IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4A2_3E-O12

Room A1

Date/Time: July 24 11:45-12:00

Eruptions on the fast track, part b): The variability of strombolian explosions from highspeed thermal and optical videos
Damien Gaudin1 , Jacopo Taddeucci1 , Piergiorgio Scarlato1 , Monica Moroni2
1

Department of Seismology and Tectonophysics, HP-HT Lab, INGV Roma, Italy, 2 DICEA , Sapienza
UniversitÃă di Roma, Rome, Italy
E-mail: damien.gaudin@ingv.it

Strombolian activity style is characterised by discrete explosions at variable frequency, lasting a few to tens ofseconds, and ejecting pyroclasts at heights of tens to hundreds of meters. New imaging techniques, including
high speed camera observations, have already shown that these explosions are complex phenomena that include
pyroclast ejection pulses, typically lasting tenths of seconds, characterized by a non-linear decay on the ejection
velocity over time and related to individual pressure release events. Here, we show the results of a new computingtechniques (described in a companion abstract, Eruptions on Fast Track, part a), based on joint applicationof Particle Image Velocimetry (PIV, providing information on the ash and gas clouds) and Particle Tracking
Velocimetry (PTV, providing information on the cm-to m-sized pyroclasts) to high-speed visible and thermal videosof Strombolian explosions at Stromboli (Italy) and Yasur (Vanuatu) volcanoes. In comparison to previous studies,
which manually analysed the videos, the techniques we use increase by ten-fold the number of identified andmeasured particles, allowing a significant advancement in the study of Strombolian dynamics.
A variable number (a few to several tens) of well-defined ejection pulses are present in all analyzed explosions.
Pulses sometimes are organised in larger ejection events, or super-pulses, lasting up to a few seconds,
characterised by a velocity decay following the same non-linear trend shown by pulses. The mean source depth ofthe pulses, modeled from the velocity decay trends, are mostly in the tens of meters range and remains constanton the whole explosion, while super-pulses seems to originate deeper (up to hundred of meters) in the conduit.
Within each pulse, we observe a strong correlation of ejection parameters: the spread of the ejection angle and thesize of particles increases while the number of particles and their velocity decreases. This correlation is tentatively
linked to the explosion dynamics of gas pockets at the surface, where maximum pressure differential is initiallyreleased through a small, ventral opening that gets wider with time, with possible effects of inter-particle collisions
also playing a role.
At the time scale of the whole explosion, ejection angle is a reflection of the overall vent orientation. The temperatureof the bombs and ash decrease over time and height above the vent, more slowly for the bombs, to be comparedwith theoretical predictions. The total mass of the pyroclasts ejected during a single pulse ranges from tens of kg toseveral tons, while the comulative one for an explosion can reach hundreds of tons. The ejection mass rate tends
to decrease during one explosion, suggesting that, despite the presence of ejection pulses, each explosion stillrepresent one well-identified and impulsive energy release event.

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