Influence of hydrothermal alteration on phreatic eruption processes in Solfatara (Campi Flegrei)

Abstract:

The strong hydrothermal activity exhibited at Campi Flegrei by the Solfatara and Pisciarelli fumaroles points to a significant risk for phreatic eruptions in this densely populated area. Phreatic eruptions, triggered by various processes are hardly predictable in occurrence time and size. Despite their hazard potential, these eruptions, as well as the influence of hydrothermal alteration on their likelihood, magnitude and style, have so far been largely overlooked in experimental volcanology.

The physical properties and the mechanical behavior of volcanic rocks are highly dependent on their original magmatic microstructure and on any eventual alteration of those microstructures due to hydrothermal reactions. We have therefore investigated the potential effects of hydrothermal alteration on rock microstructure and, as a consequence, on fragmentation dynamics.

Rock samples from the vicinity of the Solfatara fumaroles have been characterized 1) geochemically (X-ray fluorescence, X-ray diffraction), 2) physically (density, porosity, permeability and elastic wave velocity) and 3) mechanically (uniaxial compressive strength, tensile strength). We have investigated the effects of hydrothermal alteration on fragmentation processes using a shock-tube apparatus, operating with Argon gas, water vapor and superheated water at temperatures up to 400°C and maximum pressures of 20 MPa. The three different energy sources within the pores initiating fragmentation, have been investigated: overpressure by 1) Argon gas; or 2) water vapor and due to 3) steam flashing of water. Fragmentation speed, fragmentation efficiency and fragmented particle ejection velocity were measured. Our results indicate, that steam flashing provides the highest energy - resulting in increased particle ejection velocity and higher fragmentation efficiency.

Based on our results, we aim to constrain the influence of hydrothermal alteration on the dynamics of phreatic explosions and the effect on the amount of energy, released by the different sources. The results may provide valuable information for the hazard assessment of well characterized, hydrothermally active areas.