The joined interpretation of structural, geophysical, historical and DTM-extracted data collected on the Somma-Vesuvius volcano reveals the presence of a spoon-shaped paleodepression, which is masked by the well known Vesuvius subsidence caldera. Such depression was caused by failure of the Somma southwestern flank. The flank failure was triggered by dike intrusion along a pre-existing structural discontinuity. This event occurred between 20--25 and 18 ka and marked a change in the composition of erupted products and style of activity, which shifted from an effusive and strombolian activity to a plinian-type activity. The flank failure induced a reorientation of the gravity-induced stress trajectory, which, below the depression, resulted parallel to the surface. This prevented the ascent of magma to the surface allowing the formation of a shallow reservoir in which more felsic magma evolved. When the magma pressure exceeded a critical value, plinian activity began. ¿ 1999 American Geophysical Union