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VESUVIUS VOLCANO


Somma-Vesuvius

 The Somma-Vesuvius (strato volcano) is the southernmost active volcanic complex of the so-called Roman Comagmatic Province, which extends from the Vulsinian district north of Rome, and including very large volcanic complexes, such as Sabatini, Alban Hills, and Roccamonfina. In Campania, the other still active volcanic areas are Phlegraean Fields, Procida and Ischia. The volcanic activity took place mainly in the last million year in response to the strong tensional tectonics of the area, generally linked to the opening of the Tyrrhenian Sea and to the collapse of the Apennine chain after thrust tectonics in the late Miocene.



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The unusual chemical composition of the rocks of the Roman Province attracted much interest in the petrological literature. Indeed, these volcanoes of the Tyrrhenian Sea border have a very peculiar potassium-rich composition, which is reflected in the abundant presence of leucite (KAlSi2O6), and also K-feldspar (KAlSi3O8). The rocks have also a very large variation in composition, and range from mafic (potassium rich basalts, shoshonitic basalts, leucite basanites and leucitites), to intermediate (latites, leucite tephrites), to felsic (trachytes, phonolites). The rocks have been grouped in magmatic series characterised by different degree of Silica undersaturation and following distinct liquid lines of descent.

Another important aspect of the rocks of the Roman Province is the abundant presence in almost all the volcanic complexes of alternation of effusive and explosive activity. While some complexes are built up almost completely by pyroclastic rocks (e.g. Phlegraean Fields), with effusive activity confined in minor occurrences, volcanoes like Somma-Vesuvius, Roccamonfina, Alban Hills, Sabatini and Vulsini are characterised by period of quiet effusive activity, with lava flows intermittently erupted from central or lateral vents, intercalated by strong (often dominant) explosive activity, with eruption of pyroclastic rocks. Linked to powerful explosive eruptions is the formation of calderas, like in Phlegraean Fields, likely formed after the eruption of the Neapolitan Yellow Tuff. The type of eruption (whether effusive or explosive) and the possibility of forecasting the magnitude of the eruptive event has major importance for the mitigation of the volcanic hazard, bearing in mind that these volcanoes are close to densely inhabited areas.

The Somma-Vesuvius complex is actually made up of two distinct volcanoes, the older and likely much higher Monte Somma, remnants of which are easily observed in the very steep caldera rim in the northern slope of the volcano, and Vesuvius s.s., which formed after the formation of the Monte Somma caldera.

The recorded activity has been divided into at least seventeen periods, which started with powerful, mainly pyroclastic eruptions of plinian type, generally followed by open-system moderate strombolian activity, with emission of lava flows, ash and ballistic material from the vents, and then ends with a powerful mixed pyroclastic-effusive eruption. This is the case of the last activity period of the volcano started in 1631 with a devastating eruption, recorded in most of the southern slopes of the volcano, and ended in 1944, with the eruption of pyroclastic rocks and a lava lobe that destroyed the outskirts of the village of San Sebastiano.

The periods of activity are separated by quiescence that can reach also several hundreds years, and that could serve for the recharge of the feeding system of the volcano. It is useful to note that the plinian eruptions that begin the periods of activity are generally made up of very strongly differentiated volcanic rocks (mostly phonolites), that form after the prolonged differentiation in shallow reservoirs of mafic alkaline magmas ultimately coming from the upper mantle. Indeed, this is the case of the plinian eruption of 79 D.C. which destroyed Pompeii, Herculaneum and Stabiae. The differentiated compositions of the magmas are also richer in volatilise, making the magma highly explosive (initial plinian fall deposits); the subsequent interaction with groundwater make even more explosive the magma, forming the very hot and mobile nuees ardentes or ignimbrites, which were erupted after the plinian fall phase, and that covered a very large area.

The Somma Vesuvius volcanic complex is still the subject of active surveillance by the Vesuvius Observatory and several other agencies, and now it is a quiescence period. The forecasting of the next eruption, and probably more importantly, the inferred magnitude of an eventual next eruption are fundamental for problems of evacuation in such a densely inhabited area like that living on the slope of the volcano.

 

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