Rock avalanches are one of the most hazardous landslide phenomena. They involve huge, extremely rapid flowing masses, carrying a strong power of destruction and reaching high mobility. The present article shows results of back-analysis of the Potrero de Leyes case in the Argentinian Andes. This rock avalanche has travelled on an unconstrained topography showing a great mobility, even without any channelizing effects. Simulations are carried out with VolcFlow, a continuum model, first on an unconstrained granular flow experiment and then on Potrero de Leyes. Results show that purely frictional model works well for the experiment, but that it can only reproduce the longitudinal runout in the field, and this provided that the basal friction coefficient used is close to the measured Fahrböschung. Here, since the rock avalanche starts as a coherent mass that disaggregates and shatters only during its propagation, lateral spreading is overestimated and only the use of a viscous rheology produces better results.