Types and mechanisms of strombolian explosions: characterization of a gas-dominated explosion at Stromboli

Textural and chemical analyses of bombs quenched directly from a normal explosion at Stromboli volcano (Italy) were integrated with coincident seismic, acoustic, and thermal data. The data set defines a new gas-dominated type of “strombolian” eruption, named type 0. These events are characterized by high-velocity emission (150–250 m s −1 ) of a few relatively small, juvenile particles, with entrained non-juvenile clasts that previously fell back into the vent to be re-erupted. For the studied event, the explosion depth was more than 250 m deep, and the particles showed little residence time in the shallow system. Slug ascent velocities over the final 20–35 m of magma-filled conduit, and the low viscosity of the resident magma, are all consistent with simple bubble burst in a “clean” conduit. This conduit type and eruption style likely fit popular slug ascent and burst models used to explain “strombolian” eruptions. In contrast, the ballistic-dominated type of explosions (type 1) are associated with larger proportions of stagnant material in the shallow system magma mix. We argue that the additional volume of this stagnant material pushes the free surface upward. Because of the larger volume of material available for entrainment into a type 1 slug burst, which has to fragment through a thick cap of degassed material, type 1 events tend to be rich in particles. In contrast, the less spectacular, gas-rich (type 0) events have little material to entrain, thus being poor in lapilli and bombs.