Detailed analysis of particle launch velocities, size distributions and gas densities during normal explosions at Stromboli

Abstract

Using high frame rate (33Hz) thermal video data we describe and parameterize the emission and ascent dynamics of a mixed plume of gas and particles emitted during a normal explosion at Stromboli (Aeolian Islands, Italy). Analysis of 34 events showed that 31 of them were characterized by a first phase characterized by an initial diffuse spray of relatively small (lapilli-sized) particles moving at high velocities (up to 213ms−1; average 66–82ms−1). This was followed, typically within 0.1s, by a burst comprising a mixture of ash and lapilli, but dominated by larger bomb-sized particles, moving at lower exit velocities of up to 129ms−1, but typically 46ms−1. We interpret these results as revealing initial emission of a previously unrecorded high velocity gas-jet phase, to which the lapilli are coupled. This is followed by emission of slower moving larger particles that are decoupled from the faster moving gas-phase. Diameters for particles carried by the gas phase are typically around 4cm, but can be up to 9cm, with the diameter of the particles carried by the gas jet (D) decreasing with increased density and velocity of the erupted gas cloud (ρgas and Ugas). Data for 101 particles identified as moving with the gas jet during 32 eruptions allow us to define a new relation, whereby Ugas=Uparticle+a [ρgasD]b. Here, Uparticle is the velocity of bombs whose motion is decoupled from that of the gas cloud, and a and b are two empirically-derived coefficients. This replaces the old relation, whereby Ugas=Uparticle+k D; a relation that requires a constant gas density for each eruption. This is an assumption that we show to be invalid, with gas density potentially varying between 0.04kgm−3 and 9kgm−3 for the 32 cases considered, so that k varies between 54m1/2s−1 and 828m1/2s−1, compared with the traditionally used constant of 150m1/2s−1.