The Institute

Title: Chemically Driven Nano-Elastic Heterogeneities Control Fragility in Volcanic Melts

Authors: M. Cassetta,  D. Szewczyk,  G. Giuliani,  S. Dominijanni,  F. Vetere,  G. Iezzi,  F. Radica,  D. Bondar,  F. Di Fiore,  A. Pontesilli,  L. Karacasulu,  N. Daldosso,  H. Mizuno,  D. Di Genova

Journal: Advanced Science

DOI: 10.1002/advs.202512063

This study presents a structural perspective on volcanic glasses, exploring how nanoscale heterogeneities relate to the physical properties of the melts. By examining a compositional series of silicate glasses from basaltic to rhyolitic end-members, we assess the variability of three key structural parameters: correlation length (ξ), microscopic free volume (V_c), and melt fragility (𝑚) and consider how these may be linked to differences in rheological behavior.

Our results indicate that nanoscale elastic inhomogeneity helps set where degassing starts and when brittle failure can occur. The boson-peak–derived ξ (size of comparatively stiffer melt domains) and V_c (size of compliant pockets where local dilatation concentrates) act together to bias nucleation and failure thresholds in the melt, the continuous phase that accommodates deformation and ultimately breaks. Bubble embryos form preferentially in V_c-sized compliant regions and along ξ-scale interfaces where stress focuses; melts with larger V_c and ξ therefore show enhanced local nucleation. The same ξ-scale interfaces behave as effective flaws that promote cavity coalescence and crack growth when deformation outpaces relaxation, consistent with lower fragmentation thresholds in more polymerized (rhyolitic) compositions. For crystals, V_c facilitates the cooperative rearrangements needed to initiate ordering upon undercooling, while ξ provides elastic contrasts that may pin or template early clusters; in both cases, nanoscale heterogeneity shifts rates and onset conditions rather than phase equilibria.

Crystals and volatiles remain essential co-controls through their effects on effective viscosity, elasticity, and permeability. Within that broader context, ξ and V_c provide melt-scale descriptors that help integrate glass vibrational structure into models of degassing and fragmentation, clarifying how nanoscale structure contributes to eruptive dynamics.