We show slab-parallel advection and Rayleigh-Taylor instability (RTI) as two competing gravity-driven flow mechanisms in the melt-rich layer atop a subducting slab. Scaled laboratory model results, supported by CFD simulations, indicate a transition of the RTI to advection mechanism at a threshold slab-dip angle (α) between 20° and 30°. The advection mechanism results in updip flow, forming a single plume at the upper edge of the melt-rich layer. Based on lubrication approximation, we present an analytical solution of the linear stability problem to perform a criticality analysis of α for the transition between RTI and advection, as observed in the experiments. The solution indicates this transition to occur at α⁎ = 28°, in favorable agreement with the experimental values. The article finally highlights the implications of this criticality analysis in interpreting the spatial distributions of subduction volcanism.