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 field campaign at Stromboli was undertaken by Italy's Istituto Nazionale di Geofisica 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 fluidal, 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 fluidal 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.

Abstract Water diffusion experiments were performed on a trachytic melt from the Agnano-Monte Spina explosive eruption (Phlegrean Fields, South Italy). Experiments were run in a piston cylinder apparatus at 1 GPa pressure, at temperatures from 1373 to 1673 K and for durations of 0 to 255 s, using the diffusion-couple technique. Water concentration profiles were measured by Fourier transform infrared spectrometry. Water diffusion coefficients at different temperatures and water concentrations were calculated from the total water profiles, using the Boltzmann Matano technique. Over the investigated range of temperatures and water concentrations, the diffusivity of water in potassic melts ( D water ), m 2 /s can be described by Arrhenius equations that can be generalized for water concentrations between 0.25 and 2 wt% as follows: where C h 2 o is the water concentration in wt%, R is 8.3145 (J K 1 mol. 1 ) and T is the temperature in Kelvin. Water diffusivities in trachytic melts were compared with water diffusivities in rhyolitic and basaltic melts. The activation energies for water diffusivity in trachyte and basalt are comparable, and higher than in haplogranitic melt. This results in a convergence of water diffusion coefficients in all melts at lower (magmatic) temperatures.