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Supplements of Geografia Fisica e Dinamica Quaternaria
Volume V - 2001


Proceedings of the 8th Italian Glaciological Meeting


Modello genetico ed evolutivo dei mulini glaciali sulla base di osservazioni sul ghiacciaio dei Forni (Alta Valtellina, Italia settentrionale) – Genetic and evolutive model for glacial sinkholes on the basis of field observation on Forni Glacier (Valte llina, Northern Italy)

Pages 165-178


Since 1994, glacial sinkholes in Forni Glacier (Valfurva, Northern It aly) have been th e object of systematic observation , with the aim to understand their genetic and evolutive mechanisms, their geometric arrangement and the hydrodinamic s of related en- and subglacial aquifers. Field observat ions every year required to locate caves ent rances on a field-measured survey network , in order to measur e their annual movements, together with the survey of the main caves, which let us appreciate changes in shapes, dimensions, depths and sur face str eam water supply. Measurements have been made of ice discontinuities such as jointing and foliations : th ey proved to exert a strong control ‘on caves systems geometrical arrangement and morphologies. As for the or igin and evolut ion of these caves systems, some models are at pre sent being proposed ; th ese models are very similar one to the others despit e they have been carri ed on ind ependentl y by different researche rs 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 flowin g of any glaciers , pro vided they are temperate enough to allow ice melting and liquid water circulation. Th e suggested gene ral models are mainly based on local and non -continuousin-time field ob servation s on a large number of glaciers all over the world, while only very few systematic studies have been carried on for severa l 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 the oretical models. On Fo rni Gl acier about fifty main caves and man y other minor ones have been detected . The caves systems are organised in a serie s of sink – holes, or moulins , aligned along certain directions (mainly sets of main discontinuities, related to ice jointing and foliation s). They entirely or partly catch the water flowing along surface bedieres, leading water circulation from sur face to englacial, or possibly subglacial, zones. Thanks to the thin ice-sheet , which is about 100 m thick in its thicker portion , the englacial flow may be drained towa rds the subglacial zone , where englacial caves systems are cut by deep fractures and cre – vasses at the glacier front. In each system, downstream moulins are year by year fossilised by the formin g of new sinkholes in an upper stream positi on: fossilised moulins are gradually carri ed downstream by glacier flowin g, so that it is possible to observe sinkholes in all th eir different phases of evoluti on , from a proto-moulin phase, merely a fracture enlarged by ice meltin g and water friction, to large sub -circular shafts, some meter large and over 40 m deep , to rel ict, completely fossil mou lins, whose dimensions gradually decrease because of plastic collapse when water feeding stops. Every year, the formation of new sinkholes was obs erved , always in the same positi on with respect to the bedrock, probably just above rock rises, which create in the overlaying ice tensional conditions inducing open jointin g, which , in its turn, causes surface water to seep at depth. During summer season, neoformed moulins fossilise downstream sink – holes that survived during winter season. A systematic observation of the caves allowed us to estimate an age: a time span of 6 year is assessed from the moment a proto-moulin is formed, generally at the int ersect ion of two or more discontinuities sets, to the moment, after reaching its maximum size, the moulin gets fossilised and deprived of wat er feeding by the formin g of new upstream sinkholes, getting gradually narrower by ice plastic collapse, till it finally disappears. Th e obs ervation of very peculiar ice textures inside nzoulins suggests that during wint er season ice caves might get completely filled with water becau se of a raising of the water-table caused by ice closin g disch arge points at the glacier front. The drowning of the caves and the sub sequent formation of secondary ice, possibly completely filling the englacial conduits, probably cause the surviving of moulin s season by season : when depri ved of water feeding, these features are in fact very prone to collapse and disappear in the course of few weeks . Th e maximum depth of th e sinkholes is about 40 m; curiously, the depths of the explored shafts (about 30 in number ) do not show an homogeneous distribution, and three main depth classes are observed , at about 2-3 m, 10-15 m and 35-40 m dep th. Obviously, the active sha fts are the deepest ones . Proto-moulins on their way of forming often exhibit a depth 00-20 m) which is considerable if compared with th eir small diamet er (few centimetre, or decimetre).

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