Etna Volcano

Mount Etna is a complex stratovolcano which towers 3340m over the city of Catania on the Mediterranean Island of Sicily. The volcano has been frequently active in recent times and shows a combination of mild explosive activity, sometimes producing sustained ash columns, together with frequent A'a type lava flows which may reach runouts of several km. Spectacular paroxysmic lava fountaining events have been frequently observed at the summit craters in recent years.

Lava flow fed from SE crater emerging at base of Hornito, top of Valle del Bove, November 2006.	Ash eruptions from vent on E flank of SE crater, November 2006	Lava flow transporting rolling rock. Top of Valle del Bove, November 2006.

Main Features of Mount Etna

Mount Etna volcano is a massive volcanic complex with numerous features, testament to its long and varied eruptive history. Some of the main features are shown below.

Summit Craters

The summit complex consists of a main structure with two adjacent craters (Voragine and Bocca Nuova), a NE crater and the large SE crater which was rapidly approaching the height of the summit craters until activity shifted to a vent on its E flank, now known as SEC-E (or SEC-2). The SE crater is located at the top of the Valle del Bove and numerous lava flows from the crater have entered this depression in recent years. All of the summit craters have been active in recent times.

Summit craters viewed from N crater - SE crater (left), Voragine, Bocca Nuova (off to right)	SE Crater of Mt Etna	SE Crater of Mt Etna, July 2011

North crater viewed from road to SE	Ash cloud rising from Bocca Nuova crater, July 2011

Inside wall of North Crater (SW section)	North crater, view E along S rim

Summit plateau with Voragine crater	View towards SE across Voragine crater to adjoining Bocca Nuova which is largely shrouded in gas

Ash eruption in Bocca Nuova in evening light, July 2011. Sharp ridge separating Voragine and BN is visible in front left	Fumaroles on north flank of Bocca Nuova

View of summit region from east. In order from left: SE crater complex, main crater complex (BN, VOR, NE), ancient volcanic structures	View of summit region from SW. Summit craters top left.

Degassing SEC-E , with original SE crater behind to left (July 2011)	SE / SEC-E complex in October 2011 after 14th paroxysm of year. Red circle indicates tourist bus for scale

Valle del Bove Depression

The Valle del Bove depression is the site of numerous coalescing collapse features resulting from activity of the ancient Trifoglietto eruptive centers. In recent years, the large depression on the East flank of Etna has frequently channelled lava flows from eruptions of the SE crater complex. The depression is essentially horseshoe-shaped and about 5km wide and 10 long

Depression with line of craters from Montagnola, 3 craters of 2001/2 eruptions, to SE crater (left to right) on plane above	View across collection of dikes to cliffs overlooking depression and SE crater complex	SE crater complex (top right) perched above the Valle del Bove depression. View from Serra della Concazze.

SW flank of depression with La Montagnola cinder cone (1763) in top right corner, 2002 cone top right	Ancient volcanic dike (sheetlike intrusion) exposed by erosion.	Volcanic dike structures formed by magma rising into elastic cracks and solidifying

Piano Provenzano Area

Further, lava flows from Etna are frequently emitted from fissures in the area surrounding the summit craters of lower on the flanks of the volcano and the landscape has many parasitic cones as a result. Numerous craters were most recently formed in October 2002, when craters were formed south of the summit craters and along a fissure on the north flank, from which lava flowed into the Piano Provenzano ski resort, destroying much of its infrastructure.

Craters of 2002 eruption on N flank viewed from summit	Trees embedded in lava, Piano Provenzano area	Lava flows entering Piano Provenzano area, where skiing facilities and shops were destroyed

Further Features

Numerous small volcanic structures, largely monogenetic cinder-cones formed by short-lived eruptive events, are found in the summit region and on the flanks of Etna.

Craters south of summit formed during 2002 eruption	Cinder cones incl. Monti Calcarazzi (1766) and Monti Silvestri (1892) near Rifugio Sapienza tourist center	Monte Frumento delle Concazze cinder cone, NE flank of Etna

History of Eruptive Activity at Mount Etna

Over 500000 years ago, submarine basalt lavas layed the foundation of present day Mount Etna volcano. Thereafter, modest effusive structures were emplaced, which are now buried in the NE flank of the volcano. The summit of the largest of these, Monte Callana (1325m) is still visible near Zafferana. About 80000 years ago, activity became increasingly explosive and the Trifoglietto stratovolcano was gradually built up. Little is visible of this structure today, although its remains make up much of the volume of the eastern flank of Etna. The Valle del Bove depression is the last visible remnant of the activity of Trifoglietto and signifies the location of its eruptive centers. Trifoglieto consisted of a number of highly eruptive centers which erupted huge amounts of scoriae, lapilli and ash during large eruptions which dwarfed the size of present-day eruptions at the volcano. The rapid emptying of the magma chambers resulted in numerous caldera-forming collapse events. The Valle del Bove is the result of a number off coalescing collapses.

It seems that between 65000 and 35000 years ago, activity was reduced at Etna. The center of present Mongibello volcano (today generally referred to as Etna) has since grown up on the SW flank of Trifoglietto, eventually covering most of the former structure. Two eruptive phases of Mongibello are distinguished. The "Antique" phase (35000-8000 years ago) was mostly explosive like Trifoglietto. The "Recent" phase (8000-0 years ago) has seen the establishment of more effusive activity and has led to the build-up of the present day structure including all of its summit craters.

However, Etna is also capable of explosive activity, and along with Tarawera and Masaya volcanoes, is one of the few volcanoes for which basaltic plinian activity has been documented. Deposits of the 122 AD eruption reveal that it involved 2 distinct Plinian phases (Houghton et al., 2004. J. Volc. Geotherm. Res. 137, p.1-14), the first of which being preceeded by strombolian and phreatomagmatic activity. It is postulated that the first phase stopped after the conduit narrowed by accumulation of cooler, partially degassed magma on its walls. A phase of phreatomagmatic activity followed, which is suggested to have removed the degassed material allowing resumption of Plinian activity. The eruption was concluded by a weak phreatomagmatic phase. The material in the Plinian deposits is exceptionally rich in microlite crystals. This is possibly the result of cooling of the magma in the conduit. High microlite content increases magma viscosity and limits degassing until the magma reaches the top of the conduit. This can account for the unusually explosive fragmentation of the magma resulting in the Plinian eruption (Houghton and Gonnermann, 2008. Chemie der Erde 68, p.117-140). The 122 AD event emplaced at least 10cm of material over an area of 530 square kilometers and involved eruption of about 0.4 cubic km of dense rock equivalent (DRE).

Historical records at Etna date back many centuries. Most historical eruptions were essentially effusive. Their impact on the local population depended primarily on the size and path of the emitted lava flows. In the past 8000 years, lava flows have covered the whole area in a radius of 15km from the summit. In many directions, these lavas coat the landscape to a distance of 20km and more. Much of these lavas erupted from fissures in the flanks rather than the summit area. The most notable historic eruptions are those that affected inhabited areas, in particular the city of Catania which is located on the coast 25km SE of the summit craters. Lava from an eruptive fissure 6km above the city inundated the northern parts of it in 1381 and reached the sea. In 1669, a vent opened at 850m altitude and erupted 1 cubic km of lava in 122 days. The flows reached a length of 15km, covered an area of 37.5 square km and entered the center of Catania. Eruptions from fissures low on the flanks often produce large amounts of lava since they can be fed from the conduit even when the level of magma therein has dropped significantly.

Since then, the largest eruption volume was in 1991-1993, when 0.3 cubic km of lava were emitted over a 473 day period, starting on 14 December, from several vents along a huge fissure that extended from the base of the SE crater at 3000m to locations just over 2000m at the top of Valle del Bove. Up to 300m high lava fountains and strombolean and phreatomamatic explosions were observed in the first days of the eruption, after which activity settled into an effusive pattern at vents near the lowest part of the fissure. Lava flows rapidly entered the Valle del Bove, and by December had covered a distance of 7km to within 2km of the town of Zafferana. In order to save the town, attempts were made to stop the flows by building trenches and earth walls, the main trench was overflown on April 7 1992 and lava proceeded to within 1 km of Zafferana on 14 and 20 April. Explosive were deployed to try and disrupt the lava tunnels which had formed in the Vale del Bove and were feeding the lower parts of the flow. Further attempts were made by throwing concrete blocks into the lava tunnels to divert the lava out of them. These attempts were however only partially successful, slowing the flows for several days. After an initial attempt at rechanneling lava into an artificial channel brought some success, a new attempt was made. A large channel was excavated next to the main tube. After the channel had been completed, only a thin wall separated the tube from the channel. This was blown away on the 27th of May allowing a proportion of the lava to drain into the new channel. In the following days, complete diversion was achieved by blocking the natural tunnel just downstream of the channel using concrete slabs that were slid into it. This diversion was the first effective measure and together with reduced effusion rates by the end of June meant that the town of Zaferrana was saved. The eruption continued to send lava into the Valle del Bove from various vents along the fissure until ceasing on March 30, 1993.

Activity at the summit craters started again in July 1995 and spectacular ash eruptions, strombolian activity, lava flows and even intense lava fountaining episodes could be observed. Indeed in July 1997, all craters had become active in the summit region. On 4 September 1999, an unprecedented 1500m high lava fountain was observed from the Voragine (Main summit crater) and 5 Million cubic meters of material were deposited downwind. 5 cm long scoriae fell on the beach near Riposto and Giarre received a several cm thick layer of deposits. Accumulated hot pyroclasts on the S flank caused a debris avalanche to a length of 500m. Activity at the adjacent Bocca Nuova crater was elevated after 5 October 1999. On the 25th of October, an endogenous dome was formed at the S flank of the Bocca Nuova (the rim of which had already been covered with accumulating pyroclasts and breached by lava flows). Rockfalls forming from this dome had the appearance of classic pyroclastic flows reached up to 600m runout distances over a period of 40 minutes (the period during which dome growth could be observed). It is thought that magma rising up along the old crater wall pushed recently deposited and still hot pyroclastics, together with part of the crater wall upwards and outwards, leading to this short-lived dome-building event which is the first of its kind to be observed at a basaltic volcano (Behncke et al., 2003 (J. Volc. and Geotherm. Res. 124:115-128)). Up to 20 million cubic meters were erupted between 17 October and 3 November at the Bocca Nuova.

Early 2000 provided for repeated spectacular paroxysmal lava fountaining episodes and lava flows at the SE crater. In March, these eventually led to the partial destruction and burial of the Torre del Filosofo mountain hut. Fountain heights of 500-600m, pulsing to over 800m were reported.

The eruptive period in July and August 2001 is interesting since several new vents opened and threatened the touristic facilities at Rifugio Sapienza. The eruption was preceeded by 14 paraxysmic lava fountaining episodes at the SE crater in the period from 17 June to 13 July. On July 17, a strong eruption at the SE crater was followed by the opening of an eruptive fissure at 3000m near its base. Later in the day, further vents opened at an elevation of 2800 and 2700m along a SSW line. Early on 18 July a vent opened at 2100m near the Sapienza Refuge, 200m uphill from Mt Silvestri crater. Minor lava fountaining ocurred and a lava flow crossed the road between the Silvestri carter and the Rifugio, proceeding 6km down the mountain in the following week. On July 19, a new cone (Montagnola II or Piano del Lago) was rapidly built 200m uphill from the 2600m Montagnola crater overlooking the upper cable car station. This cone erupted in a violent phreatomagmatic and later magmatic fashion, producing huge amounts of ash causing disruption to the city of Catania. Flows from vents near the base of this cone threatened the cablecar facilities and Rifugio (which was protected using an earth wall). On 28 July, powerful explosions threw car-sized projectiles to a distance of 500m from the vent. The upper cable car station was seriously damaged by these events, finally being set alight by a lava flow on 30 July. Eruptive activity diminished in August and ceased by the 10th of that month. A total of over 30 million cubic meters of material was erupted.

The next major eruption occurred starting on 27 October 2002. Eruptive fissures opened on both the N and S flanks resulting in lava fountaining and flows, accompanied by intense seismic activity which severely damaged buildings at Piano Provenzano (Note: a large Earthquake on 29 Oct. also damaged hundreds of homes in Sta Verenina). Lava flows from vents along the N fissure (at an altitude of 2500-1800m) rapidly spread over the Piano Provenzano tourist facilities, completing their destruction, and proceeded into the forests below in the direction of Linguaglossa, eventually stopping days later 5km from the vents. The N vents became inactive by November 3rd. The S fissure produced two large cinder cones (eventually reaching heights of about 200m each) at around 2700 and 2750m elevation. Activity at these initially involved fire fountaining with an associated large ash plume, together with lava flows of up to 2km. These flows stopped by 3 November but fountaining continued. On 9-10 December, vents at the base of the 2750m cone emitted lava flows which crossed the road by Rifugio Sapienza on December 17. 32 people were injured by an explosion when a tank exploded inj a building that had been inundated by lava nearby. The Rifugio however survived once again, although the cable car facilities were again destroyed. Further shorter flows and strombolian activity at the new vents continued until 28 January 2003, when the eruption finally stopped. An estimated 70 million cubic meters of material was erupted, the majority of which (unusually) being pyroclasts.

	Ash eruptions from new vent - November 2002

Ash cloud from November 2002 eruption of Etna passing over the city of Catania. The city is frequently affected by ash which can lead to closure of the airport.	Earthquake damage caused by tremors accompanying the Nov.2002 eruption. Etna eruptions are frequently accompanied by minor earthquakes.

Activity including paroxysms and lava flows have also occurred since 2003 but not reaching the same intensity or level of impact. Etna was visited again by the author in Nov. 2006, during an eruptive episode centering around the E flank of the SE crater. Ash emissions from the SE crater were observed, as was an erupting hornito being fed by an underground channel and lava flows emanating therefrom.

Summit region of Etna on evening of 24. Nov. 2006. Lava flows emanating from flank of the SE crater are visible.	Lava flow fed from SE crater emerging at base of Hornito, top of Valle del Bove, November 2006.

Lava flow in Valle del Bove, Nov. 2006	Lava flow in Valle del Bove, Nov. 2006

Flow from base of hornito, Nov. 2006	Vent on SE Crater erupting, Nov. 2006 View from upper cable car station	Flow from base of hornito accompanied by mild strombolian activity, Nov. 2006

2011 has seen the return of one of the most spectacular forms of activity at Etna. Several powerful paroxysms have been observed at varying intervals from the SEC-E vent, with the most powerful to date propelling lava to a height of about 1 km on the 9th of July. During August these events were occuring weekly, yet the events have since become less frequent and predictable. By mid-october, the 15th event of this kind had been witnessed. The 24th paroxysm on April 1st, 2012, pictures of which are shown below.

Paroxysmal Eruption No.24 (2011-12 series), 01.04.2012

The paroxysm occurred in darkness during the early hours of the morning of April 1st, 2012. The eruption was preceeded by the typical precursor Strombolian activity. This began sporadically about 38 hours prior to the eruption and intensified in frequency and power in the hours leading up to the eruption.

SE Craters before onset of activity	Ashcloud from first strombolian eruptions	Powerful Strombolian eruption

	Strombolian activity gradually intensifying

	Paroxysmal Eruption

Peak of eruption	Line of vents appears visible	Steam produced by interaction of lava with snow

Lava flows progressing into Vale del Bove	Lava flows	Lava flows at dawn

After paroxysm lava continues to flow	Morning view	After sunrise, flow-field is only visible due to steam

Situation after eruption	Small volcanic bombs landed about 4km from vent	Example of volcanic bomb

Tourism on Mount Etna

Access to eruptions of Etna largely depends on the decisions of the local authorities restricting access. During the first weeks of the 2002-2003 eruption, roadblocks were set up near Nicolosi and Linguaglossa, severely restricting access. During the minor eruptions in Autumn 2006, access was regulated on an daily, sometimes hourly basis, and it was possible to get close to the lava flows entering the Valle del Bove. Lightning has caused more casualties on Etna than volcanic activity in recent times, and visitors should keep a good eye on the weather since there is little protection to be found in the summit region. The summit region is most easily approached using the cable-car from Rifugio Sapienza which takes one to an elevation of over 2600m, from where buses may take one up to an altitude of nearly 2900m. The Rifugio has a large parking lot for day tourists and comfortable accomodation is available at the Hotel Corsaro nearby.

Cable car station, Rifugio Sapienza	Tourist shops at Rif. Sapienza

Ski lift and cable cars above Rifugio Sapienza	Hikers on road from cable car station to summit region

Tourist bus parked in front of SE crater. Buried Torre del Filosofo alpine refuge in foreground.	Warning sign buried in lapilli

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