The Hidden Internal Flow Dynamics of Shear-Thinning Magma in Dikes

The viscosity of magma has a first-order control on the explosivity and hazards of a volcanic eruption, and the detection of diking within the subsurface may indicate an eruption is imminent. As magma approaches the surface it is highly likely it will have a non-Newtonian shear-thinning rheology (apparent viscosity decreases as shear rate increases), yet most dike models assume magma is a simple Newtonian fluid. Here we use laser light and particle image velocimetry to image flow within a scaled experimental dike hosting a shear-thinning fluid. The results show that the internal flow dynamics of shear-thinning magma in dikes are very different to Newtonian dikes. The velocity of shear-thinning flow radiates out toward the dike margin at similar magnitude across the dike plane; this is very different to the jet flow and recirculation characteristic of the Newtonian dike model at the same conditions. A linear relationship between tip velocity and inlet Reynolds number ${\text{Re}}_{\text{in}}$ in the viscous regime (${\text{Re}}_{\text{in}}\lesssim 0.4$) is confirmed to also apply to shear-thinning fluids, and transitional flow ($0.4\lesssim {\text{Re}}_{\text{in}}\lesssim 100$) is generated experimentally for the first time. These findings suggest that magma rheology (Newtonian or shear-thinning) cannot be recognized from external factors, such as the dike tip velocity. These results mark a step-change in dike modeling, introducing a new physical framework to test the petrological and geochemical evidence of magma ascent dynamics in dikes leading to volcanic eruptions.