The surface of an active pahohoe lava flow rapidly cools to form a thin solid crust above a still-molten, flowing interior. I consider how stresses induced in the solid crust due to the underlying fluid motion can result in wrinkling of the surface, producing a characteristic “ropy” appearance.
I begin by reconsidering buckling of a plate in pure shear, using the Foppl–von Karman equations to explore the onset of linear instability and the subsequent weakly and fully nonlinear development of patterns. I then extend this study to the case where an underlying fluid flow induces the shear stress within the plate. The theory is complemented with a suite of experiments.