  Dirt cones are meter-scale structures encountered at the surface of glaciers,
which consist of ice cones covered by a thin layer of ashes, sand or gravel,
and which form naturally from an initial patch of debris. In this article, we
report field observations of cone formation in the French Alps,
laboratory-scale experiments reproducing these structures in a controlled
environment, and two-dimensional discrete-element-method-finite-element-method
numerical simulations coupling the grains mechanics and thermal effects. We
show that cone formation originates from the insulating properties of the
granular layer, which reduces ice melting underneath as compared to bare ice
melting. This differential ablation deforms the ice surface and induces a quasi
static flow of grains that leads to a conic shape, as the thermal length become
small compared to the structure size. The cone grows until it reaches a steady
state in which the insulation provided by the dirt layer exactly compensates
for the heat flux coming from the increased external surface of the structure.
These results allowed us to identify the key physical mechanisms at play and to
develop a model able to quantitatively reproduce the various field observations
and experimental findings.
