A new model for the formation of the Somma Caldera

Summary The evolution of the Mt. Somma caldera is reconstructed by means of volcanological, geological and geomorphologic data. The present caldera shape results from two caldera forming events: (a) break-up of the upper part of the older structure, which occurred between 17 and 8Kyr B.P. through a series of external volcaniclastic debris flow depositions and internal collapses into a cored-out vent. (b) W–SW directed sector collapse of the enlarged Mt. Somma crater, caused by excess vapour pressure generated during the Avellino eruption (3.5Kyr B.P.). At this time, the morphology of the Mt. Somma crater probably resembled the demolished crater formed in 1980 at Mt. St. Helens. Flank failure of Mt. Somma during the Avellino eruption was probably caused both by a migration of the vent to the west and a drastic change of the hydrogeological conditions at depth. Flank failure processes were extended to the S–SE sector of the Mt. Somma edifice during the 79 A.D. and 472 A.D. eruptions. After the 472 A.D. eruption interplinian activity during the Middle Ages resulted in the formation of the Vesuvius cone inside the Mt. Somma caldera.