Reconsidering the glacier to rock glacier transformation problem: New insights from the central Andes of Chile

• We study a debris-covered glacier whose morphology is turning into rock glacier morphology.
• The common criteria defining a rock glacier are thoroughly tested and verified.
• Permafrost conditions exist at the site with low sub-debris ice ablation rates.
• According to potential debris-covering times, the glacier sediment store is consistent with common rock wall retreat rates.
• The glacial landscape is shifting towards a periglacial landscape.

The glacier to rock glacier transformation problem is revisited from a previously unseen angle. A striking case in the Juncal Massif (located in the upper Aconcagua Valley, Chilean central Andes) is documented. There, the Presenteseracae debris-covered glacier has advanced several tens of metres and has developed a rock glacier morphology in its lower part over the last 60 years. The conditions for a theoretically valid glacier to rock glacier transformation are discussed and tested. Permafrost probability in the area of the studied feature is highlighted by regional-scale spatial modelling together with on-site shallow ground temperature records. Two different methods are used to estimate the mean surface temperature during the summer of 2014, and the sub-debris ice ablation rates are calculated as ranging between 0.05 and 0.19 cm d− 1, i.e., 0.04 and 0.17 m over the summer. These low ablation rates are consistent with the development of a coherent surface morphology over the last 60 years. Furthermore, the rates of rock wall retreat required for covering the former glacier at Presenteseracae lie within the common 0.1–2 mm y− 1 range, assuming an average debris thickness and a range of debris-covering time intervals. The integration of the geomorphological observations with the numerical results confirms that the studied debris-covered glacier is evolving into a rock glacier.