Debris deposits on ice can either enhance or hinder glacial ablation relative to a clean ice surface. The effect of debris on ice is dependent on whether its thickness is above or below a critical thickness. This is the thickness at which ablation of the underlying ice equates to that of bare ice. It is dependent on the properties of the debris material, as well as local conditions. The small amount of published data available indicate that volcanic deposits have lower critical thickness values than non-volcanic deposits at similar latitudes and altitudes, suggesting that they are more effective insulators. Volcanic ash fall events are significant for glacial ablation because (1) they mantle topography and distribute well-sorted material over a wide area, and (2) the locations of Earth’s glaciers often coincide with regions of active volcanism; at least 280 active volcanoes have ice or snow cover. At ice-capped volcanoes, volcanic material will also be deposited on glaciers by erosion and aeolian remobilisation.
Volcán Sollipulli is an active, ice-filled caldera volcano in southern Chile (38.97°S:71.52°W) which last erupted 700 years ago. Satellite imagery and comparisons of field photographs show that the intra-caldera ice body is receding. The Sollipulli glacier is a local water resource and an increase in melting rate could produce lahars. Sollipulli lies between Llaima and Villarrica, two of the most active volcanoes in Chile, and the potential for its glacier to receive ablation-changing ash fall from other volcanoes in the Southern Volcanic Zone of the Andes is high. Indeed, it has in June-July 2011 received ash fall from Puyehue-Cordón Caulle volcano to the south. It is, therefore, important to understand the ice ablation effects of volcanic debris deposited both at Sollipulli and on glaciers worldwide.
In February-March 2011, field experiments were conducted on the surface of Sollipulli glacier, on both ice and snow, in order to determine critical thicknesses for the local volcanic debris. This debris is naturally distributed as a fine grained discontinuous covering across the glacier. Dirt cones were observed to form where patches of debris had thicknesses of 1-12 cm. Results indicate that volcanic material overlying ice at Sollipulli enhances ablation of ice where cover is discontinuous, with ablation rate increasing with debris concentration. Once debris cover becomes continuous, behaviour changes to hinder ablation, with ablation rate decreasing with increasing deposit thickness. On a snow surface, critical thickness is approximately 3 mm. The natural dusting of debris across the glacier surface observed in February-March 2011 accelerates ablation, while the dirt cones insulate the underlying frozen substrate, relative to a debris-free surface. Whilst the dirt cones are not extensive, the debris dusting is; therefore the net effect of volcanic material at Sollipulli is likely to be enhanced ablation. Any distal deposition of ash fall on the glacier, such as that of the Puyehue-Cordón Caulle ash fall deposit, either as a discontinuous covering or as a thin continuous layer, could have significant consequences for its long term preservation. These results are likely to be applicable for volcanic deposits on glaciers and ice sheets across the globe.