Mt. Etna - Recent Activity

During the last two decades, Mt. Etna revealed one of the most active volcanoes on the earth. Recent Activity can be divided in differente major eruption or eruptive phases..

The 2001 eruption

Summit activity was intense in the years preceding the 2001 eruption and was characterized by strong strombolian activity and voluminous lava overflows from the Bocca Nuova (summer – autumn 1999; Harris and Neri 2002; Behncke et al. 2003) and violent paroxysms firstly at the Voragine and later at the SE crater with a series of 66 events (since January to August 2000). In the first months of 2001 activity was exclusively present at the SE crater with slow lava effusion from a vent on its northeastern side and successively, on May 2001, with another series of 16 paroxysmal episodes culminated on 13^th of July (Behncke and Neri 2003).

The eruption occurred between July 13 and August 9, 2001 on the upper slopes of Mount Etna. The first eruptive episode of lava fountaining at the SE crater (July 13) was preceded by a swarm of shallow earthquakes located with their foci distributed between 1.5 km a.s.l and 2.0 km b.s.l., just below the SE crater area (Alparone et al. 2004). The opening of fractures on the southern slope of the volcano, between 2800 and 2000 m a.s.l., was heralded by a long-lasting seismic swarm, starting on July 12, with hypocentres distributed along a NNW-SSE oriented plane, at depths between 1.5 km a.s.l. and 4 km b.s.l. (Privitera et al. 2001; Bonaccorso et al. 2002; Patanè et al. 2002; Billi et al. 2003; Lanzafame et al. 2003).

On July 13 and 14, fractures progressed southwards to the area of Piano del Lago, forming a NNW-SSE-trending anastomized network that reached the western margin of Valle del Bove. On July 17, a NNW-SSE eruptive fissure, which opened at the southern base of the SE crater between 3040 and 2940 m a.s.l., gave rise to strombolian and effusive activity that resulted in the formation of three aligned spatter cones and to a southerly-directed lava flow. Ground fracturing propagated downslope between 2900 and 2800 m a.s.l. and formed a N160°-170°-trending graben. The lowermost end of this structure propagated along a N15°-20° direction, and reached the Piano del Lago crack field at about 2750 m a.s.l. Along this 600 m long fissure, two groups of hornitos developed at about 2780 and 2720 m a.s.l., while a well fed lava flow poured out and expanded south-westerly down to 2000 m a.s.l. This period presented a hypocentre distribution versus time characterized by a decreasing number of deeper events and by earthquakes concentrated only at depths within 2 km from the surface (Privitera et al. 2001; Bonaccorso et al. 2002; 2004; Patanè et al. 2002; Billi et al. 2003; Lanzafame et al. 2003; Monaco et al. 2005).

During the night between July 17 and 18, new N-S oriented ground fractures opened south of the Montagnola pyroclastic cone and west of Mts. Calcarazzi at about 2100 m a.s.l. Lava flows were emitted from the lowest portion of these fractures, and advanced southward. Lava crossed the main road to Nicolosi, and flowed east of the Rifugio Sapienza, reaching its lowest elevation (1035 m a.s.l.) seven days later. Meanwhile ground fracturing extended northwards from the Calcarazzi to the Laghetto area, just north of the Montagnola cone, with an overall length of about 1 km up to 2550 m a.s.l. Here, the fractures met and intersected the NNW-SSE-trending ones that were extending southward and since July 21, a pit crater was characterized by ejection of ash-laden steam and abundant lithic tephra with spectacularly high thrust columns (about 300 m above the edge of the pit crater) and a convective plume, related to phreato-magmatic explosive activity and conduit widening (Taddeucci et al. 2002; 2004; Behncke and Neri 2003). Starting on July 24, ash and steam emissions evolved to markedly magmatic lava fountaining up to 500 m high. During a two weeks period, a 60 m high cinder cone formed through explosive activity (Calvari and Pinkerton 2004; Ferlito and Siewert 2006). Lava emission started from the Laghetto vent on July 26^th, when a breach formed at the southern side of the cone. On July 27^th, two vents, located to NE and SE at the base of the cone, fed lava flows directed into the Valle del Bove, with a rather low output rate (2-6 m³/sec), whereas, in the evening of July 30, the lava output from the southern vent of the cone increased dramatically and gave rise to a large flow, 150 m wide at its front, that reached the Rifugio Sapienza area within only few hours (Ferlito and Siewert 2006). This was the climactic phase of this eruption, after which no more lava was emitted by the Laghetto vent. The explosive activity slowly decreased both at the SE crater and lower vents, with the last lava flowed out from the newly formed vent at 2100 m on August 9. This eruption produced on the whole an estimated volume of 25 ´ 10⁶ m³ of lava and 7 ´ 10⁶ m³of tephra (Behncke and Neri 2003; Lautze et al. 2004).

The 2002-2003 eruption

After the 2001 eruption, it followed a period of about 14 months of rest. Only three months before the onset of the eruption, the activity resumed at the Bocca Nuova and NE crater with sporadic strombolian explosions. On September 22, an earthquake with magnitudo = 3.7 shook the northeastern flank of the volcano causing ground fracturing near the Piano Provenzana area (Clocchiatti et al. 2004; Andronico et al. 2005). In the night of October 26, a seismic swarm preceded and accompanied the development of ground fracturing simultaneously on the upper southern flank close to Torre del Filosofo (2850-2600 m a.s.l.) and along the NE Rift (northeastern flank) with en-echelon fractures developed along 4 km (3010-1890 m a.s.l.) (Patanè et al. 2002; Acocella et al. 2003; Clocchiatti et al. 2004; Andronico et al. 2005). This eruption was then bi-radial, as fractures were directed NE-SW on the northeastern flank and N-S on the southern one.

Gas emission, strombolian explosions and lava outpouring from many vents along the fracture characterized the activity on the northeastern sector of the volcano from October 27 to November 5. In particular, in the first days of the eruption, lava flows fed by vents opened at about 2200 m a.s.l. and successively at 2000-1900 m a.s.l. buried most of the tourist facilities at Piano Provenzana area and proceeded through the secular pine forest above the Linguaglossa village. Violent strombolian activity ceased on October 31, whereas lava flows went on being emitted until November 5. On the whole, the eruption on the northeastern flank produced an estimated volume of ~8 ´ 10⁶ m³ of lavas and ~2 ´ 10⁶ m³ of tephra (INGV Official Reports; Clocchiatti et al. 2004; Andronico et al. 2005; Neri et al. 2005).

On the southern flank eruptive activity started in the early hours of October 27 with lava fountains from a vent at ~2750 m a.s.l., located just 1 km north of the 2001 Laghetto cone (Clocchiatti et al. 2004; Andronico et al. 2005). The eruption was characterized since the beginning by scarce lava flows and strongly strombolian activity with ash columns and lava fountains that, during the sustained phase, resembled a small sub-plinian column (Clocchiatti et al. 2004). This activity continued for about 1 month, leading to the growth of a significant pyroclastic cone. Lava flows emission increased on November 13 and continued almost continuously until the end of the eruption. In this period, flows were directed towards S – SW and expanded in length for about 2-3 km from the vent. On November 24 a major flow reached a length of 4 km missing by few meters the lower cable-car station. Since November 25, the explosive activity at 2750 m shifted northwards to another vent at 2800 m where a second pyroclastic cone formed, which became the main site of explosive activity in the first days of December. The eruptive activity slowly decreased and definitively ceased on January 28, 2003. On the whole, the eruption on the southern flank produced an estimated volume of ~26 ´ 10⁶ m³ of lavas and ~38 ´ 10⁶ m³ of tephra (INGV Official Reports; Clocchiatti et al. 2004; Andronico et al. 2005; Neri et al. 2005).

The 2004-2005 eruption

After the 2002 – 2003 eruption, a 20 months rest followed during which the open conduit system was filled with magma undergoing a continuous degassing as shown by explosions regularly heard from the edge of summit craters. Flank activity abruptly resumed on September 7 2004, when a radial easterly fissure opened at the base of the SE cone (~3000 m a.s.l.) with no significant seismic activity and ground deformation (INGV Official Reports; Burton et al. 2005; Corsaro and Miraglia 2005). Fissures progressed downslope for about 200 m and became eruptive pouring out a slow lava flow for about one day. In the following days, fractures progressed south-eastwards for further 200 m and reached the Valle del Bove when, on September 10, a new vent opened at 2620 m a.s.l. near Serra Giannicola Piccola (Fig. 2.4). The last development of the fractures field occurred on September 13, when another new vent opened at 2320 m a.s.l. near Serra Giannicola Grande. After September 13 only lava flows were erupted for about 6 months until the 8^th of March, without any explosive activity.

The 2006 - 2007 activity

After about 16 months rest, the activity at Mt. Etna started again in the night of July 14, 2006, when an eruptive fissure with two vents opened on the eastern flank of the South-East Crater (SEC) at ~3050 m of altitude. At the beginning, the activity was characterized only by lava effusion from these two vents, with flows emitted at low rates (<5 m³/sec) that expanded eastwards into the Valle del Bove. In the early morning of July 15, a third vent opened, located at the highest tip of the active fissure, and it was characterized by ash emission that evolved in few hours to strombolian activity with ejection of lithics and fragmented magma up to 200 m of elevation. In the following days (16/7 – 19/7), effusions continued regularly, producing a complex lava field which extended for about 2.5 km from the vents into the Valle del Bove, whereas the persistent strombolian activity produced a scoria cone about 30 m high. In the evening of July 19 an abrupt change of eruptive style occurred. A portion of the newly formed scoria cone collapsed and an explosive vent opened, located just below the previously active ones, erupting dense ash clouds and metric lithics accompanied by violent strombolian explosions. Meanwhile, the emission rate increased at the effusive vents (~10 m³/sec) and lava overflowed the previously formed channel walls. Starting from the morning of July 20 both the explosive and effusive activity slightly decreased, but in the evening of the same day a new increase of the eruptive intensity occurred, modifying the style from strombolian to sustained lava fountaining up to 250 m higher than the crater edge. In the following days (21/7 – 23/7), the activity progressively diminished; strombolian explosions reduced their frequency and lava flows were emitted at quite low rates (~3 m³/sec). In the morning of July 24, the eruptive activity, both explosive and effusive, ceased. After about 10 days of eruption, a scoria cone ~50 m high and a complex lava field, extending for about 3 km from the active vents, were formed.

After about 1 month rest, on August 31 the eruptive activity abruptly resumed at the SEC with strombolian explosions every 5-10 seconds and emission of fragmented magma and lithics. The strombolian activity, mostly intra-crateric, continued until the evening of September 4, when a lava overflow occurred from the SEC, originating a small lava flow, that progressively filled the pit-crater formed during the 2005 on its eastern flank, and advancing then eastward. On September 5, the activity did not present any significant variation, with only modest strombolian activity restarted at the SEC. Both the effusive and strombolian activity, variable in their emission rates and intensities, characterized the period until September 15. In these ten days, several rock failures and collapses affected the eastern flank of the SEC cone, which resulted already strongly destabilized by the activity of the late phases of the 2005 events and that of July 2006. Only lava effusion at very low emission rates (<1 m³/sec) took place until September 20, when the explosive activity started again at the SEC. Later, the strombolian activity progressively increased, reaching its maximum peak on September 25, when it was also associated to a new overflow from the eastern edge of the SEC directed into the Valle del Bove. These events ended in the morning of September 27, and for about 1 week the activity was characterized only by sporadic ash emission from the SEC. On October 3 strombolian and effusive activity restarted more vigorously; explosions ejected lapilli and bombs up to ~150 m above the crater edge and a lava flow covered 1 km from the vent in about 24 hours, overlaying flows emitted in the previous weeks. This activity lasted until the night between October 10 and 11, when an increase of the emission rate and frequency of explosions signed the passage from strombolian to lava fountaining episodes. The intensity slightly decreased in the next day, and on October 13 a new vent opened at 2800 m, accompanied by no explosive activity both at this vent and SEC, pouring out a lava flow at ~3 m³/sec towards the Valle del Bove. The strombolian activity at the SEC resumed on October 20 associated to a new lava flow from the summit of the cone, passing by the previous flows. Meanwhile, the effusion started on October 13 from the vent at 2800 m was constantly continuing to discharge lava into the Valle del Bove with the active front placed at ~2000 m. The eruptive theatre changed again on October 23: a new fissure on the southern flank of the SEC cone emitted flows directed to south, towards the Torre del Filosofo area, and to south-east, towards the vent at 2800 m, while effusion from this vent and explosive activity at the SEC continued. The emission from the new fissure ended on October 26. In the same day, a new effusive vent opened at ~3050 m on the southern flank of the Bocca Nuova cone, pouring out a lava flow SSW-directed at a rate of ~10 m³/sec. Until November 8, the activity was characterized by a variable frequency and intensity of strombolian explosions at the SEC and by lava effusion at quite constant rates (~5 m³/sec) from the vent at 2800 m in Valle del Bove and from the fissure at ~3050 m on the south flank of the Bocca Nuova cone. On November 8, the vent at ~3050 m stopped the lava emission and a new effusive vent opened at ~3180 m, just above it, emitting lava for only 1 day. On November 9, the lava emission from the vent at ~3050 m restarted, while strombolian activity at the SEC and effusion at the 2800 m vent continued persistently. From the early morning of November 16, an evident change of the eruptive style took place at the SEC with a strong increase in the intensity of strombolian explosions and the occurrence of repeated phreatomagmatic episodes. Several rock failures and sector collapses also occurred, causing a marked morphologic change of the eastern flank of the SEC cone. However, the most violent paroxysm took place at 3:28 p.m. (Local Time), when a new fissure opened abruptly at the south-eastern base of the SEC cone and a conspicuous sector collapse originated. In a few seconds an eruptive column was formed and it was accompanied by intense steaming caused by the fast vaporization of ground water and buried snow. The column of gas, finely fragmented magma and water steam, too dense and heavy to rise high, suddenly collapsed and gave rise to a small pyroclastic flow directed downward along a channel formed by the preceding cone failure episodes. The fissure, opened at the base of the SEC cone, was characterized by two active vents, which emitted lava for ~24 hours towards the vent at 2800 m. After the peak of the eruptive activity of the November 16 episode, four vents were variably fed: a new vent at the summit of the SEC and those previously active at 3180 m, 3050 m and 2800 m. Along with the effusive activity, the SEC presented phases of intense fumarolic activity alternating to violent ejections of ashes, producing pulsing eruptive columns 4-5 km high, such as that of November 24. On November 27, the lava effusion at the 3050 m and 3180 m vents stopped. From that moment on, only lava effusion from the 2800 m vent and from the summit of the SEC occurred, here associated to low-intensity episodes of strombolian activity. After about 2 weeks of emission at variable rates (generally <5 m³/sec), the eruptive events of the 2006 definitively ended on December 15. After about 3 months rest, characterized by fumarolic activity at the SEC, the strombolian activity restarted on March 29, 2007, at the same vent with an intermittent character; similar episodes occurred in the following months on April 11, April 29, May 7 and September 4-5. All these episodes lasted only few hours but they were characterized by a vigorous explosive activity associated to strongly fed lava flows directed into the Valle del Bove.