Disequilibrium rheology of basaltic magma: Cooling deformation experiments with one-step and two-step cooling rates

Natural lavas cool via a combination of radiative, convective, and conductive heat loss. Radiative cooling dominates initially, followed by convective cooling as the flow develops a surface crust, while conductive cooling plays a minor role. Previous experimental rheological studies have not investigated the effect of changes in cooling rate, as would be expected with the transition in heat loss mechanism, and instead implemented one-step cooling rate protocols. This work represents the first step in replicating natural cooling rates in laboratory experiments and assessing their effect on lava rheology and its temporal evolution throughout flow emplacement. Cooling deformation experiments (CDEs) were carried out on a basaltic sample undergoing a constant shear rate of 1 s⁻¹ using one of two cooling protocols: one-step and two-step. The one-step cooling deformation experiments were run at 0.5, 1, 2, 3, 4, and 5 °C/min. The two-step cooling deformation experiments started with an initial (first-step) faster cooling rate and a later (second-step) slower cooling rate once the sample reached 1165 °C, specifically 1 → 0.5, 2 → 0.5, 3 → 0.5, 4 → 0.5, and 5 → 0.5 °C/min. Two-step CDEs result in more complex, three-stage viscosity trends. This work brings us closer to more accurately replicating the cooling dynamics of natural lavas and to furthering our understanding of their evolving rheology. Importantly, we find that lavas that experience two-step, faster-to-slower cooling histories are less viscous and can therefore flow faster than those that experience a one-step slow cooling history.