Volcanic activity of Vesuvio
One of the most famous volcanic disasters was the eruption of Vesuvio in 79 A.D., which destroyed the cities of Ercolano and Pompei. The stages of the eruption were carefully described by Plinio il Grande: the eruption started with an explosive phase, with the emission of gas and burning material. Then, the catastrophic phase occurred as the underground water entered inside the volcano and rapidly evaporated when it got in contact with the magma. This determined a fast pressure increase and the volcano exploded, provoking the destruction of Ercolano and Pompei and more than 30,000 victims.
Volcanoes like Vesuvio alternate long dormancy periods, where magma and gasses accumulate underground, and sudden and destructive awakenings.