Combining 3D vesicle textural analysis and numerical modelling of magma ascent to understand eruptive style transitions at Villarrica volcano (Chile)

Style transitions are common during volcanic eruptions, but the processes that trigger them remain poorly constrained. Understanding these transitions is crucial for hazard assessment, as changes in eruptive style can alter associated risks. Vesiculation plays a fundamental role in magma ascent and eruption dynamics, and the content, size, shape, and distribution of vesicles in pyroclasts record key information on conduit processes and subsequent eruptive behaviour. In this work, we combine 3D textural analyses of basaltic and basaltic-andesite pyroclasts with conduit-flow numerical modelling to investigate the processes driving style transitions at Villarrica, Chile’s most active and highest-risk volcano. Villarrica is characterised by a constant, background Strombolian activity. In March 2015, Strombolian activity progressed quickly into a 1.5-km-high lava fountain. After that event, the activity continued shifting intermittently between Strombolian explosions and open-conduit degassing from the active lava lake.

We analysed 12 pyroclasts erupted between the March 2015 paroxysm and 2024, using X-ray microtomography (XCT) and SEM imaging. Vesicularity ranges from 0.61–0.88 in paroxysm clasts and 0.44–0.93 in post-paroxysm clasts. All samples are characterised by vesicle number densities of ~10¹²m^-3. Permeability parameters, such as vesicle tortuosity (1.54–2.16) and pore-throat ratios (0.20–0.35), show limited variation and lack clear patterns that differentiate eruptive contexts. This overlap could relate to high decompression and ascent rates, which may restrict outgassing, or suggest that other processes are likely influencing the resultant eruptive style. We used XCT-derived parameters and published eruptive conditions (composition, pressure, temperature, crystal content) to model conduit flow during the 2015 paroxysm and background activity. The 2015 lava fountaining is reproduced with a 4-m-radius conduit and 4 wt.% H₂O, showing no fragmentation within the conduit and gas ascending coupled to the melt. Background, mild-explosive eruptions were reproduced with 2.5 wt.% H₂O, showing fragmentation at ~900m depth and lower decompression rates. Our integrated results indicate that the eruptive style at Villarrica is governed not by a single permeability parameter, but by the complex interplay among outgassing, volatile content, and ascent rate.