The effect of lithology and grainsize on the ablation of glaciers

Most glaciers are retreating, i.e. the loss of snow and ice (ablation) during the summer is larger than the amount of snow fall during winter. The rate at which ablation occurs generally depends on two independent factors, 1) the atmospheric conditions, such as air moisture, solar radiation, temperature, and 2) the reflectivity which greatly depends on the characteristics of debris coverage (e.g., thickness, grain-size and colour). The atmospheric conditions may vary dramatically on any temporal time scale. In order to decipher the influence of the debris coverage on glacier melting, we prepared eleven different test fields (one without coverage) at the elevation of 3000 m on the Vernagtferner glacier in the Austrian Alps and tested the effects of 1) type of lithology (albedo), 2) grain size, and 3) debris cover thickness. The test area was inclined by less than five degrees to North. Each test field was covered by a certain lithology with controlled grain size and coverage thickness. We used three different lithologies: 1) mica schist (MS), which typifies the local lithology, 2) black, basaltic tephra (EB) of Etna volcano (Italy), and 3) grey, trachytic pumice (SC) of Sete Cidades volcano (Azores, Portugal). We used four different grain size ranges, all in the sand and gravel fraction: 1) between 1 and 2 mm (MS, EB, SC), 2) between 5.6 and 8 mm (MS, EB, SC), 3) between 2 and 3 cm (MS), and 4) larger than 5 cm (MS). The test fields were placed in very close proximity and were up to 1 m2 large. The debris cover thickness usually ranged between 3 and 5 cm, for the coarse gravel, we prepared test fields with up to 18 cm debris coverage. We placed a level in the middle of each test field and measured the individual melting rates over one month (end of June to end of July) on twelve different days, sometimes twice daily (morning and afternoon). Over the 4 weeks period, the uncovered glacier lost 1.55 m of ice thickness whereas the minimum ice loss in one of the test fields was 78 cm (average at all test fields of 96 cm). We will present data on the influence of granulometry and the physical properties (porosity, colour) of the used debris on the melting of glacier ice under identical meteorological conditions.