Pyroclast Tracking Velocimetry illuminates bomb ejection and explosion dynamics at Stromboli (Italy) and Yasur (Vanuatu) volcanoes

Abstract

A new image processing technique-Pyroclast Tracking Velocimetry-was used to analyze a set of 30 high-speed videos of Strombolian explosions from different vents at Stromboli (Italy) and Yasur (Vanuatu) volcanoes. The studied explosions invariably appear to result from the concatenation of up to a hundred individual pyroclast ejection pulses. All these pulses share a common evolution over time, including (1) a non-linear decrease of the pyroclast ejection velocity, (2) an increasing spread of ejection angle, and (3) an increasing size of the ejected pyroclasts. These features reflect the dynamic burst of short-lived gas pockets, in which the rupture area enlarges while pressure differential decreases. We estimated depth of pyroclast release to be approximately 1 and 8 m below the surface at Stromboli and Yasur, respectively. In addition, explosions featuring more frequent pulses also have higher average ejection velocities and larger total masses of pyroclasts. These explosions release a larger overall amount of energy stored in the pressurized gas by a combination of more frequent and stronger ejection pulses. In this context, the associated kinetic energy per explosion, ranging 10 3 -10 9 J appears to be a good proxy for the explosion magnitude. Differences in the pulse-defining parameters among the different vents suggest that this general process is modulated by geometrical factors in the shallow conduit, as well as magma-specific rheology. Indeed, the more viscous melt of Yasur, compared to Stromboli, is associated with larger vents producing fewer pulses but larger pyroclasts.

Viscous plugging can enhance and modulate explosivity of strombolian eruptions

“…This complexity might be manifest in the seismo-acoustic signatures of the explosions (e.g., Johnson and Lees, 2000;Lyons et al, 2012), and in the visual character of the explosions. We note, for instance, that recent high-speed videography studies have identified that gas escape during strombolian explosions is typically pulsatory Gaudin et al, 2014), suggesting greater complexity than simple bursting of an overpressured slug. Understanding the role that a viscous plug plays in modulating the dynamics of slug ascent and burst is, therefore, of considerable importance in the interpretation of the waveform and amplitude of generated pressure changes (Lane et al, 2013).…”

Recycled ejecta modulating Strombolian explosions

“…An individual Bexplosion^is characterized by multiple, second-long Bpulses^and sub-second-long Bsubpulses^, each pulse being characterized by the ejection of particles at similar velocities which then decrease in time (Taddeucci et al 2012a;Gaudin et al 2014;Bombrun et al 2015). In addition we can observe multiple emission points during a single event.…”

“…James et al 2004James et al , 2006James et al , 2008Lane et al 2013), and field observations (e.g. Chouet et al 1974;Blackburn et al 1976;Ripepe et al 1993Ripepe et al , 2005Patrick et al 2007;Harris et al 2012;Taddeucci et al 2012a, b;Gaudin et al 2014;Bombrun et al 2015).…”

The roar of Yasur: Handheld audio recorder monitoring of Vanuatu volcanic vent activity

“…Thus the vent flow was active about 25-33% of the time. Recently, Gaudin et al (2014) have used particle tracking velocimetry-i.e. analysis of high speed (500 frame per second, 14 s long) video acquired in 2011 to measure pyroclast velocities from the Yasur vents-their velocity histograms show a modal velocity of 20 m/s, a median of~40 m/s and a maximum of perhaps 150 m/s.…”