Alpi e Prealpi centro orientali: cenni di geologia e geomorfologia

Abstract

Biogeographia vol. XXVI - 2005 (Pubblicato il 30 Dicembre 2005) Biogeografia delle Alpi e Prea/pi centro-orienta/i Alpi e Prealpi centro orientali: cenni di geologia e geomorfologia ALBERTO CARTON Dzpzzrtimento di Geogmfia qG. Momndz'mq’, Universitiz degli Studi di Padova e—mail: azrt0n@ unipd. it Key word: Central-eastern Alps and Prealps; geopmorphological evolution, geological evolution SUMMARY The central-eastern Alps and Prealps of Italy correspond to more than two thirds of the whole Italian Alps. They are subdivided into: Pennine, Iepontine, Retiche, Orobie and Atesine Alps, the Dolomites, and the Carnian and Julian Alps. The Prealps of Lombardy and Veneto are located in the southern part of the chain and are separated by the furrow ofI.ake Garda. From the orographic viewpoint three areas, arranged in a longitudinal pattern, can be recognised: i) the piedmont hills which comprise moraine deposits and hills of sedimentary and volcanic origin (moraine amphitheatres of Lakes Maggiore, Como, Lecco, Iseo, Garda and the River Piave, the Berici and Euganei Hills); ii) a relief area named “Prealpi” with peaks that seldom exceed 2000 m; iii) an alpine area with “high—mountain” peaks separated by large valleys with big or small glaciers. The Alpine chain is the result of the progressive approaching and subsequent collision between the northern part of the Apula (or Adriatic) plate and the European continent. In particular, the European plate was subducted under the African one and the front of the latter was overthrust over the European margin. The Alpine chain — whose emplacement took place during the Tertiary — started to be affected in its upper parts by weathering and erosion even before the early Quaternary. The influence of Pleistocene glaciations was of paramount importance for the shaping of the landscape that to date characterises . the central—eastern Alps, in particular with reference to the large glacial valleys and large lakes. During the last maximum glacial expansion the Alps were nearly entirely covered by mountain glaciers which formed an ice field characterised by a network of interdigitated flows, which were mainly confined within the valleys but locally they developed tongues toward the plain in the form of ice lobes at the foot of the hills. After the last glacial maximum the Alps were affected by rapid glacier withdrawal. Since the early Holocene — starting from 9000 yrs BP until about 5000 yrs BP - the Alpine glaciers showed an extension prevalently less than or, at most, equal to the extension developed in the second half of the Holocene. The actual extension of the glaciers in the central—eastern Italian Alps and Prealps during the Holocene Climatic Optimum is not directly known, owing to the fact that the subsequent glacier advance (Neoglacial) obliterated the most ancient geomorphological evidence almost everywhere. Other phases of etpansion preceding the “Little Ice Age” occurred some 3000 yrs BI’ and towards the end of the first millennium AD. The Little Ice Age consisted of a series of positive fluctuations spaced out by periods of relative shrinking of the glaciers. Starting from 1860 A.D., glaciers have been aflected by a considerable withdrawal which, in some cases, has attained 2 km of linear reduction, with a surface loss up to 45%. The present morphogenesis results from all the weathering processes which characterise high-mountain areas and extend as far as low hills. They act according to century—old and/or seasonal rhythms. There are frequent events which redistribute previously deposited materials (debris flows etc.); these take place in conditions of energy concentration under the violent action of runningwater, rain or avalanches, especially where there are marked elevation differences. Processes of accelerated degradation, especially in the mid- and high mountains, are more and more frequently associated with the constant and inexorable modelling of slopes. In some cases, these extreme processes can be related to the sudden climate changes presently in progress. INTRODUZIONE Ie Alpi e le Prealpi centro orientali, rappresentano poco piii dei due terzi della catena alpina italiana e, secondo la tradizionale partizione sistematica‘ dell’Arco 1) La tradizionale suddivisione delle Alpi, articolata su tre parti (Alpi Occidentali, Centrali e Orientali), 26 sezioni e 112 gruppi E: stata introdotta in Italia nel 1926 dal Comitato Geografico Nazionale sulla base dei “Nomi lirniti delle grandi parti del Sistema Alpino” proposti da una Commissione nominata allo scopo dal IX Congresso Geografico Italiano.