Les explosions phréatiques et phréatomagmatiques terminales à l’Etna

On September 12, 1979, there occurred a fatal explosion at the Bocca Nuova on Mount Etna which killed nine persons and injured twenty three. It was entirely phreatic. This accident revealed a type of terminal explosion, sometimes only phreatic, sometimes phreatomagmatic, which is quite normal on Etna, but too often disregarded despite having been observed a hundred of times over the past century. The mechanism of these explosions and their different circumstances are discussed: in general they are caused by the clearing of a crater pit whose vent had been sealed by a collapse from the walls, with a wet material plug. High temperatures and heat flux are constant in crater pits, being maintained by persistent activity. Temperatures increase when the volcano erupts. The water within the plug is thus vaporised and vapour superheated. The internal high pressure so produced is discharged during an explosion which can be single or repetitive. This may happen without any eruption taking place, or it may occur in the framework of an eruption. The violence of these explosions can be very great: several have ejected blocks farther than one kilometer. The forecast of such events is difficult except when intracrateric collapses are observed. They represent a permanent hazard, chiefly during an eruptive phase.