Self-potential survey near the craters of Stromboli volcano (Italy). Inference for internal structure and eruption mechanism

Abstract

The presence of self potentials on the upper part of Stromboli Volcano is associated with the existence of convection cells of gases and condensed liquids whose rising parts are situated above the hot zones. A model of a convection cell fed by a hot fluids through its lower middle part was calculated and applied to the various structures of the upper part of the volcano. This model which takes account of a process which is probably fairly common in volcanic system, gives a clear explanation of the various potential zones.

Although other interpretations could be made, the fact that the positive and negative potentials are of the same importance, the limited size of the convection zone and the presence of gas under pressure beneath permeable grounds lead us to conclude that the phenomenological cause of these potentials is probably the mechanism of electro-filtration.

The results of self-potential measurements showed very close correlation with those we obtained previously by magnetotelluric profiling. These two geophysical methods were very efficient in the precise delimitation of hot zones which give rise to resistivity and potential anomalies. The depth of those zones was estimated from the aspects of the potential anomalies and from the geological environment.

The two methods were furthermore powerful tools in the investigation of volcanological processes and in particular the «Strombolian» rythmic eruption. The geometrical disposition of the feeding channels involves the obstruction of the path of the gases by the lava. Generally, the force of pressure exerted by the gas increases quickly and in consequence the plug of lava is ejected and shattered. When the gas flow lessens, the forces of viscosity increase quicker than the forces of pressure, the discharging outlets are then obstructed and thus there is great hazard that the accumulated energy will be liberated in a very violent fashion. In this case there will be a paroxysmal-type eruption.

This mechanism is the consequence of a recent accident in the volcano evolution: the landslip of a segment of the cone, probably associated with the subsidence of the central caldera; the lava spread over the collapsed sector and the rhythmic eruptions described above constructed the present craters.