mount etna 2001 eruption case study

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Volcano case study - Mount Etna (2002-2003), Italy

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Can you describe the location of Mount Etna? Could you draw a sketch map to locate Mount Etna?

Case study task

Use the resources and links that can be found on this page to produce a detailed case study of the 2002-2003 eruption of Mount Etna. You should use the 'Five W's" subheadings to give your case study structure.

What happened?

The Guardian - Sicilian city blanketed in ash [28 October 2002]

When did it happen?

Immediately before midnight on 26 October 2002 (local time=GMT+1), a new flank eruption began on Mount Etna. The eruption ended after three months and two days, on 28 January 2003.

Where did it happen?

The eruption occurred from fissures on two sides of the volcano: at about 2750 m on the southern flank and at elevations between 2500 and 1850 m on the northeastern flank.

Why did it happen?

Mount Etna is a volcano. The reasons why Mount Etna is located where it is are complex. Here are some of the theories:

• One theory envisages a hot spot or mantle-plume origin for this volcano, like those that produce the volcanoes in Hawaii.
• Another theory involves the subduction of the African plate under the Eurasian plate.
• Another group of scientists believes that rifting along the eastern coast of Sicily allows the uprise of magma.

Who was affected by it happening?

• The Italian Government declared a state of emergency in parts of Sicily, after a series of earthquakes accompanying the eruption of forced about 1,000 people flee their homes.
• A ship equipped with a medical clinic aboard was positioned off Catania - to the south of the volcano - to be ready in case of emergency.
• Emergency workers dug channels in the earth in an attempt to divert the northern flow away from the town of Linguaglossa.
• Schools in the town have been shut down, although the church has remained open for people to pray.
• Villagers also continued their tradition of parading their patron saint through the streets to the railway station, to try to ward off the lava flow.
• Civil protection officials in Catania, Sicily's second-biggest city, which sits in the shadow of Etna, surveyed the mountain by helicopter and were ready to send water-carrying planes into the skies to fight the fires.
• The tourist complex and skiing areas of Piano Provenzana were nearly completely devastated by the lava flows that issued from the NE Rift vents on the first day of the eruption.
• Heavy tephra falls caused by the activity on the southern flank occurred mostly in areas to the south of the volcano and nearly paralyzed public life in Catania and nearby towns.
• For more than two weeks the International Airport of Catania, Fontanarossa, had to be closed due to ash on the runways.
• Strong seismicity and ground deformation accompanied the eruption; a particularly strong shock (magnitude 4.4) on 29 October destroyed and damaged numerous buildings on the lower southeastern flank, in the area of Santa Venerina.
• Lava flows from the southern flank vents seriously threatened the tourist facilities around the Rifugio Sapienza between 23 and 25 November, and a few days later destroyed a section of forest on the southwestern flank.
• The eruption brought a heightened awareness of volcanic and seismic hazards to the Sicilian public, especially because it occurred only one year and three months after the previous eruption that was strongly featured in the information media.

Look at this video clip from an eruption on Mount Etna in November 2007.  What sort of eruption is it?

There is no commentary on the video - could you add your own explaining what is happening and why?

You should be able to use the knowledge and understanding you have gained about 2002-2003 eruption of Mount Etna to answer the following exam-style question:

In many parts of the world, the natural environment presents hazards to people. Choose an example of one of the following: a volcanic eruption, an earthquake, or a drought. For a named area, describe the causes of the example which you have chosen and its impacts on the people living there. [7 marks]

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The 2001 summit and flank eruption

Etna greeted the new millennium with one of its most unusual and complex eruptions recorded during the past 300 years, in July-August 2001. If ever there has been an Etnean eruption in the international mass media, then it was this one: during the dramatic days of late July, dozens of television stations transmitted the spectacular images of the erupting volcano in real time. Very often, unfortunately, the reports were unnecessarily dramaticized, so that the international public received the impression that the town of Nicolosi was doomed, its inhabitants in the grip of fear if not panic, packing their things and some of them already evacuating. Footage of processions held in Nicolosi were a common feature in television reporting, although, seen on location, they rather had quite a different character, and most of the participants were there because they thought "better safe than sorry". As one result of this enhanced media coverage of the dramatic aspects of the events, many people who planned holiday trips to Sicily were worried or cancelled their reservations, others were in fear for their relatives or friends living near Etna or making holidays in Sicily. Actually, Nicolosi (or any other town) was never at serious risk during the eruption. This was mostly because the main lava flow, emitted from a fissure at about 2100 m elevation, and about 11 km north of the town, was not as vigorously fed as to be capable of covering the whole distance. For this the effusion rate would have had to increase significantly, or the flowing lava would have had to form a stable lava tube, but the eruption ended before any major lava tube system could develop. The population of Nicolosi and surrounding towns and villages can at best be said to have lived through a period of apprehension, but most of the people took the chance to be on a gigantic "Etna Party", with thousands of people walking several kilometers to get as close as possible to the flowing lava, and, if they made it, to the nearest erupting vents. Since car traffic was interrupted by numerous roadblocks around Nicolosi and its neighboring towns, this was a long way to walk, but amazingly enough many did make it through to as far as the area of the famous mountain hut Rifugio Sapienza, which was only a few hundred meters away from the eruptive fissure at 2100 m elevation. The procession of sightseers was continuous, 24 hours a day. During many interviews with the local people, no one ever expressed any true fear to me, although everyone was conscious that if this eruption went well for them, future eruptions could well bring a much more serious threat to their homes. The photos presented on this page are mostly from newspapers. A gallery with my own photos of this eruption is under construction.

Map of the lava flows of July-August 2001

Caracausi A, Italiano F, Paonita A, Rizzo A and Nuccio PM (2003) Evidence of deep magma degassing and ascent by geochemistry of peripheral gas emissions at Mount Etna (Italy): Assessment of the magmatic reservoir pressure. Journal of Geophysical Research 198, DOI: 10.1029/2002JB002095 Lanzafame G, Neri M, Acocella V, Billi A, Funiciello R and Giordano G (2003) Structural features of the July-August 2001 Mount Etna eruption: evidence for a complex magmatic system. Journal of the Geological Society of London 160: 531-544, DOI: 10.1144/0016-764902-151

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Report on Etna (Italy) — November 2002

Bulletin of the Global Volcanism Network, vol. 27, no. 11 (November 2002) Managing Editor: Richard Wunderman. Etna (Italy) Witnesses saw N- and S-flank eruptions begin at around 0200 on 27 October

Please cite this report as: Global Volcanism Program, 2002. Report on Etna (Italy) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network , 27:11. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200211-211060

37.748°N, 14.999°E; summit elev. 3357 m

All times are local (unless otherwise noted).

After the violent flank eruption of July-August 2001, Mount Etna was rather calm for more than 10 months, except for usual fumes from the four summit craters [and minor ash emissions]. In the first days of July 2002 weak magmatic activity resumed sporadically at the NE Crater with ejection of bombs that fell on the outer slopes of the cone. On 12 September explosions occurred every 2 or 3 minutes and were violent enough to throw large spatter as far as the northern rim of the Voragine (Central Crater). However, there were many days without explosive activity and, at other times, the NE Crater emitted large clouds of brownish ash. Although a magnitude 3.7 earthquake had struck the northern flank of the volcano on 22 September, subsequent days were so calm that, to these contributors, the following events came as quite a surprise.

As the National Institute of Geophysics and Volcanology (INGV) previously reported ( BGVN 27:10), a seismic swarm began to shake Etna late during the evening of 26 October 2002. One observer, Maurice Aubert, happened to be in a hotel on the northern flank (at Piano Provenzana, 1,816 m elevation). There the seismic shocks were distinctly felt after midnight and rapidly reached hazardous levels. Hours later, at 0205 on 27 October, lava fountains began to play along a fissure 1-2 km up slope, but decreased at 0220 when lava flows expanded downwards.

The seismic intensity of earthquakes felt the night of the 26th ranged from II to VII or perhaps VIII. The approximate timing and seismic intensity was recorded as follows at 0030, II; at 0140, VI; at 0200, VI; at 0320, VII; and at 0343, VII or VIII. Maurice Aubert and his group hastily retreated shortly after 0320, exiting while cracks were developing through the mountain road. The last of the above-reported intensities was felt during their departure, when a strong earthquake shook their car.

Vents at ~2,700 m elevation on the southern flank (on the Piano del Lago) are here called the S2700 vents. These new S-flank vents lay just SE of the ancient cone of Monte Frumento Supino and ~800 m NW of the Laghetto cone, which appeared in 2001.

Watching the S2700 vents, Giuseppe Scarpinati saw two lava fountains develop after 0200, together with a large ash plume that drifted S. The eruptive phenomena were accompanied by strong detonations and rumblings together with continuous earthquakes that were felt in Acireale, a town at Etna's southeastern foot.

Lava flows from the northern vents invaded and over ran the flat area containing tourist facilities at Piano Provenzana and proceeded as two branches downwards through the pine trees towards Linguaglossa, a village ~10 km to the NE. The greatest damage was not the loss of all tourist facilities at Piano Provenzana, but was instead due to heavy ashfall S of the volcano, which led to closing of the Catania airport on the afternoon of 27 October.

On the morning of 28 October the S fissure had developed at least three explosive vents. A 100-to-200-m-high lava fountain, ~200 m downslope, fed lava flows that extended by more than 2 km toward the uninhabited area of Monte Nero degli Zappini (figures 97 and 98). During the day, however, the effusive activity significantly decreased, and on 29 October the lava fronts virtually stopped on the southern side, although violent degassing at the upper end of the fissure continued unabated. Sustained release of high pressure gas fed a voluminous SE-directed ash plume that reached to more than 5 km altitude. At the same time on the 29th, a large plume of white vapor was emitted at the summit from the central crater vents (Bocca Nuova, Voragine) and the NE Crater. The SE crater, the main site of the 2001 eruption, remained entirely calm.

Strong earthquakes on 29 October caused damage on the lower E flank of the mountain, particularly at Santa Venerina where some 1,000 people were left homeless. The main shock was recorded by Jean-Claude Tanguy in the SE region of the volcano (Trecastagni) at 17 seconds after 1102 (± 5 sec). Horizontal ground motions there lasted 7 to 8 seconds. The INGV reported the seismic event as M 4.4, located 8-9 km beneath Santa Venerina. Other strong shocks at 1739 and 1814 (M 4.0 and 4.1) caused walls to collapse along the road between Zafferana and Milo.

On 30 October soon after midday the Bocca Nuova vent began to emit large clouds of brownish ash. This activity culminated between 1310 and 1320, and the ash cloud merged into the still large, dark ash plume from the southern lateral vents. However, Strombolian explosive activity was still vigorous at the main explosive center, which included a group of about six vents near 2,000 m elevation (called the N2000 vents). These vents, which produced photogenic activity into the night (figure 99), lie just to the E of an old cinder cone known as Monte Ponte di Ferro (at 2,040 m elevation). Here the accumulation of pyroclasts had built a spatter rampart ~200 m long and 30 to 40 m high, the upper part of which reached 2,035 m elevation (± 5 m, measured from Mt. Ponte di Ferro using both altimeter and inclinometer).

On 31 October the wind gradually shifted from the N to the W and then SW, so that ashfall from S2700 vents affected localities NE of the volcano including Reggio di Calabria, whose airport also had to be closed. At the northern vents the lava effusion was on a waning stage, but violent explosions from the two upper vents of the N2000 group threw blocks of ancient material amid juvenile tephra (figure 100).

On 1 November all activity ceased on the northern side except for very small residual lava flows, but the S2700 upper vent appeared to enter a phase of sustained explosive activity resembling a small subplinian column that continued to cause disruptions around the volcano. It was not until 12 November at 1340 that the activity abruptly changed to typical Strombolian explosions of liquid lava clots with loud detonations. On 13 November at about 1600 a small lava flow began to trickle from the lower base of the S2700 cone. The lava effusion increased on 14 November, expanding downwards along the 27-28 October flows. Meanwhile ash emission recommenced at the S2700 crater.

This kind of eruption style is quite unusual at Mount Etna. The authors suggest that it could indicate that a considerable amount of magma has intruded into the S rift zone, which would account for strong degassing without any significant lava effusion between 2 and 13 November.

Geological Summary. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of 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 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: Jean-Claude Tanguy , University of Paris 6 & Institut de Physique du Globe, 94107 St. Maur des Fossés, France; Maurice Aubert , University of Clermont-Ferrand, Department of Geology, 63038 Clermont-Ferrand, France; Roberto Clocchiatti , CNRS-CEN Saclay, Lab. Pierre Süe, 91191 Gif sur Yvette, France; Santo La Delfa and Giuseppe Patané , University of Catania, Department of Geological Sciences, Corso Italia 55, 95129 Catania, Italy; Giuseppe Scarpinati ,via Muggia 7, 95024 Acireale, Italy.

The July–August 2001 eruption of Mt. Etna (Sicily)

• Research Article
• Published: 27 March 2003
• Volume 65 , pages 461–476, ( 2003 )

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• Boris Behncke 1 &
• Marco Neri 2  

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The July–August 2001 eruption of Mt. Etna stimulated widespread public and media interest, caused significant damage to tourist facilities, and for several days threatened the town of Nicolosi on the S flank of the volcano. Seven eruptive fissures were active, five on the S flank between 3,050 and 2,100 m altitude, and two on the NE flank between 3,080 and 2,600 m elevation. All produced lava flows over various periods during the eruption, the most voluminous of which reached a length of 6.9 km. Mineralogically, the 2001 lavas fall into two distinct groups, indicating that magma was supplied through two different and largely independent pathways, one extending laterally from the central conduit system through radial fissures, the other being a vertically ascending eccentric dike. Furthermore, one of the eccentric vents, at 2,570 m elevation, was the site of vigorous phreatomagmatic activity as the dike cut through a shallow aquifer, during both the initial and closing stages of the eruption. For 6 days the magma column feeding this vent was more or less effectively sealed from the aquifer, permitting powerful explosive and effusive magmatic activity. While the eruption was characterized by a highly dynamic evolution, complex interactions between some of the eruptive fissures, and changing eruptive styles, its total volume (~25×10 6  m 3 of lava and 5–10×10 6  m 3 of pyroclastics) was relatively small in comparison with other recent eruptions of Etna. Effusion rates were calculated on a daily basis and reached peaks of 14–16 m 3  s -1 , while the average effusion rate at all fissures was about 11 m 3  s -1 , which is not exceptionally high. The eruption showed a number of peculiar features, but none of these (except the contemporaneous lateral and eccentric activity) represented a significant deviation from Etna's eruptive behavior in the long term. However, the 2001 eruption could be but the first in a series of flank eruptions, some of which might be more voluminous and hazardous. Placed in a long-term context, the eruption confirms a distinct trend, initiated during the past 50 years, toward higher production rates and more frequent eruptions, which might bring Etna back to similar levels of activity as during the early to mid seventeenth century.

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Acknowledgments

The work has benefited from comments and discussions involving colleagues from the Istituto Nazionale di Geofisica e Vulcanologia and the Dipartimento di Scienze Geologiche at the University of Catania, the Alpine Guides of Etna, Luigi Tortorici, Peter Ippach, Giovanni Tomarchio, and Giuseppe Scarpinati. The authors are grateful to the pilots and technicians of the Civil Defence helicopters who made our observations possible. Andy Woods and David Pyle are acknowledged for their critical and thoughtful reviews which contributed significantly to the quality of the manuscript.

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Boris Behncke

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Correspondence to Marco Neri .

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Behncke, B., Neri, M. The July–August 2001 eruption of Mt. Etna (Sicily). Bull Volcanol 65 , 461–476 (2003). https://doi.org/10.1007/s00445-003-0274-1

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Received : 17 February 2002

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Issue Date : October 2003

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Threat of Etna eruption eases

Ashes from Mount Etna continued to rain down on the Sicilian city of Catania today but the nearby airport was able to reopen after strong winds cleared the runway of ash and dust.

However, ash from Europe's most active volcano still covered Cataniaês streets, where people had been using umbrellas the day before to protect themselves.

The volcano continues to belch columns of gas, vapours and volcanic matter into the air but the lava flow has been steadily slowing. Emergency crews have managed to divert it by building earthworks.

"There are very slight signals that the eruption activity is diminishing. I am cautiously optimistic," said Alberto Di Pace, prefect of Catania, a city at the base of the volcano.

But experts say the volcano remains highly unpredictable, making it extremely difficult to tell whether the eruptions will stop soon.

Police and firefighters have been deployed in towns on the slopes, but officials said they hoped they would not be needed. Since last week, the volcano has been at its most active for a decade.

Closest to the flow is the village of Nicolosi, which is just two-and-a-half miles from the lava front. However, residents have taken some comfort in computer simulations showing that even if the unpredictable volcano became more active, lava flow should miss them.

Scientists from the Catania Institute of Vulcanology and Geophysics said the two main arteries of lava have slowed down considerably since yesterday, when two new fractures appeared on the south side of the volcano.

The new outlets reduced pressure on two rivers of lava advancing toward Nicolosi and Rifugio Sapienza, a tourist station high on the slopes. Ski lifts have already been damaged by the magma.

Etna, which towers 3,310 metres above Sicily, springs to life every few months and its last major eruption was in 1992.

The government yesterday declared a state of emergency for the Etna area as a precaution. The local tourism industry has been knocked and farmers have complained of damaged crops.

In recent days, helicopters have doused the hot lava with water while the mountain has put on a mesmerising display, spewing out pyrotechnic lava, ash, smoke and rocks.

On Sunday, the community organised a religious procession to pray that the flow be diverted or stopped. Legend has it that in 1886 the local archbishop managed to divert a wave of boiling magma descending on the village by brandishing before it a statue of Saint Anthony.

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Original research article, mechanisms of ash generation at basaltic volcanoes: the case of mount etna, italy.

• 1 School of Earth and Environmental Sciences, University of Manchester, Manchester, United Kingdom
• 2 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Osservatorio Etneo, Catania, Italy
• 3 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Pisa, Italy
• 4 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma1, Rome, Italy

Basaltic volcanism is the most widespread volcanic activity on Earth and planetary bodies. On Earth, eruptions can impact global and regional climate, and threaten populations living in their shadow, through a combination of ash, gas and lava. Ash emissions are a very typical manifestation of basaltic activity; however, despite their frequency of occurrence, a systematic investigation of basaltic ash sources is currently incomplete. Here, we revise four cases of ash emissions at Mount Etna linked with the most common style of eruptive activity at this volcano: lava fountains (4–5 September 2007), continuous Strombolian activity transitioning to pulsing lava fountaining (24 November 2006), isolated Strombolian explosions (8 April 2010), and continuous to pulsing ash explosions (last phase of 2001 eruption). By combining observations on the eruptive style, deposit features and ash characteristics, we propose three mechanisms of ash generation based on variations in the magma mass flow rate. We then present an analysis of magma residence time within the conduit for both cylindrical and dike geometry, and find that the proportion of tachylite magma residing in the conduit is very small compared to sideromelane, in agreement with observations of ash componentry for lava fountain episodes at Mount Etna. The results of this study are relevant to classify ash emission sources and improve hazard mitigation strategies at basaltic volcanoes where the explosive activity is similar to Mount Etna.

Introduction

Explosive volcanism is characterised by magma fragmenting into particles of different size varying from micrometre to metre. In the presence of an eruption column, ash particles are volcanic fragments up to 2 mm in size that are dispersed to large distances from the eruptive centre in comparison to coarser fragments (e.g., bombs and lapilli) that fall in more proximal areas. Abundant ash has characterised most of the explosive activity at Mount Etna, Italy, since 1995 ( Figure 1 ) (e.g., La Delfa et al., 2001 ; Alparone et al., 2003 ; Andronico et al., 2009a , 2015 and references therein), often deeply affecting people’s everyday life and the overall economy of Eastern Sicily (e.g., Barsotti et al., 2010 ; Andronico et al., 2014a ; Andronico and Del Carlo, 2016 ; Horwell et al., 2017 ). At Etna ash emissions accompany different eruptive styles, from mild to moderate Strombolian explosions to high energy lava fountain activity (e.g., Andronico et al., 2008a , 2015 ), from short-lasting ash explosions ( Andronico et al., 2013 ) to long-lasting explosive eruptions like those occurred in 2001 (20 days; e.g., Taddeucci et al., 2002 ; Scollo et al., 2007 ) and in 2002–2003 (∼2 months; e.g., Andronico et al., 2005 ). All the 1995–2019 explosive activity producing significant ash emissions in the atmosphere and involving summit craters or flank areas has been summarised in Table 1 .

Figure 1. Satellite images of ash dispersal during the 2001 and 2002–2003 Mount Etna eruptions. (a) July 22, 2001; Image courtesy Jacques Descloitres, MODIS Land Rapid Response Team at https://visibleearth.nasa.gov/ view.php?id=56431 . (b) October 27, 2002; image courtesy Jacques Descloitres, MODIS Rapid Response Team at NASA GSFC, at https://visibleearth.nasa.gov/view.php?id=10376 . (c) November 12, 2002; image courtesy Jeff Schmaltz, MODIS Rapid Response Team, NASA GSFC at https://visibleearth.nasa.gov/view.php?id=10398 . Mount Etna coordinates 37°45′18′N, 14°59′42′E.

Table 1. Summary of the explosive activity (summit eruptions, i.e., lava fountains, strong Strombolian episodes, Strombolian explosions and sustained ash emissions, and flank eruptions) producing significant ash emission up to tens–hundreds of kilometres of distance away from Etna from 1995 to 2017.

Based on visual observations of the eruptive activity and textural and compositional features of ash samples, it was found that the characteristics of ash particles at Etna usually vary with the eruptive style. For example, ash emitted during Strombolian explosions and at the peak of lava fountain activity is more vesicular, less crystallised and with a less compositionally evolved groundmass than that erupted during less explosive events or at the end of a long-lasting explosive eruption, and it contains less lithic material ( Taddeucci et al., 2004 ; Andronico et al., 2008b ). Based on ground and satellite data, Andronico et al. (2009b) have attempted a first classification of ash-enriched Etnean volcanic plumes, subdividing ash emissions in the autumn of 2006 into five categories and demonstrating the utility of such classifications for volcanic hazard mitigation planners and civil protection purposes. It is worth specifying, however, that this classification is valid mostly for that period of activity in 2006 and it does not cover the whole range of explosive activity displayed by Etna.

Previous research on ash characteristics and the link between ash and eruption behaviour has improved our general knowledge on ash emissions at Etna ( Taddeucci et al., 2002 ; Andronico et al., 2013 ). A better understanding of ash formation has the potential of further improving hazard assessment and forecasting at this volcano. However, while the sources and features of ash particles at Etna have been investigated in several cases, a systematic comparison is still lacking. The present paper represents a concrete step forward in the attempt of classifying the most common mechanisms through which ash originates at Etna. In the following, first we briefly revise the characteristics of Etna ash, then we present four different case studies of ash emissions, each associated with a different eruptive style and marked by a different duration and intensity (i.e., mass eruption rate; MER) of the tephra emission. Ultimately, we link each ash emission case study to a different, peculiar mechanism of ash formation. Although based on case studies from Etna, the proposed mechanisms can explain ash generated by other basaltic volcanic systems [e.g., Paricutin, Mexico ( Pioli et al., 2008 ), and Villarrica, Chile ( Romero et al., 2018 )] which, during specific eruptive phases, resemble closely the Etna explosive activity in terms of duration, intensity and style of emission.

Characteristics of Ash at Etna

Textural and compositional features of ash particles erupted from explosive activity at Etna have been described in previous studies, and for details on the topic the reader is referred to those works (e.g., Taddeucci et al., 2002 , 2004 ; Andronico et al., 2009a , 2013 , 2014b ). Here we summarise the common aspects that can help extending from specific to general cases. Juvenile ash particles at Etna consist of two end-members: sideromelane and tachylite ( Figure 2 ). The former has fluidal to irregular morphology, is yellow to brown in colour, transparent, vesicular and generally glassy in the groundmass. The latter is blocky, grey to black, generally opaque (sometimes it can be shiny), poorly vesicular and crystallised in the groundmass. There is however, a continuous, progressive transition of textural features between the two ash types, mainly generated by the different extent of groundmass crystallisation, far more pronounced in tachylite, and by the higher content of (sub) spherical vesicles, as opposed to vesicles with complex and/or irregular shapes, in sideromelane ( Figure 2 ). As a general rule, glass composition of Etna ash overlaps with the compositional field of pyroclastic material erupted from this volcano since 1995 ( Corsaro et al., 2017 ; Pompilio et al., 2017 ). Within the same eruption, tachylite glass tends to be more compositionally differentiated in comparison to its sideromelane counterpart, with a higher silica, alkali and phosphorous content and lower magnesium and calcium ( Taddeucci et al., 2002 , 2004 ; Polacci et al., 2006 ). Compositional variations are mainly related to fractionation of a few tens of percent of crystal phases forming the groundmass. Lithic ash from Etna eruptions is not always easily distinguishable from tachylite juveniles, but is often marked by altered/oxidised surfaces and secondary minerals overgrowths.

Figure 2. Secondary (top) and backscattered (bottom) scanning electron microscope images of typical sideromelane (a,c) and tachylite (b,d) ash particles at Mount Etna. Scale bar 20 μm in all images.

Visual observations of the eruptive activity integrated with investigations of ash componentry have revealed that the proportion of sideromelane ash increases with increasing eruption intensity ( Taddeucci et al., 2004 , and this work, section “Representative Episodes of Ash Emission at Etna”). Therefore, Strombolian and lava fountain activity generally produce more sideromelane and fewer tachylite than low energy, impulsive ash explosions ( Andronico et al., 2008b ). This correlation, together with textural and compositional observations, suggested a longer conduit residence time for tachylite with respect to sideromelane ash. Hence, several authors hypothesised that tachylite ash particles at Etna are generated by the fragmentation of a cooler, viscous, crystallised and degassed magma at the conduit walls, while sideromelane represents the hotter, less viscous and vesiculating magma rising in the central portion of the conduit ( Taddeucci et al., 2004 ; Polacci et al., 2006 ; Pompilio et al., 2017 ).

Ash dispersal at Etna is mostly controlled by eruption intensity and the ensuing plume height, as expected. For example, on 23 February and 23 November 2013, two lava fountain episodes characterised by relatively high eruption columns (at least for the most common explosive activity at Etna) of up to 9–10 km a.s.l. and high MERs, generated dispersal of ash particles up to 400 km from Etna, in Puglia (Italy) ( Poret et al., 2018a , b ). Lower intensity impulsive explosions and discontinuous explosive activity generally produce lower and intermittent eruption columns above the vent, producing minor ash dispersal. However, if the intensity is low but the duration of the activity is prolonged for days to weeks, the continuous injection of relatively fine-grained ash in the atmosphere is able to form a sustained tephra column feeding an eruption cloud spreading hundreds of km away from the volcano. This occurred, for example, during the 2001 and 2002–2003 eruptions (e.g., Andronico et al., 2005 ; Villani et al., 2006 ; Scollo et al., 2007 ) ( Figure 1 ).

Representative Episodes of Ash Emission at Etna

In the following we present and revise four case studies, in order of decreasing eruption intensity, during which ash was vented at Etna from either a single, short-lasting (minutes to tens of hours) explosive event (i.e., 24 November 2006, 4–5 September 2007, 8 April 2010) or a longer period of erupted activity that involved several successive explosive events over the course of a week (i.e., the last phase of the 2001 eruption). We have chosen these specific case studies because they cover the most common eruptive styles and intensities that have characterised the explosive activity at Etna in the last decades: sustained lava fountain activity (the 2007 event), continuous Strombolian activity transitioning to quasi-steady or pulsing lava fountain (the 2006 event), isolated Strombolian explosions (the 2010 event) and nearly continuous to pulsing ash explosions (the 2001 eruption). For each case study we provide a brief description of the event (or eruption), as well as a description of the associated deposit and ash particle features. This overall information represents a basic framework for classifying ash emissions at Etna and provides the constraints necessary to model mechanisms of ash formation.

Case Study No. 1: The 4–5 September 2007 Sustained Lava Fountain

This episode of lava fountain from the Southeast Crater (SEC) at Etna is one of the best studied lava fountains occurred in the last 40 years of eruptive history of the volcano for its long duration (about 10 h) and steady, sustained, violent jets of magma and gas resulting in a continuous 2 km-high eruption plume ( Andronico et al., 2008a ) ( Figure 3 ). For the high eruptive intensity and the textural features of the erupted ejecta, this activity compares well with the most energetic episodes of the well-studied paroxysmal, cyclic fountaining activity that characterised SEC in 2000 ( Alparone et al., 2003 ; Polacci et al., 2006 ) and the New Southeast Crater (NSEC) in 2011–2012 and 2013 (e.g., Behncke et al., 2014 ; De Beni et al., 2015 ). The paroxysmal sequences at NSEC, in particular, included also very high energy episodes characterised by high eruption columns (7–8 km above the vent; Scollo et al., 2014 ) and very large spreading of tephra up to 400 km from the vent (e.g., Poret et al., 2018a , b ). This suggests that the magnitude of lava fountains at Etna may range significantly, in terms of eruption column height and MER, from small- to large-scale events ( Andronico et al., 2015 ). This terminology also well describes the four lava fountains episodes which took place in December 2015 at the Voragine crater. These episodes were, in fact, characterised by large-scale eruption columns, reaching the troposphere-stratosphere boundary and forming relatively thick tephra deposits on the volcanic slopes ( Vulpiani et al., 2016 ; Corsaro et al., 2017 ; Pompilio et al., 2017 ; Cannata et al., 2018 ). The tephra deposit ( Figures 3B,C ), resulting from the juvenile material fallout from the Eastward directed plume of the 4–5 September lava fountain, blanketed an area on the mainland extending up to 19 km away from the vent before being subsequently dispersed in the Ionian sea ( Andronico et al., 2008a ). Investigations carried out on the ash collected in medial and distal areas reveal that the whole sample is completely made up of sideromelane particles containing large sub-spherical vesicles, with a minor proportion of sideromelane particles characterised by a high number of smaller vesicles ( Andronico et al., 2008a ).

Figure 3. 4–5 September 2007 lava fountain at Mount Etna: (a) magma jets from the South-East Crater (photo at https://forum.meteonetwork.it/meteo-foto/62785-eruzione-etna-04-settembre-2007-a.html ); (b,c) images of the tephra deposit in the village of S. Alfio, ∼13 km from the vent (photo by Daniele Andronico). Reproduced with permission.

Case Study No. 2: The 24 November 2006 Continuous Strombolian Activity

The explosive activity that occurred at SEC on 24 November 2006 represents one of the paroxysmal episodes that took place at Etna during the August–December 2006 eruption. Such explosive activity developed high energy, continuous Strombolian explosions that transitioned to a quasi-sustained or pulsing lava fountain for about 13 h generating an ash plume up to 2 km high on the summit craters and rotating from SE to S ( De Beni et al., 2006 ; Nicotra and Viccaro, 2012 ; Andronico et al., 2014b ) ( Figure 4 ). Based on their steady supply, duration and large tephra fallout, Andronico et al. (2009b) classified plumes like the one generated by the 24 November 2006 explosive activity as middle eruptive intensity plumes , the most dangerous plume type, in terms of volcanic hazard, throughout the 2006 autumn Etna eruption. Long, though small-scale, eruptions lasting several hours like the 24 November 2006 event may cause greater anxiety and worry to the local population and authorities than shorter and higher intensity eruptions (e.g., 12 January 2011; Andronico et al., 2014a ). Indeed, the relatively low eruption intensity, combined with the long-lasting tephra fallout, was enough to produce a continuous deposit up to distances of 15 km from SEC. At greater distances (∼25–30 km), a discontinuous tephra layer formed in the city of Catania and surroundings ( Figure 4 ) where, although at a low sedimentation rate on the ground (5–6 g/m 2 h –1 ; Andronico et al., 2009b , 2014b ), the ash dispersal caused temporary closure of the airport infrastructures and disruption to the air traffic. During this activity, the emitted ash was composed mostly of lithic particles (typically subrounded and with weathered surfaces), while the juvenile fraction (sideromelane and tachylite) represented about a quarter or less of the ash componentry in most of the collected samples at different distances from the vent ( Andronico et al., 2014b ). Tachylite fragments were microcrystalline, blocky and characterised by fresh, angular surfaces, while sideromelane clasts were mainly glassy with irregular to elongate shape. Hybrid partially crystallised particles made of both sideromelane and tachylite were also present ( Andronico et al., 2014b ).

Figure 4. Plume of the 24 November 2006 activity (from Polacci et al., 2009 ) and diagrams illustrating the relative abundance of ash componentry at different distances from the vent. Reproduced with permission.

Case Study No. 3: The 8 April 2010 Single Strombolian Explosion

This case study is well representative of the explosive activity occurred in 2010, when the Etna summit craters produced several single or short sequences of impulsive ash release events, lasting usually less than 1 min. Most of them consisted of small failures involving the rims and/or the inner walls of the summit craters; however, some of them were much more intense and correlated with seismic and acoustic signals ( Andronico et al., 2013 ), suggesting some magmatic dynamics were involved in it. One of the major explosions took place on 8 April 2010 at SEC and produced a dark ash emission rising quickly up to 600–700 m of height above the eruptive vent, which formed a small eruption cloud dispersed to the NE ( Andronico et al., 2013 ) ( Figure 5 ). Light tephra fallout occurred at Linguaglossa, a village located 16 km from the vent, where the ash deposit had unimodal grain-size distribution (peaked at 0.25 mm) and a mass loading of 18 g/m 2 ( Andronico et al., 2013 ).

Figure 5. Image of the peak in activity of the 8 April 2010 explosion and comparison between the ash componentry of the 29 October 2006 explosion and the average value of 4 samples collected at increasing distance from the vent after the 8 April 2010 explosion (data Andronico et al., 2013 ). Photo from Thomas Bretscher at https://www.flickr.com/photos/bretscher/4508419186/in/album-72157623824783982/ . Reproduced with permission.

Another example of this activity is the ash emission that rose a few hundred metres above SEC on 29 October 2006, forming a diluted ash plume which generated modest tephra fallout on the southern slopes of the volcano. A thin, discontinuous ash layer was deposited up to Catania, reaching a maximum loading of 30 g/m 2 at Rifugio Sapienza (∼5 km from the vent) and a minimum of 5 g/m 2 in Catania (∼27 km from the vent) ( Cristaldi and Scollo, 2006 ). Lithic clasts dominated the ash componentry in both cases. On 8 April, furthermore, between 4.7 and 16 km from the vent, the ash consisted mainly of tachylite (12–27%), a variable, decreasing fraction of sideromelane particles (from 18 to 1%), abundant lithics including weathered volcanic fragments of glass and crystals (∼50–80%), and negligible amounts of crystals (2–5%) ( Figure 5 ). On 29 October the distal erupted ash mainly consisted of lithics (56%) and tachylite (34%) particles, with poor or negligible percentages of sideromelane (8%) and crystals (2%), respectively ( Figure 5 ) ( Cristaldi and Scollo, 2006 ).

Case Study No. 4: The Ash Explosions During the Last Phase of the 2001 Eruption

In comparison to case studies 1, 2, and 3, where the eruptive activity involved one of the summit craters, the July–August 2001 Etna eruption was marked by lateral activity and the ash was emitted mostly by two newly-formed vents located at 2550 and 2100 m a.s.l. on the south flank of the volcano. In the last phase of the July–August 2001 Etna eruption (1–7 August 2001), the activity at the 2550 m a.s.l. vent transitioned from spatter-forming Strombolian explosions to a series of sustained to pulsed, ash-rich explosions ( Taddeucci et al., 2002 , 2004 ). Ash erupted at first as an almost continuous plume; then the intensity and frequency of explosions decreased to isolated, cannon-like events producing discrete plumes. Plumes were easily bent by moderate wind and their height was in the 500–1500 m range. Ash from these plumes was deposited very locally, reaching a total thickness of about 20 cm at a distance of about 50 m eastward of the vent ( Taddeucci et al., 2002 ). Ash deposited in this time window presents a gradual but systematic increase (from about 64 to 73 particles %) of blocky, poorly vesicular to non-vesicular, microlite-rich tachylite particles at the expenses of sideromelane, and a small increase (from 1 to 7%) of lithic particles. These changes are accompanied by a concomitant decrease in the overall vesicularity of the ash particles ( Taddeucci et al., 2002 ) ( Figure 6 ).

Figure 6. Images (a,b) of the pulsing ash plume generated during the final phase of activity of the vent at 2550 m a.s.l. in 2001, and diagrams of ash componentry at the beginning and at the end of this final phase of activity. Reproduced with permission.

Mechanisms of Ash Formation at Etna

We present here three mechanisms of ash formation that we link with the four different types of ash emission activity described in the previous section. We anticipate however, that such mechanisms may not be exhaustive to describe and explain mechanisms of ash formation at Etna or other basaltic volcanoes characterised by ash emission activity, and that other mechanisms may therefore be feasible. We propose that variations in the magma mass flow rate are a common, key-parameter in determining variations in the style of activity of erupted ash, translating into different mechanisms of ash generation.

Mechanism No. 1 (The Sustained 4–5 September 2007 Lava Fountain and the Continuous Strombolian Activity of 24 November 2006)

The first mechanism concerns ash accompanying lava fountain activity and generated by fragmentation of fast-rising (order of 10 to 40 m/s, La Spina et al., 2016 ; Giuffrida et al., 2018 ), vesiculating magma. Previous studies have well documented that lava fountaining from SEC at Etna is driven by the superposition of two distinct outgassing mechanisms, gas foam collapse and syn-eruptive vesiculation in the glassy pockets between bubble walls ( Figure 7 ) ( Allard et al., 2005 ; Polacci et al., 2006 , 2009 ; Vergniolle and Ripepe, 2008 ). Drawing from the model of Jaupart and Vergniolle (1988 , 1989 ), we assume here that gas bubbles are exsolved and accumulated in a foam layer at the roof of a storage area below the eruptive crater ( Figure 7 ). We also assume that an increase in the magma mass flow rate into the storage area increases the bubble supply to the foam, promoting bubble coalescence and enhancing the potential for the foam layer to collapse, ascend rapidly and fragment explosively at the top of a magma column that fills the conduit almost entirely. Considering a storage area depth of approximately 1.5–1.9 km (at least for SEC; Bonaccorso, 2006 ), the rapidity and intensity with which the fountain events take place do not allow the involved magma to reside in the conduit for longer than a few minutes (see section “Analysis of magma residence time in conduits”). This greatly limits the occurrence of magma cooling and crystallisation at the conduit walls, and, as a result, decreases the generation of tachylite and its presence in the deposited ash. We suggest that the combination of the two outgassing mechanisms described above, foam collapse and syn-eruptive vesiculation, results into a more efficient and rapid magma fragmentation, providing a constant supply of vesicular sideromelane ash to the erupted plume. These observations agree with the characteristics of the sideromelane ash produced by the lava fountain episode of the 4–5 September 2007 (see section “Representative Episodes of Ash Emission at Etna”).

Figure 7. Sketch of ash formation by fragmentation of vesiculating magma (mechanism no. 1). Grey areas in conduit section and in magma storage section insets indicate liquid magma (plus crystals). Not to scale. See text for further explanation.

Tachylite ash was present in the ash samples collected after the continuous Strombolian activity that transitioned to pulsating lava fountain activity of the 24 November 2006. This episode was subdivided into three main eruption phases (resumption, paroxysmal and conclusive), similarly to what was proposed for the 2000 lava fountains by Alparone et al. (2003) . It started with high energy Strombolian explosions that increased in frequency and intensity with time originating a quasi-sustained lava fountain, where the pulsed behaviour may be explained in terms of oscillations in the magma mass flow rate, hence, in the bubble supply rate to the foam layer. Sustained lava fountaining lasts as long as the bubble supply is sufficient to sustain the critical thickness needed for the foam layer to collapse. If the bubble supply rate decreases, the foam layer critical thickness may be no longer supported and the steady fountain phase of the event ceases, progressively shifting to pulsed behaviour and/or less energetic Strombolian explosions till, eventually, the end of the activity. Potential consequences deriving from this series of events are sudden changes in the magma fragmentation depth, longer magma residence time in the conduit, increasing proportion of cooled, crystallised (tachylite-like) magma vs. vesiculating (sideromelane-like) magma, and variations in the proportion of sideromelane vs. tachylite ash particles in the deposited fragmented magma (section “Analysis of magma residence time in conduits”). Ultimately, the pressure decrease associated with the lack of a sustained magma column inside the conduit, occurring during the initial and waning phase of an eruption, can favour the collapse of conduit walls and the production of lithics ( Aravena et al., 2018 ). Similarly, pressure profiles associated with smaller conduits favour conduit walls instability and thus eruptions with a higher proportion of lithic fragments ( Aravena et al., 2017 ). This explains the high abundance of lithics in the ash componentry of the 24 November 2006 activity (see Figure 4 ).

Mechanism No. 2 (The Last Phase of the 2001 Eruption)

The second mechanism concerns ash produced by low energy explosive activity or by impulsive explosions that characterise the final period of a prolonged explosive eruption. This type of ash is generated by the fragmentation of a partially degassed magma column occupying the conduit and left from previous explosive activity. We envisage that slugs or pockets of gas rise across the magma and burst at the free magma surface progressively emptying a magma body that is increasingly cooler, crystallising and losing gas both vertically and horizontally from the conduit centre to the walls ( Figure 8 ). Here vesicles are deformed and collapsed by both magma strain rate and increasing crystallisation, eventually generating the complex vesicle shapes typical of tachylite-forming magma ( Taddeucci et al., 2004 ; Polacci et al., 2006 ). This mechanism of ash formation from a tachylite-rich magma well agrees with descriptions of the dynamics and tephra samples characterising the last phase of the 2001 Etna eruption. During this period of explosive activity, a decrease in the magma mass flow rate determined also a decrease in the eruption explosivity ( Taddeucci et al., 2004 ), allowing a proportion of magma to stagnate in the proximity of the conduit walls and promoting an increase of tachylite versus sideromelane ash in the related deposited fragmented magma ( Figure 8 and section “Analysis of magma residence time in conduits”). In comparison to the first mechanism, the ash fallout generated by mechanism no. 2 is fewer because the explosive events from which it is erupted are shorter in duration and significantly less energetic than lava fountain activity. In addition, the ash is finer than that erupted from a lava fountain because this mechanism of formation involves, besides ash from primary magmatic fragmentation, material eroded from the conduit walls and/or collapsed inside the crater, as well as recycled ash particles (e.g., ash particles that have been ejected and that fall back into the conduit; D’Oriano et al., 2014 ). This is supported by comparing grain-size data from the 4–5 September 2007 lava fountain ( Andronico et al., 2008a ) and 24 November 2006 pulsed lava fountain ( Andronico et al., 2014b ) with those from distal samples of the last phase of the 2001 eruption: the latter has a mode at 2.5–3 phi (Cristaldi and Andronico, pers. comm.), while the former have a coarser mode considering the grain-size distribution of both the total deposit and single samples collected at comparable distances. Given these particles were generated during a flank eruption, the different shallow feeding system may have affected ash generation and, consequently, componentry, changing for example the proportion of tachylite versus sideromelane with respect to the case where ash is vented from a central vent/crater along a well, already structured and relatively wider conduit.

Figure 8. Sketch of ash formation by fragmentation of partially degassed magma (mechanism no. 2). In conduit section inset arrows indicate increasing velocity gradient from conduit centre (where sideromelane is) to walls (where tachylite is). Not to scale. See text for further explanation.

Mechanism No. 3 (The 8 April 2010 Single Strombolian Explosion)

The third mechanism highlighted in this study mainly concerns ash produced by collapses of intracrateric material and conduit lining ( Figure 9 ). Visual observations and images from video cameras as well as seismic and infrasonic signals suggest that this mechanism of ash formation is often promoted by intracrateric gas jets or isolated Strombolian bursts. Such mechanism usually occurs at the end of a high energy Strombolian explosive episode or between consecutive explosive events within an eruption or within a period of explosive activity, the latter case being nicely illustrated by the April 8 2010 explosion ( Andronico et al., 2013 ). In agreement with features displayed by ash erupted from this episode and from the 29 October 2006 event, ash generated by mechanism no. 3 mainly consists of lithic particles (weathered scoria and lava fragments from the conduit walls) and microlite-rich tachylite ash from the cooling, crystallising magma skin at the conduit walls and the very top of the magma column filling the conduit. Notwithstanding the prevalence of lithic clasts in the ash componentry during ash emissions in 2010, the presence of clear seismic and acoustic geophysical signals may help to detect the presence of a pressurised magma/gas trigger and thus to hypothesise the possible involvement not only of old and/or stagnating magma but also of fresh material ( Andronico et al., 2013 ). This mechanism is similar to type 2 Strombolian activity described for Stromboli volcano by Patrick et al. (2007) .

Figure 9. Sketch of ash formation by collapses of intracrateric material and conduit lining (mechanism no. 3). Red arrows indicate collapsing material. Not to scale. See text for further explanation.

Analysis of Magma Residence Time in Conduits

Processes occurring in volcanic conduits, the pathways through which magma travels from its storage region to the surface, have a fundamental control on the nature of eruptions and associated phenomena ( Polacci et al., 2017 ). In the previous sections we have seen how the relative amount of tachylite and sideromelane can be associated with different residence times of magma within the conduit, and thus with different ascent rates that can be associated with either a different average velocity, or different ascent velocities between the centre of the conduit and the portions close to conduit walls. In this section we present an analysis, for both cylindrical conduits and dikes, aimed at quantifying the fractions of erupted mass representative of different residence times within the conduit (and thus different ascent velocities).

First of all, we introduce some notation holding for both cylindrical conduits and dikes (for further details about this analysis please refer to the Supplementary Material ). We denote with r the distance from the axis of the conduit or the dike (see Figure 10 ), corresponding to the radius of the conduit (or the semi-width of the dike), and with u z the vertical velocity, function of r . If u z,avg is the average velocity, and assuming the flow being Newtonian with kinematic viscosity ν, we can introduce the Reynolds number

Figure 10. Sketch of the geometries investigated in the analysis of conduit residence times and the velocity profiles. For both geometries, r denotes the distance from the vertical axis.

The Reynolds number is a dimensionless quantity allowing to predict if the flow regime is laminar (Re < 2300 for a cylindrical conduit) or turbulent (Re > 4000 for a cylindrical conduit). We can imagine a laminar flow as a flow where all the fluid velocity vectors line up in the direction of the flow (the vertical axis z in Figure 10 ). Below the depth of fragmentation, for typical eruptions at Etna we have R = O (10 0 −10 1 ), ν = O (10 −1 −10 0 ), and u z , avg = O (10 0 ) ( La Spina et al., 2016 ), and laminar flow characterises nearly the entire conduit. Thus, assuming that the flow is fully developed and introducing the normalised radius r ¯ = r R and the normalised velocity u ¯ = u z ⁢ ( r ) u z , max (where u z,max is the maximum velocity), we can write:

If we denote now with t ¯ = t ⁢ ( r ) t min (where t min = 1/ u z , max is the ascent time per unit length associated with the maximum velocity) the normalised ascent time, we can also write:

It is important to observe that the two equations above are independent of viscosity, flow rate values and conduit size, and that they hold for both cylindrical and dike geometries.

Now, if Q is the volumetric flow rate through the whole cross section of the conduit, and if we denote with Q ( r ) the cumulative volumetric flow rate through the portion from the centre up to a distance r from the axis (represented in grey in Figure 10 ), we can also introduce a non-dimensional normalised radius, defined as Q ¯ ⁢ ( r ) = Q ⁢ ( r ) Q , with Q ¯ ⁢ ( r ) = 0 for r = 0 and Q ¯ ⁢ ( r ) = 1 for r=R .

For a cylindrical conduit it holds:

The last two equations allow us to quantify the volume of magma associated with different ascent velocities and with different ascent times. In particular, the last one gives, for a fixed value of t ¯ , the fraction of magma residing in the conduit during the rise for a time smaller than t ¯ times the minimum ascent time.

Similarly, for a dike geometry, we have the following equation defining the normalised cumulative volumetric flow as a function of the normalised radius:

and thus, using the equations introduced above relating normalised radius, velocity and time, we obtain:

Equations (1) and (2) are independent of ascent velocity and conduit size, and thus they allow to quantify the relative proportions of volumes rising with different times also for conduits of variable width and when velocity changes during the ascent.

The two relationships between normalised cumulative volumetric flow and ascent time are plotted in Figure 11 , where on the top axis of both the plots we have also reported a temporal scale in which we assumed an average ascent time of 300 s. We remark that the average velocity for a cylindrical conduit is half the maximum velocity, while for a dike is 2/3 of the maximum velocity. Thus, for a cylindrical conduit, the fastest ascent time within the conduit (at the centre) is 150 s, while for a dike it is 200 s. The plot allows us also to quantify the portion of magma residing in the conduit for a time 10 times larger than the minimum ascent time, which correspond to 1% of the total magma for a cylindrical conduit and less than 0.4% for a dike. This is consistent with the timescales presented for mechanism 1 and the very small amount of tachylite present in the related case studies (see section “Mechanisms of Ash Formation at Etna”). In order to have a steady ascent producing a significant amount of tachylite (∼50%) at the conduit walls during the rise, it is thus needed an average ascent time one order of magnitude larger, comparable to the characteristic crystallisation times. In addition, the relationships obtained and the plots show that if the same volume of magma is erupted with the same volumetric flow rate (i.e., with same average velocity) through a dike rather than through a cylinder, the ratio between tachylite and sideromelane will be smaller. This counterintuitive result is a consequence of the higher average velocity to maximum velocity ratio of the dike with respect to the cylinder.

Figure 11. Relationships between normalised cumulative volumetric flow and normalised ascent time for a fully-developed laminar flow in a cylindrical conduit (A) and in a dike (B) . The red dots are plotted for the average ascent time, while the blue dots for a time 10 times larger than the minimum ascent time. A non-normalised time scale is also reported on the top of each plot, where the average ascent time has been fixed to 300 s.

Conclusions

Ash emissions are widespread during explosive basaltic activity, and often have a significant impact on people’s life and infrastructure. For example, between January and February 2019 Mount Etna North-East Crater produced several episodes of continuous to pulsing ash emissions, which, despite being characterised by a very low, uncommon sedimentation rate (a few g/m 2 during several hours of activity), were able nonetheless to cause disruption at the Fontanarossa International airport of Catania (29 km from the vent). To mitigate hazard at basaltic volcanoes, it is therefore imperative to improve knowledge on mechanisms of ash generation. In this study, we use Mount Etna as a general case study. Compositional and textural features of ash particles from Etna and other basaltic volcanoes have been well characterised and the explosive activity that produces them well studied. Yet, a systematic investigation of ash sources is still incomplete and mechanisms of ash generation poorly understood. With this study, we aim to fill this gap in knowledge. By revising four ash emission episodes that are representative of the most common explosive activity at Etna in the last decades, we propose three mechanisms of ash generation based on variations in the magma mass flow rate that apply to other basaltic volcanoes erupting ash and whose explosive activity is similar to Etna. Additionally, we provide an analysis of magma residence time in the volcanic conduit, which explains why different ash particles reside in the conduit for a shorter time than others. This analysis sheds light on the proportion of sideromelane and tachylite textures found in ash from lava fountaining and continuous Strombolian activity, in agreement with our first proposed ash generation mechanism. The main finding of this study is that, integrating field observations with magma residence time calculations, we are able to provide improved information on both ash sources and mechanisms of ash formation from basaltic volcanoes erupting ash. The broader implication of this investigation is that our results are significantly relevant to the wider volcanological community, particularly modellers of eruption dynamics and scientists involved with volcano monitoring and surveillance, and should be used to improve eruption forecasting and hazard assessment and to inform stakeholders on how to implement risk mitigation strategies in active volcanic areas.

Data Availability

The data generated for this study are available on request to the corresponding author.

Author Contributions

MP and DA conceived the study. DA, MP, and AC collated most of the literature data on the Etna activity discussed in the manuscript. JT provided data on the 2001 Etna eruption. MdMV provided the analysis of magma residence time in the conduit. MP wrote the manuscript, with contribution from all co-authors.

This research was funded by the RCUK NERC DisEqm project (NE/N018575/1). We also acknowledge the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement no. 654182, which has partially supported this research.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The reviewer MV declared a shared affiliation, with no collaboration, with the authors, DA, AC, to the handling Editor at the time of review.

Acknowledgments

We thank all the INGV staff at the Osservatorio Etneo in Catania devoted to the maintenance of the camera network which allows us to study the eruptive activity at Etna in great detail. We also acknowledge F. Arzilli and G. La Spina for interesting discussions on magma residence time in basaltic systems.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/feart.2019.00193/full#supplementary-material

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Keywords : basaltic volcanism, ash generation mechanisms, sideromelane and tachylite, conduit magma residence time, Mount Etna

Citation: Polacci M, Andronico D, de’ Michieli Vitturi M, Taddeucci J and Cristaldi A (2019) Mechanisms of Ash Generation at Basaltic Volcanoes: The Case of Mount Etna, Italy. Front. Earth Sci. 7:193. doi: 10.3389/feart.2019.00193

Received: 11 April 2019; Accepted: 10 July 2019; Published: 02 August 2019.

Reviewed by:

Copyright © 2019 Polacci, Andronico, de’ Michieli Vitturi, Taddeucci and Cristaldi. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Margherita Polacci, [email protected]

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Located near the east coast of Italy’s province of Sicily, Mount Etna is Europe’s most active volcano and is one of the world’s largest continental volcanoes. Among all the world’s volcanoes, Mount Etna has the longest recorded history of eruptions, dating back to 1500 B.C. Since then, the volcano has erupted about 200 times and has been very active in recent decades.

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Activity on Mount Etna

Activity at mt. etna.

In mid-January 2011, Europe’s largest and most active volcano, Mount Etna, rumbled with new energy and lit up the Sicilian night with a fountain of lava.

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Europe’s most active volcano , Mount Etna, has been hitting the headlines recently after a series of spectacular eruptions. In Etna’s first eruption of 2021, explosive lava fountains reached over 1500 m in one of the most amazing eruptions in decades.

Mount Etna, located on the island of Sicily, has been largely dormant for the past two years. The stratovolcano (composite) dominates the skyline of the Italian island, where it sits on the eastern coast.

Located between the cities of Messina and Catania, it is the highest active volcano in Europe outside the Caucasus – a region between the Black Sea and the Caspian Sea – and the highest peak in Italy south of the Alps.

The recent activity is typical of a strombolian eruption among the normal activities of the more than 3,300-metre-high volcano. The recent eruption is the strongest explosion in the southern crater since it was discovered in 1971.

On Monday 22nd February 2021, at around 11 pm, the lava fountains, surrounded by gigantic clouds of smoke, exceeded 1,500 metres (4,900ft) in height, while thousands of rock fragments, some the size of fridges, were thrown from the crater into the sky for several kilometres.

Etna is a hyperactive volcano with over 3,500 years of historically documented eruptions. The volcano has been erupting on and off since September 2013. Since September 2019, it’s been erupting from its various summit craters virtually continuously. In December 2020, Etna’s explosive activity and lava output began to spike, and in February 2021, it has been launching fluid lava skywards.

Etna is an unusual volcano in that it can produce explosive eruptions of runny lava and release slower flowing, thick lava flows. Scientists are still trying to work out why this is the case.

The magma from the latest eruption appears to be coming up from deep within the mantle . Extremely hot, fluid magma is rapidly rising through the network of conduits within and below the volcano. However, there is another factor that is contributing to the current explosive eruptions.

There are high quantities of water vapour in Etna’s magma, which makes it explosive. The water does not cool the magma. As the molten magma approaches the surface, the pressure drops, and the bubble of water vapour expands violently, leading to lava being ejected out of the volcano.

Following each explosive lava fountain , less gassy magma lingers just below the vent. This is then cleared when a new volley of gassy magma rises from below. These explosive eruptions are known as volcanic paroxysms.

Authorities have reported no danger to the nearby towns, however, local airports have been temporarily closed, as has the airspace around the volcano. Etna’s last major eruption was in 1992. Despite the explosive nature of the recent eruption, there is no risk to the population, other than from the ash that covers buildings and smoke that can, after a few hours, cause breathing problems. In March 2017 vulcanologists, tourists and a BBC film crew were injured during an eruption when a flow of lava ran into snow, producing superheated steam that sent fragments of rock flying in all directions.

Further reading For a Volcanologist Living on Mount Etna, the Latest Eruption Is a Delight  – Advisory – this article contains expletives (swear words).

In Pictures: Mount Etna eruption lights up Sicily’s night sky

Mount Etna: BBC crew caught up in volcano blast

Mount Etna illuminates night sky with 1,500-metre lava fountain

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• Published: 13 November 2023

Assessment of eruption source parameters using infrasound and plume modelling: a case study from the 2021 eruption of Mt. Etna, Italy

• Silvio De Angelis   ORCID: orcid.org/0000-0003-2636-3056 1 , 2 ,
• Luciano Zuccarello   ORCID: orcid.org/0000-0003-0094-9577 1   nAff2 ,
• Simona Scollo   ORCID: orcid.org/0000-0001-8704-8629 3 &
• Luigi Mereu   ORCID: orcid.org/0000-0003-0303-1171 4 , 5  

Scientific Reports volume  13 , Article number:  19857 ( 2023 ) Cite this article

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• Natural hazards
• Volcanology

Atmospheric injection of volcanic ash during eruptions is a threat to aviation. Reliable forecast of airborne ash dispersal relies on empirical and numerical models. Key inputs into these models are so-called eruption source parameters such as the rate at which pyroclastic material is ejected from the vent and the maximum height of eruptive columns. Here, we use infrasound data recorded during eruptive activity in June 2021 at Mt. Etna, Italy, to demonstrate its potential for assessment of eruption rates in near-real time. We calculate a time series of flow velocity at the vent using data corrected for topographic scattering, and the effect of vent geometry on the acoustic source radiation. We obtain values of flow velocity of 50–125 m/s during a period of sustained, paroxysmal, activity. We use independent estimates from other ground-based remote sensing data to validate our results. Further, we use the infrasound-derived flow velocities as input into a 1D plume model to estimate the maximum height of the eruption column. Our results suggest that infrasound technology holds promise to assess eruption rates and inform modelling of volcanic plumes. We anticipate that implementation of real-time operational workflows based on infrasound data and plume modelling will impact decision-making and risk mitigation at active volcanoes.

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Introduction

Challenges in monitoring volcanic plumes and ash transport.

Explosive eruptions frequently release significant amounts of fine rock particles and glass fragments, referred to as volcanic ash, into the atmosphere. The emission of volcanic ash, owing to its highly abrasive nature and the low melting point of its glass component, can cause severe damage to aircrafts 1 , 2 , 3 . Nine Volcanic Ash Advisory Centres (VAACs) are deputed to evaluate the presence and extent of volcanic ash in the atmosphere, and to issue ash-cloud warnings to the aviation community around the world 1 . The ability of these operational centres to forecast the movement of ash in the atmosphere, and thus identify at-risk regions, is underpinned by complex numerical models. Such models require a set of input parameters that represent the meteorology of the atmosphere at the time of eruption (e.g., wind speed and direction, vertical temperature profile) and the nature of the eruptive source (e.g., source location, time and duration of eruption, vent geometry, height of the plume, mass eruption rate, particle size distribution, ash density and shape) 4 . Constraining the extensive parametrization of atmospheric ash transport models is challenging, and thus, their outputs are affected by substantial uncertainty. Meteorological parameters are usually well-constrained by data and models with temporal and spatial resolutions of the order of hours and few kilometers, respectively (e.g., the Met Office Unified Model 5 used by the London VAAC 4 ). Conversely, larger uncertainties are linked to estimates of eruption source parameters 6 , 7 (ESP), in particular mass or volume eruption rates (MER or VER, respectively). Commonly, VER is estimated through inversion of so-called 0D eruption plume models (EPMs), that is empirical scaling relationships between VER and the height reached by the plume above the vent. The simplest, and most widely used, family of EPMs takes the general form \(H=C{V}^{n}\) , where \(H\) is the maximum height reached by the plume above the vent,  \(V\) is VER (measured as \({\mathrm{m}}^{3}\) of dense-rock equivalent per second),  \(C\) and \(n\) are constants 1 , 8 . For the purpose of informing volcanic ash transport modelling, 1D integral EPMs have also been increasingly tested owing to their ability to include processes such as the entrainment of atmospheric air into the eruption column, and to account for the effect of wind speed and direction on plume rise 4 , 9 . The main challenge in the use of EPMs for assessment of VER is that they require validation, which depends on the availability of independent measurements of both plume heights and MER/VER 1 , 10 , 11 . Independent column height estimates are available from either satellite and ground-based (e.g., camera, radar, lidar) measurements albeit with limitations posed by data latency, logistics and weather conditions. The majority of independent VER estimates are average measurements of the ratio between the total volume of erupted tephra and the duration of the eruptive event. These data can exhibit significant scatter 4 , 12 , which arises from differences in the techniques employed to evaluate the volume of tephra deposits and the duration of eruptions, and factors such as the impact of wind speed and direction on the rise of volcanic plumes. Therefore, the need for new methods to quantify volcanic emissions has emerged as a critical step towards improving our capacity to assess and mitigate hazards from volcanic ash.

Measurements of volume eruption rate

In recent years, a series of studies have proposed methods to estimate VER/MER from analyses of ground-based radar, thermal infrared (IR), and acoustic data. Gouhier and Donnadieu 13 and Freret-Lorgeril et al. 14 used echo power data from Doppler Radar measurements to estimate MER at Stromboli and Mt. Etna (Italy), respectively; Marzano et al. 15 proposed a workflow to measure MER using microwave weather radar and infrasound array data collected during eruptions at Eyjafjallajökull (Iceland, 2010), Grímsvötn (Iceland, 2011) and Mt. Etna (2013); Ripepe et al. 16 employed a combination of thermal camera imagery and infrasound array data to evaluate plume exit velocity during the 2010 Eyjafjallajökull eruption; Calvari et al. 17 calculated the exit velocity of the eruptive jet from thermal IR data during paroxysmal activity at Mt. Etna in 2020–2021. Among these methods, infrasound has emerged as a promising tool to quantify VER/MER owing to the comparatively low costs associated with sensor installation and maintenance, data availability in real-time, and its suitability for automated data processing. A substantial body of research has informed continuous development of methods for inversion of acoustic waveforms aimed at quantifying volcanic emissions. Caplan-Auerbach e al. 18 , Lamb et al. 19 , Ripepe et al. 16 used scaling laws that link the power radiated by acoustic sources to gas velocity during flow from a volcanic vent 20 . Johnson et al. 21 and Johnson and Miller 22 used a monopole source model, that is a compact source that radiates acoustic waves hemispherically, to quantify volcanic emissions at Mt. Erebus (Antarctica) and Sakurajima volcano (Japan), respectively. Kim et al. 23 introduced a waveform inversion method to calculate VER at Tungurahua volcano (Ecuador) based on the Green’s Function solution to the inhomogeneous Helmoltz wave equation in a half-space; the method, based on the representation of the acoustic pressure wavefield as a combination of monopole and dipole terms, was also applied by De Angelis et al. 24 at Santiaguito volcano (Guatemala). Progress in numerical modelling of the acoustic wavefield generated by compact volcanic sources 25 , 26 underpinned additional studies focussed on retrieval of VER via full waveform inversion of infrasound signals using 3D numerical Green’s Functions 27 , 28 , 29 , 30 . A comprehensive review of these methods and their underlying theoretical frameworks can be found in De Angelis et al. 31 . More recently Hantusch et al. 32 and Freret-lorgeril et al. 33 have shown that VER can be calculated from direct integration of the acoustic pressure wavefield after applying corrections for scattering from topography, wavefield directivity (controlled by vent geometry and the acoustic wavenumber) and reflectance at the conduit outlet. Hantusch et al. 32 accounted for topographic scattering of the acoustic wavefield at Copahue volcano (Argentina) via the insertion loss (IL) parameter in the screen diffraction approximation. Freret-Lorgeril et al. 33 assumed that topographic scattering at Mt. Etna was negligible as the position of the infrasound array used was in line-of-sight with the erupting vent. Maher et al. 34 demonstrated that the screen diffraction approximation is only appropriate under specific conditions and suggested that numerical simulations are the best tool to estimate IL; a workflow for calculating maps of IL based on numerical modelling of acoustic wave propagation over volcanic topography was proposed by Lacanna and Ripepe 26 .

Here, we build on this extensive body of research and demonstrate a methodology to obtain independent estimates of VER from the integration of attenuation-corrected infrasound data recorded at Mt. Etna, Italy. We benchmark our results with independent estimates of flow velocity at the vent obtained from analyses of ground-based thermal IR imagery. Finally, we show how VER can be used as a direct input into 1D plume models for rapid assessment of maximum column height, thus, providing key monitoring information when other observations may not be available. The results of plume modelling are further validated using ground-based X-band radar data and satellite imagery gathered during eruption.

Eruptive activity at Mt. Etna: June 2021

Mt. Etna, Italy, is one of the most active volcanoes in the world. One of its distinctive features is the frequent occurrence of so-called paroxysms, that is episodes of intense explosive activity lasting from tens of minutes to many hours 35 . Paroxysms typically occur in clusters within eruptive periods that can last from few weeks to months 36 . Between December 2020 and February 2022 Mt. Etna produced 66 paroxysms 17 . In this study, we focus on an episode on 20 June, 2021, part of a sequence of paroxysms that occurred, with striking regularity, during the second half of June 2021. These events had durations of up to few hours and all followed the characteristic pattern of paroxysmal activity at Mt. Etna. Initial rapid-fire Strombolian explosions evolved into nearly continuous lava fountaining, feeding lava flows and volcanic plumes with heights of up to several kilometers above the vent 37 . Infrasound and thermal IR data recorded during the paroxysmal activity on 20 June are shown in Fig.  1 a–f. Figure  1 c, d illustrate early Strombolian activity, consisting of discrete explosions clearly distinguishable in both the waveform (Fig.  1 c) and spectrogram (Fig.  1 d). During the later stages of the paroxysm, the acoustic signal evolves into nearly continuous tremor (Fig.  1 e) and individual explosions are only occasionally discerned (Fig.  1 f). Figure  1 b shows thermal IR images tracking the change from discrete explosions to lava fountains feeding a sustained volcanic plume.

Infrasound and thermal IR data recorded during the paroxysmal activity at Etna on 20 June, 2021. ( a ) Pressure waveform recorded through a paroxysmal event on 20 June, 2021. Infrasound data are recorded by the central sensor of a temporary 6-element infrasound array at approximately 6 km from the active vent (see Fig.  2 a–c); ( b ) Four images recorded by a thermal IR camera of the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo (INGV-OE), located approximately 15 km South of the Etna summit craters (see Fig.  2 a). The images correspond to times 1–4 marked by dashed lines in ( a ); ( c ) Zoomed waveform showing 15 min of acoustic data during Strombolian activity; ( d ) Spectrogram of the signal in ( c ). The spectrogram is calculated as the Power Spectral Density of the pressure time series over a sliding window of 10.24 s with 50% overlap; ( e ) Zoomed waveform showing 10 min of acoustic data during lava fountaining activity; ( f ) Spectrogram of the signal in e) calculated with the same parameters as in ( d ).

Infrasound array

For this study, we used acoustic data recorded by a temporary 6-element infrasound array deployed at Mt. Etna in June 2021. The array was installed and operated by the University of Liverpool and the Istituto Nazionale di Geofisica e Vulcanologia (INGV sezione di Pisa and INGV-Osservatorio Etneo) at the Mt. Conca site (Fig.  2 a, CONC), approximately 6 km from the active vent within the South East Crater (SEC) area (Fig.  2 b). The array had an aperture (i.e., largest distance between any two sensors) of approximately 70 m (Fig.  2 c) and was instrumented with IST2018 infrasound microphones 38 (full-scale range of 480 Pa peak-to-peak, flat response between 0.1 and 40 Hz). Data were recorded on-site with a sampling frequency of 100 Hz and 24-bit resolution.

Map of Mt. Etna (digital elevation model data source: https://tinitaly.pi.ingv.it/ ) and location of infrasound and thermal IR sensors used in this study. ( a ) Map showing the locations of temporary infrasound array CONC (red triangle) and thermal IR ENT camera (yellow triangle). CONC was installed at the site Monte Conca, approximately 6 km from the active vent; ENT operated by the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, and installed in the town of Nicolosi, approximately 15 km South of the active vent. The inset plot shows the location of Mt. Etna (yellow square) in Sicily (Italy). The dashed box identifies the summit area. The red dashed arrow indicates the backazimuth between array CONC and the active vent within the South East Crater area; ( b ) Detail of the summit area and craters at Mt. Etna in June 2021 obtained from an Uncrewed Aerial Vehicle survey of the summit area: Bocca Nuova/Voragine (BN/VOR), North East Crater (NEC) and South East Crater (SEC); c) Configuration of the 6-element infrasound array CONC. All maps were produced with the Python packages Matplotlib v3.8 ( https://matplotlib.org/ ) and GDAL v3.7.0 ( https://gdal.org/index.html ).

Mt. Etna is routinely monitored by INGV-Osservatorio Etneo (INGV-OE) with an extensive multi-parameter sensor network, including thermal IR cameras. Here, we used thermal imagery of the ENT camera recorded at the site located in Nicolosi (Fig.  2 a), approximately 15 km from the active summit craters. The site is equipped with a Flir A40M camera recording IR images in the \(7.5 - 13\,\upmu {\text{m}}\) band, with a field of view of 640 × 480 pixels and thermal sensitivity of \(80\;{\text{mK}}\) at \(25^\circ\) 17 , 39 . The camera records images with a sampling rate of \(0.5\;{\text{Hz}}\) , and provides a spatial resolution of 1.3 μrad ( \(\sim 15\;{\text{m}}\) at the ENT site). We chose this site as it provides the best trade-off between data availability, spatial and temporal resolution, and the ability to track the development of the eruptive plume during paroxysmal activity.

X-band radar

Volcanic plumes from Etna are also observed using a dual polarimetric X-band radar, managed by the Italian Department of the Civil Protection, part of the national weather radar network 40 . The X-band dual-polarization radar, located at the international airport of Catania (distance of about 32 km from the SEC), has a wavelength of 3.1 cm (9.6 GHz), peak power of 50 kW and half-power beam width of 1.3°. The radar scans a volume defined by an area of 160 × 160 km 2 and a height of 20 km, recording data along 12 elevation angles every 10 min.

Here, we have investigated an episode of paroxysmal activity that occurred on 20 June, 2021 starting at approximately 21:30 (UTC), lasting for about 2 h (Fig.  3 a). For this event, INGV-OE issued two Volcano Observatory Notice for Aviation (VONA) with color code Red (the highest level of concern) at 22:01 and 22:19 UTC 41 , 42 ; INGV-OE reported lava fountaining at the SEC and a plume drifting towards the East-South East. However, the height of the plume could not be estimated in real-time during the paroxysm.

Time series of VER and flow velocity at the vent (infrasound array and thermal IR), and plume height (X-band radar data) during paroxysmal activity on 20 June, 2021. ( a ) Infrasound signal through the paroxysm, recorded by the central sensor of the a temporary 6-element infrasound array at approximately 6 km from the active vent (Fig.  2 a, c); ( b ) Time series of high-quality (MCMM > 0.65) Direction of Arrival estimates throughout the paroxysm; ( c ) Time series of flow velocity at the vent estimated from integration of infrasound data (yellow circles) and analysis of thermal IR data (blue squares); ( d ) time series of thermal IR data from the ENT site (Fig.  2 a). The plot is produced from stacks across a vertical section (through the crater area) of each time-lapse thermal IR image (one image every 2 s); ( e ) Time evolution of plume height estimated from X-band radar reflectivity data (uncertainty on plume height measurements is +/− 300 m).

VER and flow velocity from infrasound array data analyses

We analysed infrasound data recorded at the CONC site (Fig.  2 a, c) during paroxysmal activity on 20 June, 2021. Infrasound arrays allow to identify windows of coherent signal and the direction of their source region, providing an effective tool for discrimination of eruptive activity from other signals. We processed the CONC data using slowness inversion 43 to obtain a time series of Direction of Arrival (DOA), that is the direction from which infrasound energy travels as it crosses the array. The results, in Fig.  3 b, show coherent acoustic energy crossing the array from a stable backazimuth in the direction of the active SEC (~ 205°–215° N). We selected data windows corresponding to high quality array locations (MCMM > 0.65) and integrated the pressure signal recorded at the CONC6 microphone (Fig.  2 c) to calculate a time series of VER (Fig.  3 c). Finally, we used a vent radius of 10 m 44 to convert VER into flow velocity and calculated its 1-min median, which is shown in Fig.  3 c (yellow circles). We also performed calculations for a second paroxysm that occurred on 24 June, 2021 (supplementary material, Fig. S1 ).

Flow velocity from thermal IR data analyses

Figure  3 c shows a time series of measurements of jet velocity at the vent (blue squares) obtained from analyses of thermal IR data recorded at the ENT site (Fig.  2 a) during paroxysmal activity on 20 June, 2021. The measurements, performed with a time step of 1 min, can be compared with the 1-min median jet velocity obtained from acoustic data (Fig.  3 c, yellow circles); the two independent time series show excellent agreement throughout the phase of sustained lava fountaining (~ 22:45–23:30). After 23:30, concurrent with waning of lava fountaining, thermal IR estimates of velocity decrease while infrasound-derived jet velocities remain high as acoustic sensors continue detecting individual explosions with no significant associated lava fountaining. Figure  3 d shows the thermal IR data from the ENT camera plotted as a time series. The figure, produced by plotting stacks across a vertical section (through the crater area) of each time-lapse thermal IR image (one image every 2 s), allows to track the evolution of the paroxysm in space and time, clearly showing its onset (~ 22:00), peak (22:45–23:40) and waning (Fig.  3 c) phases.

Radar-derived height of the plume and plume model

We retrospectively analysed X-band radar data to estimate the temporal evolution of plume height during paroxysmal activity. Figure  3 e shows radar detection of a ~ 5 km ash plume starting from 23:00 increasing to a maximum height of 10 km at 23:30. The plume top was estimated at 10 km until ~ 00:00 (21 June, 2021) when it started to progressively wane during the late stages of the paroxysm (supplementary material, Fig. S2 ). We also used flow velocity estimates from the analysis of infrasound data as input into a 1D model of plume rise 9 . The model output, displayed in Fig.  4 for values of flow velocities of 50, 75 and 125 m/s (VER of \(1.6\, \times \,10^{4}\) , \(2.3\, \times \,10^{4}\) , and \(3.9\, \times \,10^{4} \,{\text{m}}^{3} {\text{/s}}\) considering a vent radius of \(10\,{\text{m}}\) ) shows the plume drifting towards the East (Fig.  4 a), extending vertically to a maximum height of 8–10 km above sea level (Fig.  4 b). These results agree with both the radar-derived maximum height (Fig.  3 e) and observations of the plume drifting eastwards (Fig.  4 c) from satellite thermal imagery collected by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI, 8.7, 10.8, and 12 μm wavelenghts).

Model and satellite image of the ash plume during paroxysmal activity at Mt. Etna on 20 June, 2023. ( a ) Map view of the modelled plume showing ash drifting towards the East; ( b ) Cross-section (West–East) view of the modelled plume for values of flow velocity of 50, 75 and 125 m/s. Solid lines are represent the height of the plume at its centre and the dashed lines are lower and upper height of the plume as defined by its Gaussian width; ( c ) Composite thermal IR (8.7, 10.8, 12  \(\mathrm{\mu m}\) wavelengths) satellite image from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) at 23:30 on 20 June, 2021 (UTC time). The black arrow indicates the eruptive plume drifting towards the East. Panels ( a ) and ( b ) were generated by the PlumeRise model web interface ( https://www.plumerise.bris.ac.uk ). Panel ( c ) contains EUMETSAT Meteosat Volcanic Ash RGB—MSG—0 degree product derived from SEVIRI data. SEVIRI data were downloaded from the EUMETSAT data portal ( https://view.eumetsat.int/productviewer?v=default ). The map was produced with the Python packages Matplotlib v3.8 ( https://matplotlib.org/ ) and GDAL v3.7.0 ( https://gdal.org/index.html ).

Discussion and concluding remarks

The potential impact of airborne volcanic ash on aviation and infrastructure is well-known. Mitigation of the associated risks depends critically on the ability to issue timely forecasts of atmospheric ash dispersion, and thus, characterize ESP in real- or near real-time. In this study, we have demonstrated an infrasound-based methodology to obtain measurements of VER that holds potential for real-time implementation. We have analysed acoustic data gathered during a paroxysmal event at Mt. Etna on 20 June, 2021, and have benchmarked infrasound-derived measurements of VER (transformed into flow velocity assuming a vent with circular cross-section) with independent estimates of flow velocity at the vent from thermal IR imagery, one of the tools routinely employed by INGV-OE to detect the onset of eruptions, track their evolution and investigate the height of lava fountains during paroxysms.

Flow velocity at the vent and VER are important parameters to constrain eruption dynamics and to evaluate the height of the eruptive plume. Infrasound is a widely adopted tool for volcano monitoring and provides an effective means to measure VER. The main advantages of infrasound over other methods with comparable resolution, such as Doppler radar, are its low installation costs, and simple and rapid data processing that can easily be automated for real-time implementation. Unlike other methods such as those based on analyses of optical or thermal IR imagers, infrasound is not affected by cloud cover. Strong wind can, however, increase noise levels and deteriorate infrasound signal quality although its effects at the recording site can be mitigated through careful sensor installation. Strong directional winds can still severely affect along-path acoustic propagation and obscure signal; installation of multiple arrays at different azimuths with respect to the position of the active vent/s would provide effective mitigation of such adverse wind effects. We also stress that, for application of the methods presented in this study, acoustic arrays are preferred to larger aperture deployments of individual microphones in network configuration owing to their superior capabilities to separate coherent eruption signals from noise. Owing to its characteristics, infrasound is used for operational eruption early warning at Mt. Etna 45 and worldwide. Implementation of the methodology presented here for VER measurements would be a straightforward addition and a valuable complement to the existing monitoring systems. The main limitation of using infrasound to measure VER is that it requires a well-constrained source mechanism, as well as knowledge of topographic scattering and true signal attenuation, and reflectivity effects at the vent-atmosphere interface. In the case study presented here, as well as other volcanoes 32 , 33 , the acoustic source is well-represented by an isotropically radiating monopole (see “ Methods ” section for details). We adopted the formulation of Lacanna and Ripepe 46 to estimate VER, which provides a tool to characterize and correct the pressure wavefield for reflectivity at the vent-atmosphere interface in terms of signal frequency and the characteristic source dimension. Their formulation also provides a means to identify non-isotropic radiation patterns in terms of the product of two measurable parameters, that is the characteristic dimension of the source (i.e., the vent radius, a) and the acoustic wavenumber ( k ). The assumption of a pure isotropically radiating source (i.e., an acoustic monopole) holds for values of \(ka\le 0.43\) ; for values of \(ka>0.43\) additional corrections accounting for wavefield directivity are required. Finally, correction for signal attenuation from topographic scattering represented by IL is included; IL is evaluated from numerical modelling of acoustic wave propagation (see “ Methods ” section for additional details).

The acoustic and thermal IR flow velocity time series shown in Fig.  3 c show good agreement until approximately 23:15 on 20 June, 2021. After that time the acoustic data continues tracking eruption while the thermal IR signal decreases abruptly. Infrasound waveforms and array DOAs (Fig.  3 a, b) confirm that eruptive activity was still ongoing until approximately 23:45; at the same time the X-band radar detected a well-established plume (Fig.  3 e). A plausible explanation for the divergence between acoustic and thermal IR data starting from 23:15 is a migration of the fragmentation front deeper into the conduit towards the final stages of paroxysmal activity. In this scenario explosive activity would still be ongoing at depth into the conduit, as detected by infrasound data (Fig.  3 a, b), but it would only produce low-level lava fountaining at the surface (which would not be efficiently detected by the ENT camera) while still feeding an airborne ash plume. Infrasound activity declined at ~ 23:45 marking the waning of the paroxysm. A plume between 8 and 10 km persisted until at least 00:30 (supplementary material Fig. S2 ).

Finally, we have shown that infrasound-derived flow velocities can be used as input into a 1D plume rise model to obtain realistic estimates of the maximum height of the ash column. It should be noted that flow velocities ( \({v}_{jet}\) ) were calculated from the infrasound-derived VER and the cross-sectional area of the vent, \(S\) , as \({v}_{jet}=VER/S=VER/\pi\) , thus making the conduit radius, \(a\) , an important parameter. For example, for values of VER of the order of \(10^{4} \;{\text{m}}^{3} /{\text{s}}\) , thus similar to those reported in this study, an increase in conduit radius from \(10\) to \(20\;{\text{m}}\) would result in a decrease in calculated flow velocity of a factor of \(\sim 4\) . We suggest that a realistic vent radius is used for these calculations, or multiple plume simulations are conducted to assess the sensitivity of plume height to a range of plausible flow velocities at the vent. At Mt. Etna, and many other volcanoes, these issues are increasingly mitigated by the frequent availability of updated digital terrain models. These models, mostly obtained from Uncrewed Aerial Vehicle surveys, allow estimating vent dimensions with very high (< 1 m) spatial resolution e.g., 44 .

We stress that PlumeRise 9 , the 1D model used in this study, was capable of producing results for a single model in < 1 s, showing its suitability and potential for use in real-time. The results of plume modelling were benchmarked with data from a ground-based X-band radar (Fig.  3 e) and the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) satellite (Fig.  4 c), which confirmed the maximum height (~ 10 km) and direction (East) of the ash plume, respectively. We note that the one of the objectives of this study was to provide a proof-of-concept for the combined use of infrasound data and plume rise modelling to assess ESP; a full sensitivity analysis of the plume model employed here to its parametrization was beyond the objectives of this work. Plume height is a key ESP as it defines the spreading height of the volcanic cloud in atmospheric ash dispersal models 33 and it is also often used for assessment of eruption rates. At Mt. Etna plume height is presently derived from analyses of optical images 47 , ground-based radar 40 and satellite data 48 ; successful application of these methods depends on whether specific conditions are met. Methods based on visible cameras are contingent on favorable meteorological conditions (e.g., no cloud cover), can only be used during daytime, and the retrievable height of volcanic plumes is dependent on source-camera distance and the camera field-of-view. Radar data are not affected by meteorological conditions and can be used during both daytime and nighttime; however, detection capabilities depend on source-receiver distance and direction of plume dispersion. Satellite-based measurements of plume height are generally delayed compared to other methods as they require a fully developed (opaque) plume to produce reliable height estimates 47 . Numerical modelling (or alternatively selection of a scenario from an ensemble of pre-computed models) informed by real-time analysis of acoustic data offers a valuable complement to these methods for rapid assessment of plume height.

This study aimed at introducing an alternative methodology for real-time assessment of ESP during eruptions, in particular VER and the top height of the eruptive plume. A number of methods already exist for measurement of ESP; some have briefly been discussed in this manuscript, including advantages and limitations. We have introduced a complementary strategy that relaxes some of the assumptions and simplifications underpinning widely used 0D EPM and overcomes the limitations of other existing techniques. We have demonstrated how analysis of infrasound data allows reliable estimates of VER that can be used to inform 1D plume models, thus, providing real-time estimates of the height of volcanic plumes. These methods would prove especially valuable in the early stages of a volcanic crisis when other measurements are typically not available. At Mt. Etna, for example, rapid information on plume height would be of great importance when issuing warnings to aviation authorities (e.g., VONA), and as a preliminary tool to assess plume dynamics and identify the most likely ash dispersal scenario.

We envision that infrasound-based methods for assessment of ESP, along with numerical modelling, will play an increasingly important role in volcano monitoring operations, and suggest that the implementation of our methodology into an operational workflow will be transformative in influencing decision making and risk mitigation during future volcanic crises.

Thermal IR estimates of jet velocity

Thermal IR images at the ENT site (Fig.  2 a) were processed using the algorithm of Mereu et al. 49 ; they provide a time series of brightness temperature over the course of a paroxysmal event. The Incandescent Jet Region (IJR), that is the near-vent jet that feeds the volcanic plume, is identified from thermal images as the saturated portion within each frame by evaluating the gradient of the brightness temperature as a function of a given threshold. As a first-order approximation, the height of the IJR is assumed to correspond to the height of the lava fountain 39 , \(H_{lf}\) , and the jet velocity at the vent ( \(v_{jet}\) ) is then calculated as:

where \(g\) is the acceleration of gravity. Detection of the IJR and calculation of \(v_{jet}\) are performed for each thermal IR frame and the results averaged every minute (Fig.  3 c). The validity of this method depends on considering the IJR a region of non-viscous ballistic flow, a reasonable assumption for volcanic jets near the vent. Additional limitations of the method are that: (1) the correct identification of the IJR can be affected by atmospheric conditions causing variable levels of image saturation; (2) the IJR may be higher or lower than the lava fountain owing to the presence of bombs and lapilli at higher elevation than the lava fountain, or to some of the activity obscured by ash cover 49 .

Infrasound array processing

Infrasound array data were processed following the methodology of De Angelis et al. 43 . The processing workflow implements least-squares beamforming to evaluate the DOA of coherent energy and the propagation velocity of acoustic waves across the array. The method implements a simple inversion problem:

where \({\mathbf{d}}\) is a data vector of time delay measurements between each pair of sensors within the array, \({\mathbf{m}} = (s_{x} ,s_{y} )^{T} = (sin\theta /v,cos\theta /v)^{T}\) is a model vector of slowness in the East–West and North–South directions, \({\mathbf{G}}\) is a \(N \times 2\) matrix of distances between each pair of sensors ( \(N\) is the number of sensors, \(\theta\) is the DOA, \(v\) is the acoustic wave velocity across the array). Delay times for each data window analysed are measured using cross-correlation between all pairs of sensors within the array. The results in Fig.  3 b were obtained by processing 10-s sliding windows with 50% overlap; a preliminary bandpass filter (0.7–3 Hz, Butterworth, 2-pole) was applied to the data. Only high-quality DOA estimates were selected for plotting, that is those corresponding to a median value of the cross-correlation maximum (MCCM) greater than 0.65.

Infrasound estimates of VER

We estimated VER by direct integration of the infrasound pressure time series recorded at station CONC6, at central element of the CONC array, (Fig.  2 c) following the formulation of Lacanna and Ripepe 46 :

where \(r\) is the source-receiver distance ( \(5690\,{\text{m}}\) ), \(\rho\) is atmospheric density ( \(1.1\;{\text{kg/m}}^{3}\) ), \(R\) is reflectance at the vent, \(\alpha\) is wavefield directivity, \(IL\) is insertion loss, and \(\Delta P\) is the recorded pressure time series.

We used values of \(R = 0.95\) and \(\alpha = 1\) (i.e., isotropic radiation) for a vent radius of \(10m\) , sound velocity of \(330\;{\text{m/s}}\) and a dominant frequency of the acoustic waves of \(2\;{\text{Hz}}\) 26 , 32 . \(IL\) represents scattering effect of topography on the wavefield, beyond simple geometric attenuation, which can be calculated from numerical modelling of acoustic wave propagation. \(IL\) can be expressed as:

that is, the ratio between the infrasound pressure calculated at a given position from numerical modelling ( \(P_{topo}\) ) and the expected pressure considering only the effect of geometrical spreading ( \(P_{geo}\) ). Figure  5 shows a map of IL at Mt. Etna obtained from 3D Finite Difference Time Domain modelling using the software package infraFDTD 28 , 29 . InfraFDTD is a 3D linear acoustic wave propagation model over topography with an absorbing boundary (implemented using perfect matched layers) to prevent spurious reflections at the solid interface between the atmosphere and the ground. We note that there are several alternatives to finite difference modelling for acoustic wave propagation over topography, including the family of open source spectral element codes SPECFEM 50 . Similar to Diaz-Moreno et al. 29 we implement a Blackman-Harris source function with an amplitude of 10000 kg/m 3 and a cutoff frequency of 4.5 Hz to include the dominant frequency range of the infrasound recorded during the paroxysm (0.7–3 Hz). The topography of Mt. Etna was derived merging a 0.55 m resolution digital elevation model of the SEC area from UAV surveys conducted in March 2021 44 with a regional 30-m Advanced Spaceborne Thermal Emission and Reflection Radiometer DEM for the rest of the study area 29 ; the resulting area was resampled into a grid of 5 × 5 m cells and the acoustic pressure wavefield was propagated over the topography using 20 grid points per wavelength to ensure numerical stability of the procedures 51 . At each grid point we applied Eq. (3) to produce a map of \(IL\) for the study area. We calculated a value of \(IL = - 9.4\;{\text{dB}}\) at the CONC site. We note that assessment of VER using infrasound requires estimates of IL along a single or a limited number of source-receiver paths. In light of this, 3D modelling could be replaced with 2D modeling, which would greatly increase computational efficiency (and only requires readily accessible computational resources) at the comparatively minor cost of neglecting second-order out-of-plane effects on wavefield propagation. One additional key consideration on IL mapping, is that significant topographic changes, such as major edifice collapses, would require updating IL maps.

Map of insertion loss at Mt. Etna for a 4.5 Hz source positioned in the SEC area (white diamond). Insertion Loss evaluated from Finite Difference Time Domain numerical modelling of the acoustic wavefield (see “Methods” section of this manuscript). The inverted white triangle shows the position of the CONC array used in this study (IL = − 9.4 dB). Note high signal attenuation (IL ~ − 5 dB) in the area behind BN/VOR and NEC, and the effect of constructive interference from reflections of the acoustic wavefield along the edges of Valle del Bove (IL > 0).

Plume model and atmospheric data

We used the PlumeRise model of Woodhouse et al. 9 to estimate the maximum height reached by the volcanic plume. The model is a 3-dimensional description of the rise of volcanic ash columns based on the fluid dynamics of turbulent buoyant plumes in a windy atmosphere. This is achieved by combining an integral model of pure plumes in a horizontal wind field 52 with an integral model of volcanic columns in a static atmosphere 53 . PlumeRise also accounts for the thermodynamics of heat transfer between hot pyroclasts and the surrounding atmosphere and the effects of a variable atmosphere on the rise of volcanic plumes. Additional details and the full model formulation can be found in Ref. 9 . In this study, we used a model of the atmosphere obtained from the Reanalysis v5 (ERA5) dataset. ERA5, produced by the of the European Centre for Medium-Range Weather Forecasts of the Copernicus Climate Change Service, provides hourly estimates of many atmospheric, land and oceanic climate variables. These data cover the Earth on a 30 km grid and resolve the atmosphere using 137 levels from the surface up to a height of 80 km 54 ; we used data corresponding to the grid node that is closest to Mt. Etna, at 23:00 on 20 June, 2021. Values for all additional parameters required by the model are shown in the supplementary material (Table S1 ) and are based on previous work on plume modelling at Mt. Etna 47 .

Plume height measurements with X-band radar

We applied the VARR (Volcanic Ash Radar Retrieval) algorithm to the measured co-polar radar reflectivity Zhh (dBZ) to obtain information on the nature and height of the volcanic plume 7 , 55 . The algorithm performs tephra classification using a maximum a-posteriori probability criterion and estimates tephra features such as its mass concentration and number-weighted mean diameter. The top height of the plume (HTP) is estimated using a threshold algorithm on the measured Zhh of the probed plume and on the retrieved Ct. This estimate equals the central point of the range bin volume. The distance between the radar and the summit craters and the half-power beam width of the radar are used to reconstruct the radar beam cone; an uncertainty on HTP of ± 300 m is evaluated as the half-aperture (radius) with respect to the axis of the truncated cone in the proximal area above the crater 56 .

Data availability

The infrasound data and related metadata used in this study are available at https://zenodo.org/record/8207343 . The software used for infrasound array processing is hosted at: https://github.com/silvioda/Infrasound-Array-Processing-Matlab . The PlumeRise model is freely available at https://www.plumerise.bris.ac.uk/ . The infraFDTD software is available via request to the author, K. Kim ([email protected]). Thermal IR (Fig.  3 d), radar (Fig.  3 e) and high-resolution UAV digital elevation model data (Fig.  2 b) are available via reasonable request to the corresponding authors. All other digital elevation model data used in this study are available at: https://tinitaly.pi.ingv.it/ . The EUMETSAT Meteosat Volcanic Ash RGB—MSG—0 degree product derived from SEVIRI data is freely available, for research and education use, from the EUMETSAT data portal ( https://view.eumetsat.int/productviewer?v=default ) after user registration. The EUMETSAT data used in this study were obtained by SDA under EUMETSAT licenses 99999992, 99999993, 99999997, 99999999.

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Acknowledgements

SDA was supported by by NERC Grant NE/W004771/1 and project SINFONIA, progetto Bando Ricerca Libera 2021- Delibera 214/2021-INGV. LZ was supported by project SINFONIA, progetto Bando Ricerca Libera 2021—Delibera 214/2021-INGV and by the INGV-MIUR project Pianeta Dinamico—“Working Earth”—Sub-project VT—DYNAMO—2023. We thank all INGV-OE technicians who are involved in the maintenance of the thermal IR and visibile cameras operated by INGV-OE.

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Luciano Zuccarello

Present address: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via Cesare Battisti, 53, 56125, Pisa, Italy

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School of Environmental Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, UK

Silvio De Angelis & Luciano Zuccarello

Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via Cesare Battisti, 53, 56125, Pisa, Italy

Silvio De Angelis

Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania - Osservatorio Etneo, Piazza Roma, 2, 95125, Catania, Italy

Simona Scollo

Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Bologna, Italy

Luigi Mereu

CETEMPS Center of Excellence, University of L’Aquila, L’Aquila, Italy

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S.D.A. and L.Z. wrote the proposal that funded installation and maintenance of the infrasound array, designed the field experiment and installed all equipment. S.D.A. and L.Z. jointly designed the study, performed analyses of infrasound data, numerical modelling of acoustic wavefield and plume rise, and prepared all figures. S.S. and L.M. analysed thermal I.R. and radar data. S.D.A. wrote the initial draft of the manuscript and all authors contributed to the final version.

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De Angelis, S., Zuccarello, L., Scollo, S. et al. Assessment of eruption source parameters using infrasound and plume modelling: a case study from the 2021 eruption of Mt. Etna, Italy. Sci Rep 13 , 19857 (2023). https://doi.org/10.1038/s41598-023-46160-6

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Mount Etna Puffs Perfect Smoke Rings Into Sicilian Sky

The volcano, one of Europe’s most active, has been spewing volcanic vortex rings at a rapid pace. But that doesn’t mean a big explosion is on the way, one expert says.

By Elisabetta Povoledo

Reported from Rome

For just over a week, Mount Etna, one of Europe’s most active volcanoes, has been spewing circular, mostly white smoke rings into the skies over Sicily.

It’s not the first time Mount Etna has enchanted onlookers with its puffing (it’s been dubbed the Gandalf of volcanoes , after the pipe-puffing wizard in “Lord of the Rings.”) But experts there say this month Etna “has broken all previous records” with the frequency of the rings, according to Boris Behncke, a volcanologist at the National Institute of Geophysics and Volcanology of Catania, who posted about the phenomenon on Facebook .

The rings, known as volcanic vortex rings, appeared earlier this month after a small vent opened on the northwest border of the Southeast crater. The phenomenon occurs when enough pressure builds up so that magma inside the crater propels condensed gases, predominantly water vapor, through the vent.

In this case, the vent is perfectly circular, making for particularly perfect rings. “It is bellissimo,” said Simona Scollo, another volcanologist at the INGV Etna Observatory in Catania, using the Italian word for beautiful. Ms. Scollo copublished a study on the dynamics of volcanic vortex rings last year in the journal Scientific Reports .

But, she said, the activity does not mean that Mount Etna is going to erupt in a particularly spectacular way. “No, no, no,” she said.

During a telephone interview Tuesday, she said that the mechanism for the smoke rings was similar to how dolphins blow bubble rings . “They compress the water in their mouths, and using their tongue they push it out of their mouths and create such a pressure that it forms a ring,” she said.

Depending on weather conditions, the rings hang in the air anywhere from one to 10 minutes, according to the study. “If there is turbulence they fall apart more quickly,” she said.

Interviewed in The New York Times last year, Ms. Scollo said the study hoped to better understand how volcanoes functioned, “not only when they create a disaster for people or when they are very dangerous,” but in calmer times, too.

The new vent in the volcano has been spewing hundreds of rings, but another opening on the volcano has also been spewing rings, albeit more spaced out, since last year.

The phenomenon was first recorded on Etna in 1724, followed by periodic sightings, most recently last year , and quite spectacularly in 2000.

According to the description for its 2013 inscription as a UNESCO World Heritage site , Mount Etna is “the highest Mediterranean island mountain and the most active stratovolcano in the world.”

“No volcano on earth produces as many volcanic vortex rings as Etna, we knew this for some time,” noted Mr. Behncke.

Mount Etna isn’t the only volcano where the phenomenon has been registered. Volcanic vortex rings have been puffed by a number of volcanoes worldwide, from Momotombo in Nicaragua to ash-spewing plane stopper Eyjafjallajökul in Iceland, to Mount Redoubt in Alaska to another active Italian volcano, Stromboli .

Ms. Scollo said that the activity from the new vent was slowing down. And it could end altogether.

“It can stop because the properties of the conduit that allowed for the formation of these volcanic vortex rings can change, maybe with obstructions,” she said. Or the amount of gas within the conduit could decrease, she added.

Elisabetta Povoledo is a reporter based in Rome, covering Italy, the Vatican and the culture of the region. She has been a journalist for 35 years. More about Elisabetta Povoledo

How to Hike Mt. Etna, an Active Volcano in Italy

M ount Etna loomed large over everything as we arrived in eastern Sicily that early January. From miles away, I could see the steam plumes rising from the snow-topped behemoth, like blown-out candles on a hastily iced birthday cake. Appropriately, I was on a milestone birthday road trip around Sicily, from Palermo to Catania. The second-to-last stop: Mount Etna, Europe's most active volcano and also its tallest, at 10,900 feet. Scientists believe Etna has been active for more than 500,000 years, presiding over Sicily long before humans arrived.

I'm no stranger to volcano hikes. I've summited several active stratovolcanoes (composite volcanoes) in my lifetime, from New Zealand's Tongariro to Nicaragua's Concepción. Italy is the only country on mainland Europe with active volcanoes, thanks largely to its location near two tectonic plates. Vesuvius, the most notorious among them, engulfed and preserved the ancient cities of Pompeii and Herculaneum after a violent eruption in 79 A.D. Stromboli has had regular minor eruptions for thousands of years. While Mount Etna hasn't had a catastrophic eruption since an infamous incident in 1669, its many eruptions in 2023 filled the news with jaw-dropping photos of lava vaulting into the atmosphere. When I thought of Sicily, I thought of turquoise-colored waters, stunning cathedrals, small villages, ancient ruins and, of course, the wine and food (pasta alla Norma in particular). I hadn't pictured an almost-constantly gurgling volcano that locals ski down in winter.

Now, one might ask, why voluntarily climb something that could blow at any minute? It's a perfectly reasonable question, and it sits at the smoldering heart of volcano tourism, which has exploded (no pun intended) in recent years as eruptions have captured the world-Kīlauea in Hawai'i and Fagradalsfjall in Iceland, for example-even as the volcano blowouts damage or disrupt local economies.

Volcano tourism brings thousands of visitors to Sicily each year. Ten municipalities surround Mount Etna, with several villages of 5,000 residents or fewer who depend heavily on tourism from volcano visitors. Since its 2013 designation as a UNESCO World Heritage Site, Etna has been one of Sicily's top-growing attractions. The Parco dell'Etna has taken steps to keep the crowds in check to maintain the volcano's integrity: specifically, limited parking, steep access fees, and the strong recommendation to take guided visits. The cable car to the start of the trails (8,202 feet) is 50 euros per person. Navigating Etna's unmarked paths and frequently shifting landscape requires the expertise of volcano guides. Visitors are permitted to hike up to about 9,000 feet alone, after which a guide is necessary, for both safety and environmental reasons.

A mountain of many moods

On the morning of our hike, my rental car begrudgingly changed gears on the snake-like winding drive from the town of Nicolosi to Rifugio Sapienza (6,266 feet above sea level) in Parco dell'Etna. As we approached the southern slope of the volcano along a twisting road, we spotted the glistening sea, villas with citrus trees poking over the gates, and small family vineyards making use of the area's fertile volcanic soil.

After acquiring gourmet deli sandwiches for our future lunch break at the meeting point (it's Italy, after all), we took the cable car up to meet our volcano guide, Vincenzo Greco . Vincenzo is a local volcanologist and second-generation Etna guide. He studied geology at the University of Catania and became the youngest person ever to achieve certification as a volcanology guide in Italy.

We started our ascent-excited to work off days of Sicilian cannoli-and trekked through the Valle del Bove. Winter-tanned Vincenzo introduced us to the volcano. He explained which eruption created which new landscape, noting how old this new rock ledge was or how this patch of ground has changed in the past decade. Six weeks before my arrival, in November, a new vent opened on the southeast crater at nearly 9,200 feet. A second vent opened three days later, effusing the new lava . In December, the lava flows reached Valle del Leone and the greater Valle del Bove, depressions on the eastern slope, creating a lava flow field and burping up occasional ash amid the steam and gasses. The lava flows would stop in February, once again reworking the landscape of Etna.

Everything here changes. I recalled the homes we passed on the way in, squarely in the line of Etna's fire if she ever got angry enough. With the windy road and altitude, there could be no escape if something happened. You really have to love the volcano, Vincenzo said, or at least love living by it or perhaps just be a bit stubborn.

Vincenzo and Etna have a passionate Italian relationship. He loves Etna, but their quarrels are, well . . . volcanic. He studies her moods and knows her mannerisms. He flew a drone over an eruption once and her heat melted his camera. It was an expensive mistake, but the few photos he got were spectacular.

About half an hour into the hike, Vincenzo ducked toward a large crevasse between two rocks. "Feel this!" he exclaimed excitedly. "But be careful." I inched forward, waving my hand around in the air aimlessly until a burst of boiling heat hit my skin. It was a small steam vent-Etna saying hello. The power of the invisible heat surprised me. It was a reminder of how easy it is to fall into a false sense of security up here. This isn't your average molehill. Thanks to unmarked paths and a disorienting amount of regular landscape shifts, Vincenzo and his fellow guides have had to rescue tourists when Etna gets moody.

Our hiking group turned a corner and came face-to-face with a lava tunnel formed by a previous eruption. We smushed into the narrowing pathway one at a time. The rock walls towered overhead on both sides, blocking out the sun above. Following the same path as a recent river of molten red lava was a stark reminder of Etna's power.

A crunchy layer of snow covered the black lava rocks throughout the journey, turning the scene into a black-and-white photo anytime the clouds rolled in and covered the bluebird sky. Typically, this time of year, hikers might need snowshoes or cross-country skis in addition to hiking boots. Instead, it was unseasonably warm across Sicily. We sat down for lunch as a crater pumped out billowing white puffs in the distance, our jackets off, the sun blazing. Up next, the final stop: one of Etna's four main craters, at more than 10,000 feet above sea level.

In some parts, the crater rim narrowed to the width of two footsteps side by side, on loose lava pebbles. The winds whipped the smell of sulfur away. I could see the clouds off in the distance, a white blanket over the valley. Above me, the sun beat down from a bright blue sky. The black lava rocks absorbed the light, but the white snow was blinding. For a brief moment, I grew disoriented. I felt like I could walk out onto the blanket of clouds, thousands of feet above sea level. Maybe it was the combination of altitude, sulfur, and adrenaline, but I felt briefly euphoric in this otherworldly atmosphere. In that moment, I realized Etna's pull. I brought myself back to earth by looking down into the crater, the bottom of which was so deep it wasn't visible. Halfway around the crater, a dizzyingly steep drop appeared. With a flourish, Vincenzo appeared to hop off the ledge. Was my guide supposed to jump off? Would I be stuck in Etna's clutches forever?

The descent was arguably more challenging than the climb. Vincenzo took off, hopping down the steep drop of loose lava pebbles. Others followed suit. Unwilling to body surf down thousands of jagged little rocks, I attempted to switchback. My boots sank deep into the ground with each sideways step, as if I had attempted to walk across a ball pit. I felt my shoes fill with lava rocks. Time to throw caution to the wind. Hiking poles in hand, I "skied" down Mount Etna, sending mini avalanches of lava rocks flying with each skip. The lava rocks felt springy at that speed.

Once we arrived at the cable car, the world changed. We had come back down through the clouds, and everything was gray. The seaside views were gone, covered by fog. I tried to catch one last glimpse of Etna from below. She was gone, off in her own world.

Know before you go

Getting there.

Sicily has two major airports: Palermo Airport (Falcone Borsellino Airport) on the west side and Catania Airport (Vincenzo Bellini Airport) on the east. Catania is about an hour from Mount Etna and Palermo about three.

Where to stay

The town of Nicolosi is the gateway to the southern entrance of Mount Etna.

Hotel Alle Pendici is a nine-room B&B dotted with framed photos of Mount Etna throughout the seasons. The rooms are basic but cozy with a ski chalet ambience.

Blanc Maison Etna is a five-room B&B next to a beautifully manicured park. The rooms have views of the pool, the garden, or the volcano.

Tour operators we love

The Society of Guides Vulcanologiche Etna Nord offers a variety of tours with licensed volcano guides, including ours, Vincenzo.