published on 7 January 2010 in earth
Alps and Appennines
How a mountain chain is born
The term orogeny was born from the Greek words oros (mountain) and genesis (origin); it indicates all the geological processes that engender the formation of a mountain chain. Indipendently from the geographical position, climate or altitude, all mountain ranges are the result of a collision between lithospheric plates that, just like a mosaic, make up the external part of our planet. The collision takes place in the subduction zones, between plates that are constituted entirely by oceanic crust that give birth to volcanic islands, or between an oceanic crust plate that, being thicker and heavier, slides beneath a lighter continental crust plate forming cordilleras, such as the Andes or Rocky Mountains. When there is a clash between the continental crust plates, which have the same density, none of the two is willing to slide easily underneath the other and little by little, but inescapably, the huge pushes of the two continents facing each other create the most spectacular chains, higher and with more complex structures, like the immense mountain arc that goes from the Pyrenees and the Betic Chain to the Alps, from the Dinarids to the Tauri, up to the Karakorum and Himalaya. Mountain chains are therefore the enormous scars that testify the movements of the lithospheric plates and show their ancient borders. Thousand of km of the Earth’s surface are covered by these “scars”, some young and very long, very high and with rough and jagged reliefs like the Alps, Karakorum, Himalaya, other more ancient and with softer curves, almost like hills, such as the Urals, the Appalachi or the French Massif Central: the shapes observed are the combined result of orogenic processes and tectonic alterations, that lift the chains, and of erosion processes, that shape the reliefs and tend to “delete” in the course of time the height differences and reliefs created by endogenous processes, in a never-ending cycle.
A constant evolution
Mountainous reliefs constitute an important element in our country’s scenery: mountains are visible from any point of our Peninsula, even at the center of the Padana Plain, even if often hidden by fog! It’s easy therefore for us to consider mountainous reliefs like something that is fixed and unchangeable, that has always existed and always will exist, but this is not so. Geologically speaking, our mountains are very young and they have been part of the Italian scenery only for 100 million years, a relatively short time in geological history. Alps and Appennines are “live” mountains: they move, transform and continue to grow, they do it so slowly that the process is not apparently perceptible to the scale of human life. Geologists, though, know how to recognise the phenomena that testify how the growth of Alps and Appennines still continues under our eyes: measurements with high-technology instruments even allow to measure the rises and falls of mountains. Moreover, if we observe the distribution of earthquakes in Italy, it’s easy to understand how earthquakes are distributed along belts that skirt the margins of Alps and Appennines, as a testimony of the movements that still take place in these areas. The lifting movements along ranges are also one of the causes of slopes’ instability and of the numerous landslides that charcterise the mountains and hills of the Peninsula.
The structural and geological conformation of Italian mountains is influenced by a long and complex history, which still carries the marks of a very ancient chain, the Ercinic chain, which molded itself more than 300 million years ago, however the more evident reliefs, Alps and Appennines, are recent structures in geological terms. They are the result of the compression exerted by the African plate through its rotational movement on the enormous Euroasiatic plate: the edges of the two plates have thus “curled up”, “doubled over” and deformed one against the other. As the two plates slid laboriously one beneath the other, the long arcs of the Alps and, immediately after, of the Appennines were formed. The same compression movement has generated the mountainous chains of all the countries that look out on the Mediterranean (from Greece to Albania, to Croatia, up to Spain, Tunisia, Morocco and Algeria), and is responsible for the seismic and volcanic activity of the Mediterranean regions,apart from creating the deep basins of the Thyrrenean Sea, of the Balearics and of the Ionic Sea and progressively shrinking the Adriatic Sea.
How is a mountain chain made?
Mountain chains look like elongated belts, aften arched, of reliefs and successions of high peaks, bordered at the margins by flat areas. You can distinguish an “internal” zone within the chain, less deformed, and an “external” one, towards which goes the alteration process. The external zone, not yet altered, towards which moves the chain, is called foreland. Between the foreland and the chain is the foredeep, a depression below which the subduction of one of the two plates takes place: it is here, in the foredeep, that the majority of the detritus and sediments produced by the dismantling of the chain settles. The foredeep appears on the surface as a flat and untroubled plain, but in its depths it’s the most active area of the whole chain where the crust fractures and great tectonic nappes fold and pile one on top of the other, doubling the normal thickness of the crust.
Geological history of Italy
If we could observe Italy as it was 250 million years ago, we would surely have great difficulty in recognising the places and sceneries that are so familiar to us today! The continents were grouped in the big Pangea mass, in which opened a great channel, the Tethys Sea. Our country used to be in the Western corner of this big gulf, underneath the waters of a shallow sea (200-300 m), very similar to today’s Adriatic, the margins of which used to have a landscape similar to the Bahamas carbonate platforms. This ancient sea’s deposits are still found in the sedimentary sequences of the Alps and Appennines. The only emerging parts of what would become our peninsula were a small area between Pisa, the Argentario and Sardinia. The lithospheric movements created big fractures and new litospheric plates started moving again, to move away and then collide with one another. So, subduction of the Tethys oceanic crust towards N ended up provoking a movement of the African bloc that, in our region, opened a new Ocean ( the Ligure-piemontese Ocean). This ocean extended roughly in N-S direction, separated on the E from the vast Vardar sea by a peninsula that extended N from the African coasts towards the European plate, the so called African Promontory, or Adria: the greatest part of the territory that would become Italy was on the Adria plate, apart form Sardinia, which was on the opposite European margin. In the Medium Cretaceous (100 My) an extremely important event for the evolution of the Mediterranean area took place: the expansion movements ceased and the Ligure-piemontese Ocean started to close under the pressure of the African plate that started to rotate on itself in anti-clockwise direction. The dense and heavy oceanic crust went in subduction once again beneath the African continental coast until the Ligure-piemontese Ocean completely disappeared: there are still traces of the crust that formed the bottom of this ancient ocean in the ophiolitic rocks (the “green rocks”) in Corsica, in the Western Alps, in Liguria and Greece, while the sediments that covered it now form the rocks that constitute the shell of our country. At the same time, to further complicate the situation, a fragment of the European margin got unstuck, and created what are now Corsica and Sardinia.
Once the crust of the Ligure-piemontese Ocean disappeared, the European and the African plate found themselves one in front of the other and, since they had the same density, they started to undergo consistent reshapings, under the unstoppable pressure of the African rotation. Thus the formation of the Alpine chain started and, at the same time, of the chains that go through Corsica, the Balearics and Southern Spain and of the various basins that form the Mediterranean.
A short history of the Alps
The Alps stretch for about 1000 km, with a width of 150-200 km, and constitute an arc that separates geographically our country and the Mediterranean area from the rest of Europe. The Alps continue towards NE with the Carpati, in the heart of Europe, and towards SE with the Dinaride chain, that descends from Istria and Croatia towards Greece. Towards W the Alpine chain arches and comes into contact with the Appenninic chain in correspondence with an important tectonic lineament, the Sestri-Voltaggio Line. The Alps are among the most studied chains in the world and here they have seen the birth of many of the most important geological theories. This chain’s history is very complex, but in broad terms its birth is due to the collision between the European plates and the Adria Promontory. The European plate went into subduction underneath the African one and the collision deformed the rocks and sediments of both margins, that crossed one over the other to give the typical alpine structure, called by geologists “blanketing nappes” structure. The alpine chain structure is complex and can be divided in two sectors separated by an important tectonic lineament, the Insubric Line or Periadriatic Lineament: this series of very long faults runs from W to E along the whole Alpine arc and separate the Alpine domain, mainly made of metamorphic rocks, from the Southalpine domain (or Southern Alps), where the faults are mainly made of sedimentary rocks.
The Alpine chain thus has a peculiar structure, that geologists call “double vergence” structure, with nappes taken towards N and towards the foredeep and fore-European country, and also towards S and towards the foredeep of the Padana Plain and the Appenninic forecountry. The anti-clockwise rotational movement of the African plate started in the Cretaceous and, with alternate phases of varying intensity, it still continues to these days. Three peak phases can be individuated: in the Cretaceous the eoalpine phase, the most ancient, during which the Ligure-piemontese Ocean disappeared; from Eocene to Inferior Oligocene (30 My) when the real continental collison started with the mesoalpine phase; from the Superior Oligocene to (25 My) the Alps gained the current double vergence with the neoalpine phase.
As the alpine chain was being moulded in the depths of the terrestrial crust, the first reliefs started gradually to emerge. The erosive processes immediately started to modify the landscape of the newborn chain producing an enormous quantity of sediments and detritus that settled at the reliefs’ feet, in the foredeeps. In the southern foredeep the vast sedimentary basin that constituted the Padana Plain was formed, and there in a few million years very thick deposits were collected: geologists calculate that under the Padana Plain the thickness of the sediments deposited in the last 5 My (Pliocene) reach 7000 m in the area of Parma and Reggio Emilia!
Brief history of the Appennines
If the Alps constitute the Northern border of our country, the Appenninic chain forms the peninsula’s “backbone”: it extends towards NNW SSE, from Genova where it grafts with the Alpine chain along the Sestri-Voltaggio Line, until the Sibari Plain in Calabria where after a short interruption due to the wedging of the Arco Calabro bloc, it continues in the Sicilian mountains with a NE-SW trend and the connects with the Maghrebide chain and the Tellian Atlas in Tunisia, Algeria and Marocco. The Appennines’ history is also long and complex, but in short it can be reconnected to the rotational Eastern movements of the Corsica-Sardinian Bloc, contemporaneous to the collision of the European and African plates, which was creating the Alpine chain in the North. This rotation started a bit later compared to the Alps’ birth, between the Superior Oligocene and Inferior Miocene (30-16 My): the Appennines are therefore younger than the Alps.
The Corsica-Sardinian Bloc’s movement has had two important consequences: on one side it has generated a compression from W to E that caused the subduction of the Adrian Western margin under the Corsica-Sardinian Bloc itself, creating the corrugation of the primitive Appenninic chain and its progressive movement towards the Dalmatian coasts, while on the other side it has provoked the progressive opening of two deep oceanic basins: the Provence Basin and the Thyrrenian sea. Precisely the progressive expansion of the Thyrrenian sea has brought, in the course of the last 7-8 million years (starting from the superior Miocene) to the formation of the Appenninic chain as we see it today, with the Calabrian Arc bloc that detaches itself from the alpine chain and gets attached to the southern part of the Appennine.
The Thyrrenian Basin is the youngest of the Mediterranean basins and, with its 3600 m, one of the deepest: on its expanding seabeds are some of the most important Mediterranean underwater volcanoes. Its opening, which still continues today, is dismembering the Appenninic chain. The constant compression along the eastern margin induces the formation of great folds and pushes the Appennines against the Dalmatian coasts by 1 mm/y. The western margin presents a distensive tectonic domain, with deep tectonic trenches (Graben) and distensive faults, that open the way to the rise of magma and subsequent volcanic phenomena (in Tuscanay, Lazio, Campania): the western Appenninic margin is thus characterised by vast tectonic basins (Val d’Elsa, Valdarno, Florence plain, Val Tiberina, for example), once covered by sea, then the site of great lakes (of which the only testimony that remains is lake Trasimeno).
What happens beneath our feet?
From Oligocene to this day, a period of about 25 My, it has been calculated that the average rise of the Alpine chain has been of about 1 mm/y: this means that, if there had not been erosion processes, the peaks of the Alps could now reach the incredible height of 25.000 m! It also means that, during the course of a human life, mountains like Cervino or M. Bianco rise about 7-8 cm: too little to observe with a naked eye, but still sufficient for geophysical measures to quantify the deformations.
Some areas of the chain are more active than others and show rising values much higher than average: thus, for example, in Friuli, between Trieste and Tarvisio, a series of measures taken after the 1979 earthquake and confronted with previous 1952 geodetic measures has shown that the increase has taken place with the speed of a few mm/y, a value that is 10 times higher than the average of the Alpine chain. The most active areas from the seismic point of view are usually the areas where the rising values are higher: for example, in the Cuneo area, in the Brescia area and in the already mentioned Friuli. This greater rise is due to a tentative of the African plate to “squeeze” in subduction under the European plate, below the Alpine structure.
As for the Appennines, the part that is actively rising is the oriental one, from Romagna, Marche, Abruzzo, Molise until Basilicata, while along the Western part, that which geologists call the “internal” part of chains, the opening of the Thyrrenian basin is causing distensive phenomena that result in a general lowering of the area and in numerous volcanic phenomena. In both cases, the deformations come with a high seismicity, always an index of tectonic activity. Towards S, the opening of the Thyrrenian Basin combines with the contemporary subduction, in the Ionic Sea, of African lithosphere below the Calabrian Arc, thus originating the volcanic activity of the Eolian islands and to the intense tectonic and seismic activity of Calabria, from Sibari until the Messina Strait (see Special on Mediterranean seabeds): the Sicilian coast goes farther from the Calabrian one by 1 cm/y and rises by 4 mm every 10 years, against a rise of the Calabrian coast by 1,5 mm/y. Subduction of the African crust also takes place in the Aegean Sea, below Greece and this explains the seismicity and volcanicity of these area which, although are not geographically part of our country, influence its geological evolution. Geophysics keep constantly under control the movements and deformations of our territory through a network of measuring stations (instituted on a national scale in the ’80s), both with the traditional geodetic methods (geometric leveling and accurate measures of corners and distances), and with the modern methods of satellite bearing (GPS), that allow to see even minuscule movements in real time. The national network is then connected with those of other countries, particularly the countries that look out on the Alpine arc, in order to control the situation along the whole chain. Precisely these measures, repeated in time, have allowed to understand the relations between the evolution of Alps and Appennines and the seismic and volcanic phenomena that, sometimes with great intensity, characterise many zones of our country.
The Padana Plain: flat only on the surface
The Padana Plain extends to the S of the Alps and separates it from the Appennines: flat and monotonous on the surface, in reality it hides a very complex and active geological structure. It constitutes, in fact, the foredeep of the central part of the Alpine chain, but also that of the younger Appenninic chain: it is therefore the area where two important chains, still in the making, stand one in front of the other. The result is that underneath the Padana Plain, below a 300-400 m cover of river and lake sediments, starting from Pliocene (3,9 My) big folds and tectonic slices have formed, and continue to form and stack one on top of the other. This structure is of fundamental importance, not only from the geologic point of view, but also in economic terms, since this structural asset is the one that favours the formation of hydrocarbon traps, of which the Padana Plain’s subsurface is particularly rich.
Written by Paola Tognini