IL “VULCANO DI POMPEI”: NUOVI DATI GEOMORFOLOGICI E STRATIGRAFICI

Abstract

Cinque A. & Irollo G. – The “Pompeii Volcano”: new geomorphological and stratigraphical data – (IT ISSN 0394 3356,
2004)
By means of both detailed geomorphological investigations and analysis of pre-existing drilling data, the small hills occurring in the area of Pompeii (Province of Naples; Italy) have been definitely recognized as volcanic landforms and attributed to the activity of local vents.
The main relict of what we call the Pompeii Volcano consists in a crescent-shaped ridge (“Arco della Civita) that is tentatively interpreted as the remnant of an ancient volcanic ring. It stretches from Sette Termini, near Boscoreale, to the archaeological site of the ancient town of Pompeii. Further to the North, it is buried by lavas and pyroclastics forming the southern piedmont of Somma-Vesuvius volcano.
Southwards it is buried by the Late Quaternary sediments of the Sarno Rives alluvial coastal plain. The Eastern half of the ancient ring,
which does not crop out, may have been lowered by some tens of meters because of a SE-trending normal fault.
A geological section across the Arco della Civita crescent, shows that the structure has a core of tuffs and loose pyroclastic beds; suggesting that the initial activity of Pompeii Volcano was essentially explosive. This pyroclastic edifice was then locally covered by lava
flows and scoria (leucite bearing tephrites) coming from a number of local vents, among which is the small S. Abbondio cone and, probably, the one we named Via di Stabia crater. Their products (such as the typical foam lava, already known to Vitruvius) were abundantly utilized as building materials in ancient Pompeii.
On the base of their relations with other dated formations, the various emissions of lava appear to vary in age from place to place: they are older than 19 ky close to Porta Nocera and Early Holocene in age (likely 8÷6 ka) in the SW flank of the Pompeian hill where the
lava flows interfinger with the transgressive Versilian sediments of the lower Sarno River Plain and appear cut by a sea-cliff to be acribed to the peaking stage of the same Post-Glacial trasgression.
More difficult to date is the explosive activity which occurred before lava and scoria emissions. Some sub-surface stratigraphic data
prove that it is not older than the Tyrrhenian Stage (OIS 5) and possibly younger than the OIS 3. Other stratigraphical and palaeomorphological data suggest that the explosive activity of Pompeii Volcano occurred when the Campanian Ignimbrite (39 ka) had already been dissected by the Sarno River.
The existence and the position of the studied volcano can be connected to an important E-W trending normal fault which is perfectly
buried by the thick Quaternary fill of the Sarno Plain, but nicely revealed by gravimetric and magnetic anomalies. This fault lowers the
substrate of the Plain (Mesozoic to Neogene units of the Apenninic chain) by approximately one thousand meters to the North and it
can be followed from Nocera Inf. to Torre Annunziata. The Pompeii Volcano could be at the intersection of this fault with another, NW
trending regional fault or with a radial fracture of the Somma-Vesuvius volcano.