And inside? Glacier caves

Melting water on a glacier surface collects in small seasonal water streams that erode their bed in the ice, and are called bédières (a French term): in fact, ice is impermeable and does not allow the water to seep in deep. However, glaciers are characterized by a number of fractures that run across the surface. Through these fractures surface water  can filter in and flow within the glacier.
Liquid water is obviously warmer than the ice it comes into contact with, and determines its melting, creating a system of empty spaces, underground passages, shafts and galleries that are similar to the systems of caves in the rock. The difference is that caves in the rock are created by chemical processes, (dissolving limestone) while ice caves are formed due to a physical process (ice melting). The ice cavities form in all the glaciers that are “warm” enough for water in the liquid state to be present. The formation of ice caves is very rapid, and can be observed, one may say, in “real time”: the cavities form and change during the course of a few weeks or a few days and this offers the opportunity to understand analogous forms that developed, in much longer times, in the rocks. For ice caves to form, ice, which is impermeable, must be broken by fractures that enable water to penetrate deeply and, as it melts the surrounding ice, to  widen these fractures giving rise to the formation of  shafts  and galleries, which may even be several metres in diameter.
On the surface shafts and sinkholes are noted, known as glacier sinkholes or moulins (glacier mills),  because the water spins like in a water mill, through which water seeps into the depths of the glacier. Due to the effect of gravity, water tends to follow a way as vertical as possible, creating large shafts and deep gorges in the ice that is fractured due to the enormous tensions that develop within its mass, and flows slowly under the thrust of its own weight. Beyond a certain depth (approximately 150-200 m, and it is equal for all glaciers, independent of their thickness), ice becomes plastic and behaves like an impermeable barrier that prevents water from seeping more deeply in its course: thus horizontal galleries are formed, these are completely flooded and convey water from the sinkholes right to the front  where, due to the presence of deep crevasses, it can reach the base of the glacier, and then flow outside through the “mouths” of the glacier, with galleries that can be several metres wide, out of which the turbulent greyish waters of the glacier drainage channels flow. The “mouths” of a glacier often really look like large “mouths”, similar to the opening of an oven, from which the toponyms of some of the Alpine glaciers derive (oven is forno in Italian) (The Forni Glacier in the  Ortles-Cevedale group, the Forno Glacier in the Bregaglia valley, Switzerland).
The best places to observe glacial sinkholes are the plain areas,  far from the areas with crevasses, or along the medial moraines, or on the sides of the glacier. These may be found in all the Alpine glaciers, but only in some cases these reach a size that can be penetrated by man. Large sinkholes may be found, for example, on the Gorner Glacier, on the Mer de Glace and on the Forni Glacier.

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