Impact of Cooling Rate on Rheology and Emplacement Dynamics of Basaltic Lava Flows: Insights from the 2023-2024 Sundhnúksgígar Eruption (Iceland)

The 2023-2024 eruptions at Sundhnúksgígar in Iceland produced tholeiitic basaltic lavas that traveled at high velocities, affecting vast areas. In this context, disequilibrium crystallization can play a fundamental role in modulating the lava flow dynamic and inundation capacity. To investigate this phenomenon, we performed a comprehensive rheological characterization of the Sundhnúksgígar basaltic liquid and crystal-bearing suspension under both disequilibrium and near-equilibrium conditions. Compared to other basalts erupted worldwide, our results reveal unique features of the Sundhnúksgígar melt: i) exceptionally low solidification rates and ii) the ability to crystallize even at the highest cooling rates applied during the experiments. These characteristics enhance the efficiency of external crust formation, minimizing heat loss from the inner portion of the lava flow, which consequently experiences slower cooling rates. As a result, the lava is able to flow for longer times and travel greater distances than other basaltic flows. Our findings underscore the critical influence of disequilibrium crystallization on the rheological evolution and emplacement behavior of basaltic lavas, with implications for hazard assessment and risk mitigation during effusive eruptions.