Why are there volcanoes in Italy?

Italy's Volcanoes:
The Cradle of Volcanology

Italy is a country whose population has to live with active geological processes. Some of these are more gradual, such as the orogenic (mountain-building) processes that have led to the construction of the Alps and the Apenninic chain. Others, which are to be placed into the same geodynamic framework, are more dramatic, and at times pose serious hazards to people living nearby. These are earthquakes, flooding, landslides, and volcanoes. Of all these, the latter constitute one of the most famous features of this country, along with good food, a wonderful climate and a number of clichés which will not be further treated here. A closer look reveals that the Italy's volcanoes are strikingly different. Students of geology learn that they produce a number of highly different types of magmas, and most of them are alkalic, which means that they contain higher proportions of chemical elements such as potassium and sodium than many other volcanoes on Earth.

Italy is a small country, and yet it unites virtually all types of volcanoes that can be found in other areas of the world at distances of thousands of kilometers. They represent a constant threat to hundreds of thousands of Italians, but at the same time they bring benefits to many more of them, such as fertile soils, favorable climatic conditions, beautiful landscapes, and tourism. They also contributed much to the evolution of the science of volcanology since they were easily accessible to early European scientists and other educated persons and two of them, Vesuvius and Etna, erupted frequently when modern science and philosophy began to evolve. But volcanology is often said to have begun much earlier, still at an Italian volcano: Vesuvius. The devastating eruption of this volcano in A.D. 79 was so accurately described by a young Roman, Pliny the Younger, that this description is used in virtually all text books on volcanology. The name of its author has also inspired to name a type of violently explosive volcanism after him: Plinian. Two other types of volcanic activity are named after Italian volcanoes, Stromboli (Strombolian activity) and Vulcano (Vulcanian activity). And last but not least, the word volcano itself derives from Vulcano, which in ancient times impressed people living nearby with its then-frequent and violent eruptions.

Why are there volcanoes in Italy? And why are they so different? Many sources say that most Italian volcanoes are related to subduction, that is, where one plate of the earth's crust (or better say lithosphere) is pushed under another, causing its partial melting and thus generating the magmas that feed the volcanoes. Once one begins to read carefully in the extensive literature that exists on the geodynamics of Italy and the surrounding region, a very complicated picture emerges. The central Mediterranean - of which Italy is part - belies the common notions about plate tectonics, the processes believed to shape the surface of our planet. All the theories that easily explain volcanism in most parts of the globe seem to fail to supply good arguments for volcanism in Italy. In this country, geology is fooling the geologists.

Yet the volcanoes are there, and many of them are active or potentially active. These volcanoes are principally linked to five different tectonic environments: subduction, back-arc rifting, continental rifting, sea-floor spreading, and a fourth one that is very poorly understood. Generally Italy's volcanism is a result of the collision of two plates of the Earth's lithosphere (that is the solid outer portion of the planet) - the African plate to the south, and the European (or Eurasian) plate to the north. This collision is complicated by the complicated physical characteristics of the colliding plate margins - rather than being homogeneous over a wide area, these are extremely heterogeneous. In some places, there is some oceanic lithosphere left at the north margin of the African plate, which is consumed by subduction benath the adjacent European plate. Elsewhere, no oceanic lithosphere is left and the colliding plate margins consist entirely of continental lithosphere, which resists subduction and rather leads to mountain building, as is the case in the Alps and in the Apenninic mountain chain that runs along nearly all of the Italian peninsula. The presence of rigid crustal blocks that also resist largely to mountain building (they simply do not deform) in between the two colliding plates renders the situation more complicated. In the case of the Hyblean-Maltese block it is believed that the counterclockwise rotation of this block causes oblique-rifting in its rear (that is, to the southwest).

Subduction seems to be the main cause (though possibly not the only one) for the volcanism in the Aeolian Islands - some of it is calc-alkaline, typical for subduction-related volcanism. But the same volcanoes that have periodically produced calc-alkaline magmas have also produced more alkalic magmas, and sometimes emitted both simultaneously, which is not all that typical for subduction-related volcanoes. To explain this peculiarity, some scientists have invoked a nearly vertical lithospheric slab in subduction below the Aeolian volcanoes, which would allow the generation of calc-alkaline and alkalic magmas at different depths and pressures at the same time. Another hypothesis places the Aeolian Islands in an oblique-rifting context, and yet another one explains the different magmas emitted from the Aeolian volcanoes simply with heterogeneous mantle sources below the area.

Subduction is also assumed by some scientists to be responsible for the volcanism along the Tyrrhenian coast of central-southern Italy (the volcanoes of southern Tuscany, Latium, and Campania, including the Colli Albani and Vesuvius). However, these volcanoes are even less typical of subduction-related volcanoes than their Aeolian companions; their magmas are extremely potassic. In the past, this has been explained with the interaction of magmas with carbonatic crustal rocks during their ascent to the surface. More recently it has become common to favor a back-arc rifting setting for these volcanoes - while crustal contamination of the magmas is not excluded.

In the case of the volcanoes in the Strait of Sicily (the sea channel separating Sicily from northern Africa) the case seems to be fairly clear: continental rifting, although geographically, the site of this rifting lies not where most people would expect it. It lies below the sea. In the Strait of Sicily, the seafloor is constituted by continental lithosphere, which is affected by oblique-rifting. The volcanoes born from this process are typical of continental rift settings, because their magmas are similar to those along the Great East African Rift. The most peculiar of these magmas are called peralkaline magmas, or pantellerite - after the volcano of Pantelleria in the Strait of Sicily. Such magmas can produce highly explosive eruptions but are much more fluid than, for instance, andesitic or dacitic magmas, and can be deposited in strongly welded ignimbrites.

The massive submarine volcano of Marsili in the southern Tyrrhenian Sea seems to resemble rather the volcanoes formed along mid-oceanic ridges, as in the Atlantic or in the Pacific close to the western coast of the USA. Rifting is believed to have characterized the formation of the Tyrrhenian basin when the Italian peninsula rotated counterclockwise away from the place now occupied by Corsica and Sardinia, which once was attached to a portion of what is now Italy.

The largest and most active volcano of Italy, Mount Etna, is also the most difficult to explain. While one hypothesis envisages a hot-spot or mantle-plume origin for this volcano (and also for its predecessors to the south, the Monti Iblei or Hyblean Mountains), another places it into a certain, though indirect, relationship with subduction, and still another group of scientists believes that asymmetric rifting along the eastern coast of Sicily allows the uprise of magma at Etna. What seems to be clear is that the volcano lies at the intersection of several regional fault systems, and maybe the easiest thing to assume is that magma uprise is facilitated exactly by this tectonic situation. It must furthermore be considered that volcanism has occurred intermittently over more than 200 million years a few tens of kilometers to the south of Etna, in the Hyblean Mountains. Apparently there have been persistent conditions favorable for the generation of magma in this area, which in the same period went through different tectonic phases. It now seems almost certain that about 1.5 million years ago, volcanism began to shift from the Hyblean Mountains northward to arrive in the area of present-day Etna about 0.5 million years ago. The first eruptions in the Etna area were very similar to the earlier eruptive events in the Hyblean Mountains in that they were episodic and separated by long periods of quiescence, for the rapid emission of voluminous lavas from regional fissures, and for the composition of the emitted lavas. The northward shift of magmatism from the Hyblean Mountains to Etna may have been caused by the establishment of the presently active fault systems, which allowed uprise of magma more easily in this place than in the Hyblean area.

There is still much to be done to better understand not only the geological causes of Italy's volcanism, but also how these volcanoes work. What is certain is that there must be very good reasons for these volcanoes to exist, because they are there. In a period when well-established concepts in geology are subjected to serious doubt and criticism from an ever-growing number of scientists (take, for example, the mantle plume debate), geology is destined to live through a new period of intense growth and constructive discussion and far from having all the answers to pressing questions. Italy is one of the places where this is most evident, and where further progress in science is most necessary.

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