Hawaiian lava flows typically erupt as pāhoehoe and transform to ‘a‘ā as they travel away from eruptive vents, although the exact conditions that govern this transformation are not well constrained. Here we describe a set of laboratory experiments that use corn syrup and ellipsoidal rice grains as an analog to lava with crystals to examine the dependence of the transition on shear rate and particle concentration. At a particle volume fraction of 0.3, increasing the shear rate produces a sequence of deformation regimes defined by increasing amounts of shear localization. The same regimes may be traversed at constant shear rate by varying the particle concentration. At high shear rates, the onset of nonlaminar deformation corresponds to the percolation threshold, thus providing a link between suspension microstructure and deformation behavior. Together these results highlight the combined importance of shear rate and crystallinity in determining the flow behavior of basaltic lava.