Lunedì Nov 20

Karst phenomena

In the room dedicated to karstification we have two itineraries. To the right of the entrance, we have “Caves in Italy”. This itinerary provides an account of the extensive distribution of this geomorphological phenomenon in Italy. To the left of the entrance, we have the other itinerary – “What is karstification” – where the phenomenon is described, with an account of the superficial and deep effects in various environments and on various rock typologies.

The brief “Caves in Italy” itinerary includes material on some of the most well-known caves in Italy, in Lombardy and in the highlands of the Bergamo area. Special attention is paid to Lombardy, in which we may identify three territorial classes with very characteristic superficial and deep morphologies. The karstic zones of the high mountain areas are simultaneously subjected to the direct action of snow, surface water and below-zero temperatures, as a result of which the process of chemical and physical dissolution is greatly intensified. On the other hand, the areas impacted by the most recent glaciations present no superficial karstic phenomena, which were obliterated through glacial erosion and then covered over by glacial and fluvioglacial accumulations. Lastly, we have the areas of the most southerly hilly land in Lombardy, unaffected by the glaciations, in which both superficial and the more ancient, deep, karstic forms remain. The itinerary ends with surveys of three famous caves in the province of Bergamo – the Buco del Castello in Valle Brembana, the Büs di Tàcoi in Valle Seriana and the Grotta del Forgnone in Valle Imagna. The lithological samples most typical of the geological formations that host the more than one thousand caves of the Bergamo area are also exhibited.

The “What is karstification” itinerary begins with an account of the origins of this geological phenomenon, which owes its existence basically to the solvent action of water on rocks. Rainwater, rendered slightly acid through the presence of carbon dioxide in the atmosphere, dissolves the calcium carbonate present in certain minerals and rocks, changing it into bicarbonate. This leads to the formation of a variety of characteristic structures, ranging from small grooves to large dolines and caves, as such. The first cabinets host the rocks most frequently impacted by this phenomenon: limestones, dolostones, gypsums, and the main minerals constituting carbonate rocks, such as calcite, aragonite and dolomite. We find a large sample of limestone with flint of the Lower Jurassic period which clearly exemplifies differential corrosion in rocks of varying solubility. The corroded limestone features, in relief, non-soluble flint.

The water acting upon the limestone in the zone in which there is contact between the rock and the air gives rise to “superficial karstification”, with characteristic micro-forms and macro-forms. The most typical micro-forms are termed karrens, solution grooves and pitting, not to mention corrosion channels and kamenitzas, examples of which are displayed. A reconstructed sample of the karstic surface and subsurface illustrates the development of this phenomenon at our latitudes. A number of models illustrate the phenomena that lead to the formation of macro-forms, such as dolines, poljes and karstic valleys. One model illustrates the development of corrosion dolines, formed by the solvent action of superficial waters on cracked rocks. The resulting landscape is characterised by uniform vegetation cover of the field-pasture type, very widespread in the Pre-Alps of the Lombardy and Veneto regions. The other model illustrates a further type: the caved-in doline, formed through the collapse of the roof of a pre-existing cave. These structures are frequently found at altitudes of more than 2,000 metres, in the high altitude carbonate zones of the Lombard Pre-Alps .

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“Deep karstification” occurs when the water infiltrations descend along the discontinuities of carbonate rocks, absorbing calcium carbonate. Geological and climatic conditions impact the evolution of a karstic massif, in terms both of the manner in which underground water circulates and the distribution of the resulting galleries. In regard to water circulation, we may note two types of underground environment. One is the complete karst, in which we may identify an absorption zone, a percolation zone and a saturated zone, which feeds springs. The incomplete karst is a karst without a saturated zone. Caves are undoubtedly the form of deep karstification that is most striking and that we are most familiar with. These (often enormous) cavities develop deep below the surface, and owe their existence to the prolonged dissolution action of water on carbonate rocks. A cave may also be formed by two or more galleries joining. When saturate water flows inside a cave, it tends to re-deposit its dissolved calcium carbonate content. This leads to the formation not just of the concretions which we are all familiar with, known as stalactites and stalagmites, but also more extensive structures such as coralloid deposits and pisoliths. The cabinets display many samples from Italian caves, and rare gypsum and salt concretions.

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Another section of the room is dedicated to a very special phenomenon – “submarine karstification”. Sea water contains large quantities of salt, and it attacks the rocks that are more soluble than carbonate rocks. Particular conditions many lead to karstic dissolution also on the seabed. The Mediterranean nearly dried up completely some 5 million years ago, during the Messininian period. On the seabed, deposits formed of potent evaporitic series or large quantities of minerals and rocks originating from water evaporation leading to precipitation of the salts dissolved in this water. Normal sedimentation on the seabed then came about again. A record of this major phenomenon can be seen in the exhibited large, rare gypsum crystal. The crystal was dredged from the Bannock basin, in the Mediterranean, during a research campaign conducted on the floor of an underwater doline off the Sicilian coast, at a depth of some 3,000 metres. The section then examines the spread of pollution in caves. As with all other natural environments, caves too have undergone significant deterioration caused by pollution. Pollutants spread very rapidly inside karstic massifs, and contaminate enormous volumes of water. Chemical and physical alterations of underground water and the disappearance of cave animal populations represent the first symptoms of gradual deterioration of the underground environment, with grave consequences for this major source of drinking water.

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The last part of this section concerns the theme of “Life in caves” and the age-long bond between Man and these naturally occurring cavities. The life forms present in caves are conditioned by very particular environmental conditions, characterised by considerable dampness, modest temperature shifts, and darkness). Certain animals, generation after generation, have adapted to life under these conditions. They have developed specific characteristics such as depigmentation, a shrinking or absence of the eyes, and a lengthening of the legs and antennae. Some species also display reduced juvenile stages. We also note that Man has used caves for thousands of years. Caves have served various purposes, such as housing, defence, storage, shrines and tombs. Important mineral deposits are also to be found in karstic cavities. Over the centuries Man has sought out the most effective methods for exploiting these environments, abounding in mineralization. During the Quaternary period, many mammals of our regions took refuge in caves. Locally, a highly typical case in point is the cave bear. The skull is displayed of one of these large plantigrades, now extinct, from a cave in Lombardy. Thanks to the very many findings made in the Bergamo area, scholars, over the last few years, have come to confirm that, while the cave bear was as large as today’s grizzly, its diet was mainly vegetarian.