Report on Etna (Italy) — January 2004
Bulletin of the Global Volcanism Network, vol. 29, no. 1 (January 2004)
Managing Editor: Richard Wunderman.
Etna (Italy) Additional details and interpretation of the 2002-03 eruption; space-based photographs
Please cite this report as:
Global Volcanism Program, 2004. Report on Etna (Italy) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 29:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200401-211060.
37.748°N, 14.999°E; summit elev. 3295 m
All times are local (unless otherwise noted)
Although Etna's 2002-3 eruption was previously discussed in BGVN 27:10-27:12 and 28:01, Boris Behncke subsequently posted a report to his website highlighting some further observations and interpretations. His report presented many photos, including two taken by astronauts aboard the International Space Station (figures 102 and 103). His report drew on published or soon-to-be published material (Acocella and others, 2003; Behncke and Neri, 2003; Branca and others, 2003; and Dellino and Kyriakopoulos, 2003). Extracts from Behncke's summary follow.
Behncke puts the eruption's start date a few hours earlier than previously reported, enough to shift the start date from early 27 October to late on 26 October. While mountain guides who visited the craters on Saturday 26 October noted nothing unusual, an intense earthquake swarm at shallow depth within the volcano began at about 2125 that day. Tremors woke hikers at the Hotel "Le Betulle" at Piano Provenzana, a spot at about 1800 m elevation on the NE flank; they abandoned the seriously damaged building. At dawn on 27 October those who still remained at Piano Provenzana noted large E-W trending fractures cutting across the paved parking lot of the tourist complex. According to Behncke, the first eruptive vents to become active during this eruption opened at 2345 h on 26 October, on the upper southern flank, and immediately began to produce intense explosive activity.
Italy ended daylight saving time at 0100 UTC on Sunday 27 October (0200 local time). The change moved the clock back one hour, from 0200 to 0100 local time. Few if any important events were noted during the 60-minute interval of the time change. Later, however, at approximately 0230 h on 27 October a few persons watching the summit of Etna noted the sudden onset of high lava fountaining from the northern base of the Northeast Crater. The fountaining lasted only a few minutes.
Etna's 2002-2003 eruptions have been described as unusual, differing in four key respects. First, this was one of the most explosive eruptions in recent times. Pyroclastic material comprised more than half of the total volume of erupted products, contrasting with Etna's most recent eruptions, which mainly extruded lava.
Second, the 2002-2003 eruption discharged two different types of magma. The fissures on the NE rift produced magma of the sort normally produced by Etna over the past few centuries, while the vents on the S flank produced a magma rich in amphibole, a water-bearing mineral relatively rare in Etna's recent products. Amphibole also clearly appeared in the 2001 lavas (BGVN 26:10), and in products erupted in 1892 at Monti Silvestri, and 1763 at Montagnola. All of these vented on the S flank.
Third, some scientists have correlated large-scale flank slip and magma uprise. Specifically, on 22 September, the E flank of the volcano began to slip toward the Ionian Sea, with movement occurring mainly on the Pernicana fault system (Neri et al., 2003). This displacement is thought to have allowed magma to migrate from the central conduit system into the NE Rift, although a second, larger flank slip event was necessary to permit magma uprise to the surface. This occurred not only on the NE rift, but also on the S flank, where magma came from the eccentric reservoir previously active in 2001. Thus, the eruption can be interpreted as having been triggered by the flank slip, in contrast with the 2001 eruption, which was preceded by the forceful uprise of a dike from the eccentric reservoir, and possibly was triggered by regional tectonic compression (Acocella et al., 2003). One of the most important discoveries during the events of late 2002 and early 2003 was that the Pernicana fault system is not only 9 km long, as previously believed, but extends from the NE rift down to the Ionian Sea, and possibly continues offshore, over a distance of at least 18 km (Neri and others, 2003).
Fourth, N flank lava flows invaded a tourist complex and adjacent forest for the first time in the historic record. An important landmark in this region is the northern Monte Nero, which sits ~ 6 km N of Etna's summit, includes a crater of the 1646 eruption, and is a local high at 2,049 m elevation. It is easily confused with S- and SE-flank features bearing identical or similar names. The northern Monte Nero lies ~ 2 km W of Piano Provenzana and the surrounding forest called Ragabo. These latter environs had not been invaded by lava flows for many centuries. Eruptions on the NE Rift, which looms above the Provenzana plain, had occurred as recently as 1911, 1923, and 1947, but their lava flows had taken a more westerly course, leaving Piano Provenzana and the forest unharmed.
Tourist facilities were constructed at Piano Provenzana during the late 1960s to early 1970s. The associated ski area became popular among locals because, lying on the northern flank of the volcano, it received and preserved more snow than its southern-flank counterpart, and owing to wind patterns, it received fewer tephra falls. On Christmas 1985 an earthquake along the Pernicana fault (a few kilometers to the northeast of Piano Provenzana) destroyed the Hotel "Le Betulle," killing one person. That earthquake accompanied an eruption in the Valle del Bove, but the latter was very small and did no damage. Hotel "Le Betulle" was rebuilt to sustain higher seismic loads and it became the hub of Etna's N-flank tourism.
The 2002-3 eruption produced a number of small scoria cones along the fissure on the NE Rift and a cluster of huge cones at the vents on the S flank. The latter have completely changed the topography of what was once known as the "Piano del Lago" ("Plain of the Lake", indicated as "PDF" on the map in BGVN 26:10). Until 27 October 2002 this area bore the scars of the 2001 eruption, most of it being covered with virtually inaccessible lava flows and various pyroclastic cones. In early November 2002 the formerly rugged surface was transformed into a rolling plain of ash, which facilitated excursions on foot. The area was subsequently covered by rugged new lava flows. Etna's old abandoned cable car station (partially destroyed by an eruption in 1983) was buried by the lowermost of the new cones. A new souvenir shop and bar erected by mountain guides early in 2002 at 2,760 m elevation vanished under the cover of pyroclastics, although the Torre del Filosofo mountain hut at 2,900 m elevation is still standing. In the past, the Piano del Lago offered a splendid panorama of the summit cone complex, but this view became largely concealed by the new cones along the fissure at 2,700 m elevation. The largest of these cones, formed at a spot that was 2,750 m in elevation prior to the eruption, now stands about 200 m higher, and a second cone slightly up slope is nearly as tall. The sheer size of these new cones dwarfs that of Monte Josemaria Escrivà, which formed in 2001 and had been a prominent feature.
Behncke and Neri (2002) presented initial estimates for the volume of products emitted during the 2002 eruption. They indicated ~ 30 million cubic meters of lava, nearly two-thirds of which was emitted on the S flank. They also indicated that 40 million cubic meters of pyroclastics, were nearly exclusively emitted from the S-flank vents. In terms of magma volume, this is not an enormous eruption for Etna, but it ranks among the more significant of recent decades.
Outlook and interpretations. The 2002-2003 eruption came 1.27 years after the beginning of the 2001 eruption. Between 1971 and 1993 flank eruptions occurred at a mean interval of 1.7 years (Behncke and Neri, 2003). Etna's historical eruptive behavior has undergone significant fluctuations during the 400 years of reasonably complete documentation. It seems that flank eruptions tend to occur in series, and the 2001 eruption could be interpreted as the first in a new eruptive series, analogous to the sequence of 13 flank eruptions witnessed between 1971 and 1993. According to this interpretation, more such eruptions will occur at relatively brief intervals over a period of 10-20 years. By this model, the 2002-2003 eruption was the second in the new series, implying a third in the not too distant future.
The volcano was extensively fractured during both eruptions, which may allow magma to rise much more easily under the flanks of the edifice, and has grown progressively more active during the past 50 years. It is erupting more frequently and more vigorously than during the 280 years before 1950, and its productivity, or output, is increasing. Flank eruptions must thus be expected to occur at intervals ranging from 1 to 3 years, and some of them might be much more voluminous and potentially hazardous than the latest two eruptions.
The 2002-2003 eruption was one of the most explosive flank eruptions in the past 150 years, and it shows that Etna is a potentially explosive volcano, as it has been historically. Nearly all of the flank eruptions of the past 100 years have been relatively benign with mostly lava emissions; thus the local population has been lured into the belief that Etna is a "good volcano." But a short look at the record of its historically documented eruptions shows that the rather effusive, non-explosive behavior of the 20th century was, in fact, unusual.
Part of the explosivity of the 2002-2003 eruption might be due to the water-rich nature of the magma rising from a reservoir located below the S flank. This magma was comparatively gas rich, in contrast to the magma erupted from the central conduit system (and on the NE flank), which had degassed to some degree during the months before the eruption. The fact that this magma has appeared for the second consecutive time might be taken as an indicator that future eruptions will be fed from the same reservoir and therefore could be as explosive. However, an additional factor controlling the explosivity of an eruption is the interaction of magma with external water, such as a shallow aquifer. This was the case during the 2001 eruption (at Monte Josemaria Escrivà), and it was probably again the case at the new cones at 2750 and 2800 m elevation on the S flank. In fact, ash from the first day of the 2002-2003 eruption that arrived on the Greek island of Cefalonia during the following 24 hours was determined to be of phreatomagmatic origin (Dellino and Kyriakopoulos, 2003)
Eruptive activity at Etna quieted on 28 January, but in terms of seismicity, the volcano remained restless. Some of the continuing seismic activity was due perhaps to the adjustment of the edifice to the major displacements at the beginning of the eruption, or it may have started to recharge for its next eruption. Seismicity dropped to relatively low levels in the spring of 2003, but since mid-June 2003 was again slightly elevated.
References. Acocella, V., Behncke, B., D'Amico, S., Maiolino, V., Neri, M., Ursino, A., and Velardita, R., 2003, The 2001 and 2002-2003 eruptions of Mount Etna (Italy): Evidence for different triggering mechanisms: Abstract presented at the Annual Workshop 2003, Pantelleria, Sicily (23-28 September 2003) on Seismic phenomena associated with volcanic activity.
Behncke, B., and Neri, M., 2003, Cycles and trends in the recent eruptive behaviour of Mount Etna (Italy): Canadian Journal of Earth Sciences, v. 40, p. 1-7.
Behncke, B., and Neri, M., 2003, The July-August 2001 eruption of Mt. Etna (Sicily): Bulletin of Volcanology, v. 65, p. 461-476.
Branca, S., Carbone, D., and Greco, F., 2003, Intrusive mechanism of the 2002 NE-Rift eruption at Mt. Etna (Italy) inferred through continuous microgravity data and volcanological evidence: Geophysical Research Letters, v. 30, p. 2077.
Dellino, P., and Kyriakopoulos, K., 2003, Phreatomagmatic ash from the ongoing eruption of Etna reaching the Greek island of Cefalonia: Journal of Volcanology and Geothermal Research, v. 126, p. 341-345.
Neri, M., Acocella, V., and Behncke, B., 2003, The role of the Pernicana Fault System in the spreading of Mt. Etna (Italy) during the 2002-2003 eruption: Bulletin of Volcanology (in press but published on-line 5 November 2003).
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; Earth Sciences and Image Analysis, NASA-Johnson Space Center (URL: http://eol.jsc.nasa.gov/).