A series of photographs taken by Mauro Coltelli (INGV-Catania) before and during an explosion within the Bocca Nuova, on the forenoon of 22 June 2011. (a) View of the vent area on the crater floor; red arrows show the two active vents that were seen to emit ash on 22 June 2011. (b) An intense ash emission; the red arrow shows the more northerly vent, which is emitting a dark jet containing a significant amount of juvenile material; the yellow arrow denotes the position of the southerly vent. Flying blocks can be distinguished against the dark background.  (c) Photo taken a few seconds after (b), showing the same ash emission event; the yellow arrow indicates the more southerly vent which is now emitting a jet rich in juvenile material; the red arrow shows the position of the more northerly vent.

During the following days, the ash emissions from the central part of the BN continued, producing numerous reddish to brownish gray plumes. In the absence of wind, these plumes rose up to several hundred meters above the crater rim. Fallout from these plumes consisted of very fine-grained material of pink to brownish orange color. The onset of all emissions was impulsive, their frequency varying from a few minutes to several hours. In particular, during the evenings of 22 and 23 June, a few more intense ash emissions were observed, whose plumes rose hundreds of meters above the summit, and were then dispersed southward. This caused a strong optical thickening of the gas and vapor plume from the volcano, which extended nearly to the periphery of the volcano near Catania.

Direct observations made from the northern and eastern rims of the Bocca Nuova-Voragine complex by INGV-Catania staff, the Etnean guides and J.-C. Tanguy, revealed the presence of two vents that were the source of the explosive events, which besides ash also ejected (presumably lithic) blocks within the crater depression. Strong hissing sounds accompanied these emissions, and prolonged roading or banging sounds were heard during a field visit on the morning of 25 June.

During a field visit carried out by M. Coltelli and E. Privitera (INGV-Catania) on 22 June, the two active vents had dimensions of a few meters, and were located within a shallow depression about 30 m wide. Both vents were obstructed by lithic material (old rock), which was launched into the air during the episodes of degassing illustrated in the images at left, which show the ejection of darker, probably juvenile material first from the more southerly vent, and a few seconds later from the more northerly vent. These events did not produce any loud explosion sounds, and blocks were thrown only a few tens of meters high to fall back onto the crater floor. The frequency of the explosions varied from about 10 to 30 minutes, and their duration was commonly a few tens of seconds; only the strongest explosions produced plumes that rose significantly above the crater rim, but most plumes expanded within the crater, with only a small portion being carried outside with the volcanic gas plume.

A first analysis of the ash emitted from the BN on 17 June shows that it consists mostly (80%) of lithic fragments of various nature (grayish and reddish clasts, fragments of altered glass, and milky-white clasts derived from secondary alteration). Furthermore, the material contained significant quantities of tachylite (10%), which shows a black color, and a blocky morphology. More rare sideromelane (2%) shows a transparend brown color and a more irregular clast shape. Finally, loose crystals, mostly plagioclase, constitutes 8% of the sample.

Between 26 amd 29 June, the ash emissions from the BN diminished both in intensity and frequency, but resumed more vigorously on the morning of 30 June, with characteristics essentially identical to those of the period between 14 and 25 June. Also in this case, the images recorded by the thermal surveillance camera at the Montagnola did not show any hot material in the emissions.

At the SEC, gas emission from the crater on the lower east flank of the cone was present since the beginning of the month, but showed a conspicuous increase on 22 June, and was accompanied by a series of small emissions of grayish brown ash. This degassing, often producing dilute ash plumes, continued until 27 June, and then nearly completely ceased.

STROMBOLI