Insights into explosion dynamics and the production of ash at Stromboli from samples collected in real-time, October 2009

Abstract

Rapid characterization of tephra from ongoing explosive eruptions can provide valuable insights into eruptive mechanisms, especially when integrated with data from other monitoring systems. Here we gain perspective on Stromboli’s eruptive processes by linking the characteristics of ash collected in real-time with videos of each explosion. A 3 day, multifaceted fi eld campaign at Stromboli was undertaken by Italy’s Istituto Nazionale di Geofi sica e Vulcanologia in October 2009. At this time, activity was at a moderately intense level, with the occurrence of an average of 4– 5 explosions per hour at each of the SW and NE craters. Eight ash samples were analyzed using binocular and scanning electron microscopes to gain data on the components, grain size and morphology distributions, and surface chemistry of ash particles within each sample. Monitoring video of each explosion enabled an estimation of the duration and height of each sampled explosion. In each sample, the proportion of fl uidal, glassy sideromelane (as opposed to blocky, microcrystalline tachylite plus lithics), the degree of “chemical freshness” (as opposed to alteration), and the average size of particles appear to correlate with the explosion “type” described in previous studies, and the maximum launch height of the corresponding explosion. Our observations suggest that more violent explosions (i.e., those driven by the liberation of larger and/or more pressurized gas volumes) can be associated with type 2a conditions and the fragmentation of hot and low- viscosity magma, while weaker type 2b explosions erupt predominantly ash-sized particles derived from the fragmentation of colder, more outgassed magma and passive integration of lithic wall debris. The formation of fl uidal sideromelane ash particles (up to Pele’s hair) requires the aerodynamic deformation of a relatively low-viscosity magma and demonstrates unequivocally that ash at Stromboli is not derived entirely from wall rock and/or brittle fragmentation of stagnant magma. We suggest that this ash-sized material forms through rapid acceleration and breakup of larger magma fragments, as supported by evidence from high-speed video of two of the sampled explosions.