Lingering beneath crumbling walls: The origin of Holocene rock glaciers

Abstract Rock glaciers may appear to be stubby, slow-moving, debris-mantled cousins of glaciers, yet the controls on their formation, size, and persistence differ from those of glaciers. The defining rock glacier characteristic is a debris cover sufficiently thick to protect its icy core from ablation. We therefore model the controls on debris thickness, incorporating debris input, ice accumulation, and ice speed. We show that rock glaciers exist in a narrow range of conditions that provides both enough debris from eroding headwalls and enough ice from focused snow avalanches to create a small area of net accumulation. The deformation speed of the ice-rich rock glacier core sets the time available for accumulation of the protective cover. Quantitatively, a 100-m-tall headwall backwearing at 1 mm/yr produces a thick-enough debris cover only if rock glacier surface speeds at the exit of the avalanche zone are <1 m/yr. This low speed in turn limits the length of rock glaciers. Where surface ages have been constrained, rock glaciers are found to have originated in the early Holocene. There is only so far, typically hundreds of meters, rarely more than a kilometer, that rock glacier lobes can travel in the Holocene. The handover from glacial to rock glacial occupation of alpine valleys may involve complete deglaciation before rock glacier conditions are met. Finally, that the required rapid headwall retreat far outpaces vertical lowering rates of summits can explain observed strong ridgeline asymmetry.