How dirt cones form on glaciers: field observation, laboratory experiments and modeling

TL;DR

This study combines field observations, laboratory experiments, and numerical modeling to understand how dirt cones form on glaciers, revealing that insulation effects cause differential melting and shape evolution.

Contribution

It introduces a comprehensive model that explains dirt cone formation through thermal insulation and grain flow, validated by field, lab, and simulation data.

Findings

Dirt cones form due to insulation reducing melting underneath.

Differential ablation causes surface deformation and grain flow.

Cones reach a steady state where insulation balances heat flux.

Abstract

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.