Strombolian activity and lava production, late July-23 September. Strombolian activity began from Northeast Crater at the end of July and continued with periods of greater (19 August) and lesser intensity. In early September, 20-30 explosions occurred/minute, with ejection of bombs and scoria to 100-250 m above the vent. A scoria cone ~ 40 m high formed inside the crater. Strombolian activity increased in the late afternoon of 13 September, and lava overflowed Northeast Crater's W rim early the next morning, feeding a modest-sized lava flow that moved NW. By 16 September, the flow had descended to 2,920 m altitude (~ 1.5 km from the main effusive vent at 3,190 m altitude) after crossing a trail (maintained by the Società Turistica Star). During the following days, small lava flows advanced a few hundred meters NW, NNW, WNW, and W, beside or on top of the first flow. This slow and discontinuous activity built a small lava field, with a volume that was estimated at a maximum of 0.25 x 10⁶ m³. On 22 September, the temperature of the flowing lava near the vent was 1,094°C at 40 cm below its surface. During the effusive phase, Strombolian activity varied in strength but generally remained at rather low levels. While sampling high-temperature gases and measuring oxygen fugacity on 22 September, Patrick Allard and others noted that the floor of the crater oscillated, rising by as much as 1 m with every explosion. During a summit-area leveling traverse the next day, a very strong tremor made automatic level readings difficult. Strombolian activity increased considerably, with bombs rising to 200 m above the vent and falling 300 m away. Strombolian activity also occurred at the bottom of the E and W vents (The Chasm and Bocca Nuova) of the central crater. This activity was of variable intensity, with changing numbers and locations of vents. Only rarely were lava fragments ejected above the vent rims.

Cessation of activity, early 24 September. During the early morning of 24 September, the scoria cone and a portion of the wall inside Northeast Crater collapsed into the vent area, leaving a fuming pit ~50 m deep. Effusive and explosive activity ended almost immediately, between 0600 and 0700. That morning, activity was limited to expulsions of reddish ash and white vapor. Steaming fissures and a small graben (~ 1 m wide and 1 m deep in places, wider nearest the crater) opened to the SW and NE (figure 23), with fissures reaching the Valle del Leone (1.5 km ENE of Northeast Crater) and the Piano delle Concazze, 2 km to the ENE. Activity from Bocca Nuova had also ceased. A leveling team in the summit region and N of Northeast Crater detected some intermittent tremor and measured substantial deflation that had occurred since the previous day.

Figure 23. (left) Sketch map of Etna's summit area, showing the new fissures and graben of 24 September and the area of 24 September bomb fall. Numbers along the route taken by observers show positions of (1) Allard, Benhamou, and Pennisi; (2) Bond, Décobecq, Kilburn, Murray, and Obreski; and (3) Sagot. (center) Provisional sketch map showing depth and percentage of ejecta cover after the 24 September explosion. (right) Maximum particle size with distance from Northeast Crater. Courtesy of J. Murray, C. Kilburn, D. Décobecq, and A. Bond.

Because of the abrupt cessation of summit activity and the possibility of violent vent-clearing explosions, the summit area was closed to tourists.

Resumption of explosive activity, afternoon of 24 September. A prolonged period of ash ejection accompanied by thunder and lightning started at about 1215, causing a light ashfall on the upper SE flank (figure 23). At 1312, large dark ejecta were seen rising to ~ 40-50 m above the vent, with stronger expulsion of small brown ash clouds, but no discrete explosions were heard. Episodic eruptions continued for the next 3.5 hours. Periods dominated by quiet emission of pink-brown ash alternated with periods characterized by ejection of black cypressoid and columnar ash jets, and expulsion of large tephra to ~200-350 m above the vent (figure 24). Electric discharges were common. Thick white convecting vapor clouds emerged separately from the S part of the vent area, at increasing rates. Continuous tremor and occasional stronger shocks were felt after 1515 by a gas sampling team working ~ 200 m S of Northeast Crater. There was no apparent time relation between the seismicity and eruptive activity.

Figure 24. Evolution of activity from Etna's Northeast Crater on 24 September. Courtesy of J. Murray, C. Kilburn, D. Décobecq, and A. Bond.

The following preliminary observations have been extensively updated by J.B. Murray.

Ash emission began to increase gradually at about 1645, changing to violent phreatomagmatic explosions. Black clouds, again cypressoid and columnar, initially rose to 400 m above the vent, merging by 1743 to produce a convective cloud that reached a relatively consistent height of ~ 1,000 m. Stronger falls of ash and small lapilli occurred SE of the summit, and bombs reached heights of 600-700 m. No [sounds] were heard, [apart from frequent dry cracks of] electrical discharges in the cloud [and the rushing of air in the rising column]. Incandescent ejecta were first seen at about 1800, and within 40 minutes lava fountains had developed at the center of a much wider column of dark ash. Heat from the fountains could be felt from a distance of ~ 800 m. ... Between 1815 and 1843, bombs fell progressively farther SW from the vent, with maximum ejection distance increasing from ~300 to 700 m, and maximum heights building from ~600 to 1,500 m.

Paroxysmal explosion, evening of 24 September. The paroxysmal phase began about 1845 with continuous louder rumbling and the sudden rise of a vertical lava fountain, ~ 300 m across, to 800-1,000 m. A dense black cloud surged down the E flank of Northeast Crater into the Valle del Bove. [Dense bombs up to 50 cm in diameter fell as far as 2,700 m to the SW, suggesting to Murray that they reached 2-3 km height]. ... The eruption cloud moved SSE, and ... the tephra fallout [rapidly] advanced ~2 km, showering observers [fleeing in vehicles] on the S flank (near the Torre del Filosofo; figure 23) with a dense blanket of ... scoria and ash [with occasional heavy bombs falling from 2-3 km height]. In a short time, the entire SE flank of the volcano was covered by pyroclastic material of sizes that varied with distance from the vent (figure 23). [Differential winds at higher altitude carried some scoria SW as far as Biancavilla (~ 15 km SW), where fragments measuring a few cm fell]. Tephra fall continued for almost 20 minutes.

[Subsequent topographic mapping] showed that pyroclastic material (scoria, lava fragments, and lithic blocks) [reached 4 m thick near the edge of Northeast Crater, and 5 cm thick] at a distance of 2 km (Torre del Filosofo). Scoria and lithic blocks up to a meter in diameter could be found within a radius of 0.5 km; at 2 km, scoria a few tens of centimeters across were observed. Hot scoria 11 cm in diameter melted roof tar 4 km S of the crater (at Piccolo Rifugio); hot lapilli to [7.5] cm across [scratched car windshields at] the tourist complex around Rifugio Sapienza and the base station of the cable car system 6 km from the vent; some scoriae as large as 15 cm were found 7 km from Northeast Crater at 1,700 m altitude (Serra La Nave); and 1.9-[cm] lapilli fell at Nicolosi (17 km to the SSE). Lapilli [0.5 cm in size reached Catania, 27 km away, where light ashfalls also occurred]. Catania airport (35 km from the summit) was closed from that evening until late the next morning.

[Bond, Décobecq, Kilburn, Murray, and Obreski] estimate that the tephra column reached 10-13 km altitude during the paroxysmal phase, in agreement with an independent estimate by Mueller [from Nicolosi. However, photos by J.P. Delouche, approaching the volcano from Siracusa, show maximum column height to be 6-7 km above the summit (9-10 km altitude)]. The press reported that the tephra column was visible from the Aeolian Islands (95 km N), and Agrigento (100 km WSW). At [1900] on 24 September, the explosive activity ceased completely. During the following days, Northeast Crater slowly emitted gas, vapor, and (rarely) reddish ash.

[Nearly 200 depths measured by Murray, Décobecq, and Bond yield a tephra deposit volume of 2.6 x 10⁶ m³, mostly of pumice density, or 0.4 x 10⁶ m³ dense rock equivalent (DRE)]. The volume of material emitted was estimated by IIV geologists at around a few million cubic meters ... . If most of the tephra were assumed to have been erupted during the final phase, the mean paroxysmal eruption rate was [~500] m³ DRE/s (2.7 x 10⁶ kg/s). Kilburn notes that the independently determined values for eruption rate and column height are consistent with the column height model of Wilson et al., 1978. Total energy release was [~9 x 10²¹] ergs. Vigorous explosive eruptions are relatively uncommon at Etna; [this century only four comparable events have occurred, in 1917, 1940, 1947,and 1960].

Minor activity, late September-early October. Strombolian activity from the two central crater vents, which ended 24 September with the collapse in Northeast Crater, resumed on the morning of 29 September. Gas, at times under pressure, was also emitted from Southeast Crater and a nearby gas vent before, during, and after the paroxysmal explosion. Emission of gas with fragments of old lava and/or incandescent lapilli was observed at times. The frequency of these ejections was very irregular, varying from a few minutes to several hours. As of 9 October, no significant changes in the activity of the summit craters have been noted.

Seismicity. Beginning 13 September, the seismic activity consisted only of explosion earthquakes probably related to the onset of the increased Northeast Crater activity. During the night of 22 September, three isolated seismic shocks with a maximum magnitude of 2.9 were recorded on the NW flank. Minor shocks were recorded until the night of 2 October, when a swarm of 30 events (maximum magnitude 3.3) occurred on the lower NW flank (between Maletto and Randazzo) at variable depths of ~ 20 km. A second swarm started during the morning of 5 October, again on the W flank, at the same depth. The strongest shock (M 3.8) occurred at 1228, and was felt at numerous locations on the volcano. The swarm ended that night, after ~ 40 weak shallow events were recorded on the W flank (Monte Minardo area). Another swarm of ~ 10 shocks occurred the morning of 7 October, also on the W flank, with a maximum magnitude of 3.3.

Harmonic tremor increased during the first phase of Northeast Crater eruptive activity (13 September). Similar tremor energy values were observed until ~ 5 hours before the 24 September eruptive event, when tremor energy began a gradual increase of about an order of magnitude. Energy values returned to normal around 2000 and no significant variations occurred in the following days.

Reference. Wilson, L., Sparks, R.S.J., Huang, T.C., and Watkins, N.D., 1987, The Control of volcanic column heights by eruption energetics and dynamics: JGR, v. 83 p. 1829-1836.

Further References. Amore, C., Giuffrida, E., Scribano, V., Lowenstern, J., and Müller, W., 1987, Emplacement and textural analysis of some present-day pyroclastic deposits of Mt. Etna (Sicily): Boll. Soc. Geol. It., v. 106, p. 785-791.

Murray, J., Décobecq, D., and Bond, A., 1989-90, L'Eruption paroxysmale du cratère Nordest de l'Etna du 24 Septembre 1986: LAVE Bulletin, no. 22, p. 11-23; no. 23, p. 5-18; and no. 24, p. 11-21.

Geological Summary. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: R. Romano, G. Budetta, T. Caltabiano, D. Condarelli, O. Consoli, and E. Lo Giudice, IIV; G. Luongo, IIV and OV; G. Ricciardi and G. Forgione, OV; S. Gresta, Univ di Catania; R. Clocchiatti, P. Gillot, G. Kieffer, J. Murray, and J. Tanguy, PIRPSEV; P. Allard, G. Benhamou, and M. Pennisi, Centre de Faibles Radioactivites; A. Bond, Univ of Lancaster; D. Décobecq, Univ Paris-Sud; C. Kilburn, Univ di Napoli.