Evolution du Roccamonfina (Italie Centrale)

V. Rouchon, P.Y. Gillot, X. Quidelleur, S. Chiesa

 

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Structural and geochemical evolution of the Roccamonfina volcano, Campania, Southern Italy.

The Roccamonfina is a Pleistocene volcanic complex which developed within the extensional structures of the Tyrrhenian basin, in the Campania province, Southern Italy. It is located 50 km north of the Somma-Vesuvius complex, and started its construction about 500 ka ago.

New geochonological and geochemical data, together with field observations and airborne-imagery, allowed the elaboration of a detailed geochemical and structural evolution of the Roccamonfina volcanic complex.

While the Vesuvius (< 35 ka) is in an active phase of tephritic stratocone construction, the Roccamonfina ended its volcanic activity since about 150 ka.

For a better forecasting of the evolution of the Somma-Vesuvius complex, the Roccamonfina should be regarded as a good analogue and its evolution should therefore be well understood.

The Somma-Vesuvius and the first half of the activity of the Roccamonfina share common volcanological evolutions : the succession of stratovolcano construction and destruction cycles, via cataclysmic plinian explosions. Both are characterized by mostly under-saturated, High-K magmas.

Following this first stage of activity (from 500 to 350 ka), the products of the Roccamonfina underwent a drastical geochemical change from High-K to relatively low-K, within an alkaline-basalt to trachyte series. The Low-K series initiated after the last High-K plinian eruption, the Brown Leucitic Tuff event (the BLT, 350 ka), by the eruption of effusive basaltic lavas. Activity at Roccamonfina became dominantly strombolian with occasional large plinian (the White Trachytic Tuff event at 331 ka) and phreato-plinian to sub-plinian crises without any apparent construction of a massive stratocone.

The morphological dissymmetry of the Roccamonfina resulted from the BLT event with a directed blast of the whole eastern half of the last tephritic stratocone, creating a main central depression. We have evidence that part of the present day morphological features preserved the consequences of explosive crises older than the BLT. Post-BLT activity increased the dissymmetry by preferential deposition of pyroclastic falls and flows throughout the opened half of the main depression.

Late (330-150 ka) activity was restricted to the formation of massive latitic domes within and outside the central depression and minor strombolian emission of pyroclastic products.

Satellite view of the Roccamonfina ENEA Digital Imagery
The Vesuvius is presently characterized by a stratocone made of under-saturated products. It is growing within the main depression created by the 79 A.D. or 472 A.D. explosion of the previous stratocone, the Monte Somma, contributing to this explosive morphological dissymmetry.

The absence of a terminal phase of activity similar to the one observed in the later half of the evolution of the Roccamonfina indicates by analogy that the Vesuvius remains in a strato-construction/plinian cyclic process, and that the end of its volcanic activity has not been reached yet.

Satellite view of Somma-Vesuvius Complex Nasa