An accumulation of aerosol that, when deposited on the surface of a glacier, modifies the mass balance through its effect on surface albedo. Saharan dust, for instance, sometimes has a substantial impact on the mass balance of European glaciers. Volcanic eruptions can deliver dust and ash to nearby, and sometimes to distant, glaciers. In extreme cases the added material can turn the glacier into a debris-covered glacier. Dust can help to define the summer surface, and a dateable dust layer in firn or glacier ice can be useful as a marker horizon.