# The February–April 2021 sequence of lava fountain paroxysms on Mt Etna: source mechanism deciphered from ground-based and satellite survey of volcanic gas emissions

Giuseppe Salerno, Alessandro La Spina, Patrick Allard, Lorenzo Guerrieri, Stefano Corradini, Giuseppe Di Grazia, Luca Merucci, Pietro Bonfanti, Dario Stelitano, Roberto Maugeri, et al.

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Giuseppe Salerno, Alessandro La Spina, Patrick Allard, Lorenzo Guerrieri, Stefano Corradini, et al. The February–April 2021 sequence of lava fountain paroxysms on Mt Etna: source mechanism deciphered from ground-based and satellite survey of volcanic gas emissions. Bulletin of Volcanology, 2025, 87 (11), pp.95. 10.1007/s00445-025-01877-2. hal-05514924

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# The February–April 2021 sequence of lava fountain paroxysms on Mt Etna: source mechanism deciphered from ground-based and satellite survey of volcanic gas emissions

Giuseppe Salerno<fnref n="1" /> · Alessandro La Spina<fnref n="1" /> · Patrick Allard<fnref n="1" />,<fnref n="2" /> · Lorenzo Guerrieri<fnref n="3" /> · Stefano Corradini<fnref n="3" /> ·

Giuseppe Di Grazia<fnref n="1" /> · Luca Merucci<fnref n="3" /> · Pietro Bonfanti<fnref n="1" /> · Dario Stelitano<fnref n="3" /> · Roberto Maugeri<fnref n="1" /> · Filippo Murè<fnref n="1" /> ·

Paolo Principato<fnref n="1" />

Received: 11 August 2024 / Accepted: 3 September 2025 / Published online: 7 October 2025

© The Author(s) 2025

## Abstract

Between 16 February and 1 April 2021, the Southeast Crater (SEC) of Mt Etna produced a spectacular sequence of 17 lava fountain paroxysms, separated by 2.5 days on average, which generated ≥ 10 km high eruptive columns and heavy tephra fallout over populated surroundings. We examine the magmatic processes responsible for these events based on pre- and syn-eruptive data for the mass flux and chemistry of Etna gas emissions, surveyed both from the ground (scanning DOAS, OP-FTIR spectroscopy) and from space (TROPOMI, SEVIRI), and comparing with the seismic tremor. Bulk plume SO₂ emission rates determined from the ground and from space are consistent with one another. We show that after several months of background summit activity, sustained since June 2020 by open-system degassing of ~29×10⁶ m<fnref n="3" /> (DRE) of magma through the central volcano conduits, an influx of deeply derived primitive magma led to a pressure build-up phase from early December 2020 to 13 February 2021, marked by a rapid increase in the SO₂ flux and tremor (the former interpreted to represent an ~3 times higher magma degassing rate) and decreasing SO₂/HCl plume ratio. A series of 17 lava fountains began immediately after a shallow seismic cluster and a sharp drop in the SO₂ emission rate from the summit craters, reflecting the lateral transfer of pressurized primitive magma to beneath the SEC. The fountain paroxysms were characterized by sharp increases in tremor amplitude, intense SO₂ release, and higher volcanic gas SO₂/HCl ratios. The magnitude of SO₂ emission rate correlates with the proportion of primitive magma in co-erupted products during the first half of the sequence. The estimated total gas discharge, compared to the co-erupted tephra mass, suggests a large excess gas release for most events, which is proportional to the length of the repose interval. Combining these observations with models of S and Cl degassing from Etna trachybasalt, we infer that the February–April 2021 lava fountain series resulted from the recurrent accumulation of H₂O-CO₂-rich bubble foams at ~2–3 km depth beneath SEC, whose periodic collapse promoted fast magma ascent and fragmentation associated with essentially syn-eruptive degassing of S and Cl. Our study thus provides further insight into the complexity of magmatic processes determining lava fountain paroxysms at Mt Etna and, possibly, other basaltic volcanoes.

**Keywords** Mt Etna · Lava fountain · SO₂ flux · TROPOMI · SEVIRI · OP-FTIR · Volcano monitoring

## Introduction

Basaltic volcanoes, though fed by low viscosity magma of relatively monotonous composition, can display a wide range of eruptive manifestations, from purely effusive to highly explosive, as well as fast transitions in eruptive style during a single event (e.g. Sigurdsson et al. 2015). These transitions are believed to result from a complex interplay between temporal changes in the magma properties (amount and depth of storage, volatile and crystal content, viscosity and density), the dynamics of magma decompression and

<figure><img src="image_2.png" /></figure>

<footnote>Giuseppe Salerno
giuseppe.salerno@ingv.it</footnote>

<footnote n="1">¹ Istituto Nazionale Di Geofisica E Vulcanologia, Osservatorio
Etneo, Piazza Roma 2, Catania 95125, Italy</footnote>

<footnote n="2">² Institut de Physique du Globe de Paris, Université Paris Cité,
1 Rue Jussieu, Paris 75005, France</footnote>

<footnote n="3">³ Istituto Nazionale Di Geofisica E Vulcanologia, Osservatorio
Nazionale Terremoti, Via Di Vigna Murata, 605,
Rome 00143, Italy</footnote>
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>
◄Fig. 1 Map of Mount Etna showing its location in Sicily (bottom left) and a topographic zoom of its four summit craters (Voragine: VOR, Bocca Nuova: BN, North-East Crater: NEC, and South-East Crater: SEC; upper left). The map details the respective locations of the FLAME scanning DOAS stations (blue-solid circles), the ECPN (Etna Cratere del Piano) seismic station whose tremor recording is used in our study (red-solid triangle), and the sites from which SO₂/HCl ratios in the bulk plume emissions (yellow area) and during lava fountain degassing (purple star) were measured using OP-FTIR spectroscopy. a One typical lava fountain at SEC in February–April 2021; the photo was taken from Linguaglossa during the 16 February 2021 fountaining event (V. Greco—Guida Vulcanologica dell'Etna). b Both the height above Etna and dispersal of a fountain eruptive column during the 4 March 2021 paroxysm (F. Zuccarello)

degassing (determining the extent of pre-eruptive gas–melt separation), and the feeding system geometry (e.g. Head and Wilson 1989; Sparks 2003; Spilliaert et al. 2006a). Gas bubbles formed through the exsolution of magmatic volatiles (H₂O, CO₂, S, Cl, F) exert a crucial influence on both the rheological properties of the magma and eruptive potential and style through their degree of coupling with the melt upon decompression (e.g. Jaupart and Vergniolle 1988; Sparks 2003; Wallace et al. 2015). Gas bubbles can ascend differentially through low viscosity melts and either accumulate as a foam at structural discontinuities or rise directly towards the surface (e.g. Sparks 2003; Jaupart and Vergniolle 1988; Menand and Phillip 2007; Gonnerman and Manga 2007). At the same time, their chemical composition dramatically evolves during decompression, owing to the contrasted pressure-dependent solubility behaviour of the volatile species (e.g. Giggenbach 1987; Baker and Aletti 2012; Edmonds and Wallace 2017). Therefore, sensitive information about the magmatic processes controlling temporal changes in eruptive style on basaltic volcanoes can be retrieved by surveying the chemistry and mass flux of gas emissions, in complement to geophysical and petrological monitoring.

Mount Etna, in Sicily, is a very active 3.4 km high basal-
tic strato-volcano with recurrent eruptions superimposed on
continuous open-conduit summit crater degassing. Its erup-
tion record over the past 3500 years (Branca and Del Carlo
2005) and in recent decades (e.g. Allard et al. 2006) docu-
ments a wide spectrum of eruptive manifestations, ranging
from long-lasting lava effusions, mild-explosive Strombolian
activity (e.g. Patanè et al. 2013), and powerful lava fountains
(e.g. Allard et al. 2006; Andronico et al. 2021; Bonnacorso
et al. 2021b) to scarcer sub-Plinian and Plinian eruptions
(Coltelli et al. 1998). Given its intense activity, relatively
easy access, and continuous monitoring with multiparamet-
ric networks, Mt Etna is thus a unique natural laboratory
in which to study the magmatic processes determining the
diversity in eruptive styles at basaltic volcanoes. In addition
to seismic and geodetic monitoring, its SO₂ emissions have
long been surveyed (e.g. Allard et al. 1991; Caltabiano et al.

2004; Salerno et al. 2009b, 2018). These data have revealed precursory gas signals of impending eruptions (e.g. Malinconico 1979; Caltabiano et al. 2004; Allard et al. 2006; Patane et al. 2008), as well as close temporal correlations between the intensities of magma degassing and seismic tremor, tracking the dynamics of turbulent gas-magma flow in the volcano conduits (e.g. Salerno et al. 2018). Ranking amongst the strongest persistent volcanic gas emitters worldwide (e.g. Allard 1997; Aiuppa et al. 2008; Carn et al. 2017), Mt Etna is fed by alkali basalts-trachybasalts that are highly enriched in H₂O, CO₂, S, and Cl whose initial contents and pressure-related solubility behaviour are well established (Kamenetsky and Clocchiatti 1996; Métrich et al. 2004; Spilliaert et al. 2006b; Corsaro et al. 2009; Lesne et al. 2011).

Over the past four decades, Mt Etna has produced sev-
eral series of impressive lava fountains at its summit craters,
principally the South-East Crater (SEC, Fig. 1). These lava
fountains, consisting of pulsed and/or sustained jetting of
gases and lava clots, typically last from a few tens of minutes
to a few hours and often reach several hundreds of metres
(up to ≥ 1 km) in height. The lava fountains commonly gen-
erate 8 to 15 km high convective ash-gas plumes resulting
in heavy tephra fallout over the inhabited surroundings and
temporary air traffic closure (e.g. Guffanti et al. 2010; Sell-
itto et al. 2023). Based on a combination of field observa-
tions, physical modelling, and laboratory experiments, two
distinct models have been proposed for the triggering mech-
anism of lava fountains: (i) the rise speed model (Parfitt &
Wilson 1995; Parfitt 2004), in which a lava fountain results
from the syn-eruptive (closed system) degassing and frag-
mentation of magma decompressing at high rate in a con-
duit; or (ii) the foam collapse model (e.g. Jaupart & Vergn-
iolle 1988), in which a fountain is driven by the sudden or
gradual collapse of a coalesced bubble foam accumulating at
the roof of a reservoir or a conduit discontinuity. In this latter
mechanism, open system degassing prevails, allowing gas
segregation and accumulation in the pre-eruptive stage, then
a gas-dominated discharge during lava fountain as the foam
is emptying. These two models were concurrently shown to
be able to explain the dynamics of lava fountains at Kilauea
Volcano (Hawaii, USA; Parfitt and Wilson 1995; Vergniolle
and Jaupart 1990). At Etna, the foam collapse model was
demonstrated to closely account for the peculiar chemical
composition of the magmatic gas phase driving two series of
recurrent lava fountains that occurred at the SEC in 2000 and
2001, measured using open-path Fourier transform infrared
(OP-FTIR) spectroscopy (Allard et al. 2005; La Spina et al.
2015). The modelled results were consistent with the peri-
odic emptying of a CO₂-rich bubble foam layer accumulated
on top of a shallow reservoir emplaced at about 1.5–2 km
depth below SEC (Allard et al. 2005; La Spina et al. 2015).
This same mechanism was advocated to explain further
lava fountaining events at SEC in 2006–2018, based on associated ground deformation (Bonnacorso et al. 2021b), gravimetric variations (Carbone et al. 2015), magma ascent rate (e.g. Calvari and Nunnari 2022), and the petrology and vesicularity of ejecta (e.g. Polacci et al. 2006, 2019; Andronico and Corsaro 2011). However, careful examination of the vesicularity and 3D structure of some ejecta suggested that a lava fountain triggered by bubble foam collapse could also involve syn-eruptive melt degassing during its climax (e.g. Polacci et al 2006), which means that both open and closed system degassing may contribute to some extent in driving a lava fountain.

Here, we focus on another spectacular sequence of 17 paroxysmal lava fountains that occurred at the SE crater of Etna between 16 February and 1 April 2021. These events were well documented for their dynamics (Andronico et al. 2021; Calvari and Nunnari 2022) and energy (Marchese et al. 2021), their tephra and lava flow budget (Andronico et al. 2021; Proietti et al. 2023), the petrology-geochemistry of their products (e.g. Corsaro and Miraglia 2022, 2024; Mollo et al. 2022), and their associated geophysical signals (Bonaccorso et al. 2021a; De Gori et al. 2021; Bruno et al. 2022). Aiuppa et al. (2023) published a first study of SO₂ emissions during this period, but their use of one single field SO₂ camera, positioned on the south flank, prevented capture of the entire volcanic plume during passive degassing and did not allow quantification of SO₂ emission rate during the fountaining events owing to dense ash loads. Here, we report on a more comprehensive data set for (i) the SO₂ emission rate and SO₂/HCl ratio (and hence HCl flux) in the bulk volcanic plume during the period from June 2020 to April 2021, i.e. before and during the lava fountain sequence, and (ii) the SO₂ budget and SO₂/HCl ratio of single lava fountaining events. Bulk plume SO₂ emission rates were retrieved both from the ground and from space, using a DOAS monitoring network covering the whole volcano (Salerno et al. 2009b) and TROPOMI satellite sensing (Theys et al. 2017), respectively. The SO₂ mass in the co-eruptive gas plumes produced by the lava fountains was retrieved from SEVIRI satellite imaging (Guerrieri et al. 2023). Finally, SO₂/HCl ratios of volcanic gases during both passive and eruptive degassing were measured using OP-FTIR spectroscopy. Temporal trends in gas composition and flux are compared with the co-variations of seismic tremor. From the bulk plume SO₂ emission rates and assumptions regarding the melt sulphur content, we derive the bulk amounts of magma that were degassing to produce the SO₂ during both pre-and syn-eruptive phases. From the SO₂ budget of each fountain paroxysm and the composition of previous lava fountain gas at SEC, we assess the total gas discharge and then the gas/magma mass ratio for each fountaining event. The equilibration depth of the gas phase driving the fountains is estimated from the measured SO₂/HCl ratios and pressure-related

solubility data for S and Cl in Etna trachybasalt (Spilliaert et al. 2006b; Lesne et al. 2011). Based on this multidisciplinary approach, we demonstrate that the February–April 2021 sequence of paroxysmal lava fountains resulted from increasing pressurization of the shallow feeding system of Etna by a fast uprising batch of primitive magma that initially degassed in open system through the central volcano conduits then erupted and degassed in pulsed mode through periodic bubble foam growth and collapse in a shallow reservoir beneath SEC. Our study provides further insight into the pre-eruptive and syn-eruptive magmatic processes, leading to paroxysmal basaltic lava fountains on Mt Etna, which may apply to other basaltic volcanoes.

## Volcanic activity framework

Over the last two decades, Mt Etna has displayed intense eruptive activity at both its summit craters and flank fissure vents. Between 2011 and 2018, a period of vigorous explosive activity included more than 40 lava fountains, which took place principally at SEC but also at the central Voragine Crater (e.g. Cannata et al. 2018). Periods of quiescence were episodically replaced by effusive eruptions from fissures opening at about 3000 m elevation in 2015 and 2018. Thereafter, Mt Etna remained in a period of quiescence until the second half of 2019, when the volcanic edifice started to inflate again and eruptive activity resumed at the summit craters (De Beni et al. 2021). This inflation, which was associated with enhanced volcano-tectonic seismicity at mid-crustal depth (4–9 km bsl) beginning during the summer of 2020, was interpreted to represent replenishment and gradual pressurization of Etna’s feeding system by deeper-derived, volatile-rich magma (Bruno et al. 2022; De Gori et al. 2021). The primitive (more mafic) composition of this magma was subsequently verified through petrological analysis of erupted products (Corsaro and Miraglia 2022; Corsaro et al. 2024). The volcano’s deformation pattern was episodically interrupted by brief deflations, associated with periods of enhanced eruptive activity at the summit craters (Fig. 1) in May–June 2019, July 2019, December 2019, then April–May 2020 (Bruno et al. 2022). This eruptive activity consisted of mild Strombolian outbursts at SEC and NEC craters and intermittent ash emissions from the Voragine (VOR) and Bocca Nuova (BN) craters (Fig. 1).

In December 2020, three main seismic swarms occurred at different depths beneath Etna: a first one (local magnitude—ML 1.0–2.3) on 1 December at 1.3–4 km depth bsl beneath the central part of the volcano, a second one (ML 1.6–2.9) on 6 December at ≥ 16 km bsl (below sea level) beneath the western and north-eastern flanks, then a third one (ML 1.3–2.6) on 9–12 December at intermediate depths of 8 to 2 km bsl. These closely spaced swarms occurring

<figure><img src="image_7.png" /></figure>
within a large depth range indicated a rapid ascent of mafic magma (Bonaccorso et al. 2021; Bruno et al. 2021; De Gori et al. 2021). While intra-crater explosive and effusive activity developed at VOR, eruptive activity at SEC escalated into four discrete lava fountain events on 13 December, 21–22 December, then 18 January 2021 (Bonaccorso et al. 2021). Three of these events exhibited a ‘transitional’ character between pulsed Strombolian outbursts and sustained lava fountains (Andronico et al. 2021). Then, soon after a shallow cluster of energetic (ML 2.4–3.1) volcano-tectonic earthquakes that occurred at 2.6 km depth (700 m above sea level) beneath the summit area on 11–13 February, a major eruptive phase began. Starting on 16 February and continuing until 1 April 2021, a sequence of 17 powerful lava fountain episodes took place at SEC. Each episode was separated by brief repose intervals (2.5 ± 1.6 days on average) and reached heights of 240 to 1375 m (Calvari and Nunnari 2022). Each fountain was heralded by increasing volcanic tremor, growing Strombolian activity, and lava effusion and was repeatedly associated with sharp negative strain (decompression) signals (Bonnaccorso et al. 2021). This paroxysmal eruptive sequence at SEC was immediately associated with a rapid deflation of the entire volcanic pile that continued until 19–24 March (Bruno et al. 2021). Then, after 1.5 months of relative quiescence, another series of 34 lava fountains occurred at SEC between 19 May and 23 October 2021 (Calvari and Nunnari 2022). Here, we present data relating to volcanic gas emissions during the pre- and syn-eruptive stages of the first paroxysmal sequence between 16 February and 1st April 2021.

## Methods

### Ground-based survey of SO₂ plume fluxes

Ground-based monitoring of Etna's bulk SO₂ plume flux is carried out using the FLAME network of scanning DOAS

spectrometers (Salerno et al. 2009b; 2018). Nine scanner stations are installed on the volcano flanks at a mean elevation of 800 m asl and mean distance of 10 km from the summit craters (Fig. 1). SO₂ column amounts are retrieved from the open-path ultraviolet spectra using the DOAS method and (e.g. Platt and Stutz 2008) applying a modelled background reference spectrum (Salerno et al. 2009a). Total SO₂ amounts in a cross section of the volcanic plume are then converted into mass fluxes by scaling to the plume (wind) speed. The calibrated uncertainty on SO₂ emission rates derived from our FLAME network, mostly due to uncertainties on the wind speed at the altitude of the plume, typically ranges between -22 and +36% (Salerno et al. 2009b). However, error may occasionally be much higher during ash-rich eruptive phases, which generally leads to lower radiative transfer and underestimated SO₂ flux (e.g. Andres and Schmid 2001; Campion et al. 2015). We further emphasize that 76% of the 2020–2021 lava fountains investigated in this study occurred between late afternoon and early morning, therefore preventing SO₂ flux quantification with the DOAS FLAME array which only operates during daylight time by using absorption of the ultraviolet radiation across the plume. This limitation was overcome by using SEVIRI satellite data to quantify SO₂ emissions during the lava fountains (see below). All SO₂ fluxes are reported in ton per day (t/d; Table 1) and times are Greenwich Mean Time (GMT).

### Satellite sensing of SO₂ plume fluxes and SO₂ eruptive budgets

Space-based surveys of Etna's bulk plume SO₂ fluxes during the overall 2020–2021 pre- and post-eruptive periods were carried out via the TROPOspheric Monitoring Instrument (TROPOMI) sensor onboard of the Sentinel-5P (S5P) polar satellite (Theys et al. 2017). TROPOMI is a hyperspectral instrument that covers a spectral range from UV to Short Wave InfraRed (SWIR), has a spatial resolution of 3.3 × 3.3 km² at nadir (~4 × 4 km²), and a revisit time of 1 day at

<table><caption>**Table 1** Time-averaged values of the bulk SO₂ plume flux, measured both from the ground (FLAME network) and from space (TROPOMI), of the bulk plume SO₂/HCl ratio and HCl flux, and of volcanic tremor RMS (m/s) during the three distinct periods of Mt Etna's activity identified between 1st June 2020 and 1st April 2021 (see text)</caption><thead><tr><th>Period</th><th>Dates</th><th>Number of days</th><th>Statistics</th><th>Bulk plume SO<sub>2</sub> flux FLAME (t/d)</th><th>Bulk plume SO<sub>2</sub> flux TROPOMI (t/d)</th><th>Bulk plume SO<sub>2</sub>/HCl molar ratio</th><th>Bulk plume HCl flux (t/d)</th><th>Volcanic tremor (m/s)</th></tr></thead><tbody><tr><td rowspan="2">1st</td><td>1 June 2020</td><td rowspan="2">187</td><td>Mean</td><td>1800</td><td>2000</td><td>3.6</td><td>280</td><td>3.7 × 10<sup>-6</sup></td></tr><tr><td>5 Dec 2020</td><td>St. dev</td><td>700</td><td>1050</td><td>0.7</td><td>100</td><td>1.0 × 10<sup>-6</sup></td></tr><tr><td rowspan="2">2nd</td><td>5 Dec 2020</td><td rowspan="2">70</td><td>Mean</td><td>5100</td><td>3400</td><td>3.3</td><td>780</td><td>2.5 × 10<sup>-6</sup></td></tr><tr><td>13 Feb 2021</td><td>St. dev</td><td>2900</td><td>2400</td><td>0.6</td><td>120</td><td>3.2 × 10<sup>-6</sup></td></tr><tr><td rowspan="2">3rd</td><td>13 Feb 2021</td><td rowspan="2">47</td><td>Mean</td><td>2400</td><td>1600</td><td>3.3</td><td>410</td><td>4.1 × 10<sup>-6</sup></td></tr><tr><td>1 Apr 2021</td><td>St. dev</td><td>1100</td><td>1140</td><td>0.8</td><td>150</td><td>6.9 × 10<sup>-6</sup></td></tr></tbody></table>
Etna's latitude. A special feature of the TROPOMI sensor is its high sensitivity, about 30 times higher than that of multispectral satellite sensors, which allows it to detect SO₂ columnar abundance of only 0.02 g/m² (0.7 DU; Corradini et al. 2021). Here, we considered the TROPOMI sulphur dioxide level 2 Near Real Time products (NRT; Theys et al. 2017). SO₂ column density was obtained by interpolating at 3 km asl the two standard 'sulphur dioxide total vertical column' products at 0.5 km asl and 7 km asl. To discard the noise and what is not volcanic SO₂, only the pixels with values greater than 1 DU and contained within a specific 'plume mask' (drawn manually for each image) were considered. Knowing the SO₂ columnar abundances, SO₂ fluxes were obtained by applying the traverse procedure, based on the definition of multiple circular transects around the volcano and multiplying the integrated cross-section by the wind speed at the height of the plume itself (e.g. Pugnaghi et al. 2006; Bluth et al. 2008, Corradini et al. 2021). This makes it possible to obtain an SO₂ flux trend for several hours before the satellite image acquisition. Here, we computed Etna's SO₂ flux over the 6-h average prior to satellite image acquisition in order to minimize the influence of gas dilution in the atmosphere. All wind speeds were obtained from the 6-hourly NCEP datasets (Kalnay et al. 1996) at Etna position (37.5 N, 15.0 E) interpolated at 3 km asl.

In order to determine the syn-eruptive SO₂ discharge of each fountaining paroxysm, we used satellite data recorded in the thermal infrared domain by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI). The SEVIRI sensor, on board the Meteosat Second Generation (MSG) geostationary satellite, is a multispectral instrument with 12 channels ranging from visible (VIS) to thermal infrared (TIR). It has a spatial resolution of 3×3 km² at nadir (about 4×4 km² at Etna coordinates) and a temporal resolution ranging from 5 (Rapid Scan) to 15 min (Full Disk). Compared to TROPOMI, the SEVIRI instrument has a lower sensitivity but a much higher temporal resolution (Corradini et al 2021) and, therefore, was much more appropriate to quantify the SO₂ budget of Etna's short-lived fountaining events.

SEVIRI data have been inverted in SO₂ columnar abundance using the Look Up Table procedure (LUTp, e.g. Corradini et al. 2009). In this case, the LUTp-SO₂ emitted mass for each of the 21 studied lava fountains were obtained from Guerrieri et al. 2023 (Table 2). The uncertainties on SEVIRI and TROPOMI SO₂ column abundance respectively average ~40% and ~35% (Corradini et al. 2021).

## OP-FTIR sensing of SO₂/HCl ratios

The chemical composition of Etna's gas emissions is routinely surveyed by using an open-path Fourier transform infrared (OP-FTIR) MIDAC M4406 spectrometer. The instrument has a field of view of 20 mrad, a 9-micron cut-off,

and is equipped with a Sterling cooler Mercury Cadmium Telluride (MCT) detector sensitive between 800 and 5000 cm⁻¹. Infrared spectra were collected at a resolution of 0.5 cm⁻¹ with the Essential FTIR software. The molecular column amount of each gas species in absorption spectra was retrieved by using the custom-made FTIR_FIT tool, the HITRAN2020 spectral database (Gordon et al. 2022) and following previously described procedures for spectra processing (e.g. Allard et al. 2005; Burton et al. 2007; La Spina 2010; La Spina et al. 2015). The standard error on measured molar gas amounts averages about 5% for most of the species. Gas ratios were derived from the linear regression of gas amounts in scatter plots.

Absorption spectra of the bulk plume chemistry were collected passively by using the solar occultation method (e.g. Francis et al. 1998). At least 100 spectra were routinely recorded about three times a week after driving the instrument to a location where we could observe the volcanic plume in front of the sun and using a mobile external mirror to track the sun's motion. Most of the measurement sites were located at an altitude of ~1000 m and at distances of 10–16 km from the volcano summit (Fig. 1). These measurements allowed us to quantify the molar abundances and chemical ratios of SO₂, HCl, and HF, which are purely volcanic gas components. In this configuration, the high atmospheric background of H₂O and CO₂ prevents detection of the volcanic contribution of these two species.

OP-FTIR sensing of magmatic gases powering SEC lava fountains in 2021 was attempted by recording absorption spectra of the infrared radiation directly emitted by molten lava fragments (e.g. Allard et al. 2005; La Spina et al. 2015). This potentially allows one to determine the molar amounts and ratios of both H₂O, CO₂, SO₂, HCl, and HF (e.g. Allard et al. 2005; La Spina et al. 2015). However, during the 2021 lava fountains, reliable quantification of H₂O, CO₂, and HF was hampered by the combination of access difficulties under harsh winter conditions and logistical limitations linked to the nighttime occurrence of a majority of the fountains. Instead, the recorded FTIR spectra allowed accurate determination of the molar abundances and ratios of both SO₂ and HCl during four lava fountains.

## Seismic tremor

The permanent seismic network of INGV comprises 45 three-component broadband (40 s) digital stations with continuous data acquisition and transmission at a sampling frequency of 100 Hz (e.g. Di Grazia et al. 2006; Patanè et al. 2008). In this work, we use data from the station ECPN (Etna Cratere del Piano), positioned at 2900 m a.s.l. and ~1 km from the summit craters on the southern flank of Etna (Fig. 1). The ECPN station offers the best signal-to-noise ratio, and its proximity to the summit craters makes

<figure><img src="image_8.png" /></figure>
<table><caption>**Table 2** Main features of the lava fountain events which occurred at the Southeast Crater (SEC) of Mt Etna between December 2020 and April 2021. Pre-eruptive repose interval (day), duration (hour), and mean height (m) of the lava fountains are from Calvari and Nunnari (2022). Erupted volumes of tephra (Andronico et al 2021; Calvari and Nunnari 2022) and lava flows (Proietti et al. 2023) are expressed as dense rock equivalents (DRE). SO₂ mass discharge (kton) from each lava fountain was inferred from space-borne SEVIRI imaging (Guerrieri et al. 2023 and this work). SO₂/HCI molar ratios were measured using OP-FTIR spectroscopy (this work). *Total gas discharge during the fountains is computed by scaling their SO₂ mass discharge to the mean gas composition of analogous lava fountains at SEC in 2000–2001 (Allard et al. 2005; La Spina et al. 2015) whose mean SO₂/HCI ratio (7.1 ± 1.9, see Table 4) was remarkably similar to the mean SO₂/HCI ratio (7.1 ± 0.9) measured for the fountains series between 16th February and 1st April 2021. The ratios of total emitted gas versus the erupted mass budget of tephra and bulk magma (including lava flows), computed with a 2.7 ton/m³ density for Etna trachybasalt (Armienti et al. 1994), are reported as wt% and with one decimal despite a large uncertainty (±20–30%) on both parameters</caption><thead><tr><th></th><th>Lava fountain<br>date (mm/dd)<br>val (day)</th><th>Repose inter-<br>Duration<br>(hour)</th><th>Height mean<br>(m)</th><th>Tephra (10<sup>6</sup><br>m<sup>3</sup>)<br>DRE</th><th>Lava flow (10<sup>6</sup><br>m<sup>3</sup>)<br>DRE</th><th>SO<sub>2</sub> release<br>(10<sup>6</sup> m<sup>3</sup>)<br>DRE (kton)</th><th>SEVIRI molar ratio<br>FTIR (r<sup>2</sup>)</th><th>Total gas*<br>(kton)</th><th>Deg. S/tephra<br>(wt%)</th><th>Tot. gas/tephra<br>(wt%)</th><th>Tot. gas/bulk<br>(wt%)</th></tr></thead><tbody><tr><td colspan="12"><strong>2020</strong></td></tr><tr><td>12/13</td><td>-</td><td>3.7</td><td>231</td><td>0.44</td><td>0.6</td><td>1.04</td><td>3.0</td><td>130</td><td>0.13</td><td>10.7</td><td>4.6</td></tr><tr><td>12/21</td><td>8.0</td><td>8.0</td><td>1295</td><td>0.58</td><td>1.1</td><td>1.68</td><td>13.3</td><td>576</td><td>0.42</td><td>36.1</td><td>12.7</td></tr><tr><td>12/22</td><td>1.0</td><td>4.5</td><td>295</td><td>0.72</td><td>3.0</td><td>3.72</td><td>5.3</td><td>230</td><td>0.14</td><td>11.6</td><td>2.3</td></tr><tr><td colspan="12"><strong>2021</strong></td></tr><tr><td>01/18</td><td>27</td><td>2.5</td><td>343</td><td>0.52</td><td>1.1</td><td>1.62</td><td>6.3</td><td>273</td><td>0.22</td><td>19.1</td><td>6.2</td></tr><tr><td>02/16</td><td>29</td><td>2.2</td><td>757</td><td>0.46</td><td>1.6</td><td>2.06</td><td>6.3</td><td>275</td><td>0.26</td><td>21.7</td><td>4.9</td></tr><tr><td>02/17–18</td><td>1.3</td><td>1.5</td><td>343</td><td>0.82</td><td>3.8</td><td>4.62</td><td>4.8</td><td>207</td><td>0.11</td><td>9.2</td><td>1.6</td></tr><tr><td>02/19</td><td>1.3</td><td>2.5</td><td>358</td><td>0.78</td><td>3.0</td><td>3.78</td><td>10.1</td><td>435</td><td>0.24</td><td>20.3</td><td>4.2</td></tr><tr><td>02/20–21</td><td>1.5</td><td>6.2</td><td>492</td><td>1.43</td><td>2.6</td><td>4.03</td><td>7.8</td><td>336</td><td>0.10</td><td>8.5</td><td>3.0</td></tr><tr><td>02/22–23</td><td>1.9</td><td>4.5</td><td>686</td><td>1.19</td><td>2.7</td><td>3.89</td><td>10.6</td><td>460</td><td>0.17</td><td>14.0</td><td>4.3</td></tr><tr><td>02/24</td><td>1.6</td><td>4.5</td><td>649</td><td>1.37</td><td>3.1</td><td>4.47</td><td>9.7</td><td>418</td><td>0.13</td><td>11.1</td><td>3.5</td></tr><tr><td>02/28</td><td>3.5</td><td>7.0</td><td>1375</td><td>0.70</td><td>1.3</td><td>2.00</td><td>16.1</td><td>696</td><td>0.43</td><td>36.2</td><td>12.9</td></tr><tr><td>03/02</td><td>2.2</td><td>3.0</td><td>278</td><td>1.03</td><td>1.9</td><td>2.93</td><td>13.6</td><td>591</td><td>0.25</td><td>20.9</td><td>7.5</td></tr><tr><td>03/04</td><td>1.7</td><td>3.5</td><td>204</td><td>2.33</td><td>2.1</td><td>3.68</td><td>16.3</td><td>706</td><td>0.19</td><td>16.3</td><td>5.9</td></tr><tr><td>03/07</td><td>2.9</td><td>3.5</td><td>638</td><td>1.07</td><td>1.4</td><td>2.47</td><td>15.6</td><td>675</td><td>0.27</td><td>22.9</td><td>10.1</td></tr><tr><td>03/9–10</td><td>2.7</td><td>3.2</td><td>655</td><td>1.44</td><td>1.4</td><td>2.84</td><td>20.2</td><td>873</td><td>0.26</td><td>22.1</td><td>11.4</td></tr><tr><td>03/12</td><td>2.2</td><td>4.2</td><td>1140</td><td>1.63</td><td>1.9</td><td>3.53</td><td>15.0</td><td>85 (0.82)</td><td>0.17</td><td>14.5</td><td>6.8</td></tr><tr><td>03/14–15</td><td>1.6</td><td>2.5</td><td>670</td><td>0.53</td><td>1.4</td><td>1.93</td><td>10.6</td><td>460</td><td>0.37</td><td>31.6</td><td>8.8</td></tr><tr><td>03/17</td><td>2.0</td><td>6.0</td><td>538</td><td>1.58</td><td>1.1</td><td>2.68</td><td>15.1</td><td>652</td><td>0.18</td><td>15.0</td><td>9.0</td></tr><tr><td>03/19</td><td>2.1</td><td>2.5</td><td>740</td><td>0.75</td><td>1.0</td><td>1.75</td><td>29.1</td><td>1259</td><td>0.72</td><td>61.1</td><td>23.6</td></tr><tr><td>03/23–24</td><td>4.5</td><td>9.0</td><td>456</td><td>0.38</td><td>1.5</td><td>1.88</td><td>9.3</td><td>7.4 (0.86)</td><td>402</td><td>0.45</td><td>38.5</td></tr><tr><td>04/01</td><td>7.7</td><td>12.5</td><td>241</td><td>4.10</td><td>2.0</td><td>6.10</td><td>1.8</td><td>6.4 (0.90)</td><td>77</td><td>0.01</td><td>0.7</td></tr><tr><td colspan="12"><strong>17 Lava fountains</strong></td></tr><tr><td>Mean</td><td>2.4</td><td>4.6</td><td>9.0</td><td>1.3</td><td>2.0</td><td>3.3</td><td>12.5</td><td>7.3</td><td>540</td><td>0.25</td><td>21.4</td></tr><tr><td>St. dev</td><td>1.7</td><td>2.8</td><td>2.4</td><td>0.8</td><td>0.8</td><td>1.2</td><td>6.4</td><td>0.9</td><td>276</td><td>0.17</td><td>14.2</td></tr><tr><td>Total</td><td></td><td></td><td></td><td>21.6</td><td>33.8</td><td>55.4</td><td>212</td><td></td><td>9173</td><td>0.17</td><td>21.4</td></tr><tr><td colspan="12"><strong>21 Lava fountains</strong></td></tr><tr><td>Mean</td><td>3.8</td><td>4.6</td><td>9.4</td><td>1.1</td><td>1.9</td><td>3.0</td><td>11.4</td><td>6.8</td><td>494</td><td>0.25</td><td>21.0</td></tr></tbody></table>

<figure><img src="image_9.png" /></figure>
<table><caption>**Table 2 (continued)**</caption><thead><tr><th></th><th>Lava fountain Repose interval (day)</th><th>Duration (hour)</th><th>Height mean (m)</th><th>Tephra (10<sup>6</sup> m<sup>3</sup>) DRE</th><th>Lava flow (10<sup>6</sup> m<sup>3</sup>) DRE</th><th>SO<sub>2</sub> release (10<sup>6</sup> m<sup>3</sup>) DRE (kton)</th><th>SEVIRI/molar ratio FTIR (°)</th><th>SO<sub>2</sub>/HCl (kton)</th><th>Total gas* (kton)</th><th>Deg. S/tephra (wt%)</th><th>Tot. gas/tephra (wt%)</th><th>gas/bulk (wt%)</th></tr></thead><tbody><tr><td>St. dev</td><td>5.8</td><td>2.7</td><td>2.6</td><td>0.8</td><td>0.8</td><td>6.4</td><td>1.4</td><td>275</td><td>0.16</td><td>13.5</td><td>6.0</td></tr><tr><td>Total</td><td></td><td></td><td></td><td>24.0</td><td>39.6</td><td>63.5</td><td>240</td><td>10,382</td><td>0.25</td><td>21.0</td><td>129.3</td></tr></tbody></table>

it especially sensitive to eruptive activity. Volcanic tremor spectral amplitude is calculated using the root mean square (RMS expressed in arbitrary units) of the seismic signal recorded on the vertical component. In this study, the daily RMS spectral amplitude is calculated in the frequency range 0.5–2.5 Hz, and tremor amplitude is obtained by averaging the RMS values over a 10.24 s time window. Dispersion of individual values around the mean affects the relative standard deviation of the RMS average, with values ranging between 10 and 25% for the short-5 min intraday recordings, enveloped between 6 and 77% for the daily averaged RMS.

## Results

Figure 2 shows the data time series for the gas emissions and seismic tremor during the period from June 2020 to April 2021. Figure 2a, b describes the variations of the bulk plume SO₂ flux respectively measured from the ground (FLAME network) and from space (TROPOMI), Fig. 2c shows the SO₂ discharge from each of the lava fountaining events (SEVIRI), and Fig. 2d depicts the SO₂/HCl ratios measured with OP-FTIR during both passive and eruptive degassing.

As a preliminary observation, we highlight the fairly good consistency in the overall values and temporal variations of the bulk plume SO₂ fluxes recorded through ground-based DOAS survey (Fig. 2a) and TROPOMI satellite sensing (Fig. 2b) during both pre- and syn-eruptive periods. TROPOMI sensing of SO₂ emissions from the lava fountains themselves is not included in our time series because it was prevented by the nighttime occurrence of most (70%) of the lava fountains but also by the high ash load of eruptive plumes, leading to strong underestimation in SO₂ retrieval for the other fountains that occurred in the daylight. Apart from a few occasions where the two flux series appear to differ somewhat (e.g. in August 2020 and between late January and early February 2021), the temporal trends in both datasets are in excellent agreement. Over the whole investigated period, the time-averaged SO₂ flux derived from the FLAME network and from TROPOMI compares very well (2660 ± 2000 and 2270 ± 1600 tons per day, respectively; Table 1). Day-by-day differences with low linear correlation (Pearson's coefficient of 0.10) can easily be accounted for by the combination of different factors, such as the higher data sampling rate of FLAME compared to TROPOMI, detection time lags, and differences in the field of view/air mass between the two methods. Specifically, TROPOMI data can be affected by geometrical spreading of the volcanic plume and associated SO₂ dispersion, with consequent lowering of SO₂ concentrations per pixel, possibly down to the detection limit of the satellite sensor, as a function of the dispersion time. Such effects mainly influence the intra-day diurnal

<figure><img src="image_10.png" /></figure>
**Fig. 2** Temporal variations of SO₂ emissions, SO₂/HCI ratio, and volcanic tremor in function of eruptive activity at Mt Etna during the period from June 2020 to April 2021. Eruptive activity at the summit craters is depicted on top of the figure. **a** 7-day moving averages of the bulk SO₂ plume flux (tons/day) measured from the ground (DOAS FLAME network, blue-solid line) and of the spectral amplitude of volcanic tremor (in m/s, frequency band 0.5–2.5 Hz; red solid line). Daily SO₂ flux values are displayed in light grey; **b** daily and 7-day moving average of the bulk SO₂ plume flux (tons/day) measured from space (TROPOMI sensor; grey and pink solid lines, respectively). Note: the overall very good matching of SO₂ fluxes determined from the ground and from space; **c** bulk SO₂ amounts (ktons) released during each of the 21 lava fountains that occurred at SEC between December 2020 and April 2021, as derived from SEVIRI satellite sensing (cyan-solid line); and **d** SO₂/HCI molar ratios in the bulk volcanic plume (green squares) and in magmatic gas driving the lava fountains (orange dots) measured with OP-FTIR spectroscopy. The dashed black lines distinguish three periods characterized by different eruptive activity, degassing regime, and geophysical signals (see text). The purple star marks the onset of major geodetic and seismic indications of a new magma influx, while the yellow star indicates the occurrence, on 11–13 February 2021, of clustered energetic (M2.4–3.1) volcano-tectonic earthquakes at shallow depth (700 m above sea level) beneath Etna summit

<figure><img src="image_11.png" /></figure>
variability. Therefore, in order to minimize this bearing, we compared the 7-day averaged records between the two time series, which provides an improved linear Pearson correlation strength of 0.43°.

Considering the temporal changes in volcanic activity, degassing regime, and seismic tremor, we distinguish three distinct periods (Table 1) in the time series depicted in Fig. 2.

Period 1, from 1 June to 5 December 2020, was characterized by episodic mild Strombolian activity at SEC and NEC summit craters and occasional ash emissions at VOR main crater (Fig. 1). We observe that during this period, the bulk plume SO₂ flux and seismic tremor displayed moderate oscillations (Fig. 2a), except for a short-lived SO₂ flux increase by the end of September. The time-averaged SO₂ plume flux, 1800 ± 680 tons/day (t/d), is typical of Mt Etna's background degassing activity over the past two decades (Salerno et al. 2018). TROPOMI data verify a comparable mean SO₂ flux and moderate (within a factor 2) temporal oscillations (Fig. 2b). In this period, the bulk plume emission preserved a relatively stable SO₂/HCl molar ratio, with a mean value of 3.6 ± 0.7 (range: from 2.4 to 4.9; Table 1), though with a tendency to upper values from mid-October to November (Fig. 2d).

Period 2, extending from 5 December 2020 to 13 February 2021, corresponds to a pressure build-up phase marked by several seismic swarms from deep to shallow depths, continued volcano inflation, and increasing eruptive activity at the central craters (e.g. Bruno et al. 2022). Paradoxically, it started with a drop of volcanic tremor and the bulk SO₂ flux (Fig. 2a) in correspondence to the three first discrete fountain events of transitional character which occurred at SEC on 13 and 21–22 December (Andronico et al. 2021). These events, excepting for the 21 December one, released modest amounts of SO₂ (3.0–5.3 kt; Fig. 2c and Table 2). Afterwards, both volcanic tremor and the bulk SO₂ flux started to rapidly increase (Fig. 2a), evidencing an accelerating pressurization of the upper feeding system, as also indicated by geophysical data (e.g. Bruno et al. 2022). At the same time, a mixed explosive and effusive activity developed at the Voragine (VOR) summit crater and rapid eruptive escalation at SEC led to another discrete lava fountain of transitional character and modest SO₂ release (Fig. 2c) on 18 January (Andronico et al. 2021). Despite two brief waxing-waning phases in correspondence with these discrete events at SEC, both FLAME (Fig. 2a) and TROPOMI (Fig. 2b) data reveal a sustained increase of the bulk plume SO₂ flux until 13 February. The daily SO₂ flux occasionally reached over 10,000 t/d (on 7 and 23–24 January, then 9 and 13 February; Fig. 2a), and its time-averaged value during this second period (5100 ± 2900 t/d, reaching 7000 ± 2700 t/d between 21 January to 13 February) is 3 times higher than during period 1. In the same time, the SO₂/HCl ratio of the bulk

plume tended to slightly decrease (Fig. 2d), its mean value being 3.3 ± 0.6 (Table 1).

Period 3, extending from 13 February to the end of April 2021, covers the sequence of 17 lava fountaining paroxysms at SEC between 16 February and 1 April. It is worthy of note that the fountaining sequence started shortly after a very abrupt, synchronous drop of seismic tremor and the bulk SO₂ flux (Fig. 2a), almost concomitant with the shallow cluster of energetic (M2.4–3.1) volcano-tectonic earthquakes on 11–13 February. The drop in bulk SO₂ flux was synchronously recorded from the ground (Fig. 2a) and from space (Fig. 2b). This abrupt decrease of the two parameters continued until 27 February, while six of the 17 lava fountains at SEC had already occurred (Fig. 2a and Table 2). Both modestly re-increased during the most powerful fountaining phase until 19 March, then decreased again more gently until the end of April. Superimposed on this overall declining trend, the 17 lava fountaining paroxysms were marked by sharp peak increases of the tremor (Fig. 2a) and intense SO₂ release (Fig. 2c). SEVIRI data indicate a SO₂ discharge ranging from 4.8 to 29 kt (mean: 12.0 ± 6.4 kt) during single paroxysms (Table 2), with an apparent increase culminating on 19 March (Fig. 2c and Table 2). The last two paroxysms on 24 March and especially on 1 April happened after increasing repose intervals and were of declining magnitude (Table 2). Note that the increasing SO₂ magnitude of the paroxysms between 28 February and 19 March coincides with the modest re-increase of the bulk SO₂ flux and seismic tremor in that same interval (Fig. 2a, b). As a whole, the 17 paroxysmal fountains in February–April 2021 released 212 kt of SO₂ (±30–40%; Table 2).

During the whole period 3, the bulk plume SO₂/HCl ratio showed high-frequency oscillations between ~2 and ~4.8 (Fig. 2d), even though we cannot entirely exclude that some of the highest values were influenced by the dispersion of the fountain gas column. The eruptive gas phase powering the lava fountains was indeed characterized by distinctly higher SO₂/HCl ratios, ranging from 6.0 to 8.5 (mean: 7.1 ± 0.9; Table 2). Scaling the mean SO₂/HCl ratio to their cumulative SO₂ discharge, we infer a total HCl release of 17 ± 2 kt from the fountains (Table 2). Finally, we note that during the 45-day length of the paroxysmal phase from 16 February to 1 April, the bulk plume SO₂ emission rate from Etna averaged 2400 ± 1100 t/d, which is only about 30% more than its background level during period 1. However, the cumulative SO₂ contribution from the 17 paroxysmal events is equivalent to an additional time-averaged SO₂ emission of ~4700 t/d over this same interval. Hence, the total SO₂ release from Mt Etna during this eruptive period averaged about 7000 t/d, 2/3 of which is due to paroxysmal degassing at SEC. Such a total emission rate is similar to the average bulk SO₂ plume flux during the previous pressure build-up phase between 21 January and 13 February (period 2). These data therefore
support the idea of continued extensive gas release, but in discontinuous (pulsed) mode, from the same source of over-pressure, i.e. the shallowing primitive magma.

Discussion

Enhanced SO₂ release, associated with increasing seismic tremor (e.g. Leonardi et al. 2000; Salerno et al. 2018; Aiuppa et al. 2023), has frequently been observed to be a precursor of impending eruptions at Etna (e.g. Malinconico 1979; Allard et al. 1991, 2006; Caltabiano et al. 2004; Salerno et al. 2018). We show here that this was again the case before the series of paroxysmal lava fountains at SEC in February–April 2021. Our results for the three distinct periods described above, combined with published geophysical and petrological data, provide further insight into the magmatic processes, which controlled these events. In particular, our data set provides two specific constraints on these magmatic processes:

(i) The measured SO₂ flux variations not only track temporal changes in the degassing regime in relationship to the underground magma dynamics and surface volcanic activity but also permit quantification of the DRE (Dense Rock Equivalent) amounts of degassing magma ($V_d$), using the following relationship:

$$V_d = 1/2fSO_2[(S)_d \times \rho_m \times (1 - X_c)]$$

where $(S)_d$ is the degassed mass fraction of sulphur, and $\rho_m$ and $X_c$ are the mean DRE density and crystal content of the magma (e.g. Allard 1997). Dividing the SO₂ mass flux ($fSO_2$) by two accounts for the twice higher molar mass of SO₂ (64 g) compared to elemental S (32 g). $X_c$ averaged 25% in products erupted during the investigated period (Corsaro and Miraglia 2022), and we use a DRE density of 2.7 tons/m<fnref n="3" /> for Etna trachybasalt (Armienti et al. 1994a). In contrast to H₂O and especially CO₂, sulphur starts exsolving from modern Etna trachybasalts at $\leq 140$ MPa (Spilliaert et al. 2006a and 2006b; Lesne et al. 2011) where its initial content averages 0.32 wt% (Spilliaert et al. 2006b). Sulphur degassing thus develops from $\leq 2$ km depth below sea level up to the summit craters, i.e. within the shallow plumbing system of the volcano. Whether the trachybasalt reaches sulphide saturation or not, 95% of its initial S content is generally lost at 1 atm. (e.g. Métrich et al. 2004; Spilliaert et al. 2006a; Corsaro et al. 2009), so $(S)_d$ averages ~0.3 wt% (e.g. Allard et al. 2006 and references therein). We here use this value to compute the minimal bulk DRE amounts of magma that were degassing during the three periods described above. The results are reported in Table 3 and depicted in Fig. 3.

<caption>**Table 3** Cumulative degassed magma volumes and time-averaged magma degassing rates computed from both passive and eruptive SO₂ emissions at Mt Etna during the three distinct periods of activity between 1st June 2020 to 1st April 2021. Calculation procedure is described in text. Listed for comparison are the bulk amounts of</caption>

erupted magma from lava fountaining events between December 2020 and April 2021, quantified from field/mapping observations (¹ Andronico et al. 2021, Calvari and Nunnari 2022;<fnref n="2" /> Proietti et al. 2023) or inferred from strain variations (³Bonaccorso et al. 2021). All values are DRE magma amounts

<table><thead><tr><th>Activity periods/degassing mode</th><th>Cumulative volume (10<sup>6</sup> m<sup>3</sup> DRE)</th><th>Degassing rate (m<sup>2</sup>/s)</th><th>Error (±%)</th></tr></thead><tbody><tr><td><em>Degassed magma volumes</em></td><td></td><td></td><td></td></tr><tr><td>Period 1: bulk plume degassing</td><td>28.7</td><td>1.8</td><td>-27+32</td></tr><tr><td>Period 2: bulk plume degassing</td><td>26.5</td><td>4.4</td><td>-27+32</td></tr><tr><td>Period 2: bulk plume and eruptive degassing</td><td>28.8</td><td>4.8</td><td>-48+51</td></tr><tr><td>Period 3: bulk plume degassing</td><td>8.4</td><td>2.1</td><td>-27+32</td></tr><tr><td>Period 3: eruptive degassing</td><td>17.3</td><td></td><td>40</td></tr><tr><td>Period 3: bulk plume and eruptive degassing</td><td>25.7</td><td>6.5</td><td>-48+51</td></tr><tr><td>Post-eruptive period (April: 2–30) bulk plume</td><td>2.9</td><td>1.2</td><td>-27+32</td></tr><tr><td>Total degassed magma (1 Jun 2020–1 Apr 2021)</td><td>86.1</td><td>3.0</td><td></td></tr><tr><td><em>Erupted magma volumes</em></td><td></td><td></td><td></td></tr><tr><td>Total tephra<sup>1</sup></td><td>23.1</td><td></td><td>20</td></tr><tr><td>Total lava flows<sup>2</sup></td><td>42.0</td><td></td><td>20</td></tr><tr><td>Cumulative estimate from strain changes<sup>3</sup></td><td>59.0</td><td></td><td>n/a</td></tr></tbody></table>

<footnote n="1">¹ Calvari and Nunnari 2022</footnote>

<footnote n="2">² Proietti et al. 2023</footnote>

<footnote n="3">³Bonaccorso et al. 2021</footnote>
<figure><img src="image_12.png" /><figcaption>**Fig. 3** Temporal evolution of the cumulative volumes of degassing magma which supplied Etna's SO₂ emissions between June 2020 and April 2021. The black curve refers to the amount of magma supplying the bulk plume SO₂ emission, while the red curve includes the contribution of degassing magma involved in the lava fountain paroxysms. Note the change in slope and hence in time-averaged magma degassing rate (in m³/s) between the three distinguished periods, in particular between the background degassing period 1 and the pres-</figcaption></figure>

sure build-up period 2. Also shown (secondary Y axis) are the instantaneous and cumulative volumes of magma erupted during SEC lava fountains (period 3), with the indication of the relative amounts of lava flow (blue) and tephra (red) during each paroxysm (Andonico et al. 2021; Calvari and Nunnari 2022; Proietti et al. 2023). Note the difference in scale between the two Y axes. Refer to the text for the discussion

(ii) The SO₂/HCl ratio is a sensitive indicator of degassing pressures. As shown by melt inclusion studies (e.g. Métrich et al. 2004; Spilliaert et al. 2006b; Corsaro et al. 2009) and petrological experiments (Lesne et al. 2011), chlorine starts exsolving in Etna trachybasalt at lower pressure (≤100 MPa) than sulphur and, for its greatest part, degasses at ≤20 MPa, i.e. at very shallow depth (~0.8–1 km) beneath the summit. Moreover, in contrast to S, a variable but limited fraction (between ~20 and 55%) of its initial content of 0.19 wt% at 100 MPa is eventually lost at the surface depending on the magma degassing mode and dynamics (e.g. Métrich et al. 2004; Spilliaert et al. 2006b). During magma decompression, the S/Cl (or SO₂/HCl) molar ratio of the magmatic gas phase thus evolves from a very high initial value (≥30) at 100 MPa down to low values at the surface, the latter depending on the degassing conditions, the actual Cl loss, and the degree of sulphide saturation of the melt for S (Spilliaert et al. 2006b; Moretti et al. 2018). Following the approach developed by Spilliaert et al. (2006b), we present in Fig. 4 the modelled evolution of the molar S/Cl ratio of the magmatic gas phase during either closed or open system decompresion and degassing of Etna trachybasalt, saturated or not with sulphide, from 100 MPa to atmospheric pressure. Note that the influence of either open or

closed system conditions upon the S/Cl ratio of the magmatic gas phase becomes significant from only around 80 MPa (Fig. 4) or 3.2 km depth beneath the summit craters, i.e. right at the sea level. The ranges in SO₂/HCl (or S/Cl) molar ratio we measured in the bulk plume of Etna and during the lava fountains are compared with the model.

Thereafter, we use this specific information provided by SO₂ fluxes and SO₂/HCl ratios in tandem with other data types to examine the magmatic processes which, during the three periods identified above, preceded then determined the February–April 2021 lava fountain sequence.

## Background degassing activity during period 1

A key framework to interpret our data set for Etna gas emissions during the period June 2020–April 2021 is provided by geophysical and petrological evidence of a new influx of volatile-rich primitive magma into the mid-crustal then shallow feeding system of the volcano (Bonaccorso et al. 2021; De Gori et al. 2021; Bruno et al. 2022; Corsaro et al. 2024). This influx was first manifested in a marked inflation of the volcanic edifice since 2019, associated with renewed eruptive activity at the four summit craters then increasing volcanic seismicity since the summer 2020 (Bonaccorso et al. 2021; De Gori et al. 2021; Bruno et al. 2022). Based
<figure><img src="image_13.png" /></figure>

**Fig. 4** Pressure-related evolution of the S/Cl molar ratio of the magmatic gas phase in equilibrium with Etna trachybasalt during either closed system (CSD) or open system (OSD decompression from 100 MPa (~4 km lithostatic depth beneath the volcano summit) to the surface. S/Cl molar ratios were computed following the approach of Spillaert et al. (2006b), using melt inclusion data for the initial abundances S and Cl in the trachybasalt (0.32 wt% S at 140 MPa and 0.19 wt% Cl at 100 MPa) and both melt inclusion and experimental data for the solubility behaviour of these two elements during magma decompression (Spilliaert et al. 2006b; Lesne et al. 2011). Distinct S/Cl evolution curves result from two effects: (i) the possibility that the magma be at sulphide saturation (indicated with asterisk) or not—which influences S degassing, and (ii) the outgassing extent of more

on GNSS data, Palano et al. (2024) estimated an influx of $27 \pm 9 \times 10^6$ m³ of magma between 21 Nov 2019 and 15 Feb 2021, from a main pressure source located at 7–8.5 km depth bsl, i.e. at the bottom of the upper crustal (0–9 km) plumbing system of Etna.

As described above, the activity of Etna between 1 June and 5 December 2020 (period 1 in Fig. 2) was representative of the background regime of the volcano over the past decade. Sporadic mild Strombolian activity (SEC and NE) and episodic ash emissions (VOR) at the summit craters (Fig. 1) were associated with relatively stable, moderate levels of the bulk SO₂ flux (av. $1800 \pm 680$ t/d; Table 1) and seismic tremor (Fig. 2a). Eruptive manifestations at the summit craters produced an unknown but likely trivial amount of magma with a composition typical of evolved trachybasalt commonly filling the shallow feeding system of Etna (Corsaro and Miraglia 2022). This means that, while the edifice was continuing to inflate (Bruno et al. 2022), the shallowing batch of primitive magma had not yet mixed with the resident magma filling the shallow reservoirs and conduits, except perhaps beneath SEC, from which the ejecta were somewhat less evolved (Corsaro and Miraglia 2022). From the cumulative SO₂ emissions during this ~6-month

soluble Cl (from ca. 20% and 55%) which depends on the magma degassing mode and dynamic (Spilliaert et al. 2006b). The black arrow indicates the shift in S/Cl ratio during possible transitions from closed system to open system degassing at constant depth in SEC reservoir (CDS and OSD, respectively). The ranges in S/Cl (or SO₂/HCl) molar ratios measured in the bulk plume emissions and during the lava fountains between June 2020 and April 2021 are shown for comparison and for estimating the degassing source depths during the three activity periods defined in our study. Also depicted is the depth interval (~1.5–2.5 km) inferred for the source depth of previous lava fountains at SEC (e.g. Allard et al. 2005; La Spina et al. 2015; Carboni et al. 2015). See the text for discussion

period 1, we compute the overall degassing of ca. $29 \times 10^6$ m³ (DRE) of trachybasalt, at a time-averaged rate of 1.8 m³/s (Table 3 and Fig. 3). Since little of this magma was erupted, we conclude that Etna gas emissions during period 1 were entirely sustained by gas transfer decoupled from the magma across the central volcano conduits.

The central conduits of Mt Etna are ~2.0–2.3 km in height. The mean SO₂/HCl ratio of the bulk plume emissions ($3.6 \pm 0.7$; Table 1) during period 1 sheds light on the possible source depth of gas phase transfer decoupled from the magma. Under pure open system conditions, this ratio indicates that separate gas escape may have occurred after gas and melt attained equilibrium at between ~50 MPa (~2 km depth) and ~20 MPa (~0.8 km) beneath the craters, depending on whether the magma was or was not at sulphide saturation (Fig. 4). Moreover, because the vesicularity of Etna trachybasalt is already 60 vol% at 2 km depth (Spilliaert et al. 2006b), open system degassing in the form of gas percolation across the magma-filled conduits (e.g. Colombier et al. 2021) is also feasible from that depth. In contrast, in the case of closed system decompression of sulphide-saturated or unsaturated trachybasalt, the above SO₂/HCl ratio requires a separate gas escape (system opening)
at shallower conduit depths of between 0.8 and 0.2 km (~20 to 5 MPa; Fig. 4), followed by efficient downward convective recycling of the degassed magma in order to maintain the SO₂ flux at the surface (e.g. Allard 1997; Kazahaya et al 1994). An intermediate scenario between these two end-members is possible. Lacking other independent constraints, we simply infer that SO₂ emissions during period 1 were likely sustained by separate gas ascent and release through the central volcano conduits from between 2 and ≤ 1 km depth beneath the craters. Such a depth range corresponds to the centroid source depth interval of persistent volcanic tremor during background activity at Mt Etna (e.g. Di Grazia et al. 2006). The two occurrences of short-lived SO₂ flux increase recorded in late September 2020 (Fig. 2a), coupled with a ~30% decrease of SO₂/HCl ratio (down to 2.4; Fig. 2d), suggest a temporary shallowing of the gas source, leading to enhanced peak emission of HCl (900 t/d) compared to the time-averaged HCl flux (280 ± 100 t/d) over the whole period 1 (Table 1).

## Pressure build up phase (period 2)

During this period, from 5 December 2020 to 13 February 2021, several lines of evidence demonstrate an increasing pressurization of Etna's shallow feeding system by the uprising batch of primitive magma. In addition to continued volcano inflation, these lines of evidence include the succession of earthquake swarms occurring at both deep and shallow depths in December 2020 (Bonaccorso et al. 2021), the rapidly increasing SO₂ flux and volcanic tremor (Fig. 2a, b), the development of explosive and effusive activity within VOR central crater, and, finally, the advent of the first four transitional fountaining events at SEC, on 13 and 21–22 December 2020 then 18 January 2021 (Andronico et al. 2021). Petrological analysis of the products from these latter events confirmed an increasing influence of the shallowing primitive magma compared to previously erupted products (Corsaro et al. 2022, 2024). The two brief waning phases of bulk SO₂ flux and tremor, corresponding to these discrete events at SEC (Fig. 2a), particularly those in December, indicate a temporary pressure shift from the central volcano conduits, which are usually Etna's main gas drains, towards the more off-centre conduit system feeding the SEC. Similar temporal shifts in gas emission and seismic tremor have already been observed at Mt Etna during other lava fountain sequences that occurred at the South-East Crater between 2014 and 2018 (Ferlito et al. 2017; Calvari et al. 2020). From the total passive and eruptive SO₂ emissions in this 70-day period, we compute a cumulative total of 28 × 10⁶ m³ (DRE) of degassing trachybasaltic magma (Table 3), at a mean rate of 4.8 m³/s (26 × 10⁶ m³ and 4.4 m³/s, respectively, from only the plume emissions). As seen in Fig. 3, the time-averaged magma degassing rate during

this pressurization phase was thus about 3 times higher than during period 1, and both periods are well distinguished from the change in slope of their respective degassing curves. The quantity of magma erupted in period 2 includes 8.1 × 10⁶ m³ DRE produced as lava flows and tephra during the four transitional fountain events at SEC (Table 2), plus an unknown but likely modest amount (only a few 10⁵ m³ DRE according to our contemporaneous field observations) produced by intra-crater explosive and effusive activity at the Voragine (VOR). We thus assess a maximum erupted magma volume of ~8.5 × 10⁶ m³ DRE. Accordingly, about 3.4 times more magma was degassed than erupted during this pressure build-up phase, implying a 70% prevalence of segregated gas transfer through the central volcano conduits. In addition, we note that the total amount of magma erupted during the four transitional events at SEC in December 2020 and January 2021 is about four times larger than the total degassed amount we infer from SEVIRI data (Table 3), which implies the eruption of prevalently degassed magma during these events, in agreement with their transitional character (Andronico et al. 2021).

As shown in Fig. 2d, the bulk plume SO₂/HCl ratio broadly tended to decrease during this period. Its time-averaged value (3.3 ± 0.6; Table 1) is somewhat lower than during period 1, suggesting a shallower decoupling and transfer of the magmatic gas phase, in coherence with both tremor and SO₂ flux evidence of an enhanced magma-gas flow rate within the central conduits. From the total SO₂ emissions and the mean plume SO₂/HCl ratio, we compute a total HCl discharge of 54 ± 8 kton (mean: 780 ± 120 t/d) over this period (Table 1).

## The sequence of paroxysmal lava fountains (period 3) and its source mechanism

As previously described, the onset of this eruptive period was marked by a spectacular drop of the seismic tremor energy and of the bulk plume SO₂ flux. The latter sharply decreased from 8000 to 10,000 t/d on 10 to 13 February down to ~1500 t/d, i.e. values typical for period 1, by 27 February (Fig. 2a, b). Such a sharp co-decrease of the SO₂ emission rate and the tremor indicates a sudden reduction in the dynamics of gas transfer across the main central conduits of Mt Etna, even though mild eruptive activity persisted at the central summit craters and shallow degassing (from ~10 to 15 MPa; Fig. 4) is suggested by the initially low SO₂/HCl plume ratios (~2.5; Fig. 2d). Because it started in close concomitance with the shallow (700 m above sea level) cluster of energetic volcano-tectonic earthquakes beneath the summit area on 11–13 February and only 3 days before the first paroxysmal lava fountain at SEC on 16 February, we interpret this sudden reduction in degassing through the central volcano conduits as likely due to shallow system fracturing
and a preferential migration of the pressurized primitive
magma towards the SEC feeding system. This interpreta-
tion is supported by the continued drop in SO₂ flux and seis-
mic tremor during the whole paroxysmal phase and till the
end of April 2021, except for a modest re-increase between
27 February and 19 March (Fig. 2a, b). However, we also
emphasize that passive SO₂ degassing from SEC during
this period, including the intervals between lava fountains,
remained at a very low rate (~40 t/d; Aiuppa et al. 2008)
and, hence, contributed negligibly to the bulk plume SO₂
flux. The amount of magma involved in supplying Etna's
plume SO₂ emissions from 13 February to the end of April
is assessed as only ~8 × 10⁶ m³ DRE (Table 3). It implies a
time-averaged magma degassing rate of 1.9 m³/s, very close
to that in period 1 (Fig. 3).

The 17 lava fountain paroxysms that occurred at SEC
from 16 February to 1 April were associated with sharp
peak increases of the seismic tremor superimposed on the
otherwise declining trend in tremor amplitude (Fig. 2a).
Their mean recurrence time (2.5 ± 1.6 days), mean height
(630 ± 290 m), and mean duration (3.8 ± 1.6 h; Table 2) evi-
dence a pressure source mechanism releasing intense energy
in a pulsed mode. All the more so since, between the foun-
taining events, the SEC displayed very weak activity and

minor SO₂ emission (~40 t/d; Aiuppa et al. 2008). SEVIRI
data (Guerrieri et al. 2023, and this study) show that the
SO₂ discharge of the lava fountains varied from 4.8 to 29
kt (Table 2), with a tendency to increase until March 19
(Fig. 2c). Looking into more detail, the SO₂ magnitude of
the fountains poorly correlates with their respective intensity
in terms of height and maximum tremor amplitude but tends
to closely parallel temporal changes in the composition of
co-erupted products (Fig. 5). Corsaro and Miraglia (2022,
2024) showed that the influx and mixing of primitive magma
beneath SEC was manifested by enhanced CaO/Al₂O₃ ratio
(0.53) in SEC products from the first transitional fountains
in December 2020 compared to products erupted in period
1. This imprint of the primitive magma gradually increased
until the first lava fountain on 16 February then rapidly esca-
lated up to a maximum (0.60) during the 28 February parox-
ysm (Fig. 5), one of the most powerful fountains (Table 2).
After this event, the broad decrease of CaO/Al₂O₃ ratio,
except for two new pulses on 10 and 17 March, indicates a
lowering input of primitive magma and the eruption of more
and more differentiated trachybasalt (Corsaro and Miraglia
2022, 2024). The good matching between the temporal vari-
ations in SO₂ discharge and CaO/Al₂O₃ ratio during the first
part of the lava fountains sequence, until 4 March at least

<figure><img src="image_14.png" /><figcaption>**Fig. 5** SO₂ discharges (ktons) from SEC lava fountains between December 2020 and April 2021 (red triangles; Guerrieri et al. 2023 and this work) compared to temporal variations of CaO/Al₂O₃ ratio in co-erupted solid products (blue squares; Corsaro and Miraglia 2022, 2024). Increasing CaO/Al₂O₃ ratio tracks the increasing influx and mixing proportion of the primitive (more mafic) magma in SEC feeding system. The broadly parallel evolutions of the fountains’ SO₂ discharges and the magma composition since December 2020 and dur-</figcaption></figure>

ing the first half of the February–April 2021 lava fountain sequence, until March 4 at least, suggest a causal relationship between the extent of SO₂ release and the proportion of primitive magma in erupted products. This relationship became less obvious during the second part of the sequence, as the fountains emitted proportionally more SO₂ while erupting more evolved magma (decreasing CaO/Al₂O₃)
(Fig. 5), suggests a direct relationship between the extent of SO₂ release and the proportion of primitive magma in co-erupted products. This relationship is less obvious during the second half of the sequence, as the fountains emitted proportionally more SO₂ while erupting more evolved magma.

The SO₂ discharge of each fountain paroxysm can use-
fully be compared to the DRE amount of co-erupted tephra
(Andronico et al 2021; Calvari et al. 2022). As a matter of
fact, the tephra mass is the most directly related to magma
degassing and fragmentation during a lava fountain and, in
the specific case of 2021 events, the most closely related
to SO₂ detection by SEVIRI. From the data reported in
Table 2, we compute a degassed mass fraction of S varying
from 0.10 to 0.72 wt% during the 17 lava fountains, with a
mean of 0.25 ± 0.17 wt%. Given the large uncertainties on
both SEVIRI data and tephra volume estimates (Andron-
ico et al. 2021), the differences between individual values
are likely dominated by the error and therefore difficult to
interpret. Rather, we find that the mean degassed fraction of
0.25 ± 0.17 wt% S for the whole fountain series compares
fairly well with the maximum (S<sub>d</sub>) value of 0.3 wt% we con-
sidered for the bulk degassing of Etna trachybasalt. Such
a consistency makes it plausible that syn-eruptive degas-
sing of sulphur dominated during most of the fountains.
This mechanism requires a fountain source depth at which
limited S was already exsolved. Geophysical, geochemical,
and petrological evidence point to a shallow magma stor-
age system feeding SEC lava fountains, whose estimated
depth ranges from ca. 2 km (Allard et al 2005; Spilliaert et al
2006b; La Spina et al 2015; Carbone et al 2015) to ~3–4 km
beneath the crater (0–1 km b.s.l.; Bonnacorso et al. 2021b;
De Gori et al. 2021). From the negative (decompression)
strain changes associated with the 17 lava fountains in
February–April 2021, Bonnacorso et al. (2021b) inferred
an oblate, vertically elongated shape for this storage zone,
centred at ~1 km depth b.s.l., which probably extends to the
base of the volcanic edifice at ~2 km beneath SEC. At this
level, which corresponds to the maximum centroid source
depth of persistent seismic tremor recorded between the
2021 fountains (INGV-OE reports, 2021), we envision that
the SEC feeding system may include a smaller storage cavity
where gas and magma can accumulate during pre-eruptive
explosive phases. This is illustrated in Fig. 6, which shows
a sketch of the upper magma feeding system of Etna and the
shallow reservoirs respectively feeding the central conduits
(CC) and SEC. According to melt inclusion data and labora-
tory experiments, at 3–2 km beneath SEC (~80–50 MPa) lit-
tle sulphur and no Cl are yet exsolved from Etna trachybasalt
(e.g. Spilliaert et al 2006b; Lesne et al 2011). The high SO₂/
HCl ratio of the magmatic gas phase driving the 2021 lava
fountains (from 6.0 to 8.5; Table 2) is compatible with syn-
eruptive degassing of S and Cl during fast decompression
of a magma that had reached these depths while degassing

<figure><img src="image_15.png" /><figcaption>**Fig. 6** W-E section of Mt Etna magma plumbing system as inferred from a recent seismic tomography based on temporal Vp changes (De Gori et al. 2021). The sketch shows the location of an intermediate reservoir ($R_{int}$) emplaced at between ~4 and 9 km b.s.l., as also inferred from deformation data (Aloisi et al. 2018), and two shallower reservoirs: one feeding the central volcano conduits ($R_{cc}$) and one feeding SEC ($R_{sec}$). The latter was also constrained from strain data recorded during SEC lava fountains in 2020–2022 (Bonnacorso et al. 2021, 2025). We hypothesize here the existence of a shallower storage cavity ($S_{sz}$) at 2 km depth beneath SEC, right at the interface between the volcanic pile and its sedimentary substratum, where pre-eruptive gas-magma accumulation may occur. Temporary shifts in eruptive activity from the Central Craters to SEC likely reflect temporal changes in the distribution of pressurized volatile-rich magma within the upper feeding system of Mt Etna (Modifed from De Gori et al. 2021)</figcaption></figure>

in either an open system (i.e. losing its exsolved gas phase)
or in a closed system at sulphide saturation (Fig. 4). A
fast magma decompression rate agrees with mineralogical
observations of a rapid disequilibrium growth of clinopy-
roxene microlites/microcrysts in the 2021 fountain products
(Mollo et al 2022). However, if rapid magma decompres-
sion and syn-eruptive (closed system) degassing were the
prevalent mechanisms responsible for the 2021 lava fountain
sequence, one also expects a simultaneous bulk release of
both dissolved and exsolved H₂O and CO₂ coexisting with
the magma at the source depth of the fountains. We examine
this aspect in detail below.

Etna trachybasalt has a measured initial water content of
3.4 ± 0.2 wt% at 400 MPa (e.g. Métrich et al. 2004; Spilliaert
et al. 2006a; Corsaro et al. 2009) and an estimated original
CO₂ content of 1.2–1.5 wt% (Allard et al. 1997; Caltabiano
et al. 2004; Spilliaert et al. 2006a). Including S and Cl, the
initial volatile content of the trachybasalt thus totals ~5–5.5
wt%. Modelling of closed-system decompression of the tra-
chybasalt from 400 MPa in the presence of 1.5 wt% initial
gas shows that at 4–2 km beneath SEC (~100–50 MPa),
CO₂ is almost entirely exsolved, and the magma coexists
with 3.2 to 3.6 wt% of a gas phase composed of 74 to 77 mol% of H₂O and 26 to 23 mol% of CO₂ (Métrich et al. 2004; Spilliaert et al. 2006a). This gas phase constitutes 55 to 60% in volume of the magmatic mixture and contains very little sulphur and no Cl (Spilliaert et al. 2006b; Lesne et al 2011).

The high SO₂/HCl ratios measured for the 2021 lava fountains (from 6.0 to 8.5, mean: 7.1 ± 0.9; Table 2) are remarkably similar to the SO₂/HCl ratios previously measured with OP-FTIR spectroscopy during analogous lava fountains at SEC: from 6 to 10 (mean: 7.1 ± 1.9) in 2000–2001 (Allard et al. 2005; La Spina et al. 2015), 6.8 on 25/09/2009, 7.1 on 12/08/2011, and 6.1 on 12/04/2013 (A. La Spina, unpublished data). Such a remarkable consistency of the S/Cl signature of SEC lava fountains over two decades points to reproducible magma degassing processes occurring in a feeding system of quite steady geometry. Based on this chemical consistency and the relatively uniform geochemistry of Etna lavas erupted over these two decades (e.g. Andronico and Corsaro 2011; Corsaro and Miraglia 2014, 2022, 2024; Mollo et al 2015; Giacomoni et al. 2018), we use here the overall mean gas composition accurately measured during SEC lava fountains in 2000–2001 as a proxy to estimate the total gas release during the 17 paroxysms in 2021. Assuming a similar gas composition in 2000–2001 and 2021 is a realistic extrapolation given the similar pattern and similar inferred source mechanism of recurrent lava fountain paroxysms at SEC in 2000–2018 (e.g. Allard et al. 2005; Andronico et al. 2011; Bonnacorso et al. 2011; La Spina et al. 2015; Carbone et al. 2015; Ferlito et al. 2017; Calvari et al 2020). For this calculation, we thus scaled the SO₂ discharge of each 2021 fountain to the mean weight ratios of H₂O, CO₂, and HCl versus SO₂ in the gas phases of the 2000–2001 SEC lava fountains (Table 4)—we neglect here the minor contribution of HF. The computed values range from ~75 to 1260 kton (mean: 540 ± 275 kton; Table 2) and, for a mean duration of 3.8 ± 1.6 h, imply a mean gas emission rate of ~3.5 ± 1.5 × 10⁴ kg/s. Scaling the

total gas discharge of each fountain to the co-erupted amount of tephra (Andronico et al 2021; Calvari et al. 2022), we find that the mass fraction of emitted gas for all the fountains varies from 8.5 to 61 wt% (mean: 25.2 ± 13.8 wt%; Table 2). It thus widely exceeds not only the exsolved gas amount at 4–2 km depth beneath SEC (3.2–3.6 wt%, see above) but also the original bulk volatile content (ca. 5 wt%) of Etna trachybasalt. Scaling the total gas discharges to the bulk amounts of erupted magma, including lava flows, has a less straightforward significance because, in addition to syn-eruptive lava overflowing, the lava flow budget of the fountains (Proietti et al. 2023) also includes pre-fountain effusion of degassed magma left in the SEC conduit after the previous event (e.g. Allard et al. 2005, 2006; Andronico and Corsaro 2011); the relative proportion of which is unknown. Nevertheless, while yielding lower values (mean: 8.0 ± 5.9 wt%), this second comparison still reveals a large excess gas mass fraction (from 7.9 to 26.6 wt%; Table 2) during a half (8) of the fountains.

Hence, we infer that most of the 17 lava fountain paroxysms at SEC in February–April 2021 likely involved a large excess gas release with respect to the co-erupted magma, even when syn-eruptive lava effusion is considered. Such an observation is fully consistent with the hypothesis of a pre-eruptive accumulation of H₂O and CO₂-rich gas beneath SEC before most of the fountains. As a matter of fact, we find that the computed amount of excess gas emitted during the 2021 fountains not only correlates with the CaO/Al₂O₃ ratio of the erupted products (Supplementary material) but also correlates with the length of the repose interval between each fountain (Fig. 7). Such a correlation further supports the idea of a process of pre-eruptive gas accumulation beneath the SEC, the extent of which is proportional to the interval of quiescence between the fountains. We omit two events from this correlation: the first event on 16 February, which happened almost 1 month after the isolated event on January 18 and represents the ‘time zero’ of the

<table><caption>**Table 4** Molar gas ratios of SO₂, H₂O, CO₂, HCl, and HF in magmatic gases driving Etna's lava fountains at SEC in 2000–2001 (Allard et al 2005; La Spina et al 2015). The corresponding mean</caption><thead><tr><th rowspan="2">Lava fountains</th><th colspan="4">Molar gas ratios</th></tr><tr><th>SO₂/HCl</th><th>H₂O/SO₂</th><th>CO₂/SO₂</th><th>HF/SO₂</th></tr></thead><tbody><tr><td>14 June 2000</td><td>10.0</td><td>92</td><td>7.3</td><td>0.05</td></tr><tr><td>13 June 2001</td><td>6.0</td><td>108</td><td>10</td><td>0.08</td></tr><tr><td>17 June 2001</td><td>6.1</td><td>160</td><td>10.3</td><td>0.05</td></tr><tr><td>07 July 2001</td><td>6.4</td><td>146</td><td>11.6</td><td>0.05</td></tr><tr><td>Mean (mole)</td><td>7.1</td><td>126</td><td>9.8</td><td>0.06</td></tr><tr><td>St. dev</td><td>1.9</td><td>32</td><td>1.8</td><td>0.01</td></tr><tr><td>Mean (weight)</td><td>12.7</td><td>36</td><td>6.7</td><td>0.02</td></tr><tr><td>St. dev</td><td>3.4</td><td>9</td><td>1.2</td><td>0.005</td></tr></tbody></table>

weight ratios were used to compute the total gas discharge during each SEC lava fountains in 2021 by scaling to its measured SO₂ budget (see text)
<figure><img src="image_16.png" /></figure>

**Fig. 7** Total gas/tephra mass ratio versus the length of pre-eruptive repose interval for the 15 lava fountains which occurred between 18 February and 24 March 2021. The first and the last fountains of the series were left apart for the following reasons: (i) the first fountain, on 16 February, represents the sequence's initiation ("time zero") and cannot be related to any pre-eruptive interval of gas accumulation; and (ii) uncertainties on the starting time and duration of the last event, on 1 April 2021, introduce a very large error on its erupted tephra amount (see text). Most fountains (blue dots) define a remarkable linear correlation between the two parameters ($R^2=0.98$). The pink area delineates the 95% confidence interval of the regression line. Only three fountains (19 February, 15 and 19 March) define a distinct steeper linear trend (green dots), whose possible explanation is discussed in the text. Because the total gas/tephra ratio (i.e. the emitted gas mass fraction) for almost all fountains widely exceeds the original volatile content of Etna trachybasalt (see text), the observed linear correlations strongly support a pre-eruptive gas accumulation process beneath SEC whose magnitude was proportional to the repose time interval before each lava fountain. The intercept of linear fits was constrained to zero since no detectable $SO_2$ (SEVIRI) nor any tephra were emitted at SEC outside of the fountains

paroxysmal sequence, as well as the ultimate event on the 1st of April, whose uncertain start time and much longer duration (from which the tephra amount was estimated) involve large uncertainties on the gas/tephra ratio. Figure 7 also reveals a distinct, steeper correlation for three of the fountains (on 19 February then 15 and 19 March). While both groups ultimately support a process of pre-eruptive gas accumulation, the distinct trend defined by these three fountains suggests a greater gas-magma decoupling at depth, i.e. enhanced pre-eruptive gas accumulation, linked to transient changes in the magma viscosity or supply rate.

Therefore, we propose that the February–April 2021 series of paroxysmal lava fountains may have involved the combination of two distinct processes:

(i) A pre-eruptive separation and accumulation of $H_2O-CO_2$-rich gas beneath the SEC, promoted by the pressurized influx of volatile-rich primitive magma, which led to the growth of bubble foams

whose periodic collapse and emptying triggered the precursors of the fountains (rapidly increasing tremor and Strombolian activity, outpouring of resident conduit magma) and then the fountains themselves (e.g. Jaupart and Vergniolle 1988). This mechanism determined the periodicity of the lava fountains and controlled the magnitude of their gas discharge (Fig. 7). This same triggering mechanism was previously proposed for the recurrent lava fountain paroxysms which have occurred at SEC since 2000 (e.g. Allard et al 2005; Andronico et al. 2011; La Spina et al 2015; Carbone et al 2015). We suggest that its recurrence results from a specific control of the branching geometry of the SEC feeding system with respect to the main plumbing system feeding the central volcano conduits. Moreover, the alternance of waning/waxing phases of the bulk $SO_2$ plume flux and seismic tremor when the SEC is either highly or weakly active (Fig. 2a) is compatible with the concept that SEC, born in 1971, acts as a kind of secondary pressure valve of Etna's upper magmatic system, where overpressure is released in pulsed rather than continuous mode due to geometrical constraints.

(ii) A syn-eruptive degassing of the more soluble volatile species (S and Cl) upon fast magma decompression and fragmentation during the fountains themselves. This mechanism is strongly supported by the mean $SO_2$/tephra mass ratio and by the close similarity between the total volume of S-degassed magma computed for period 3 with the total volume of erupted tephra ($25 \times 10^6 \text{ m}^3$ versus $\sim 24 \times 10^6 \text{ m}^3$ DRE; Table 3). It is also consistent with studies of the vesicularity and 3D-structure of ejecta from previous SEC lava fountains (Polacci et al 2006) and with the extent of chlorine degassing during closed-system magma degassing and fragmentation (La Spina et al. 2015). The $SO_2$/HCl ratios from 6.0 to 8.5 for the 2021 lava fountains are compatible indeed with syn-eruptive degassing, from between $\sim$2 km and 3 km depth ($\sim$50–80 MPa) beneath the SEC, of a magma that may have reached these depths in intermediate conditions between pure open and closed system, allowing the segregation and gradual accumulation of its exsolved $CO_2-H_2O$-rich gas phase (Fig. 4).

Finally, we emphasize that our $SO_2$ flux data for period 2 support the idea that the shallowing batch of primitive magma was first degassing in an open system through the central volcano conduits during the pressure buildup phase from 5 December 2020 to 13 February 2021. The corresponding amount of total degassed magma ($28 \times 10^6 \text{ m}^3$ DRE; Table 3) was no longer able to provide any $SO_2$ emission during period 3 but may have been partly erupted
together with undegassed primitive magma during the sequence of lava fountains at SEC, in particular as lava flows. This hypothesis is compatible with petrological evidence of magma mixing (Corsaro and Miraglia 2024) and our computation of a relative degassed S deficit for some of the fountains. We note that the cumulative amount of S-degassed magma erupted over both periods 2 and 3 (53×10⁶ m³ DRE) is comparable to the observed or inferred total amount of erupted magma during only period 3 (ca. 60×10⁶ m³ DRE; Table 3), thus raising the possibility that about half of the latter might have consisted of magma previously degassed through the central conduits during period 2.

## Conclusions

We report a comprehensive data set for the SO₂ emissions and SO₂/HCl ratios during passive and eruptive degassing at Etna in the period from 1 June 2020 to 30 April 2021, obtained from both ground-based and space-borne remote sensing tools and compared with simultaneous recording of the seismic tremor. Our data provides further insight into the magmatic processes which preceded then determined the spectacular series of 17 powerful lava fountain paroxysms that occurred at the SEC crater of Etna in February–April 2021.

We show that this paroxysmal sequence was preceded by a pressurization phase that lasted ~2 months, linked to the deep influx of a batch of volatile-rich primitive (more mafic) magma, which led to a rapid increase of the bulk SO₂ plume flux and seismic tremor. This pressure build-up phase was associated with clear geophysical signals and characterized by a 3 times higher magma degassing rate compared to background activity over the previous 6 months. We emphasize that the series of lava fountains started in close conjunction with a sudden drop of the bulk SO₂ flux and seismic tremor, indicating a sudden (seismogenic) shift of the overpressure from the central volcano conduits towards the off-centre branched feeding system of SEC. The SO₂ discharge of the lava fountains partly matches with the proportion of the new primitive magma in co-erupted solid products and, scaled to the co-erupted amount of tephra, is compatible with a synchronous degassing of both sulphur and chlorine upon rapid magma ascent then fragmentation in SEC feeding conduit. In addition, the total gas discharge of the fountains, estimated from geochemical data for previous SEC lava fountains in 2000–2001 whose SO₂/HCl ratios were remarkably similar to those measured in 2021, significantly exceeds the initial volatile content of Etna trachybasalt, implying a pre-eruptive accumulation of H₂O-CO₂ rich gas on top of SEC reservoir prior to most of the 17 events. We find indeed that the mass ratio of total emitted gas with respect to the co-erupted amount of tephra closely correlates with the length of repose

interval between the fountains, which strongly supports a pre-eruptive gas accumulation process before each fountain paroxysm. Hence, we conclude that the February–April 2021 series of paroxysmal lava fountains at Etna resulted from the combination of two distinct degassing processes: a (i) pre-eruptive accumulation of H₂O-CO₂ rich bubble foams on top of the SEC reservoir, whose periodic collapse and emptying triggered the lava fountains, and a (ii) syn-eruptive (closed-system) degassing of more soluble S and Cl during these events. Thus, our study further documents the potential complexity of magma degassing processes controlling lava fountains at Etna in particular and, possibly, at basaltic volcanoes in general.

Finally, our data permit assessment of the cumulative DRE volumes of trachybasaltic magma which were supplied and degassed at Etna during the three distinct activity periods identified in our study. Except for eruptive period 3, these degassed magma volumes generally significantly exceed those concurrently erupted, further evidencing a broad prevalence of the transfer of a free gas phase decoupled from the magma across the central conduits of the volcano (e.g. Allard 1997; Allard et al. 2006). The cumulative amount of degassed magma during both the pressure build-up phase (period 2) and the lava fountaining sequence (period 3), from early December 2020 to April 2021, is twice as large as the total influx of mafic magma estimated from GNSS data over the period from 21 Nov 2019 to 15 Feb 2021 (Palano et al. 2024). Such a discrepancy, previously observed at Etna, goes far beyond what might be explained by magma compressibility and, therefore, deserves future investigation.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s00445-025-01877-2.

**Acknowledgements** We thank Christoph Kern and the anonymous reviewer for their thorough and constructive comments on an earlier version of the manuscript. We are also grateful to a second anonymous reviewer and to Marie Edmonds for their valuable suggestions, which significantly improved the paper. We thank the editor A. Bonforte for handling the manuscript. We acknowledge the technicians and technologists of the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Sezione di Catania, for maintaining the seismic network and enabling the acquisition of seismic data. This study was supported by the IMPACT Project (INGV Department Strategic Projects – 2019). Gas geochemical and Seismic monitoring at INGV-OE was funded by the Italian Civil Protection Department (DPC) within the framework of agreements between INGV and DPC.

**Funding** Open access funding provided by Istituto Nazionale di Geofisica e Vulcanologia within the CRUI-CARE Agreement. This research received funding from the National Institute of Geophysics and Volcanology of Italy (INGV), through the Departmental Strategic Project IMPACT.

**Data availability** The data presented in this article will be made available by the corresponding author upon request.
## Declarations

**Ethics approval** No ethical question is raised by this study.

**Consent for publication** All authors and co-authors have given their consent to be included in this manuscript and to publish it.

**Conflict of interest** The authors declare no competing interests.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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