The paroxysm of Stromboli on 3 July 2019 - a summary
One of the largest explosions since the 1930 eruption of Stromboli volcano occurred in the afternoon of 3 July 2019. The eruption came with no detectable warning signs in the form of a sudden extremely large magma bubble explosion, a so-called paroxysm that showered the upper slopes with incandescent material and generated a tall ash column that rose almost 5 km above the volcano.
The large eruption column from Stromboli's eruption today rising approx. 5 km, and ash plumes from a pyroclastic flow down the Sciara. Image taken from neighboring island of Panarea, courtesy of Marco Ortenzi via twitter (@mortenzi)
Unfortunately, the event also claimed at least one fatality. A hiker was caught in the rain of hot stones and lapilli near Ginostra village and lost his life.
This event, in the middle of the tourist season, could have been much worse and lead to a catastrophe if it had occurred a few hours later in the evening when many people climb the volcano in guided groups. It likely would have killed dozens if not more.
Typical (moderately strong) activity of Stromboli as seen a few months ago during an excursion to the summit.
Reconstruction of the events - timeline:Nothing very unusual had been observed at the volcano during the previous weeks and months. After a phase of strongly elevated activity in Dec 2018 - Jan 2019, activity at Stromboli had dropped to mostly low to sometime moderate levels.
Overall activity increased in June and since 12 June had been at "moderate" including the day before the eruption. Such phases of elevated activity have been very common in the past years. Thus, everything seemed normal and promised to become a very good volcano tourist season allowing many people to enjoy the fireworks of the volcano.
A new vent appears on the upper slope of the Sciara del Fuoco
25 minutes later (16:24), the lava flow has reached maybe around 100 m length.
Location of the new vent (image: Google Earth annotated by Culture Volcan)
At 15:57 local time (13:57 UTC), almost exactly one hour before the paroxysm, a new vent appears on the upper break in slope of the Sciara del Fuoco, a few 100 m northwest from the crater terrace. Hot rocks break off from there and shortly after, a small lava flow starts to issue from it and slowly travel downslope. Its location can be seen in the attached image published on a detailed analysis by Culture Volcan.
The appearance of this new vent was likely the only precursor so far known: a possible explanation (see more detailed info further below) could be that the magma in the conduit was experiencing a surge from underneath and opened a small side-wise vent. However, we currently do not know if instruments had detected significant deformation at that time, which would give much more significance to such an observation and early interpretation. This will likely come in publications to be expected in the coming months.