Report on Etna (Italy) — April 2002
Bulletin of the Global Volcanism Network, vol. 27, no. 4 (April 2002)
Managing Editor: Richard Wunderman.
Etna (Italy) Nine months of relative quiet follow mid-2001 flank eruption
Please cite this report as:
Global Volcanism Program, 2002. Report on Etna (Italy) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 27:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200204-211060.
37.748°N, 14.999°E; summit elev. 3295 m
All times are local (unless otherwise noted)
This report discusses Etna following the July-August 2001 eruption and through 25 April 2002. According to Boris Behncke, the chief source for this report, this 9-month interval was an unusually quiet one and marked the longest quiet interval since 1995.
A visit to the summit craters on 30 January 2002 revealed low levels of activity and no evidence of energetic outbursts. Loud explosions occurred at intervals of 5-30 minutes within the NW pit of Bocca Nuova, but no solid material was ejected. The rims of the pit were covered with brown lithic ash (which had been emitted in December-January) but there were no blocks or fresh scoriae indicating recent ejections. The pit appeared much the same as in September 2001, with a crescent-shaped flat terrace surrounding a deep, degassing vent in the SE part of the pit.
Most of the present degassing at the summit craters is occurring from a vent in the SW part of the Voragine, which had been much less active during the past 1.5 years. Northeast Crater emitted a fairly dilute plume, and at Southeast Crater, fumarolic activity was concentrated at its W rim where numerous degassing vents lie in a fracture. Mechanized access remained limited after the demise of the cable car and the ski lifts on the S flank during the July-August 2001 eruption (BGVN 26:08 and 27:03). In order to access the summit area one has to hike from ~1,900 m elevation, a trip that takes several hours and leads across the July-August 2001 lava fields.
Numerous small earthquakes, some of which were felt by the local population, were recorded on the S flank (in the area of the largest of the July-August 2001 lava flows), and were interpreted to result from the cooling of the lava. Near-continuous, pulsating emissions of reddish-brown lithic ash began around 9 March at the NW vent of Bocca Nuova, generating a plume that trailed for dozens of kilometers downwind. The same source vent has been the site of deep-seated explosions during the past six months. The emissions may have been caused by collapse within the conduit, which occurred repeatedly after the end of the July-August 2001 eruption, and does not necessarily indicate an intensification of eruptive activity or uprise of fresh magma. On the other hand, the volcano had been quiet for some 8 months at this time, and renewed magmatic activity at the summit was to be expected in the near future.
During the third week of March, emissions of lithic, pink-colored ash continued at Bocca Nuova. These were accompanied by voluminous degassing from Northeast Crater and minor fumarolic activity from Voragine and Southeast Crater. During days without strong wind, these emissions rose vertically to form a spectacular plume that might easily create the impression of true eruptive activity at the summit. However, there is no evidence that fresh magma has risen to near the surface, because no incandescence can be seen at night.
A mid-March summit visit by Giovanni Tomarchio, a cameraman of the Italian television RAI (who is responsible for much of the television footage of Etna in recent years), revealed frequent loud explosions at the SW vent of Bocca Nuova. Although the floor of this vent was not visible, it seemed that the explosions originated somewhere immediately below the visible part of the pit. All recent ejecta were fine lithic ash, which accumulated to form a thick, soft deposit in the summit area. Similar emissions occurred for months at Bocca Nuova during the spring and summer of 1999, prior to the vigorous eruptions at Voragine and Bocca Nuova during September-November of that year.
In late March, after nearly three weeks of ash emissions from Bocca Nuova, Northeast Crater began to emit dark brown to gray ash. The emissions appeared to follow a series of small SE-flank earthquakes during 24-25 March. At least three of the shocks were felt by the local population. On 27 and 28 March the ash emissions from both Bocca Nuova and Northeast Crater rose as distinct puffs to several hundred meters above the summit and seemed more energetic, denser, and darker than during the previous weeks. To a passing airplane pilot they appeared so spectacular that he sent out a warning of an eruption. On 28 March, light ash fell over the S flank as far as Catania (~25 km SSE).
Whether Etna is back in magmatic eruption is the subject of debate. The ash that came from the two craters consisted of fine-grained fragments of rock and was derived from the conduit walls and thus contained no new magmatic material. The ash that fell in Catania on 28 March was distinctly darker than the ash that fell in the summit area during the previous weeks and may contain a certain proportion of juvenile magmatic material, although microscopic examination has not been conducted to confirm this. No glow has been seen so far at the summit during night observations, so it seems unlikely that magma has reached the surface. On 29 March two impressive columns bearing dark ash rose nearly continuously from the two craters to several hundreds of meters (~800 m at one point) above the summit. Shifting winds carried the plume E, S, and W.
During late March through 2 April ash emission continued without interruption from Bocca Nuova, while at Northeast Crater it had apparently stopped. Light ashfalls occurred in downwind areas, at times extending as far as Catania. The emissions took the form of billowing brown plumes, which at times rose several hundred meters above the summit. No incandescence was seen at night. Weather prevented observations after the afternoon of 2 April.
The summit became visible again on 6 April. Bocca Nuova continued to produce weak expulsions of brown-colored (probably lithic) ash, while Northeast Crater emitted only white vapor. Two small (M ~3) earthquakes occurred under the SE flank on 4 April. On 13 April two earthquakes (M 2.7-3) were felt by residents on the SE flank (between the towns of Zafferana and Santa Venerina), their epicenters lying in an area named "Salto della Giumenta," located ~5 km NW of Zafferana. Press sources citing scientists of the Istituto Nazionale di Geofisica e Vulcanologia of Catania gave focal depths of ~4 km below the surface. Numerous earthquakes had occurred within the past few weeks in this area, although their correlation with magma movement within the volcano remained unclear.
Ash emissions continued almost constantly at Bocca Nuova. On 14 April these appeared to be dark gray, and at times were emitted forcefully enough to form plumes several hundred meters high. No incandescence was seen during night observations. A dense plume of brownish-gray ash drifted from Etna's summit across the E sky of Catania as Bocca Nuova emitted pulverized rock from its SE vent. Voragine and Northeast craters gave off dense steamy plumes.
In late April heavy snow fell on Etna; snow-cover reached down to ~1,400 m elevation and access to the summit area was reduced. The snow provided a good opportunity to observe the hot areas at the summit and to confirm that no recent lava outflows have taken place. Snow was melting rapidly on the cones of the summit craters and along the fracture that extends NNE from Southeast Crater. Since 23 April, Bocca Nuova's ash emissions, which had been nearly continuous since early March, decreased markedly. The only visible summit activity during 24-25 April consisted of apparently ash-free gas emissions, mostly from Bocca Nuova and Northeast Crater. Nine months after the climax of its most recent flank eruption, Etna continues its unquiet slumber.
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: Boris Behncke, Dipartimento di Scienze Geologiche (Sezione di Geologia e Geofisica), Palazzo delle Scienze, Corso Italia 55, 95129 Catania, Italy.