Ablation of debris-covered ice: some effects of the 25 September 2007 Mt Ruapehu eruption

Abstract Melt rates of ice surfaces are strongly influenced by the existence of debris-cover. Dependent on thickness, climate and physical properties of debris, a debris layer can enhance or reduce ablation, compared to bare ice conditions. Ice ablation on bare ice and under varying thicknesses of tephra was measured using a network of 12 ablation stakes drilled into the Summit Plateau ice field, Mt Ruapehu, from 17 January to 2 February 2008 in order to study how tephra from the 25 September 2007 eruption was affecting ablation. Observation of air temperature allowed the application of a simple positive-degree-day approach to the calculation of ablation rate factors, which for bare ice was 7.8 mm d−1 °C−1, with a rate of 6.0 mm d−1 °C−1 mm for ice under 10 cm of tephra. Mean daily ablation rates varied from 116 mm d−1 on bare ice to 78 mm d−1 under tephra cover. Debris covers thicker than 70 mm reduced ablation.