Italy's Geological Substratum

Even if it is not very extensive,theItalian territory is distinguished by the considerable variety of its substratum rocks. The Alps are largely formed from crystalline rocks (granites, gneisses, mica-schists, porphyries, etc.) but there are also sedimentary rocks (limestones, dolomites and sandstones) that are widespread in the eastern sector and the pre-Alpine belt. Sedimentary rocks are also prevalent throughout the Apennines (limestones, dolomites, sandstones, clays, marls, etc.), including Sicily, and are found in Sardinia too, where crystalline and volcanic rocks predominate. There latter (formed from ancient and recent lava and tufa) also appear in Sicily and along the peninsula's Tyrrhenian margin (where there is a considerable concentration of volcanic phenomena, in part still active) as well as in the Alps. Finally, the flat areas, including the great Po-Venetian Plain, are basically formed of mixed deposits that are mainly fluvial in origin (conglomerates, gravels, sands, clays). The great variety of rock types characterizing the Italian framework is mainly the result of a complex geological past, distinguished by marked environmental alternations - now marine, now continental - as well as frequent changes in climatic conditions. Furthermore, even if present mountain forms are considered to be rather recent, Italy does contain extremely old rock formations. Some of the metamorphic outcrops in the Alpine arc and in the Sardinian-Corsican and Calabrian-Peloritan massifs were formed before the Palaeozoic era, that is more than 600 million years ago, and therefore do not contain significant traces of organisms. During the Palaeozic era (lasting from circa 570 to 230 million years ago) the area now occupied by Italy was largely covered by a tropical sea (called Tethys by geologists) from which must have emerged some mountain folds, as those of the Caledonian period, begun some 500 million years ago and whose traces remain in southwestern Sardinia (Iglesiente and Sulcis). The next mountain building period, the Hercynian, occurred during the last 100 million years of the Palaeozoic era and was accompanied by considerable volcanic activity. This provoked the formation of the original nucleus of the Alpine chain together with the emergence of the Calabrian-Peloritan mountains (Aspromonte and Sila in Calabria and Peloritan in Sicily) and the Sardinian-Corsican massif. The volcanic activity of this period also affected the Alpine arc (porphyry effusions in the Adige Valley), as well as in the northern Apennines (Garfagnana and Apuan Alps) and Sardinia and Corsica. Following the Hercynian orogenesis, the mountains formed by it were subject to intense erosion. Thus at the end of the Palaeozoic era there emerged from the waters of the Tethys (the extensive oceanic basin separating the Euro-Asiatic continental plate from the African) the remains of the palaeo-Alpine chain, part of the northern section of the peninsula - probably connected with the Sardinian-Corsican massif, and, further south, the other great island fold of the Calabrian-Peloritan massif. During the course of the succeeding Mesozoic era, lasting for over 160 million years, almost all the present area of Italy remained covered by a large marine basin on whose bottom (which varied considerably in depth) was deposited on different occasions material of various types. This was to produce, following a process of compaction and orogenesis, disparate rock formations: limestones, dolomites, sandstones, marls, etc. In particular, in the northeastern area there formed extensive coralline reefs from which the present Dolomites are derived. Towards the end ot the Mesozoic era the progressive moving together of the African and European continental plates reduced their common marine space and caused a folding of their respective margins and part of the bed of the Tethys. This was to produce the Alpine and Apennine chains whose curvature reflects the anticlockwise movement of the contact line between Europe and Africa produced by the particular forces of their respective plates. Their collision took place some 40 million years ago (between the Eocene and Oligocene periods) in the first-half of the Cenozoic era, which is considered to have lasted from circa 65 million to 2 million years ago. lc>The formation of the Alps and the Apennines continued throughout the Cenozoic, slackening in the succeeding Miocene and Pliocene periods in which however some uplifting continued. This was accompanied by intense volcanic activity that has left traces in the Lessini Mts. (Venetian pre-Alps), Euganean Hills, Sardinia, Tuscany and Sicily (Iblei Mts.). Already, however, during the Miocene period erosion had considerably increased on the Alpine and Apennine peaks and this also continued in the Pliocene period, resulting in the depositing at the feet of the chains of huge deposits of sand, gravel and clay. There then followed a phase of general increased marine predominance, lasting a good part of the Miocene and all the Pliocene. At the end of this latter period, circa 1.8-2 million years ago, with the withdrawal of the sea and the filling up of the great Po depression the shape of the present-day Italian region and particularly the peninsula and islands began to gradually appear. The Neozoic era, which is still in progress, was characterized in its early part (corresponding to the Pleistocene period) by alternating warm and cold climatic phases, which resulted on several occasions in the expansion and retraction of the Alpine and Apennine glaciers with a consequent alteration in sea level. The last glaciation ended circa 10-12 thousand years ago, giving way to the current Holocene period characterized in Italy by temperate climatic conditions. During the Neozoic era, usually called the Quaternary, volcanic activity has re-occurred very intensely especially on the Tyrrhenian side. Surface erosion followed the relief modelling, filling in with detritus the internal Apennine depressions previously occupied by lakes (Val d'Arno, Val Tiberina, etc.) and also forming the plains at the edges of the peninsula and islands. At the same time, while our present flora and fauna were evolving, there appeared the first known representatives of the human species in Italy, whose traces have recently been found near Isernia (La Pineta) and date to some 730,000 years ago.