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The paroxysm of Stromboli on 3 July 2019 - a summary

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)
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)
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.
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.
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
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.
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)
Location of the new vent (image: Google Earth annotated by Culture Volcan)
3 July:
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.

The paroxysm:

Lava flows emerge from several vents at 16:44 (image: INGV Catania Pizzo webcam)
Lava flows emerge from several vents at 16:44 (image: INGV Catania Pizzo webcam)
Seismic signal on 3 July afternoon (image: INGV Catania)
Seismic signal on 3 July afternoon (image: INGV Catania)
16:44 local time (14:45 UTC):
Immediately before the onset of the paroxysm, lava flows emerge from all vents: the speed of the magma being pushed out from the conduit dramatically increases. Seismic activity, until now quite inauspicious, starts to quickly increase as well at this point.
The bursting lava bubble during the first explosion at 16:46 (image: Skyline Webcam)
The bursting lava bubble during the first explosion at 16:46 (image: Skyline Webcam)
16:46:
A minute later or so, an extremely large gas bubble arrives at the surface and generates two lateral blasts (as the detailed report of INGV mentions, translated below) and a few seconds later the paroxysmal explosion. In the time-lapse video, the giant lava bubble bursting out from the crater is truly incredible. Most of the summit area is under rain of lava bombs of all sizes. Survival chances in the summit area would have been slim.

The explosion seen from the sea at 2 km distance (image courtesy: Francesco Rinauro / facebook)
The explosion seen from the sea at 2 km distance (image courtesy: Francesco Rinauro / facebook)
The initial explosion caused by the giant gas bubble seen from the sea at approx. 2 km distance. The lava fragments, some of which are enormous in size, can be seen red in plain daylight, meaning that the temperature must be very hot, certainly above 1000 deg C.
Image courtesy: Francesco Rinauro (facebook)
The first explosion seen from the Pizzo webcam (time seems to be wrong and should be 14:46 UTC) (image: INGV Catania)
The first explosion seen from the Pizzo webcam (time seems to be wrong and should be 14:46 UTC) (image: INGV Catania)
The same explosion a split second later (time indicated must be wrong, proably due to some malfuntion; camera destroyed immediately after) (image: INGV Catania)
The same explosion a split second later (time indicated must be wrong, proably due to some malfuntion; camera destroyed immediately after) (image: INGV Catania)
The explosion also appears on the Pizzo thermal webcam until it is hit by lava bombs and destroyed. Note: The time indicated on the camera pictures is off by 8 minutes and the second image should be the same explosion a fragment of second later.
Here's a video of the thermal cam on the summit, until it was destroyed:

Partial collapse of crater:

Comparison of the crater before (l) and after (r) (image: Vulcano a Piedi webcam)
Comparison of the crater before (l) and after (r) (image: Vulcano a Piedi webcam)
While the paroxysm took place in the western or central part of the crater platform, much of the northern portion of the crater, including the prominent cinder cone at the northern end, disappeared. It obviously had collapsed during the eruption.
The pyroclastic flows traveling about 1 km above the water (image: Sailactive I Segelreisen & Yachtcharter / facebook)
The pyroclastic flows traveling about 1 km above the water (image: Sailactive I Segelreisen & Yachtcharter / facebook)

In the time-lapse video of Skyline camera, one can see that during the minutes before the paroxysm degassing becomes more and more intense, and rockfalls increase from the slopes of the cone. This suggests that its stability decreased. However, it is not easy to know what exactly caused its collapse:
- It could have been simply "pushed aside" when the large gas bubble arrived and exploded, tipping its stability over a critical point.
- It could have become destabilized by the sudden emplacement of lots of fresh material during the eruption.
Most likely, it is a combination of both and possibly other factors.

The pyroclastic flows generated from the crater collapse traveling over the sea:
The above picture, taken from a yacht nearby, shows how the pyroclastic flows traveled over the water, reaching probably more than 1 km in length.
Credit: Sailactive I Segelreisen & Yachtcharter (facebook)

Detailed description of the events(Lees meer)

The ash plume, pyroclastic flows and ballistic impacts

The rising eruption column seen from Scari (east side of island) (image: Francesca Utano / VolcanoDiscovery)
The rising eruption column seen from Scari (east side of island) (image: Francesca Utano / VolcanoDiscovery)
The eruption column seen from Panarea (image: courtesy of Marco Ortenzi via twitter @mortenzi)
The eruption column seen from Panarea (image: courtesy of Marco Ortenzi via twitter @mortenzi)
The explosion generates an eruption column that quickly rises more than 2 km, probably reaching approx. 4-5 km height before spreading into a mushroom cloud. VAAC Toulouse issued a warning of ash in possibly up to 10 km altitude.
Ballistic ejecta landing into the sea (image: LBZ webcam)
Ballistic ejecta landing into the sea (image: LBZ webcam)
Pyroclastic flow traveling down the sciara and some hundred meters over the sea (image: LBZ webcam)
Pyroclastic flow traveling down the sciara and some hundred meters over the sea (image: LBZ webcam)
The ballistic ejecta cover much of the summit area and reach considerable distances with many falling in the area of Ginostra village and the sea beneath the Sciara. The material falling back onto the slope generates a small hot avalanche (pyroclastic flow) that reaches the sea. In addition, a portion of the crater itself, in particular the prominent cinder cone in the northern crater terrace, collapses as well into this avalanche.
The eruption column seen from an airplane, courtesy of eolianodoc / instagram
The eruption column seen from an airplane, courtesy of eolianodoc / instagram
The eruption column seen from an airplane, courtesy of eolianodoc / instagram

The impact of the eruption - panic, heavy ash fall, bush fires, one fatality and injuries

The southern slopes of the volcano engulfed in bush fires ignited by the lava bombs (image: Gianluca Giuffrè)
The southern slopes of the volcano engulfed in bush fires ignited by the lava bombs (image: Gianluca Giuffrè)
The area most hit by the eruption was the western sector including the village of Ginostra where many still hot scoria and bombs as well as large quantities of ash fell.
Eruption claims one life:
Unfortunately, it claimed one victim (although this could have easily been more): a hiker from Sicily along with a friend were on the trail above Punta Corvo, still in the allowed zone beneath 400 m. When they were caught in the rain of ballistics, only one survived.
At the time of this update, it is not entirely clear what exactly happened. However it seems that the one who died, was not killed by a direct hit from a falling bomb (as initially assumed as a likely cause), but from a combination of inhalation of smoke (from bush fires) and an internal injury afflicted when he fell onto some sharp rock during their attempt to escape. His companion who managed to dodge falling blocks survived with light injuries and in shock. He later gave a short interview to Italian news, telling the sad story how they tried to escape the burning area where they were located.
His video also shows the moment of the giant bubble explosion, which is beyond words:

In the village of Ginostra, panic broke out as hot material started to fall from the sky. Many jumped into the sea while others seeked refuge inside buildings. Afterwards, a significant part of the visitors staying evacuated with the first available hydrofoil.
A large number of tourist from the less affected main village of Stromboli was also reported to have left the island, abandoning their holiday, after the event and during the following days.
Fires: a large number of bush fires broke out in areas on the lower to middle western, southern and eastern slopes where incandescent bombs had fallen into the at this time of year very dry dense vegetation of mostly shrubs, bushes and tall grasses. The picture attached, published in an article on Eolie News shows how dramatic the situation was. A number of firefighter planes arrived at the island soon afterwards and began to extinguish the fires. People in Ginostra felt they were neglected by the government.

No evacuation order was given by Civil Protection who is evaluating the situation with volcanologist to determine whether the volcanic risk on the island has increased or remains the same. It seems, from preliminary information, that the eruption is seen as an isolated event, similar to others that occur a few times every century on average.

The cause of the eruption:

The paroxysmal eruption started when the giant gas bubble exploded at the surface, generating the spectacular blast described above. It opened the conduit and caused a catastrophic degassing and explosive fragmentation of the magma stored in the conduit, generating the vertical ash plume that rose 4-5 km.
The question what caused the eruption is not easy to solve:
1) Was the eruption the result of a batch of new, hot, very gas-rich magma that arrived on that day?
2) Or was it caused by the same magma that had been erupting during the previous weeks?
Until geochemical analysis of the erupted lava before, during and after the eruption are available, it is impossible to answer this question. In case of 1) there should be a significant difference in the chemical and/or physical (e.g. gas content, temperature) properties of the erupted magma of the paroxysm.
While the first option (1) seems more likely, Culture Volcan proposed a possible explanation for the second scenario: the intense activity of the last two weeks could have caused a rapid accumulation of material and made the summit area unstable and fragile. As the structure thus weakened, it became permeable to the magma which pierced it at several places (which explains the small lateral lava flow). The destabilization of the upper part of the rocks surrounding the conduit could thus have lead to a chain-reaction of increasing decompression of the magma, the paroxysm.
These speculations will need to wait until more detailed studies have been made based on available geophysical and chemical data.

Similar eruptions of Stromboli volcano:

The following is an adapted translation of the report published by INGV:
The ordinary activity of Stromboli consists of strombolian explosions that occur at an average frequency of 10-20 min from the summit vents. This typical activity is varying in intensity and is occasionally interrupted by more energetic explosions or paroxysmal explosions.
The latter are discreet violent events that generate eruptive columns that rise for several kilometers above the crater with the fallout of coarse material that can reach even the inhabited centers of the island and occasionally form pyroclastic flows and lava flows that are emplaced along the slopes of the volcano.

The strongest explosive events can launch bombs and large blocks in the summit area (the Pizzo sopra la Fossa). During the strongest paroxysms, the fallout of incandescent pyroclastic material can cause fires in the vegetation on the outer slopes of the volcano. Among the strongest paroxysms, documented in the last 100 years, there are those of 1919, 1930, 2003 and 2007, of which the eruption in 1930 generated pyroclastic flows.
 

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