As of the 26th of April, it was reported to SWVRC by Boris Behncke, et al, that more detail is emerging about the 26 April eruptive episode at the SE Crater and its effects, which were more far-reaching than usual. Additional information was furnished by Charles Rivière and Robert Clocchiatti, who witnessed the event from close distance, Giuseppe Scarpinati (Italian delegate of the Association Volcanologique Européenne, "Lave", seated in Paris; Scarpinati lives in the town of Acireale on the SE flank of Etna), and other sources.
The first signs of a revival of the activity at the SE Crater were observed by Scarpinati at around midnight (during the night of 25-26 April) when he noted "a very small incandescent lava extrusion, which was barely visible" at the N base of the SE Crater cone. By 0530 h on the 26th, the lava flow had grown significantly and now was about 500 m long; at its source there was vigorous Strombolian activity. The next thing that happened was the emission of white vapour mixed with brown ash plumes at 0615, as observed by Rivière. Fifteen minutes later Scarpinati, who had returned to sleeping, was awakened by loud detonations and saw the beginning of strong explosive activity at the summit vent of the SE Crater. According to Rivière, the true paroxysmal phase started at 0655 h, when lava fountains rose hundreds of meters from the crater; a tall eruption column rapidly rose several kilometers above the summit of the volcano, forming an impressive dark mushroom of gas and ash. The plume then was carried to the SE, in the direction of towns like Viagrande (which received a heavy shower of scoriaceous lapilli) and Acireale, where pea-sized lapilli fell abundantly. From Catania, the view of the plume passing just slightly to the north was awe-inspiring; it filled about half of the sky and blotted out the rising sun.
The strongest activity occurred shortly after 0700 when large bombs were thrown to more than 1 km distance of the SE Crater summit. Many bombs up to 0.5 m in diameter fell around the Torre del Filosofo building, 1 km S of the crater. At that time of the day the guided tours of tourists to the building, which is now the point of arrival of the tours, had not yet been initiated, so that there very few people in the area, and no one was hurt. During this phase of activity, a vent burst open low on the S flank of the SE Crater cone, just above the saddle which stands between the cone and the newly grown Sudestino. Lava was emitted from this vent to the SW, forming a short flow; no lava appears to have flown from there in other directions. Invisible to observers on the S side of the erupting cone, lava was also emitted through a large breach on the N side of the cone, where it advanced for a few hundred meters.
The activity subsided rapidly after 0720 h and was essentially over five minutes later. However, the lava flow to the SW remained active for more than one hour afterwards, stopping at the N side of the 1971 "Observatory" cone.
It was shortly after the end of the main paroxysmal
phase, at 0739h, when an airbus of the AirEurope, which had departed
from the Fontanarossa international airport of Catania in the direction
of Milano, entered the fallout zone of the plume at an altitude of
about 1000 m. Apparently the aircraft received windshield damage by
the violent impact of scoriaceous lapilli and was forced to return
immediately to the airport of Catania. Windshields were scratched
but apparently not broken. Passengers reported to news reporters that
it seemed that the airplane entered a zone of turbulence, causing
it to vibrate strongly, and then it seemed as though something was
scratching one of the side windows, "as if it were hit by a sharp
object". According to some news reports, some fear was aroused among
the passengers, but the pilot soon informed the passengers about a
"technical problem" and told them everything was under control,
and that they were to return to Catania airport. Other sources report
that the passengers did not note anything unusual until the pilot
advised of the return to Catania.
The circumstances of the 26 April incident are still not clear. It is not well explainable why the airliner ended up under the plume. The eruption had been perfectly visible from Catania airport as well as from all over eastern Sicily, and it occurred quite some time before the airplane took off. Yet the pilot said that he did not see the plume "it was invisible, certainly not a black cloud" - yet the plume was quite dark as seen from Catania by residents of the city), and to his knowledge it had been drifting in the opposite direction (that is, towards W; however, since about one week the wind had been constantly blowing from W). Certainly an investigation will follow and hopefully teach an important lesson which will help avoid similar incidents in the future, as air traffic at Catania is intensifying with a rapid growth of the tourist flux, and more explosive eruptions of Etna are to be expected in the near future.
Later on 26 April Boris Behncke (Dipartimento di Scienze Geologiche, University of Catania) and others visited the summit area of Etna. Very detailed observations could be made at the Bocca Nuova and the Voragine, and good views were obtained of the SE Crater from the platform in the SE part of what once was the Central Crater of Etna.
The Bocca Nuova, accessible only from SE, S or SW, which in any case means an agonizing 300 m climb up the steep flank of the main summit cone, is a most varied, colorful little world on its own. There have been no remarkable changes at this crater since visits made in the past three months, and the activity is the same as observed on earlier occasions: gas emissions from a vent in the E part of the crater, at times more explosively, but without ejections of any solid material. The explosions are generally very short, producing sharp reports, and often resulting in the formation of gas rings.
Little activity is apparently occuring in the second large vent in the NW part of the crater. This vent is a pit about 150 m in diameter with vertical, in part even overhanging, walls, from which several internal avalanches were observed during the stay at the Bocca Nuova. The interior of this pit is invisible due to a dense, but passive gas plume.
It was possible to walk around what remains of the NW rim of the Bocca Nuova to the area where this crater once intersected the SW rim of the Voragine. Until 1998 the two craters were separated by a thin wall of rock (which demonstrated a surprising degree of stability), known as the "diaframma". Now in its place there is a kind of a saddle separating a knob on the SW rim of the Voragine from the N side of the partially collapsed cone that had built around the NW vent in the Bocca Nuova in October-November 1999. The area is cut by numerous degassing fractures, some up to 1.5 m wide, with beautifully colourful sublimates on their rims.
The structure of the adjacent Voragine is amazingly simple in comparison with that of the Bocca Nuova. The crater has a large pit (about 200 m in diameter) in its W part, which was formed during the violent 4 September 1999 eruptive episode. To the N, NW, E and SE this is surrounded by a very flat terrace up to 150 m wide, which lies some 20 m below the E rim of the crater.
The SE Crater, as seen from the flat area in
the SE part of the main summit cone, shows a deep notch on its N side,
which reaches almost down to its base. The notch and the lavas that
had issued at its lower end (where the effusive vents active before
the 26 April paroxysm are located) were well illuminated by the sun,
so that no incandescence (if there was any) could be perceived there.
However, a fan-shaped field of numerous overlapping lava flows was
seen to extend from there towards NE and E, into the upper reaches
of the Valle del Leone and the Valle del Bove. Only very weak gas
emission was seen in some locations along the N flank notch. The summit
of the SE Crater cone consists of two very sharp crests lying on the
E and W sides of the elongate summit vent area, which are of approximately
equal height. The highest points of the cone stand several meters
above the elevation of the main part of the terrace from where observations
were made, that is, at approximately 3270 m. The highest point of
Etna presently lies at about 3310 m elevation, so that the SE Crater
would have to grow only 40 m to become the new summit of Etna. If
the activity continues at a similar rate to that of the past two years,
the cone might reach that height within one year or two.