Real-Time Geophysical Monitoring of Particle Size Distribution During Volcanic Explosions at Stromboli Volcano (Italy)

Abstract

Of all the key parameters needed to inform forecast models for volcanic plumes, real-time tracking particle size distribution (PSD) of pyroclasts leaving the vent coupled with plume modeling has probably the highest potential for effective management of volcanic hazard associated with plume dispersal and sedimentation. This paper presents a novel algorithm capable of providing syn-emission horizontal size and velocity of particles in real time, converted in mass discharge rates, and its evolution during an explosion, using thermal infrared videos. We present data on explosions that occurred at the SW crater of Stromboli volcano (Italy) in 2012. PSDs and mass eruption rate (MER) data, collected at frequencies of 40 Hz, are then coupled with particle and gas speed data collected with traditional image analysis techniques. The dataset is used to quantify for the first time the dynamics of the explosions and the regime of magma fragmentation. We find that explosive evacuation of magma from a Strombolian conduit during a single explosion proceeds at a constant rate while the explosive dynamics are marked by a pattern that includes an initial transient and short phase until the system stabilizes at equilibrium. These stationary conditions dominate the emission. All explosions begin with a gas jet (onset phase), with maximum recorded vertical velocities above 150 m/s. These high velocities are for small particles carried by the faster moving gas or pressure wave, and larger particles typically have slower velocities. The gas jets are followed by a particle-loaded plume. The particles increase in number until the explosion dynamics become almost constant (in the stationary phase). MER is either stable or increases during the onset to become stable in the stationary phase. The shearing at the interface between the magma and the gas jets controls fragmentation dynamics and particles sizes. Quantification of the Reynolds and Weber numbers suggests that the fragmentation regime changes during an explosion to affect particle shape. The algorithm proposed requires low-cost thermal monitoring systems, and low processing capability, but is robust, powerful, and accurate and is able to provide data with unprecedented accuracy. In general terms, its applicability is limited by the size of individual pixels recorded by the camera, which depends on the detector, the recording distance, and the optical system, particle temperature, which has to be significantly higher than the background.

The Birth of a Hawaiian Fissure Eruption

“…The currently documented weakest events, the 2008 explosive eruptions of Halema‘uma‘u, Kīlauea (Houghton et al., 2013) also had MER values of 10 2 –10 4 kg s −1 . Estimates of MER for five normal explosions at Stromboli in 2012, analyzed by Pioli and Harris (2019) are 7.4 × 10 3 to 6.5 × 10 4 kg s −1 . Strombolian paroxysms are two orders of magnitude higher, e.g., 2.1 × 10 6 kg s –1 (Pistolesi et al., 2011) and 1.1 × 10 7 kg s –1 (Rosi et al., 2006), and high Hawaiian fountains (e.g., Figure 3) have time‐averaged discharge rates of 6 × 10 3 kg s –1 to 1.5 × 10 6 kg s –1 (Wolfe, 1988).…”

“…Other attempts at in‐flight grain size determinations have been made for Strombolian explosions but not Hawaiian fountains. A variety of approaches have been previously applied to determining in‐flight TGSDs for Stromboli and Yasur (e.g., Bombrun et al., 2015; Gaudin et al., 2014; Pioli & Harris, 2019), mostly to video from infrared cameras. These show convincingly the shift in particle size and dynamics between single explosions but have a lower size limit set by the pixel resolution of the cameras (1.5–5 cm for these studies).…”

Magma fragmentation and particle size distributions in low intensity mafic explosions: the July/August 2015 Piton de la Fournaise eruption

“…Magma viscosity, diameter, density, surface tension and velocity are recognised as relevant parameters in magma fragmentation. Fragmentation of low viscosity magmas is controlled by surface tension, viscous and shearing forces; among the main dimensionless parameters used to constrain the dynamics of the breakup regimes are the Reynolds and Ohnesorge numbers 21 , 24 , 25 which are quantifying the relative role of these forces 26 – 28 ; however, very few data are available from real eruptions 29 .…”

“…among the main dimensionless parameters used to constrain the dynamics of the breakup regimes are the Reynolds and Ohnesorge numbers 21,24,25 which are quantifying the relative role of these forces [26][27][28] ; however, very few data are available from real eruptions 29 .…”

“…These methods rely on the ability to sample a deposit, an often difficult to impossible procedure in small-scale eruptions where deposits are ephemeral and/ or rapidly covered by subsequent lava or scoria, and thus rarely documented 40,41 . For this reason, a more efficient assessment of GSDs can be done at the vent by video and thermal monitoring 29,42,43 .…”