Modello genetico ed evolutivo dei mulini glaciali sulla base di osservazioni sui Ghiacciaio dei Forni (alta Valtellina. Italia Settentrionale)

Genetic and evolutive model for glacial sinkholes on the basis of field observation on Forni Glacier (Valtellina, Northern Italy)

Abstract

Since 1994, glacial sinkholes in Forni Glacier (Valfurva, Northern Italy) have been the object of systematic observation, with the aim to understand their genetic and evolutive mechanisms, their geometric arrangement, and the hydrodynamics of related en- and subglacial aquifers. Field observations every year required locating cave entrances on a field-measured survey network, in order to measure their annual movements, together with the survey of the main caves, which let us appreciate changes in shapes, dimensions, depths, and surface stream water supply. Measurements have been made of ice discontinuities such as jointing and foliations: they proved to exert a strong control on caves systems’ geometrical arrangement and morphologies. As for the origin and evolution of these cave systems, some models are at present being proposed; these models are very similar to one another despite they have been carried on independently by different researchers in different glaciers all over the world: this suggests the existence of a general process, which is valid independently of the dimensions and the characteristics of flowing of any glaciers, provided they are temperate enough to allow ice melting and liquid water circulation. The suggested general models are mainly based on local and non-continuous-in-time field observations on a large number of glaciers all over the world, while only very few systematic studies have been carried out for several years on the same glacier, both in Italy and abroad: the data on Forni Glacier are therefore interesting, because they are field corroboration of mainly theoretical models. On Forni Glacier about fifty main caves and many other minor ones have been detected. The cave systems are organized in a series of sinkholes, or moulins, aligned along certain directions (mainly sets of main discontinuities, related to ice jointing and foliations). They entirely or partly catch the water flowing along surface bédières, leading water circulation from surface to englacial, or possibly subglacial, zones. Thanks to the thin ice-sheet, which is about 100 m thick in its thickest portion, the englacial flow may be drained towards the subglacial zone, where englacial cave systems are cut by deep fractures and crevasses at the glacier front. In each system, downstream moulins are year by year fossilized by the forming of new sinkholes in an upstream position: fossilized moulins are gradually carried downstream by glacier flowing, so that it is possible to observe sinkholes in all their different phases of evolution, from a proto-moulin phase, merely a fracture enlarged by ice melting and water friction, to large sub-circular shafts, some meters large and over 40 m deep, to relict, completely fossil moulins, whose dimensions gradually decrease because of plastic collapse when water feeding stops. Every year, the formation of new sinkholes was observed, always in the same position with respect to the bedrock, probably just above rock rises, which create in the overlaying ice tensional conditions inducing open jointing, which, in its turn, causes surface water to seep at depth. During summer season, neoformed moulins fossilize downstream sinkholes that survived during winter season. A systematic observation of the caves allowed us to estimate an age: a time span of 6 years is assessed from the moment a proto-moulin is formed, generally at the intersection of two or more discontinuity sets, to the moment, after reaching its maximum size, the moulin gets fossilized and deprived of water feeding by the forming of new upstream sinkholes, getting gradually narrower by ice plastic collapse, till it finally disappears. The observation of very peculiar ice textures inside moulins suggests that during winter season ice caves might get completely filled with water because of a raising of the water-table caused by ice closing discharge points at the glacier front. The drowning of the caves and the subsequent formation of secondary ice, possibly completely filling the englacial conduits, probably cause the surviving of moulins season by season: when deprived of water feeding, these features are in fact very prone to collapse and disappear in the course of few weeks. The maximum depth of the sinkholes is about 40 m; curiously, the depths of the explored shafts (about 30 in number) do not show a homogeneous distribution, and three main depth classes are observed, at about 2-3 m, 10-15 m and 35-40 m depth. Obviously, the active shafts are the deepest ones. Proto-moulins on their way of forming often exhibit a depth (10-20 m) which is considerable if compared with their small diameter (few centimeters, or decimeters). Every system exhibits a regular distance between its moulins, the distance being evidently equal to the annual movement of the ice-sheet. This distance varies from 4-5 m to 15-20 m, and differs from one system to the others. We may therefore suppose the glacier to have different velocities in its different portions: this causes the existence of portions submitted to strong shear stresses, which may act as an impervious barrier, and this probably causes the existence of parallel and independent cave systems. On the glacier surface, two different areas have been detected, to the W and to the E of the main middle moraine. The western area exhibits a regular slightly inclined topography, broken by a series of transverse steeper slopes. Here series of parallel cave systems are observed: they are regularly aligned along a 330° N striking foliation, the same direction guiding the arrangement of the surface bédières feeding the moulins. All sinkholes are formed at the intersection of the 330° N foliation with a set of orthogonal 250° N striking joints. As all sinkholes in the same evolution phase are aligned at the same height along this direction, it is easy to infer the formation of the sinkholes to be related with the existence of a sill stretching transversally to the direction of glacier flowing. The eastern area on the contrary exhibits a more complex topography, with systems of bédières whose arrangement is not parallel, but seems to converge towards a central depression of the ice sheet. In this area ice thickness exceeds 80 m. In the eastern area the bedrock may form an amphitheater sill, or, more probably, an irregular topography, with series of small rock rises, whose surface traces are marked by groups of aligned moulins. In this area distances between moulins are more irregular and closer than in the western area: this proves the existence of an irregular bedrock surface resulting in differential movements and stresses inside the ice mass. After these short preliminary considerations, it is evident that the study of ice cave systems may be of great interest in the comprehension of the englacial water drainage system and may give interesting information about ice flow dynamics.