by the INGVvulcani editorial staff
During the past few months, the persistent explosive activity of Stromboli – which is indeed called “strombolian activity” (see this article, in Italian, on the explosive activity of Stromboli) – has been rather intense, with 4-5 distinct vents erupting on the crater terrace. This activity reached a first culmination on 25 June 2019 at 23:03 h UTC (=local time -2), with a major explosion, which originated from the central-southern area of the crater terrace (Figure 1). Video recorded by the surveillance cameras showed that pyroclastic material fell onto the Sciara del Fuoco and onto the crater terrace, with a few blocks being thrown also beyond its rim.
During the following days, the activity continued in a rather vigorous manner (well evident in this video recorded on the evening of 29 June by Micol Todesco); in particular, the southernmost of the vents in the central-southern area of the crater terrace frequently produced jets of incandescent scoriae that rose higher than the Pizzo [the panoramic summit plateau above the crater terrace].
Reports by persons who observed Stromboli’s activity on 1-2 July indicate that it had slightly diminished compared to the previous days; however, such fluctuations are normal and cannot be considered a precursor to the 3 July paroxysm. There were no significant variations in the activity until the afternoon of 3 July. A small lava flow was emitted from the central-southern crater area onto the upper portion of the Sciara del Fuoco at about 14:00 UTC.
The 3 July 2019 paroxysm
The first signal that can be interpreted as clearly precursory to the paroxysm was recorded as a significant variation at the dilatometer SVO, located at the COA (advanced operations center), at 74 m elevation, starting at about 14:38 UTC . Images of the thermal surveillance camera on the Pizzo (SPT) show the start of emission of a small lava flow from vent “C” (see Figures 1 and 2a) at 14:43:15. During the following two minutes, all vents started extruding lava flows (Figures 2b-d); infine, followed, at 14:45:43, by the rapid expansion of a huge bubble of lava in the area of vents S1 and S2, which initiated the sequence of paroxysmals explosions that lasted about one minute.
The sequence of explosions was quite complex, generating amongst others two partly directed explosions. The most violent event, at 16:46:40 UTC, affected the entire crater terrace. Portions of the eruption column collapsed onto the Sciara del Fuoco, generating at least two pyroclastic flows that advanced on the sea about 1 km beyond the coastline (Figure 3a). The eruptive column rose about 4 km above the summit of the volcano (at 924 m elevation; Figure 3b). The fall of incandescent pyroclastic material rapidly set the vegetation on the slopes of the volcano on fire (Figure 3c), mostly on the side of Ginostra, the smaller of the two villages on the island, which lies in its southwestern sector (Figure 4). Unfortunately, a 35-years-old man from Milazzo, who was in the Punta dei Corvi area (about 100 m above the sea-level) with a friend, was killed, while his friend was injured.
On the Pizzo, the fall of large pyroclastics completely buried the terrain and destroyed all monitoring instruments there. On the upper outer (southeastern) flank of the Pizzo, the abundant fall of still fluid lava bombs generated a small “rheomorphic” flow, constituted by still-fluid lava fragments which, coalescing and agglutinating are capable of flowing. This flow advanced about 200-300 m, burying a portion of the path normally used by excursion groups when descending from the Pizzo (Figure 5).
Pyroclastics of smaller dimensions (ash and lapilli) affected mostly the western sector of the island including the village of Ginostra (Figure 6). Much of this material is comprised by golden-yellow scoriae (also known as “golden pumice”), which show mixing with black lava – a phenomenon typically observed in the products of Stromboli’s paroxysms. These scoriae can be extremely porous and float on water (Figure 6a); during the following days, conspicuous rafts of such scoriae were carried southward by marine currents and nei giorni successivi, beyond the island of Vulcano.
Post-paroxysm activity through 13 July 2019
After the paroxysmal sequence, explosive activity rapidly diminished, and within one hour was back to the common Strombolian activity, though still occurring at rather high intensity and being accompanied by small lava overflows from various vents on the crater terrace. In particular, the central-southern crater area was the site of nearly continuous explosive activity from several vents and emission of small lava flows that expanded onto the southern portion of the Sciara del Fuoco, reaching about halfway down the slope (Figure 7).
On 9 July, scientiests of the INGV-Osservatorio Etneo and of the GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, conducted an aerophotogrammetric survey using a hybrid WINGCOPTER drone equipped with a Workswell (WIRIS model) and a RGB Sony 40 Mp photocamera. This was used for constructing a preliminary thermal 3D model of the island, and in particular, of the area affected by the ongoing eruptive activity. Figure 8a allows distinction of the active portions of the lava flow-field and the active summit vents (all shown in red). Figure 8b shows an image recorded by the Sentinel-2 satellite, with three strong thermal anomalies corresponding to the explosive vents, and the active lava flow on the upper slope of the Sciara del Fuoco.
As of 13 July 2019 evening, intense explosive and effusive activity was continuing at Stromboli. On the evening of 12 July, the activity of the northern crater area increased and led to the formation of a series of small lava overflows onto the upper central portion of the Sciara del Fuoco (Figure 9a), while modest lava emission also continued from the central-southern crater area (Figure 9b). Similar activity also continued on 14-15 July.
We thank all our colleagues of the INGV, who in this period have been carrying out observations, sampling, and repair of damaged or destroyed instruments on the island of Stromboli, and who have shared their photographs. Collaboration with the German colleagues of the GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, has been rather productive and pleasant. A particolar “Thank you” goes to Mr Egon Karcher, who generously shared his precious photographs of the pyroclastic flows on the Sciara del Fuoco of 3 July 2019.
Compiled by Boris Behncke, using data and information acquired by the monitoring networks and staff of the INGV involved in the field work and the Surveillance Rooms of the Observatories.