Report on Etna (Italy) — January 1989
Scientific Event Alert Network Bulletin, vol. 14, no. 1 (January 1989)
Managing Editor: Lindsay McClelland.
Etna (Italy) Strombolian activity from summit craters: inflation
Please cite this report as:
Global Volcanism Program, 1989. Report on Etna (Italy) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 14:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198901-211060.
37.748°N, 14.999°E; summit elev. 3320 m
All times are local (unless otherwise noted)
After the October 1986-February 1987 eruption, activity has been confined to the summit craters. In the following report, frequent observations by IIV personnel with help from Italian Alpine Club guides are supplemented by data from geologists who reoccupied ground deformation networks 20 May-24 June and 16 September-7 October.
Bocca Nuova. Since the end of April 1988, Strombolian activity and/or degassing has continued at varying intensity. Explosive vents were active at different points on the crater floor, 150-250 m below the rim. Lava fragments only rarely reached the crater rim. Ash and lapilli ejections were occasionally observed, especially during periods of stronger and more continuous activity. In May and June, small explosions or gas bursts could be heard deep within the crater at a rate of 30-50/minute, with larger explosions about every 5 minutes. Loudness of the explosions varied considerably; they usually were not audible 400 m from the crater, but on 23 June, 4 were heard from Rifugio Sapienza, 5.5 km S, between 1214 and 1226. In September and October, the explosion rate was 30-40/minute. Strombolian explosions from three active vents on the crater floor, roughly 150 m below the rim, were observed about once every 5 minutes. Louder series of explosions were heard up to 3 km away on 23 and 28 September and 1 October. During the night of 1-2 October, a few fresh bombs landed just outside the crater rim and the next day bombs were rising to ~ 10 m below the rim. Violent activity that included lava fountaining was observed from a distance on 31 January 1989 from 1930 to 2100. Bocca Nuova was relatively quiet in early February.
La Voragine. The vent was partially obstructed until July, with activity limited to weak gas emission. Strombolian activity of variable intensity then started at the bottom of the crater, at more than 250 m depth. Ejecta only rarely reached the rim. On 23 August between 1610 and 1650, a series of pyroclastic explosions ejected juvenile material (lava fragments, scoria, lapilli, and ash) to a few hundred meters height. The tephra fell mainly on the E flank, to 400 m from the rim. Explosive activity ceased completely from the end of August until the beginning of September. During visits on 28 September and 6 October, Strombolian explosions were occurring from vents that appeared to be higher in the crater than those at Bocca Nuova. About 30 explosions/minute were counted on the 28th, with ~1/minute strong enough to eject bombs visible to the geologists, who could see 50-80 m into the crater. At night on 2 October, no noises could be heard from the rim, although faint glow was visible. In early February 1989, little activity was occurring from the crater.
Southeast Crater. Gas emission, sometimes under pressure, continued from a vent near the center of this complex of cones and depressions. By 4 October the gas emission had strengthened, ejecting blocks, apparently from the wallrock, and sublimates. Weak and irregular Strombolian activity began in early October. A small new vent opened between 24 and 25 November E of the previous vent, ejecting ash and other tephra. Some of the explosions were fairly vigorous, as on 27 November. The activity changed gradually from ash ejection to a nearly continuous explosive Strombolian activity, with periods of greater and lesser intensity. At times, lava fragments reached 250 m height, falling within a few hundred meters of the crater. Similar Strombolian activity was continuing in early February 1989.
Northeast Crater. Only fumarolic activity was observed until the end of July, varying in intensity with weather conditions. A small vent opened inside this crater on 24 July and grew with time. Very hot pressurized gases emerged, particularly soon after the vent opened. The vent was glowing at night in September. Smoke rings were occasionally emitted in September and October, similar to those observed from Bocca Nuova early in its development.
The following deformation and seismic data are from J.B. Murray.
Despite the increased seismicity and deformation recorded February-April 1988 (13:03), the June levelling traverse showed an unusually small amount of movement since September 1987. Subsidence of recent lava around Northeast Crater was <2 cm, an order of magnitude less than usual and the smallest recorded since the line was installed in 1975. Inflation of 1.6 cm measured 3.5 km NE of the summit was also unusual. Recently active areas generally showed subsidence, with downward movement of slightly > 1 cm at the 1983, 1985, and September 1986 dikes, and isolated larger movements of -6.6 cm at the 1986-87 fissure and -19 cm at the edge of Southeast Crater. The very large amounts of subsidence measured in recent years in the Valle del Leone have diminished considerably.
A very different pattern was measured during the September traverse, with broad inflation centered just SW of Bocca Nuova. Inflation extended over the entire traverse, reaching +5.3 cm at the center when compared to the southern reference station 2.5 km away, or +8.5 cm compared to stations at the N end of the traverse, 5 km away. This inflation was the largest recorded at Etna when no eruption had occurred between measurements. Most dry-tilt stations showed typical small, more or less random, tilts of apparently local origin. However, 4 stations 4-9 km to the SW showed westerly tilts of 20-30 µrad, suggesting that inflation may extend farther in that direction. Murray noted that summit inflation preceding summit eruptions was measured in May 1976, June and September 1977, September 1978, July 1980, October 1983, September 1984, and June 1986. Inflation episodes centered just SW of the summit have tended to precede Southeast Crater activity.
Small local shocks that were apparently related to summit activity were detected optically during measurements of the levelling line and dry tilt stations. Using the automatic level, relative amplitudes were obtained by noting the amount of crosshair movement against the levelling rod and correcting for distance. In June, seismicity was very quiet, with only four events noted in a week of levelling, all within 450 m of Bocca Nuova. The largest had an amplitude of ~ 60 µrad.
In September and October, seismicity had increased dramatically. On 23 September, 73 shocks were noted in 1 hour while levelling W of Bocca Nuova at a mean distance of 330 m from the rim. The shocks were much stronger than in June, with largest amplitudes reaching 250 µrad. Seismic events were detected as much as 4.6 km from the summit. W of Piccolo Rifugio, ~ 3 km from the summit, three shocks were detected in 1 hour, the largest of 35 µrad.
Geologic Background. Mount Etna, towering above Catania, Sicily's second largest city, has one of the world's longest documented records of historical 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 horseshoe-shaped 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, T. Caltabiano, and O. Consoli, IIV; J. Murray and D. Norman, Dept of Photogrammetry and Surveying, Univ College London; D. Décobecq, Paris; J. Miller, T. Elliott, and B. Van Wyk de Vries, Open Univ; B. Bone, Lancaster.