10 years ago: The July-August 2001 eruption of Etna

Eruptive activity from multiple vents and lava flows on the south flank of Etna on the evening of 22 July 2001, seen from the area above the twon of Nicolosi. The largest glow, just right of the center of the image, comes from the vents at 2100 m elevation, near the tourist facilities around the Rifugio Sapienza, also known as "Etna sud". A dark plume of ash just above the glow from the 2100 m vents is emitted from a large explosive vent at 2570 m elevation; a few days later this vent will grow into a huge new cone. Photo taken by Boris Behncke, INGV-Catania

Prelude

Following the long-lived and voluminous 1991-1993 flank eruption, eruptive activity resumed at the summit craters of Etna in late July 1995, and continued for six years, involving all four summit craters of the volcano. Repeated overflows of lava from the summit craters - most spectacularly in October-November 1999 from the Bocca Nuova - and an unprecedented series of violent paroxysmal eruptive episodes, including two sub-Plinian events from the Voragine in 1998 and 1999, were clear visible evidence that the volcano was receiving substantial input of magma from depth. Geophysical and geochemical data, however, indicated that much more magma than was erupted at the surface, accumulated below the volcano and made the mountain swell at a notable rate. Frequent swarms of earthquakes at different depths below different sectors of the volcano were caused by the intrusion of magma, a part of which apparently did not, as usually, use the open central conduit system of Etna, but rather prepared to open a new pathway through the volcano's flank.

During the first half of 2001, most activity occurred at the Southeast Crater, first with slow lava emission from a vent located on the northeastern flank of its cone (informally named "Levantino"), and then, from early May on, at ever increasing effusion rates punctuated by a series of violent eruptive episodes at irregular intervals of several days to a few weeks. Fifteen paroxysms occurred in this period; the latest of these, on the early morning of 13 July, was also one of the most violent.

At the same time of this latest paroxysm, vigorous seismicity began to shake the volcano and its surrounding areas, including shocks that were felt as far as Acireale and Catania. Between the early morning of 13 July and the early morning of 17 July, approximately 2600 tremors, mostly unfelt, were recorded by the seismic network of the INGV-Catania on the volcano. During this same time interval, severe ground cracking and faulting occurred in various places on the W rim of the Valle del Bove and near the "Cisternazza" pit crater on the Piano del Lago, a flat area at about 2500 m elevation to the north of the Montagnola. Magma was forcefully breaking its path vertically upward through the south flank of Etna, virtually tearing the mountain apart.

Shortly after midnight on 17 July, the lull before the storm ended with yet another paroxysmal eruptive episode at the Southeast Crater. This was similar to its predecessor on 13 July, with vigorous lava fountaining from the summit vent and the "Levantino". Possibly during this paroxysm, a small fissure (F1) opened just a few tens of meters to the west of the "Levantino". This was the last event that could be considered part of the "prelude" to the main eruption.

Eruption chronology

The eruption lasted nearly 24 days, from early 17 July to late 9 August 2001. It began a few hours after the latest paroxysmal eruptive episode at the Southeast Crater at about 0500 h GMT (= local time -2) on 17 July, with the opening of an eruptive fissure (F2) at the SSE base of the SE Crater, at an elevation of approximately 2950 m. Vigorous lava spattering occurred at a number of vents, while lava flows advanced toward SE, in the direction of the Valle del Bove rim. At 1500 h GMT, another fissure (F3) opened between 2890 and 2690 m, a bit to the W of a panoramic point on the W rim of the Valle del Bove known as "Belvedere", and lava began to extend S from there, across the Piano del Lago, and in the direction of the highly vulnerable area of the cable car and ski lifts that constituted the main ascension route to the upper S flank of Etna.

At 0020 h GMT on 18 July a new N–S eruptive fracture (F4) became active still farther south, below the Montagnola and slightly uphill from the Monti Calcarazzi, at an elevation of about 2100 m. The opening of this fracture was accompanied by a magnitude 2.7 earthquake. From the lower part of this fracture a lava flow began to extend southward, invading after a few hours the parking area between the Rifugio Sapienza and the Monti Silvestri, interrupting the road to Zafferana (SP92; see the aerial view above), and continuing its downslope advance in the direction of the town of Nicolosi. Differently from the lavas emitted from F2 and F3, these lavas contained abundant inclusions of sedimentary rock, mostly white to pale yellow sandstone but also fragments of orange, red, green, and blue sediments. Some of these inclusions showed evidence of interaction with the surrounding magma. Furthermore, the water-bearing mineral amphibole was found in the lavas from F4.

During the remainder of the day and on 19 July, the lava flow fed by the vents at 2100 m elevation continued to advance rapidly down the S flank, passing between numerous older cinder cones along the E margin of the 1983 lava flow-field. By the late evening of 18 July the flow front was at 1800 m elevation. At the same time eruptive activity continued unabated at all eruptive fissures. On that evening a man from Belpasso, a nearby town, who had come with friends to witness the spectacle of the eruption, was seriously injured near the eruptive vents at 2100 m elevation. The man was brought to a hospital in Catania and treated for injuries on his dorsal spine. This incident led the local authorities to close all access roads to Etna to the public.

On the evening of 19 July at about 1900 h, a new vent (F5) opened on the Piano del Lago at about 2570 m elevation, about 500 m north of the Montagnola. The opening of these vents was characterized by violent (mostly phreatomagmatic, with minor Strombolian) explosions and the formation of dense ash and lapilli columns rising 200–300 m high. Ash emitted from F5 was dispersed over the E sectors of the volcano to tens of kilometers away, and fell in heavy showers over numerous population centers on the flanks. The coarser-grained magmatic products contained the same sedimentary xenoliths observed in the lavas emitted from F4.

Steaming vents and lava flow at the eruptive fissure (F6) that opened on the northern rim of the Valle del Leone, just below the Pizzi Deneri (crest at left), seen from the air on 22 July 2001. The building in the lower portion of the image is the Volcanological Observatory, and the elongate object next to it, a water tube tiltmeter. Photo taken by Marco Neri, INGV-Catania Violent explosion from the main vent of the new cone growing at F5, in a place that one week earlier was still a flat plain known as "Piano del Lago" (plain of the lake), 28 July 2001. A pole of a ski lift at left gives a vague impression of the scale. Many of the incandescent lava fragments in the air are up to the size of a truck. Photo taken by Boris Behncke, INGV-Catania

In the early morning of 20 July, yet another fracture system (F6) began to propagate from the N base of the Southeast Crater cone toward the NE, in the direction of the Valle del Leone. Strombolian activity began at the lowermost part of this fracture system, accompanied by a small lava outflow. During the following hours this fracture system continued to propagate downslope until it reached the base of the Pizzi Deneri where it abruptly bent SE and reached 2,600 m altitude. At its lower end a new eruptive vent formed, from which lava was emitted. In the meantime, F4 showed a marked increase of its effusion rate and produced “curtains of fire” up to 40 m high. Still on 20 July, the Southeast Crater reactivated to display a weak Strombolian activity at its summit vent. All other eruptive fissures remained fully active. Violent phreatomagmatic activity continued at F5, presenting a major threat to civil and military aviation of the Fontanarossa airport of Catania and Sigonella Air Base, disrupting car traffic, and damaging cultivations. As a consequence, on 21 July the Fontanarossa airport was temporarily closed. On 22 July the eruption rates at F4 reached a peak: in the lower part of the fissure lava fountains rose 100–200 m high, while frequent powerful phreatomagmatic explosions occurred in its upper part. The most advanced lava front reached a relatively flat area at 1060 m elevation. At the same time the lava flow from F3 reached 2070 m elevation, burying several poles of the cable car. On 23 July, at about 0700 h GMT, a new NW–SE eruptive fissure (F7) opened on the lower SE flank of the Southeast Crater cone and emitted a small lava flow that extended toward the E for a length of about 1.6 km. On 24 July there was a temporary decrease in the eruptive activity and no substantial growth of the lava flow-fields occurred. However, a change began to be noted in the eruptive style at F5, which attained a more and more magmatic (Strombolian) character. The definitive change from phreatomagmatic to essentially magmatic activity was seen on 25 July, when a fairly regular scoria cone began to grow around the principal vent. All eruptive fissures were still active on that day, but the intensity of the Strombolian activity at F4 was weaker than before, and the front of the lava flow emitted from this fissure was almost stagnant at 1030 m elevation. On the next day, 26 July, the new cone (F5) forming on the former Piano del Lago showed impressive levels of activity. Like on the previous day, it produced near continuous explosions accompanied by loud detonations that could be heard to tens of kilometers away. The cone grew rapidly by the accumulation of the enormous volumes of huge bombs ejected from at least three vents within it. The shift from phreatomagmatic to magmatic activity at this cone had serious consequences: lava was now flowing from vents on its southern and northern sides. These new flows rapidly spilled around the N and NW sides of the Montagnola and down the steep slope on the W side of that cone toward the Rifugio Sapienza area. As a precaution, earth barriers were constructed further downslope to protect the departure station of the Etna cable-car and the nearby Rifugio Sapienza from potential invasion by lava flows.

On 26 July four of the poles of the cable-car were overwhelmed by the new lava from F5, before the flow passed on the E side of the departure station of the cable-car, covered another portion of the Provincial Road 92 and advanced a few hundred meters further. The upper cable car station was surrounded by the same flow further upslope but remained intact for another few days. Desperate efforts were made to reinforce the earth barriers which until now had prevented the flow from directly invading the tourist complex at the Rifugio Sapienza, but it seemed that the battle was all but lost. Vigorous eruptive activity continued on that day at the eruptive fissure at 2950 m elevation (F2) while the fissure at 2700 m elevation (F3) continued to emit lava but pyroclastic activity there had ended. Weak activity continued at the F6 (Valle del Leone) fissure, with lava flowing toward Monte Simone (in the N part of the Valle del Bove), and the fissure at 2100 m elevation (F4) showed no significant changes in its activity with respect to the previous days.

Mild Strombolian activity and emission of lava flows is continuing from the vents at 2950 m elevation (F2), seen in the center and left foreground, on the tenth day of the eruption, 26 July 2001. Just behind the glowing vent at the largest of the spatter cones (hornitos) formed at F2, steam is rising from an about 20-m-tall cone that lies in the central portion of F3, at about 2720 m elevation; a bit further to the right, the "Torre del Filosofo" building is seen barely out of reach of the lava flows from F2. Behind the roof of the building, a broad new cone with a wide crater containing several violently explosive vents is emitting a dilute ash plume; this is F5, at 2570 m elevation, and the white streak seen to the right of it is a lava flow that is about to reach the arrival station of the Etna cable-car. Still further behind looms the silhouette of the Montagnola cone, formed in 1763. Photo taken by Boris Behncke, INGV-Catania

27-30 July were the most dramatic days of the eruption in terms of the impact on human structures. While the fissure in the Valle del Leone (F6) showed ever lower levels of activity and the fissures at 2900, 2700 and 2100 m elevation continued to erupt in a manner almost identical to the previous days, the new-born cone (F5) at the former Piano del Lago and several nearby vents not only provided an awesome show, but vents located on its N and S flanks delivered vigorous lava flows that spilled down the steep slope to the W of the Montagnola in surges, presenting a continuous threat to the Rifugio Sapienza area. These days were characterized by frantic attempts to contain the lava, to keep it away from the lower cable car station and the Rifugio Sapienza, and the souvenir shops around them. Yet on the last of these days, on the evening of 30 July, the eruption claimed its most prominent victim. Before the eyes of the operators of the cable car and the lenses of television cameras from all over the world the upper cable car station burst in flames as a tongue of lava had invaded its interior. Behind it, the rapidly growing cone of the Piano del Lago sent its lava fountains hundreds of meters into the sky, producing ground-shaking detonations. Never during the entire eruption was the atmosphere as apocalyptic as on that evening. What nobody knew yet was that this culmination was the end of the worst.

Lavas of the first group were erupted from the all vents above 2600 m elevation (that is, F1, F2, F3, F6, and F7). These fissures were apparently closely associated with the central conduit system. In contrast, the vents at 2570 (Piano del Lago) and 2100 m elevation (F5 and F4, respectively) produced the amphibole-bearing alkalic basalts, which furthermore contained abundant xenoliths derived from the sedimentary basement of Etna (Numidian Flysch). Most of these xenoliths were several centimeters in diameter, but some were 10-15 cm across.

Several larger and smaller pyroclastic edifices grew during the eruption. The largest, the F5 cone at about 2570 m elevation, is a nearly symmetrical cone, about 80 m high, which grew mostly between 25 and 31 July. It is crowned by a large crater about 150 m across and 50-60 m deep with near vertical walls. A spectacular dike was exposed at the base of the E crater wall, another, less conspicuous dike is visible at the base of the WNW crater wall. Furthermore there were several faults or groups of faults visible in the E and NE crater walls; a narrow graben developed on top of the conspicuous dike in the E crater wall. This cone was formally named "Monte Josemaría Escrivá" (after the founder of Opus Dei, a somewhat controversial choice of name), but it is commonly called "Laghetto" cone (Calvari & Pinkerton, 2004), referring to the former site of the Piano del Lago and the small meltwater lake that used to form there during the spring snowmelt.

Smaller cones formed at the F4 (2100 m) vents and in the lower portion of the F3 fissure, at around 2720 m elevation, whereas a cluster of steep-sided spatter cones (also known as hornitos) built around the vents at F2 (2950 m) and in the central portion of the fissure in the Valle del Leone (F6).

The total volume of lava emitted during the 2001 eruption is given by Behncke & Neri (2003) as 25 million cubic meters (dense-rock equivalent), whereas Coltelli et al. (2007) give a bulk volume of 40 million cubic meters, which corresponds to approximately 32 million cubic meters of dense-rock equivalent. The bulk volume of tephra emitted during the eruption was estimated at 5-10 million cubic meters, of which about 4.6 million cubic meters make up the "Laghetto" cone alone (Fornaciai et al., 2010).

Significance and legacy of the 2001 eruption

The July-August 2001 eruption of Etna marked a distinct change in the dynamics of the volcano. Firstly, it thoroughly disrupted the central conduit system of the volcano: the Southeast Crater would erupt again only five years later, and the Bocca Nuova would remain largely silent even for 10 years (except for a brief episode of weak Strombolian activity in June 2002). Ten years after the 2001 eruption, the Voragine remains silent. Only the Northeast Crater showed little change in its activity; as a matter of fact, it went through a period of vigorous Strombolian activity in the summer of 2002, before its conduit was ruptured and drained early during the 2002-2003 eruption, but it seems to have recovered soon thereafter, and has been the site of deep-seated explosive activity and intense degassing ever since.

The 2001 eruption was the first example of so-called "eccentric" activity at Etna that was documented with modern methods of volcano surveillance and scientific analysis. It had been suggested nearly 40 years earlier, by Rittmann (1964), that there were eccentric eruptions at this volcano, which were fed by conduits well distinct from the central conduits and which were more explosive because such eccentric conduits were closed systems until eruption. However, during the 20th century, there had been only one single eccentric eruption, in early 1974, when Etnean volcanology was still in its infancy, and by the turn of the millennium, no volcanologist thought of distinguishing eccentric from the more common "lateral" eruptions of Etna.

This changed with the 2001 eruption, which demonstrated how the forceful uprise of a magmatic dike would not only result in an eccentric eruption but also lead to extensive fracturing all across Etna's summit area, leading to the draining of magma from the central conduits in the style of a typical lateral Etnean eruption. In this sense, the 2001 eruption can be called "two-eruptions-in-one", which furthermore affected two different sectors of the volcano, the southern and northeastern flanks. The dual nature of the eruption was also evident from the two different chemical and mineralogical compositions of the emitted magmas. For the first time, the ascent of an eccentric dike was also documented with modern geophysical methods (Bonforte et al., 2009). It is evident that the forceful eccentric dike intrusion in 2001 caused a remarkable acceleration of the slow seaward displacement of Etna's eastern flank sector, which culminated with the eruption of 2002-2003 and is continuing to the present day. The 2001 eruption thus is a pivotal event in the recent history of the volcano, which strongly altered the dynamics of the volcano and brought Etnean geoscience a quantum leap forward. Only now, ten years on, do we see Etna return to some of its classical, pre-2001 behavioral patterns, such as the onset of persistent, mild intracrater activity within the Bocca Nuova in mid-July 2011.

References

[Note: The scientific literature dealing with Etna's 2001 eruption is abundant and probably contains nearly as many publications as those dedicated to the famous eruption of Mount St Helens (United States) in 1980. The following list, which will continue to grow, for the moment cites only those publications that deal with specific aspects discussed in this article.]

Behncke, B., Neri, M. (2003) The July-August 2001 eruption of Mt. Etna (Sicily). Bulletin of Volcanology, 65: 461-476; doi: 10.1007/s00445-003-0274-1

Bonaccorso, A., Aloisi, M., Mattia, M. (2002) Dike emplacement forerunning the Etna July 2001 eruption modeled through continuous tilt and GPS data. Geophysical Research Letters, 29, 13, doi: 10.1029/2001GL014397

Bonforte, A., Gambino, S., Neri, M. (2009) Intrusion of eccentric dikes: The case of the 2001 eruption and its role in the dynamics of Mt. Etna volcano. Tectonophysics, 471: 78-86, doi: 10.1016/j.tecto.2008.09.028

Calvari, S., Pinkerton, H. (2004) Birth, growth and morphologic evolution of the ‘Laghetto’ cinder cone during the 2001 Etna eruption. Journal of Volcanology and Geothermal Research, 132: 225-349, doi: 10.1016/S0377-0273(03)00347-0

Coltelli, M., Proietti, C., Branca, S., Marsella, M., Andronico, D., Lodato, L. (2007) Analysis of the 2001 lava flow eruption of Mt. Etna from three-dimensional mapping. Journal of Geophysical Research, 112, F02029, doi: 10.1029/2006JF000598

Fornaciai, A., Behncke, B., Favalli, M., Neri, M., Tarquini, S., Boschi, E. (2010) Detecting short-term evolution of Etnean scoria cones: a LIDAR-based approach. Bulletin of Volcanology, 72: 1209-1222, doi: 10.1007/s00445-010-0394-3

Rittmann, A. (1964) Vulkanismus und Tektonik des Ätna. Geologische Rundschau, 53: 788-800, doi: 10.1007/BF02054564