How does a glacier work?

The mass of ice that forms a glacier is not a static and homogeneous mass: the ice has different characteristics in various points of the glacier, and behaves differently depending on the compactness, density, temperature within and at the base of the glacier, and the characteristics of the bedrock the glacier rests on. On the surface of every glacier, therefore, it is possible to identify different areas, where different processes are taking place, which mould the shape of the glacier and determine its behaviour. In every glacier it is possible to identify two fundamental areas : the accumulation area where the snow that falls in winter is preserved during the hot season, which is the area where the glacier receives the supply of snow that is necessary for its survival, and the ablation area, where, instead, there is a loss of ice, mainly due to the melting of the snow that fell during the previous winter season, and the ice left bare after snow melting, but also due to collapse and detachment of material from the glacier body, as in the case, for example, of the formation of icebergs. Therefore there are areas of the glacier where ice is produced, and areas where, on the contrary, ice is destroyed and moved away. The two areas are easy to recognize in summer : the accumulation area in fact has a white surface covered with snow and firn, while the ablation area shows weathered ice, whose appearance is generally “dirty” due to the presence of rock debris that appear on the surface of the ice. The extension and importance of these two areas characterize every glacier and condition their behaviour. The extension of the two areas is not fixed in time – in fact they are separated by an equilibrium line that coincides, approximately, with the perennial snow limit. Since this limit varies greatly depending on the climatic conditions, short term and long term variations in the climate greatly influence its position, and consequently the magnitude of the accumulation and ablation area. The body of a mountain glacier is usually confined by the rocky mountain slopes surrounding it, generally on almost all the sides, however there usually is a non-confined side, where the glacier is free to expand or retreat. This is the glacier front, that indicates the limit beyond which the glacier cannot exist, because here, simply, ablation destroys all the ice. One of the most evident characteristics of a glacier, which differentiates it from a snow deposit, is that ice moves, sliding toward the valley, under the pressure of its own weight. In this manner the ice lost in the ablation area is continually replaced by new ice that forms in the accumulation area and is transported downstream by glacier movement.

Special reports

From the Multimedia section