Internal shear instability in layered lava flow may initiate the pahoehoe to 'a'a lava transition

Most basaltic lavas initially erupt as low viscosity and low crystallinity pahoehoe flows, but sometimes transition rapidly and suddenly into high viscosity and high crystallinity `a`a flows. Exceptional observational and laboratory studies provide a detailed characterization of the field appearance and rheological properties of these flow types. However, the process governing when and why the transition from pahoehoe-to-`a`a occurs, and its dependence on magma properties, eruption characteristics, and slope geometry, remains unclear. We hypothesize that the pahoehoe-to-`a`a transition may be initiated by shear instability within a pahoehoe flow. At the interface between a cooler, more viscous but more buoyant, upper layer and a warmer, less viscous but less buoyant, lower layer, there can be small perturbations. These perturbations can become unstable and exponentially grow, leading to rapid mixing of the two layers. The resulting drop in temperature from lava mixing would lead to rapid crystallization, explaining the abruptness of the transition and the high crystallinity of `a`a flows. We test our hypothesis through a linear stability analysis and compare our predictions for stable flow configurations against prior field data, finding good agreement. Better understanding the pahoehoe-to-`a`a is not merely of fundamental value. It could also have implications for managing the risk of fast-approaching lava flows if it becomes possible to intentionally trigger a transition from less viscous pahoehoe to more viscous `a`a flow. Increasing viscosity would lower the speed of fast-approaching lava, ultimately gaining more time for evacuation. With the onset of new eruptions at Kilauea (USA), Nyiragongo (DRC), and Etna (Italy), lava flows currently pose a direct risk to many communities around the world. Figure caption: Our model set-up assumes a two-layered pahoehoe flow of which both are pahoehoe lava rheology. The lower layer is denser, , and less viscous, , top layer. We use non-dimensional terms, where viscosity ratio is m, layer thickness ratio is n, density ratio is r, and the Froude number is Fr. The flow field depends on the pressure gradient of the upper and lower flows (Ku,l). We model two different scenarios to match the initial (A) and later (B) stages of a flow. (A) Shows a free-slip surface condition with similar viscosity values between the two layers (m=1) and (B) shows a no-slip surface condition with a higher difference in viscosity values between the two layers (m<1). Given the model setup, the greatest flow speeds are in the upper layer for (A) and the lower layer for (B).