour provided insights into the propagation of hydrothermal fluid-filled fractures within the unique tectonic context of the island. This understanding enhances our ability to identify conditions that promote instabilities and drive volcanic phreatic and phreatomagmatic unrest phenomena at Vulcano.