Nanoscale chemical heterogeneities control the viscosity of andesitic magmas

Explosive volcanic eruptions, driven by magma fragmentation, pose significant geohazards due to their rapid energy release and widespread dispersal of pyroclasts. High magma viscosity promotes brittle fragmentation by limiting volatile escape and enhancing internal pressure buildup. Although recent studies have recognized that iron-titanium oxide nanocrystal formation increases melt viscosity, the mechanisms underlying this effect remain poorly constrained. Here we quantify the influence of nanocrystallization on magma viscosity by developing viscosity models that incorporate iron-titanium variations, calibrated against nanocrystal-free andesitic melts. Using time-resolved imaging, we show that nanocrystals form within seconds within synthetic andesitic melts. This process generates nanoscale chemical heterogeneities, including silica enrichment in the surrounding melt and aluminum-rich shells embedding the nanocrystals. These heterogeneities result in viscosity increases of up to 30-fold at eruptive temperatures. Our findings indicate that nanocrystallization modulates magma rheology during early crystallization, with direct implications for the dynamics of andesitic eruptions.