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 The 11–13 January 2011 eruptive episode at Etna volcano occurred after several months of increasing ash emissions from the summit craters, and was heralded by increasing SO2 output, which peaked at ∼5000 megagrams/day several hours before the start of the eruptive activity. The eruptive episode began with a phase of Strombolian activity from a pit crater on the eastern flank of the SE-Crater. Explosions became more intense with time and eventually became transitional between Strombolian and fountaining, before moving into a lava fountaining phase. Fountaining was accompanied by lava output from the lower rim of the pit crater. Emplacement of the resulting lava flow field, as well as associated lava fountain- and Strombolian-phases, was tracked using a remote sensing network comprising both thermal and visible cameras. Thermal surveys completed once the eruptive episode had ended also allowed us to reconstruct the emplacement of the lava flow field. Using a high temporal resolution geostationary satellite data we were also able to construct a detailed record of the heat flux during the fountain-fed flow phase and its subsequent cooling. The dense rock volume of erupted lava obtained from the satellite data was 1.2 × 106 m3; this was emplaced over a period of about 6 h to give a mean output rate of ∼55 m3 s−1. By comparison, geologic data allowed us to estimate dense rock volumes of ∼0.85 × 106 m3 for the pyroclastics erupted during the lava fountain phase, and 0.84–1.7 × 106 m3 for lavas erupted during the effusive phase, resulting in a total erupted dense rock volume of 1.7–2.5 × 106 m3 and a mean output rate of 78–117 m3 s−1. The sequence of events and quantitative results presented here shed light on the shallow feeding system of the volcano.
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Parfitt and Wilson  have suggested that the primary difference between the so-called Strombolian and Hawaiian events lies in the ability of bubbles to coalesce and grow. They argue that in Hawaiian eruptions there is little coalescence due to fast ascent rates, so that eruptive activity is controlled by the exsolution of many small bubbles at the fragmentation surface. In contrast, they propose that Strombolian activity is fed by the bursting of large gas bubbles, or slugs, at the magma free surface, where the slugs can form by coalescence in more slowly ascending magma. This model differs from that of Vergniolle and Jaupart , who explain the transition from Strombolian and Hawaiian activity using a series of conduit flow regimes, which change depending on gas content, bubble size, and magma viscosity. They suggested that, rather than remaining as a homogeneous flow, Hawaiian lava fountaining may involve transitions from bubbly flow to annular flow, in which there is a central stream of gas bounded by liquid moving along the conduit walls. Vergniolle and Jaupart  also argued that Strombolian eruptions involve transitions from bubbly flow to slug flow, in which large bubbles of gas develop and rise through the residual bubble-poor melt. Using a combination of theoretical models and laboratory experiments, Jaupart and Vergniolle  also showed that ascending bubbles can form a foam layer at the roof of a magma reservoir. When the foam layer reaches a threshold thickness, the bubbles coalesce and the foam collapses, generating a slug that enters and ascends the conduit to burst at the free surface. After some time, a new foam layer forms, thickens and collapses to repeat the cycle. Analyses of magmatic gas measurements during lava fountain events at Etna volcano suggest activity is fed by a gas bubble layer that accumulates prior to the event at a depth of about 1.5 km below the erupting crater [Allard et al., 2005]; thus supporting a gas-melt separation model rather than a bulk degassing (rise-speed-dependent) model [Parfitt, 2004]. This has been confirmed by other recent multidisciplinary data comprising petrochemistry of ejecta, gravimetry, seismicity, ground and deformation measurements [Andronico and Corsaro, 2011; Bonaccorso et al., 2011a, 2011b].
 Etna's activity recently moved toward more explosive styles of eruption, which have characterized activity especially since 2000 [e.g., Behncke et al., 2006; Andronico and Corsaro, 2011; Harris et al., 2011]. Thus, the need to understand, recognize and predict such explosive activity is becoming increasingly important at this basaltic system. In this paper we present an integration of remote sensing data collected from a ground-based camera network installed on Etna by Istituto Nazionale di Geofisica e Vulcanologia of Catania (INGV-CT), with that collected by satellite-based sensors. The ground-based cameras provide both thermal and visible images and, for our study, were supplemented by use of thermal images collected during ground-based surveys carried out to map the lava flows. The satellite data are taken from MSG's SEVIRI sensor, a sensor that provides infrared data every 15 min and allows us to obtain the time-averaged discharge rates (TADRs) during short-lived lava fountain events [Harris et al., 2011; Vicari et al., 2011]. In addition, SO2 released by open vent degassing at the summit craters, and recorded by the FLAME scanning ultraviolet spectrometer network [Salerno et al., 2009a, 2009b] were available, along with FTIR measurements collected before and after the eruptive phase. These near-infrared-to-ultraviolet remote sensing measurements were used to estimate degassing rates and the volume of magma intruded within the system, as well as to track the gas flux and composition before, during, and after the 11–13 January 2011 lava fountain event. This integrated remote sensing data set allowed us to reconstruct the eruptive sequence, quantify the erupted volume, and compare the erupted and intruded magma volumes, thus allowing us to constrain the eruptive processes taking place in the feeder conduit and to improve our ability to forecast and track future explosive events.
2. Recent Eruptive Events at Etna
 The recent eruptive history of Etna volcano has been characterized by frequent effusive episodes, with more than 40 flank eruptions occurring during the 20th century [e.g., Andronico and Lodato, 2005; Branca and Del Carlo, 2005]. Explosive eruptions have also been common, and are of some concern due to the hazard posed by the associated ash plumes: the 2001 and 2002–2003 eruptions both caused severe disruption to Catania's international airport. During both events eruptive columns spread ash all over southern Italy [Research Group of the Istituto Nazionale di Geofisica e Vulcanologia-Sezione di Catania, Italy, 2001; Behncke and Neri, 2003; Andronico et al., 2005], with some ash reaching Cefalonia in Greece, 500 km away [Dellino and Kyriakopoulos, 2003]. The last effusive event occurred in 2008–2009, and comprised an initial phase of lava fountaining that fed an eruptive column, and was accompanied by lava flows that spread within the Valle del Bove (VdB), reaching over 6 km in length [Bonaccorso et al., 2011a, 2011b]. Following this eruption, the volcano remained largely quiescent until 2010, during which time its summit craters were actively degassing. A first explosive phase occurred at the SE-Crater (SEC; Figure 1c) on 8 April 2010, and produced an ash plume that covered the uppermost NE sector of the volcano with ash, and a small pyroclastic flow. On 25 August 2010 another intense explosive phase occurred at the Bocca Nuova crater (Figure 1c). This was characterized by an ash emission that lasted several seconds and spread ash over the uppermost flanks of the volcano. Weaker explosive phases were recorded during December 2010 from a depression (pit crater) on the east flank of SEC at 3050 m a.s.l. (Figure 1c), and on 2 January 2011 a further explosive event was observed at this site. Explosions continued until 3 January, and were characterized by pulsating gas bursts, red-glowing at night and sometimes accompanied by small ash emissions. This explosive activity stopped on 3 January and resumed on 11 January, when a short but intense eruptive phase began at SEC. The eruptive activity that occurred between 11 and 13 January is the topic of the present paper.
3.1. Thermal and Visible Camera Network
 Mount Etna's camera network consists of thermal and visible cameras that allow continuous, real-time ground-based imaging of the volcano activity every 1–2 s [e.g., Andò and Pecora, 2006; Behncke et al., 2006, 2009]. The network consists of two thermal cameras, EMOT and ENT, and five visible cameras: EMOV, ENV, EMV, ESV, and ECV (see Figure 1 and Table 1 for locations).
Table 1. Details on the INGV-CT Network of Monitoring Camerasa
Distance From Etna's Summit
Field of View/Range (m)
Here (v) = vertical; (h) = horizontal. See Figure 1 for site location.
2600 m a.s.l.
18.8° (v) - 25° (h)
2600 m a.s.l.
3° to 47.5°(h)/170 to 2860 m (h)
1985 m a.s.l.
2.8° to 48°(h)/260 to 4720 m (h)
770 m a.s.l.
3° to 47.5°(h)/592 to 9944 m (h)
730 m a.s.l.
18° (v) - 24° (h)
730 m a.s.l.
3° to 47.5°(h)/785 to 13,200 m (h)
Catania - CUAD
35 m a.s.l.
3° to 47.5°(h)/1388 to 23,320 m (h)
 EMOT and ENT are equipped with an A320 and an A40M Thermovision Forward Looking InfraRed (FLIR Systems) camera, respectively. Both record in the 7.5 and 13 μm spectral range, providing 320 × 240 pixel images with a spatial resolution of 1.3 μrad. The A320 and A40M have thermal sensitivities of 70 mK at 30°C, and 80 mK at 25°C, respectively. While EMOT thermal images are displayed with a fixed color scale that ranges between −20 and 60°C, ENT images are displayed with a fixed color scale with a range of −10 and 60°C. Radiometric data, recorded between 0 and 500°C, are processed in real-time by custom written code (NewSaraterm) [Behncke et al., 2009]. The visible cameras at EMOV, ENV, ECV, and EMV consist of a Canon VC-C4 with a 16 × optical zoom lens. This camera provides a horizontal field of view (FOV) of between ∼3 and 47.5° (Table 1). The visible camera at ESV is a Sony FCBEX 480 CP with FOV of between ∼2.8 and 48° (Table 1). Given the variable focus of the visible cameras (Table 1), to calculate the size of any object within the FOV we used reference distances between known targets within each image. For nighttime images recorded by ECV, we used the vertical distance between the pit crater and the Rifugio Sapienza tourist facility (1920 m a.s.l.; Figure 1). This yielded a vertical distance of 1130 m and was used to estimate the ash column height as well as the length of the lava flows spreading toward the east until they reached ∼2170 m a.s.l. At this elevation the VdB rim hides the lava flows from the ECV camera view.
 EMOT (Figure 2), EMV (Figure 3) and ECV (Figures 4d–4f) provided the best quality information and images during the 11–13 January eruptive event. In particular, using images from EMOT we derived the frequency of the Strombolian activity by manually counting the number of explosions across 15 min time windows (the duration of each archived video clip). The frequency of Strombolian bursts increased to a point at which the discrete bursts became uncountable. At this point we measured the height, width and area of the saturated portion of the explosive cloud. Images provided by EMV allowed us to derive the area covered by the upper portion of the lava flow field (up to ∼2200 m a.s.l.) and to track its stagnation and cooling (Figures 3a–3h and 5b). They also allowed us to observe the ash emissions that followed the end of the fountaining event (Figure 3i). Images from EMOT and ECV allowed tracking of the explosive activity (Figure 5a), with ECV providing an almost complete view of the ash column emitted by the lava fountain. ENT also showed both the lava fountains and associated ash columns, as well as dust clouds rising above the active lava flow fronts (Figures 4g–4j).
3.2. Thermal Surveys
 Ground-based thermal surveys, carried out on 13 and 14 January 2011, allowed imaging of the lower part of the lava flow field, i.e., that spreading below 2200 m a.s.l. within VdB (Figure 6). The camera used was a FLIR SC660 handheld thermal camera. This camera consists of a 640 × 480 uncooled microbolometer-detector array sensitive across the 7.5–13 μm spectral range. It has a 18 × 24°FOV, records temperature with a precision of ±1% (±1°C) and has a sensitivity of 0.08°C at 30°C. The camera allows recording of images in three temperature ranges: −40 to 120°C, 0 to 500°C and 350 to 1500°C, at time steps of up to 30 Hz. Thermal imagery of the lava flow field was collected using the middle range (0–500°C) at a frame rate of 4 images per sec. Air temperature and relative humidity were recorded simultaneously with thermal imagery and used to apply a first-order correction for atmospheric effects. For emissivity we have used 0.98 [Buongiorno et al., 2002]. See Spampinato et al.  for full review of thermal camera operation and data processing for volcanological applications.
3.3. Gas Flux Measurements
 SO2 flux at Mount Etna is measured by the Flux Automatic Measurement (FLAME) network of scanning ultraviolet spectrometers [Salerno et al., 2009a]. The network consists of eight stations spaced ∼7 km apart and installed at an altitude of ∼900 m a.s.l. on Etna's southern, eastern and northern flanks (Figure 1). During daylight, each device scans the sky in a vertical plane over 156° (almost horizon-to-horizon) intersecting the plume at a distance of ∼14 km from the summit region. In each scan, 104 open-path ultraviolet spectra are collected. SO2 slant column densities are reduced from each spectrum following the Differential Optical Absorption Spectroscopy (DOAS) methodology [e.g., Platt and Stutz, 2008] using a modeled reference spectrum [Salerno et al., 2009b]. SO2 column densities are then transmitted by Free Wave radio-modem to INGV-CT, where SO2 mass flux (in megagram per day, Mg d−1) is computed in real-time.
 HCl and HF fluxes were calculated by combining the SO2 flux with the molar ratios of SO2/HCl and SO2/HF measured during daily surveys. Ratios were determined from solar occultation open-path Fourier Transform InfraRed (FTIR) spectra, in which the infrared source was the sun and the gas plume was interposed between the sun and the spectrometer, following the methods of Francis et al. . Spectra were collected with a Bruker OPAG-22 spectrometer with a ZnSe beam splitter and a 0.5 cm−1 resolution. The detector was a liquid nitrogen-cooled Mercury-Cadmium-Telluride (MCT) sensitive between 1000 and 6000 cm−1. The gas column amounts were retrieved using a nonlinear least square fitting program based on the Rodgers optimal estimation algorithm [Rodgers, 2000] and the Oxford Reference Forward Model (RFM) radiative transfer model (http://www.atm.ox.ac.uk/RFM/), using line parameter data from the HITRAN96 molecular spectroscopic database [Rothman et al., 1998]. Solar occultation mode provides information on the concentrations of SO2, HCl and HF, which are three gas species with negligible concentrations in the free troposphere, but which are abundant within volcanic plumes [e.g., Sparks et al., 1997]. The uncertainty on retrieved gas amounts was calculated using the residual of the least square fitting, and was ∼4%. Ratios were determined by measuring 100 or more spectra. The retrieved amounts of SO2 were then plotted against HCl and HF. The gradient of the resulting linear regression plots give the ratios of SO2/HCl and SO2/HF [e.g., La Spina et al., 2010].
3.4. Satellite Data
 The satellite time series comprised the full archive of MSG-SEVIRI data acquired during the lava fountaining phase by the direct reception capability at the Observatoire de Physique du Globe de Clermont-Ferrand (OPGC, Clermont Ferrand, France). The SEVIRI (Spinning Enhanced Visible and InfraRed Imager) sensor is flown on the Meteosat Second Generation (MSG) satellite. This flies in a geostationary orbit above the Equator over Africa at an altitude of 35,000 km. From its equatorial location, SEVIRI can image Etna once every 15 min. Thus we built a time series of 96 images for the 24 h period spanning the main lava fountain phase. We use data collected in SEVIRI's IR3.9 (3.48–4.36 μm) and IR12 (11.00–13.00 μm) channels. While the wavelength of the IR3.9 channel is sensitive to sub-pixel hot spots, that of IR12 is useful for characterizing the temperature of the ambient background [e.g., Wright and Flynn, 2004]. Both channels have a spatial resolution of 3 km. Radiance data from all hot spots identified in the IR3.9 channel, corrected for atmospheric, surface emissivity and reflection effects, were used to estimate the heat flux and lava discharge rate for the active, and cooling, flow field following a modified version of the methodology of Harris et al. . The full methodology as applied to the SEVIRI data is described by Gouhier et al.  and Vicari et al. .
4. The 11–13 January Eruptive Episode
Figure 6 shows the main area impacted by the eruption, showing the SEC, the pit crater responsible for the eruptive events described here, the lava flow field emplaced within the VdB, and the summit area of Mount Etna (in the background). The eruptive activity began at the pit crater on 11 January, though poor weather conditions permitted its observation only from EMOT and then only after 17:50 (all times reported are UTC). The eruption fed Strombolian activity that was initially confined to the pit crater (Figure 6), as evident from observed glow (red in Figure 2a). Between 20:30 and 21:45, the number of explosions increased and the frequency became quite regular (Figure 5a), with explosions reaching a height of ∼30 m above the pit rim at 23:30. After this peak, both the frequency and intensity of the events decreased to a lower, but steady level, before decreasing further between 00:45 and 01:30 on 12 January. The frequency of explosions increased again later in the morning (especially between 09:45 and 10:00; Figures 2a and 5a) with spatter being erupted from two vents. Strombolian activity intensified further between 17:45 and 18:00 with ejecta being emitted in several directions and bombs falling well beyond the pit crater rim (Figures 2b and 5a). By 18:38 we could not distinguish the vents; this suggested that the temperature within the pit crater was so high that the two vents had formed a unique saturated area as visible from EMOT (Figure 2c). After ∼19:15 the lower sector of the pit also began to produce occasional explosions, with activity at up to three explosive vents. At 20:20, lava began to flow from the lower rim of the pit crater (Figure 3a), which slowly spread toward the SE. At 20:49 a second lava flow covered the upper part of the lava channel feeding the initial flow, and the explosion frequency and ejecta height increased (Figure 3b).
 After 21:15 explosions became almost continuous (Figures 2c–2d), suggesting shift from Strombolian to a transitional eruptive style [e.g., Parfitt, 2004; Spampinato et al., 2008]. At 21:27 explosive activity increased further (Figure 3c), and a third lava flow appeared, while spatter were covering the upper part of the lava flow field. About 15 min later, the new lava flow was followed by a fourth flow. This new flow spread over the uppermost portion of the channel that fed the previous flows, and large incandescent blocks detached from the flow front to roll-off downslope. Between 21:44 and 21:47, spatter began to cover most of the northern outer flank of the SEC. Coverage was sufficient so as to form a rheomorphic or rootless lava flow [Head and Wilson, 1989]. Meanwhile, the pit crater began to feed a fifth lava flow (Figure 3d). At this point, the velocity and forward propagation of the lava flow fronts was observed to increase steadily (Figure 5b), and the height of the ash column grew (Figures 4a–4b and Table 1).
 At ∼21:50, spattering from the pit crater became steady and the emission style evolved to fountaining (Figures 2e, 4g, and 4j). Two minutes later, both the height of the lava fountains and ash column increased significantly, with most of the tephra spreading SSW (Figures 2e, 3e, and 4c). Increased explosive activity was accompanied by collapses on the southern flank of the SEC. By 21:59 a thick ash plume was apparent (Figures 2f, 3f, and 4e), and the lava flow field within the VdB continued to spread in three branches (Figure 3f). A second rootless flow also began to form due to tephra remobilization in the same site as the previous rheomorphic flow (Figure 3f). Explosive activity peaked between 22:00 and 23:00 on 12 January when the maximum height of the lava fountains reached ∼800 m above the pit (compare Figures 2 and 3), and the apparent temperature recorded by ENT increased from ∼80°C to 120–160°C (Figure 4j). By 22:05 four lava flows were spreading down the western flank of the upper VdB. At this time, the lava fountains also expanded eastward (Figures 2e–2f) and became higher. Increased explosive and effusive activity was accompanied by erosion of the eruptive vent. By 22:20 lava fountains widened further and also became taller (Figures 2f–2g and 5b). At 22:29 two main jets could be distinguished suggesting that two explosive vents were still active within the pit crater (Figure 3g). Successively, more flows covered the upper part of the lava flow field which, between 22:00 and 23:30, reached its largest active area (Figure 5b). Three more lava flows erupted at ∼22:42, 22:59, and 23:13, to feed a final total of nine lava flows (Figures 2g–2i).
 After 23:10 the lava fountaining intensity decreased substantially (Figures 2j–2l, 3h, 4j, and 5b), with the last small vertical jet of lava showing an apparent eastward displacement of the explosive vent by ∼110 m to the east, due to widening of the pit rim. This modified vent location was also ∼30 m lower than that of the initial vent (Figures 2b–2l). This outline pit crater enlargment was directly observed during an overflight in the following days. After 23:24 lava output also declined and the distal portion of the lava flow field began to cool (Figures 3h, 4j, and 5b). Lava flows were still active on the north flank of SEC, though, probably fed by collapse of hot tephra emplaced during the lava fountaining phase. Slow movements of the lava flow fronts continued until ∼23:59.
 At midnight, the eruptive activity returned to Strombolian style, with a few discrete explosions feeding small lava flows that covered the proximal lava flow field to the east and south. At ∼00:25 on 13 January shallow explosions at the pit crater were observed. These produced a hot gas cloud that rose several hundred meters. Collapses were apparent at the lava flow fronts of the shorter south and north flows emplaced on the flanks of the SEC. These were probably caused by destabilization of the eruptive products on the steep slopes of the SEC, as has often been observed at Etna after major explosive phases [e.g., Calvari and Pinkerton, 2002; Behncke et al., 2003].
 After 00:55 the flow field within the VdB displayed considerable surface cooling (Figure 5b), although the north and south flows on the SEC flanks were still slowly moving as the channels drained. At 01:00 glow from the pit crater also waned significantly. By 02:17, all lava flow movement had halted (Figure 5b), even if localized flows and collapses of the flow fronts were observed until ∼06:00; representing post-emplacement reorganization of the lava flow field. Explosive activity at the pit crater was over by 04:15.
 Between 06:15 and 06:45 only impulsive degassing (gas puffing) was observed, and between 07:22 and 07:35 pulsating dark ash plumes were emitted from the pit. These were likely from collapses inside the crater, but were possibly associated to deep explosive activity. After 08:17 ash became reddish and more dilute, with emission continuing until 09:22 and suggesting collapses within the pit crater following drainage [e.g., Bertagnini et al., 1990; Calvari and Pinkerton, 2004]. No further emissions from the pit crater were detected after ∼13:00 on 13 January.
5.1. Gas Flux Between 2 and 19 January 2011
Figure 7 shows both long and short-term variations in the 7-day-running mean of the SO2, HCl and HF fluxes, with the long-term plot spanning May 2010 to January 2011. Overall, the three gas species showed correlated behavior, though sometimes they displayed decoupling. Note that, in Figure 7a, in order to plot both the HCl and HF fluxes together on the secondary y axis, we have had to multiply the HF flux by 5. Hence, in Figure 7a the real values of HF are actually a fifth of the fluxes plotted. Between May 2010 and July 2010, SO2, HCl and HF fluxes showed trends which remained steadily confined within 1300–2400 Mg d−1, 130–260 Mg d−1, and 118–136 Mg d−1 for the three species, respectively (Figure 7a). From the second half of July 2010, the three geochemical signals displayed pulsating but increasing trends, that concordantly climaxed in November 2010, when fluxes reached 4800, 963, and 640 Mg d−1 for SO2, HCl and HF, respectively (Figure 7a). After this period the three emission rates declined to values of 1500, 314, and 123 Mg d−1 by January 2011 (Figure 7a).
Figure 7b is a zoom that details the gas flux temporal variations between 2 and 19 January 2011, a period including the eruptive phase. During these 18 days of observations, the SO2 fluxes were constantly recorded, except on 1, 3 and 4 January when the wind (and thus plume) direction was toward a sector of the volcano not covered by the FLAME network. The daily averaged SO2 emission rates varied between a minimum of 500 Mg d−1 (on 19 January) and a maximum of 3400 Mg d−1 (on 8 January), with the mean daily SO2 emission rate being 2000 Mg d−1 (standard deviation, 1σ = 800 Mg d−1). Figure 7b shows three main peaks on 8, 11 and 13 January, when mean daily SO2 emission rates of over ∼3000 Mg d−1 were recorded. These are followed by a generally decreasing trend, with SO2 fluxes decreasing to ∼500 Mg d−1 by 19 January.
 The HCl and HF fluxes were obtained by FTIR measurements on 11 and 14 January, respectively (i.e., before and after the 11–13 January 2011 eruptive episode). The SO2/HCl and SO2/HF molar ratios were 2.5 and 6.6 on 11 January, and 2.9 and 16.3 on 14 January, resulting in HCl and HF fluxes of 470 and 100 Mg d−1 on 11 January, and 300 and 30 Mg d−1 on 14 January. Both HCl and HF fluxes then show a marked decline between 11 and 14 January.
Figure 7c displays a further zoom, plotting the daily averaged SO2 fluxes measured (during daytime) between 10 and 14 January, and thus recorded before and after the main eruptive event of 12 January. Over this period, the SO2 flux shows a cyclic pattern, with maxima recorded on 11 and 13 January (when values peaked at 5000 and 4200 Mg d−1, respectively) and a minimum of 650 Mg d−1 on 12 January (Figures 7c and 8). On 10, 12, 13, and 14 January the variance was approximately half that measured on 11 January. Knowing the total elemental sulphur released between 1 August 2010 and 10 January 2011 by SO2 flux measurements, the cumulative quantity of degassed magma was calculated following Allard . This yielded a volume of ∼32 × 106 m3.
5.2. Strombolian and Lava Fountain Activity
 During the morning of 11 January, the pit crater on the east flank of the SEC started to show pulsating degassing, with explosive activity first observed at 17:50 the same day. Figure 5a displays the number of explosions with time. An overall generally increasing trend can be seen after 00:45 on 12 January, which is overprinted by cycles of waxing and waning activity lasting 3–4 h and increasing in wavelength with time (Figure 5a). No data were available between 10:30 and 16:00 due to obscuration by thick meteorological clouds. After 20:15 the Strombolian activity passed to transitional, with explosions being so continuous that they were almost uncountable (Figure 4j). After ∼21:50 the transitional style changed to lava fountaining (Figures 2d–2e, 3d–3e, 4b–4c, and 4j). Thus, from 21:15 onwards we measured the height, width and area occupied by the lava fountains as seen from the two positions of EMV and EMOT (Figures 3c–3h, 5b, and 9a–9c). We note that, although not measuring exactly the same parameter, the heights measured from the two locations show comparable values, although at times EMOT recorded lower values. We interpret this as being due to ash fallout obscuring the fountain from the EMOT thermal camera view. Fountain activity ceased at 23:50, having lasted 2 h and 35 min. The maximum height reached by the lava fountains was between 750 m (measured from EMV) and 830 m (measured from EMOT) and occurred at 22:21 on 12 January (see also Figure 5b). Maximum fountain width was recorded at between 420 m (measured from EMV) and 516 m (measured from EMOT), with peaks of up to 550 m (Figure 9b). During the peaks the presence of fallout was adding to the apparent width of the eruptive column, making the EMOT measurement larger than the EMV measurement (which was not so affected by fall out). Between 21:56 and 22:59, the estimated height of the ash column rising above the fountain, measured from ECV, was ∼6 km (Figures 4d–4f). This value has to be considered a minimum because this height marks the upper limit of our FOV, and the ash continued to rise upwards and out of the camera FOV. The ash column then drifted SSW in the wind. By 23:00 the lava fountaining was declining, with heights to ∼200 m. We selected 53 frames from EMV and the corresponding 53 frames recorded from EMOT at time intervals of 180 s. We then extracted the lava fountain heights from the mean value of each pair of frames. We use these values to estimate initial velocity at the vent (v0) to account for the measured height (h), using
Derived velocities span 33 m s−1 at 21:15 to 125 m s−1 at 22:21. After this time there was a gradual decrease in the velocity until midnight, corresponding to the decline in the maximum height of the lava fountains. Using ground- and helicopter-based photos collected by INGV-CT during surveys following the eruptive episode, we estimated a diameter for the vent at the bottom of the pit crater of ∼30 m. We used this value to estimate vent area assuming a circular shape which, with the exit velocity of the ejecta, allows us to calculate the volume flux of magma passing through the vent to feed the lava fountains. The total erupted volume of vesiculated material is then obtained from integrating these volume fluxes through time. Given that this was vesiculated material, and that the lava fountain jets comprised a mixture of pyroclasts and gases, to obtain the dense rock equivalent (DRE) erupted volume we assumed that the jet comprised 0.35% of magma. This value is suggested by Parfitt  as being typical for lava fountains, such as those occurred at Kīlauea in 1983 [Wolfe et al., 1988]. The resulting DRE volume erupted during the 2 h and 35 min of lava fountaining is ∼0.85 × 106 m3, giving a mean output rate of ∼92 m3 s−1 for just the pyroclastic portion of this eruptive event. This result is in good agreement with estimates obtained during previous lava fountaining events at Etna [e.g., Behncke et al., 2006] and has to be considered a minimum value for the entire episode, because it does not take into account the tephra erupted during the 26 h of Strombolian activity that preceded the lava fountaining phase.
5.3. Lava Flow Field
 The emplacement of the upper lava flow field across the western headwall of the VdB was tracked and quantified using images recorded by EMV (Figures 3a–3i and 5b). In carrying out this analysis, we have to bear in mind that the portion of the lava flow field that spread beyond the EMV FOV was not included in the image (Figure 3). Thus, the flow field area below the ∼2200 m elevation was not accounted for in this analysis. Using NI Vision Assistant software we selected a fixed color threshold to crop the whole lava flow field in each image. We then used this to calculate the area covered by active lava flows, and thus to quantify the change in area covered by active lava flows with time (Figure 5b). The length of the active lava flows was obtained from ECV, which provided a side view until ∼22:00, when the flows began spreading within the VdB. Thus, although the complete development of the lava flow field could not be observed, we do have a rather good description of the growth of the uppermost three-quarters of it (Figure 5b). The area covered by the active lava flows displays two peaks at 22:25 and 22:40. Considering that lava fountain area peaked at 22:15 and 22:24 (Figures 5b and 9c), it is thus possible that each peak in the lava fountain area is related to a subsequent peak in the lava flow field area. In fact, we observed that rapid accumulation of spatter on the upper part of the SEC cone, and on the upper portion of the lava flow field, was followed by remobilization of this loose material to form rootless flows which increased the supply to the flow field.
 The first lava flow emerged from the lowest point on the pit crater rim at 20:20 on 12 January, and lava flow fronts stopped final movement at 02:17 on 13 January, giving a total emplacement time of ∼6 h. Ground-based thermal imagery collected during the mornings of 13 and 14 January 2011 from the south rim of the VdB showed that the stationary flow fronts were located at ∼1650 m a.s.l. (Figures 6 and 10). At the time of the thermal surveys, the lava flow field displayed low temperatures, with maximum temperatures across the distal area being between 330° and 430°C on 13 January and ∼160°C on 14 January. The higher temperatures recorded on 13 January were due to lava channel drainage that locally disrupted the lava crust. At that time the lava front also experienced lateral spreading, promoted by the low topographic gradient of the lower section of the VdB, as the flow field underwent post-emplacement reorganization before finally solidifying. Oblique thermal imagery of the proximal area recorded on 14 January showed maximum apparent temperatures of ∼200°C. During both surveys, no explosive activity from the SEC pit crater was observed, and maximum temperatures recorded on the eastern flank of the SEC on 14 January did not exceed 170°C.
 The lava flow field displayed simple morphological structures, and lacked ephemeral vents, lava tubes or tumuli, in agreement with the morphology expected for short duration, high effusion rate flow [e.g., Calvari and Pinkerton, 1998, 1999; Duncan et al., 2004]. During the initial stages of lava output, the lava flow spread at low rate, and displayed surface structures similar to large-scale folds. The supply to the lava flow field then increased due to the contribution of the fallout from the lava fountain (Figure 5b), as well as by overflow from the pit crater that eventually formed nine lava flow units that overlapped along the upper part of the south lava channel. These flow units overlapped with the rheomorphic or rootless flows resulting by flowage of the proximal spatter covering the flanks of SEC (Figure 11).
 Maximum (final) lava flow field area and length were derived using thermal and visible images obtained from a helicopter survey on 19 January, and were 1.07 km2 and 4.3 km, respectively [Behncke et al., 2011] (Figures 6 and 10). Using this area, with a minimum and maximum bound on the mean flow field thickness of 1 and 2 m, and an average vesicularity of ∼22% [Harris et al., 2005], we obtained a DRE volume of the lava flow field between ∼0.83 and 1.77 × 106 m3, resulting in mean output rates for the lava flow field of between ∼38 and 77 m3 s−1. Considering also the DRE volume of pyroclastics erupted (∼0.85 × 106 m3), we obtain a total erupted DRE volume (for all products: lava + pyroclasts) of between ∼1.7 and 2.5 × 106 m3, of which the pyroclastic component comprised ∼20%. The total volume yields a mean output rate for pyroclastics and lava at between 78 and 116 m3 s−1.
5.4. SEVIRI-Derived Heat Flux Trend and TADR Measurements
 The onset of effusive activity was apparent in the SEVIRI data from a hot spot that developed from 20:00 onwards on 12 January. The heat flux continued to wax through 21:00 when the hot spot became obscured by the plume associated with the most explosive phase of the episode (Figure 12). By the time the plume cleared to allow the hot spot to be detected once more, lava flow activity had reached such an extent that the IR3.9 data were saturated. Termination of saturation at 01:00 coincides with the termination of supply to the lava flow field from the vent. Thereafter we recorded a cooling curve, as the flows stagnated and began to cool, with the hot spot becoming unresolvable by 11:00 on 13 January (Figure 12). This trend has also been reported by Vicari et al. . Perturbations in the otherwise smooth cooling curve, such as those apparent during 03:00 and 04:00, may be due to late stage flows as the lower sections of the channels drained and the flow field underwent a final re-organization.
 Converting the heat flux to a TADR for the eruptive period of the time series, i.e., between 20:00 and 01:00, yields a TADR that climbs to ∼15 m3 s−1 during the first hour and a half of effusion (Figure 12). Thereafter, the record has a gap, during which time the flow field was obscured by the overlying plume until ∼23:00. At this point we record a minimum possible value (capped by saturation) of ∼30 m3 s−1. Obscuration by the plume, as well as saturation, during the period of peak discharge mean that time-integration of TADRs to obtain total effused volume will yield an underestimate [Gouhier et al., 2011]. Therefore, we modified the approach of Wooster et al.  to estimate the total effused volume. Wooster et al.  integrated heat fluxes obtained from satellite thermal data during the cooling phase of Etna's 1991–1993 flow field to obtain the total power generated by the lava during cooling. Given a cooling curve, we can thus integrate the heat flux through time to estimate the total power released (in Joules) by the cooling lava. This can, in turn, be converted to the mass or volume of lava that needs to be cooled in order to liberate that power [see Rowland et al., 2003]. This conversion methodology, as applied to the SEVIRI data, is explained by Gouhier et al. .
 By integrating the power under the cooling curve we obtain a value for the total power release during cooling of 460 GJ. Converting this to a volume of lava that needs to be cooled by 50°C, we obtain a lava volume of 1.2 × 106 m3 [Gouhier et al., 2011]. Distributing this volume over the period of effusion, and removing the volumes known to be erupted during the ash-cloud-free phase, we find that 83% (or 106 m3) of the total volume was erupted during the period of peak effusion that spanned 21:30 – 01:00. This gives a TADR over this 3.5-h-long period of peak effusion of 80 m3 s−1. By comparison, ground-based thermal camera measurements yielded a total DRE volume of 1.7–2.5 × 106 m3, which is roughly in agreement with that calculated using the satellite data, as well as that calculated by Vicari et al.  using the same SEVIRI data set. As discussed by Gouhier et al. , the discrepancy may be explained by some uncertainties on parameters used at the input of the satellite-based retrieval scheme, or error in the thickness assumption used for the thermal-camera-based extraction. However, these results show that comparable volumes are obtained using three independent methods. This lends confidence to the measurement.
 Intermittent explosive events at Etna are usually preceded and/or accompanied by major changes in the volcanic gas composition and flux rates [e.g., Caltabiano et al., 1994, 2004; Andronico et al., 2005; Burton et al., 2005; Salerno et al., 2009a]. Magma contains dissolved volatiles (H2O, CO2, S, Cl, F, etc.) with different solubilities, each of which gradually reaches a saturation pressure and exsolves into a separate magmatic gas phase (bubbles) during magma ascent [e.g., Anderson, 1975; Carroll and Holloway, 1994; Oppenheimer, 2003]. Thus, during magma ascent toward the surface, the chemical composition of the gas phase changes following the pressure-controlled solubility of each volatile species, but also as a function of the dynamics of magma supply and ascent [Sparks, 2003]. Melt inclusion studies indicate that S, Cl and F start to exsolve at confining pressures of ∼140, 100, and <10 MPa, respectively. These pressure values are equivalent to depths of ∼4–5, 3, and 1 km, respectively [e.g., Carroll and Webster, 1994; Spilliaert et al., 2006]. At Etna, prior to the 11–13 January 2011 lava fountaining event, significant temporal changes were observed in SO2, HCl and HF fluxes beginning in August 2010 (Figure 7a), with the daily mean SO2, HCl and HF fluxes displaying an increasing trend that peaked in November 2010. The progressive increase in gas fluxes likely marked the gradual supply of gas-rich magma into the shallow feeding system (1–4 km depth). After the November peak, gas fluxes gradually decreased reflecting a decline in the magma supply rate and the end of intrusion of volatile-rich magma into the shallow feeding system [Allard et al., 2006; La Spina et al., 2010]. Over the following two months, gas exsolution and bubble nucleation in the stored batch of magma would have led to volatile saturation and overpressure, the January 2011 eruptive activity being the result.
 The January 2011 eruptive episode lasted ∼32 h, with the first 26 h being characterized by increasing Strombolian activity, passing into a six-hour-long transitional phase, to culminate with lava fountaining. Such behavior is quite common at Etna, and normally marks the gradual transition from a two-phase slug flow regime to annular flow [e.g., Jaupart and Vergniolle, 1989; Vergniolle and Mangan, 2000; Parfitt, 2004]. Combining acoustic and experimental data, Vergniolle and Ripepe  proved that, at Etna's SEC, a lava fountain results from a closely spaced sequence of Strombolian explosions that reflect the dynamics of a foam trapped in the crater reservoir, which is located at a depth of ∼2 km [Allard et al., 2005; Spilliaert et al., 2006; La Spina, 2010], with more active foam coalescence occurring during lava fountaining when compared to Strombolian activity.
 The 11 January 2011 Strombolian phase was characterized by linear growth in the number of events (by ∼20 explosions every 4 h). The constant rate of growth in the explosive activity with time implies a steady increase in gas exsolution from the conduit magma. Following the onset of Strombolian activity, the SO2 flux showed a pulsing decline until 11:00 on 12 January. This behavior was independent from the explosive activity observed at the surface, which passed to transitional at 21:15 of 11 January. The period of low SO2 flux marked a phase of poor degassing efficiency of the feeding system, as previously observed during Etna's 2002–2003 south flank eruption [Spampinato et al., 2008]. On that occasion, the November 2002 lava fountaining phase was preceded by a decrease of the SO2 flux that then increased to record the highest peak ever measured at Etna only after the onset of lava fountaining [e.g., Andronico et al., 2005; Spampinato et al., 2008; Steffke et al., 2011]. Similarly, in 2011 the eruptive activity climaxed with lava fountaining, which was followed by high SO2 flux values recorded the next day (on 13 January). The lava fountaining phase, in fact, may have renewed the degassing efficiency of the system. Once the degassing regime came back to its equilibrium, the SO2 flux, as well as the HCl and HF fluxes, gradually declined to the pre-eruptive rates levels. This model suggests that the January 2011 eruptive event was fed by a small batch of gas-rich magma that had been stored in the shallow feeding system for at least two months prior to the eruptive event.
 The 11–13 January fountain was also accompanied by lava output. This suggests that degassing of the stored magma also resulted in a volatile-depleted magma that accumulated in the shallow conduit. This magma was pushed out of the conduit, filled the pit crater and, eventually, overflowed from its lower rim. In fact, expansion of the lava volume stored in the shallow system, might have forced the degassed lava at higher levels in the conduit out of the vent and into the pit, in a process similar to that envisaged for gas-piston-related lava flow emplacement [Johnson et al., 2005]. Unloading of the magma column may have been aided by further decompression of the shallow feeding system. This likely led to development of trains of more closely spaced slug sequences, that would have induced the change from Strombolian, to transitional and, then, to lava fountaining activity. Similar to other short-lived eruptive events at Etna, the final stage of the activity was then characterized by crater wall collapses to produce reddish ash emissions, revealing conduit drainage, removal of crater-wall support, and collapse.
 Based on previous eruptive cycles at Etna, the high intensity and the brief duration of the 11–13 January 2011 event leads us to propose that it was only the first episode of a sequence of paroxysmal events that will, if the pattern of previous cycles repeats itself, culminate in a longer effusive eruption. Our assessments are supported by an estimate of the volume of degassed magma available for eruption, and that actually erupted so far. The amount of magma degassed between 1 August 2010 and 10 January 2011, and thereby assumed resident in the shallow system and available for eruption, is ∼32 × 106 m3. If we compare this with the total volume (lava + tephra) erupted during the 11–13 January 2011 event (1.5–2.7 × 106 m3), we obtain a ratio between the intruded and erupted magma of between 10:0.5 and 10:1.0. Thus, only 5–10% of the available magma was erupted by the 11–13 January 2011 event, and ∼32 × 106 m3 remains stored in the volcano's shallow supply system. At Etna, Allard  and Allard et al.  showed that, over long time-scales, only a small part of the total degassed magma is erupted, with the long-term ratio between degassed to erupted magma of ∼4. Hence, considering the difference between the intruded and erupted magma volumes estimated here we propose that, in order to obtain the intruded/extruded ratio that characterizes Etna, further paroxysmal eruptive events and/or a longer eruptive phase are required.
7. Conclusive Remarks
 The extremely short duration of Etna's 11–13 January eruptive episode meant that remote sensing data collected at high temporal resolutions from ground-based cameras and geostationary satellites were ideal for tracking its explosive and effusive character. Using a range of remote sensing data sets, we calculated that a total volume of ∼1.7–2.5 × 106 m3 DRE was erupted in ∼6 h of intense explosive and effusive activity, with a TADR in the range ∼78–116 m3 s−1. The building, pulsing pattern of Strombolian activity observed in the preceding hours, suggested release of a small gas-rich batch of magma from a body previously intruded into the shallow system. The similarity between this event and others previously tracked on Etna, leads us to suggest that the January 2011 event may be the prelude to a new phase of activity that may terminate in a longer, higher volume eruption. This hypothesis was partially supported by three additional eruptive events, all characterized by an initial explosive phase and followed by a short-lived fountaining and lava flow that occurred on 18 February, 9–10 April, and 8–12 May 2011; while this manuscript was in review. These eruptive episodes were all fed by the same vent, i.e., the pit crater on the east flank of SEC. The January 2011 event was the strongest of these four events. If we make a rough estimate of the total erupted volume between January and May as being four times that erupted in January, we are still well below the ∼4 ratio proposed by Allard  and Allard et al. , and thus can still expect for further eruptive events to follow shortly (three occurred on 9, 18–19, and 24–25 July 2011 while this manuscript was undergoing final checking by the co-authors).
 The authors wish to thank Alfio Amantia for kindly providing his excellent airborne photos, Stefano Branca for the photo of Figure 9a, and Michele Prestifilippo for his assistance with the handling of camera videos and images. This paper benefited of discussions and data comparison with Alessandro Bonaccorso, Ciro Del Negro, Tania Ganci, and Anna Vicari. We acknowledge the precious work of the INGV-CT technical staff on the installation and maintenance of the camera network used here: Michele Prestifilippo and Francesco Ciancitto, and of those dealing with the FLAME network and FTIR measurements: Filippo Muré, Tommaso Caltabiano, and Daniele Randazzo. This manuscript benefited greatly from the encouragement, comments, and suggestions of Alessandro Bonaccorso, Thor Thordarson, an anonymous reviewer, and the Editor André Revil.