Impact of cooling rate on rheology and emplacement dynamics of basaltic lavas: Insights from the January 2024 Sundhnúksgígar eruption (Iceland)

The 2023-2024 eruptions at Sundhnúksgígar in Iceland produced tholeiitic basaltic lavas that traveled at high velocities, affecting vast areas. Under such conditions, disequilibrium crystallization can play a fundamental role in modulating the lava flow dynamics and inundation capacity. To investigate this phenomenon, we carried out a comprehensive rheological characterization of the Sundhnúksgígar basalt, analyzing both the liquid phase and the crystal-bearing suspensions under disequilibrium conditions (cooling rates ranging from 0.1 to 10 • C/min) and near-equilibrium conditions (at fixed temperatures between 1242 and 1180 • C). Our results show that the cooling rate critically influences the extent and timescale of disequilibrium crystallization, thereby shaping the rheological evolution of the melt, leading to distinct deformation response of the crystal-bearing magmatic suspension. Compared to other basalts erupted worldwide, the Sundhnúksgígar melt exhibits two main features: i) an exceptionally low rate of viscosity increase induced by crystallization and ii) the ability to crystallize even at the highest cooling rates applied during the experiments. These characteristics may contribute to enhancing the efficiency of external crust formation, limiting heat loss from the inner portion of the lava flow, which consequently cools more slowly. Thermal insulation effects reduce the rate of viscosity increase during lava emplacement, a key factor in determining lava flow inundation potential. Our findings underscore the critical role of disequilibrium crystallization in governing the rheological evolution and emplacement dynamics of basaltic lavas, offering new insights into lava flow behavior.