In situ nanoscale insights on magma viscosity and explosive eruptions

Explosive volcanic eruptions pose significant threats to populated areas by injecting substantial amounts of gas and ash into the atmosphere. These events, resulting from magma fragmentation, are triggered by factors such as limited bubble expansion and relatively high strain rates during ascent, predominantly controlled by chemical composition. Less evolved melts, like andesites and basalts, present challenges in achieving fragmentation conditions due to their inherently low viscosities. Despite this, explosive activity of these magmas occurs. Recent studies highlight the role of Fe-Ti-oxide nanocrystals (nanolites) in increasing viscosity during laboratory measurements. Interestingly, nanolites have been found in natural volcanic products erupted during explosive events. However, the mechanisms and the extent to which nanolite formation affects magma viscosity remain a subject of ongoing debate. Here, we present the first in situ imaging observation of nanolite formation in andesitic melt and thoroughly quantify the impact on melt viscosity. To establish a robust point of comparison, we develop multiple novel viscosity models exclusively using viscosity data from nanolite-free samples. Our findings reveal that above the glass transition temperature, iron oxidation and nanocrystallization readily occur, inducing structural heterogeneities in the nanoscale. The precipitation of magnetite nanocrystals induces a heterogeneous distribution of elements in the residual melt, generating a relatively SiO₂-enriched matrix and Al-enriched shells around the nanolites. This phenomenon results in a substantial, up to 30-fold, surge in magma viscosity at eruptive temperatures. This noteworthy increase in magma viscosity has profound implications for the physical properties of andesitic plugs and domes and could play a critical role in driving the magma towards fragmentation during eruption.