The Yih Instability in Layered Lava Flow May Initiate the Pāhoehoe to ‘a‘ā Lava Transition

Abstract

Most basaltic lavas begin flowing as pāhoehoe but sometimes transition into ‘a‘ā. Field observations and previous models have demonstrated that the rheology of smooth, liquid-like pāhoehoe is distinct from rough, pasty ‘a‘ā, but the cause of this dramatic and rapidly occurring change in rheology has remained unclear. Here, we propose that the pāhoehoe to ‘a‘ā transition could be initiated by the Yih instability, an internal shear instability, in layered pāhoehoe flow. We show that the conditions under which instability arises depend on both extrinsic and intrinsic factors and derive a non-dimensional regime diagram. We test the model's prediction of stable flow configurations against field observations of solidified lava flows.