Rock microstructural and mechanical constraints on the 2021-ongoing hydrothermal unrest at La Fossa Caldera (Vulcano Island, Italy)

Abstract

Volcanic unrest associated with increased hydrothermal activity is often difficult to interpret, yet it may culminate in hazardous steam-driven explosions or slope failures without magma intrusion. At La Fossa Caldera (Vulcano Island, Italy), recently affected by the 2021–ongoing unrest episode, we identify a porous scoriaceous horizon at ~ 615–655 m depth that is consistent with the inferred hydrothermal reservoir and confined by mechanically competent lava units. We combine field observations and borehole stratigraphy with high-resolution X-ray microtomography, petrophysical analyses, and in-situ uniaxial mechanical testing within the micro-CT system to characterise the pore structure, transport properties, and geomechanical behaviour of representative lithologies. Our results indicate that the porous scoriaceous horizon provides favourable conditions for fluid accumulation, whereas the surrounding low-porosity lava units act as an effective mechanical and hydraulic seal, allowing overpressure to build up to values comparable to the tensile strength of the overlying rocks measured on fresh samples. These conditions may promote fracture propagation and episodic gas release without the need for shallow magma intrusion. Our findings highlight the fundamental control exerted by the physical and mechanical properties of volcanic rocks on fluid storage, pressurisation, and migration. We propose a physically based framework for interpreting hydrothermal unrest during the 2021–ongoing episode by linking subsurface rock properties with fluid overpressure and geophysical observations. This framework provides a transferable approach for integrating geological, laboratory, and monitoring data to improve the interpretation of hydrothermal unrest in active volcanic systems.