The Somma-Vesuvius volcanic complex consists of an older building, Somma, characterized by a caldera, and a younger cone, Vesuvius, which grew up within the caldera after the eruption of Pompei in 79 AD.
The eruptive history of Somma-Vesuvius, whose age is less than 39,000 years, can be divided into distinct periods, each characterized by different types of activity.
Between 39,000 and 20,000 years ago, the activity was characterized mainly by effusive eruptions and then explosive eruptions of low energy. The formation of the ancient volcano, the current Monte Somma, dates back to this period.
A major change in the type of volcanic activity occurred about 19,000 years ago, when an activity predominantly effusive was replaced by an explosive activity. Around 18,000 years ago, in fact, after a long period of rest, the first and largest Plinian eruption (Basic Pomici) took place. Other large Plinian eruptions, all preceded by long periods of inactivity, there have been up to the famous eruption of Pompei in 79 AD (Pomici of Mercato, 8,000 years; Avellino pumice, 3,500 years).
The eruption of 79 AD occurred after a three century rest of the volcano,and was one of the most violent and destructive in the history of Vesuvius. It has been defined plinian after Pliny the Younger, who gave us a description of the event. The eruption, which lasted less than two days, emitted into the atmosphere about 4 cubic km of ash and lapilli; the activity was characterized by several phases that produced different effects on the territory, up to distances of hundreds of kilometers from the volcano and with catastrophic consequences for Pompei, Herculaneum and Stabia.
After the eruption of 79 AD there have been numerous strombolian eruptions and effusive that led to the gradual building of the Great Cone of Vesuvius and the emplacement of lava flows on the slopes south and west of the volcano. The eruptive activity has since experienced two significant periods of rest, followed in both cases by explosive events of great energy, such as the eruption of 472 and that of 1631, of subplinian nature. During the 1631 eruption the whole range of countries between the town of Pollena in the northern area, and that of Torre Annunziata in the south-west, was devastated by the flow of pyroclastic flows that killed more than 5 thousand people.
In the period between 1631 and 1906, when there was one of two events of increased energy of the last century, the volcano showed almost continuous strombolian activity, associated with effusive activity. The event of 1906 was characterized by explosive and effusive activity of varying intensity and caused numerous deaths and injuries to the collapse of roofs as a result of the accumulation of ashes.
The 1944 eruption closes a period of more or less continuous in open tube, the event is characterized by effusive and explosive activity, caused the death of 21 people in the collapse of the roofs and the almost total destruction of the countries of San Sebastian, Massa di Somma and Terzigno.
Scholars believe that the eruption of 1944 marked the end of a period of activity in open tube and the beginning of a period of quiescence duct obstructed. From 1944 to date, Vesuvius has only fumarolic activity and seismic swarms of moderate energy, without ground deformation or significant changes in physical and chemical parameters of the system.