Processes delivering debris to glacier surfaces include debris flows, snow avalanches, rockfalls, and rock avalanches (Gordon and Birnie, 1986). In tectonically active regions, earthquake-generated rock avalanches are significant (Post, 1964; Hewitt, 1988), but many rock avalanches have no obvious trigger (Gordon et al., 1978; McSaveney, 1992). The input of debris to glacier surfaces by mass movement processes is partly controlled by catchment topography, which determines the gradient, area and distribution of debris source areas. Additionally, bedrock lithology can introduce contrasts in supraglacial debris supply between otherwise similar catchments. For example, granite batholiths overlooking the Baltoro Glacier (Karakoram Mountains) yield little debris, whereas fissile sedimentary rocks in the same region are associated with high debris supply. The importance of the relative rates of debris and snow/ice delivery to the glacier surface is often overlooked. Glaciers with high debris concentrations tend to occur where rockfall rates are high and/or snowfall is low. For example, on the main Himalayan chain in Nepal, where precipitation is low (~500 mm year-1) and the rapid uplift of young sedimentary rocks and extreme topography encourages very high rates of rockfall delivery to glacier surfaces, the majority of glaciers have debris-mantled ablation zones, and rock glaciers are widespread (Müller, 1980). In contrast, in western Norway, where crystalline rocks yield low rockfall rates, and precipitation is in the range 2,000-3,000 mm year-1, glaciers have little or no surface debris and rock glaciers are absent.
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