Evolution of a cave

A moulin forms in a precise point of the glacier where fractures are favourable, and like whatever is on top of or inside the glacier, it is then slowly dragged downstream by the movement of the ice: next spring a new moulin will form in the same point, and the old moulin, deprived with water, which has been captured by its younger upstream companion, will slowly close, due to the plastic swelling of the ice, till it finally disappears after a few years, while new moulins continue to form further upstream. For this reason moulins are almost always in groups, aligned in a precise direction and always in the same point of the glacier: from upstream, moving downstream, it is possible to observe all the stages of the life of a moulin, from moulin “embryos”  that are fractures slightly widened by water, to baby moulins, cylindrical holes with a diameter of only a few centimetres, which are often many metres deep,  up to large shafts dozens of metres deep, and a few metres wide, with complex forms, and finally old inactive shafts, fossil and silent, that year after year become inexorably narrower, till they disappear without leaving a trace.
We are used to thinking of geological phenomena as processes that are mostly slow, even though inexorable, and it is surprising to see the speed at which glacier caves form, modify, and disappear: when returning, even only few days later, to observe the same moulins, it is possible to note deep changes in shape, size, quantity of water supply, so much so that at times one may even doubt one is observing the same structure. In order to study these kinds of cavities therefore it is necessary to mark  them with stakes so as to be able to recognize them year after year, and to draw their topographical features, in order to monitor the  variations in  their shape and depth. In this way, for example, it has been possible to study the moulins of the Forni glacier and to establish that moulins have an average life of at least 6 years,  of which the first three are necessary to reach the maximum size and the following years show the progressive decline. In larger glaciers, as in the Svalbard islands, old moulins over 25 years old were observed. In any case, independently from the ice thickness, the maximum depth of ice caves does not exceed 200 m (203 m, to be precise, in Greenland): no cave in fact can exist below this depth, considering the limit of  fragile ice.

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