Abstract

At present, in many low- to mid-latitude mountain ranges, the few small bare ice glaciers that do exist are found where snow is focused by avalanches and where direct exposure to radiation is minimized. Valley heads are more likely to be occupied by rock glaciers, which can number in the hundreds. Geomorphically, rock glaciers serve as conveyor belts atop which rock is pulled away from the base of cliffs. We show how rock glaciers emerge in numerical models that include ice dynamics, debris dynamics, and the feedbacks between them. On debris-covered glaciers (i.e., glaciers with a partial rock mantle), rock delivered to the glacier from its headwall, or from sidewall debris swept into the glacier at tributary junctions, travels englacially to emerge below the equilibrium line altitude (ELA). Debris accumulation on the surface damps the rate of melt of underlying ice. The termini of debris-covered glaciers extend well beyond debris-free counterparts, decreasing the ratio of accumulation area to total area of the glacier (AAR). In contrast, rock glaciers (i.e., glaciers with a full rock mantle) occur where and when the environmental ELA rises above the topography. They require both avalanches and rockfall from steep headwalls. That ice-cored rock glaciers depend on avalanche sources makes them most common on lee sides of ridges and peaks where wind-blown snow enhances the strength of the avalanche source. To maintain positive mass balance, the avalanche cone developed in the winter must be sufficiently thick not to melt entirely in the summer, thus providing an ice accumulation area for the rock glacier. In the absence of rockfall, this would support a short cirque glacier. The presence of debris, however, facilitates the development of rock glaciers with lengths of hundreds of meters, thicknesses of tens of meters, and speeds of order meter per year. Numerical models suggest that climate warming can transform a debris-covered glacier into a much shorter rock glacier, leaving in its wake a thinning ice-cored moraine. Rock glaciers will persist in landscapes for much longer than their debris-free counterparts. Valley heads with steep headwalls will therefore oscillate between glacier and rock glacier occupation over glacial-interglacial cycles, maintaining a means by which rock from the headwall can be conveyed away. This enhances the asymmetry of alpine ridgelines, with downwind valleys biting headward into range crests, as originally noted by G.K. Gilbert.