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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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