The A.D. 472 eruption of the Somma volcano

In 472 A.D. an explosive eruption took place on November 6 from the summit caldera of the Somma. The stratigraphy of the pyroclastic deposits suggests that the eruption involved a complex series of events, which may be grouped into four main phases. The first phase was a small magmatic explosion that resulted in a well vesiculated pumice fall layer (La) in the north-northeast. La products contain ∼20% lithic fragments and represent the vent opening stage of the eruption. Subsequent activity resulted in layered fall deposits, subdivided into layers Lb and Lmax. The products contain a much higher proportion of lithic fragments (∼50%) and are inversely graded, coarse-grained, poorly vesiculated and interbedded with thin, lithic-rich fine ash beds. These characteristics indicate that the Lb–Lmax events were most likely phreatomagmatic, caused by influx of groundwater in the magma chamber–conduit system. They resulted in a fall deposit to the north-northeast (Lb) and northeast–southeast (Lmax) of the volcano. The volume of this deposit is estimated at 1.2 km3 (0.52 km3 DRE). Component analysis indicates that lithics make up about 0.22 km3 of the deposit. Therefore the magma emplaced as tephra fall is ∼0.3 km3. The fall phase was ended by block and ash flow of the second phase, emplaced on the northern flanks of the Somma edifice. The largest block and ash flow descended to the northwest, reaching a distance of 5 km from the summit. Increasing pressure due to strong magma–water interaction is the most likely cause of the volcaniclastic debris flow of the third phase. The deposits, identified in the northern and eastern sectors of Mt Somma, comprise a large volume of lithic breccia containing a high proportion of tephritic–phonolitic xenolithic blocks up to 0.5 m across, in a grey, poorly sorted, coarse, sandy matrix, containing only a trace of juvenile pumice. The fourth phase represents the final activity, producing surge deposits, rich in accretionary lapilli and ash fall deposits, that are clearly related by phreatic–phreatomagmatic interaction of water table and magma in the chamber–conduit system. The reconstruction of the A.D. 472 eruptive phases shows that a violent explosive eruption took place in historic times. Based on these data conclusions may be drawn on explosive eruptions which may occur in the future at Somma-Vesuvio.