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Section of the emerged part of Stromboli volcano, redrawn after Apuani et al. (2005) and the model of its inner structure with the displacement of the shallower section of Northeast Crater conduit. Abbreviations are as follows: PS – Paleostromboli unit; PS II – Paleostromboli II unit; NS – Neostromboli unit; PI – Pizzo Unit; SC – Sciara Unit; MV – Middle Vancori; UV – Upper Vancori. The shallow landslides affecting the volcanic deposits located between the summit craters and Pizzo Sopra la Fossa might significantly contribute to the inclination of some volcanic jets. See the text for further details.
Source publication
Twenty eruptive events from the Northeast Crater of Stromboli volcano recorded by a thermal monitoring camera in early 2004 were analysed in order to understand the eruptive dynamics.
Selected events were chosen to be typical of explosions that characterize the steady activity of Stromboli
in terms of jet height and duration. Most of the explosions...
Contexts in source publication
Context 1
... of sound) nor a perturbation produced by the arrivals of P-seismic waves at the soil–air boundary (the halo is observed only around the volcanic jet and not around the profile of the crater), they should be interpreted as air perturbation linked to the eruptive jet. The rapid release of pressurized gas during the burst of a gas slug prompts an air wave (e.g. Braun & Ripepe, 1993; Johnson, 2002), which propagates above the crater and precedes the development of the main volcanic jet. The thermal peak shown in Figure 4 may be explained by the sudden compression and decompression of the plume mixture crossed by the wave. This flashing air wave is evident in Figure 5, where a low-temperature halo, enveloping the eruptive jet and lasting only a few fractions of a second, is shown. When the air wave reaches the crater rim, it initiates a rapid lateral expansion, moving horizontally, at a velocity that is, on average, twice that of the volcanic jet. This phenomenon is thought to be always present, but it was evident only in the most powerful explosions showing a vertical blast. On the contrary, it was not observed in low fountain-like explosions. The presence of an air wave during explosive events has been reported during observations of Vulcanian explosions (e.g. Perret, 1912; Nairn, 1976; Livshits & Bolkhovitinov, 1977; Yokoo, Ichihara & Taniguchi, 2004; Yokoo & Taniguchi, 2004), but not previously reported from Strombolian-type explosions. A few ideas can be put forward as to the nature of the other peaks shown in Figures 3 and 4, in the absence of synchronized high-resolution visible images. To be cautious it may be noted that the flashing wave is always soon followed by the expansion of a dense cloud of magmatic gases and quite small ( < 138 cm) incandescent ejecta, which constitute most of the visible erupted material. The velocity values measured for the eruptive jets are in agreement with those reported by Chouet, Hamisevicz & McGetchin (1974); Blackburn, Wilson & Sparks (1976); Weill et al. (1992); Ripepe, Rossi & Saccorotti (1993), Ripepe, Ciliberto & Della Schiava (2001); Ripepe (1996); Hort & Seyfried (1998); Hort, Seyfried & Voge (2003); Patrick (2007) and Patrick et al. (2007). Ejecta of an appreciable size ( > 138 cm) become visible only after 1.5 s after the onset of the event (Fig. 2g–q). The ejection of this coarse material can last for a few seconds, forming something like a very small-volume lava fountain. The presence of large hot bombs cannot be excluded during the first development phases of an eruptive jet, due to the high density of the particles contained in it. Single clasts reach heights that are comparable with those of the mass erupted some instants before. The material hurled from the vent, considered as a whole, becomes buoyant at a height between 70 and 115 m, commonly 2 seconds (s) after emission from the conduit. Once the maximum height allowed by the kinetic energy of the explosion is reached, the volcanic mixture remains stable for a few fractions of a second, and then starts to expand laterally and to rise, because of the development of thermals above the crater (Fig. 4a). The minimum diameter of the base of the volcanic jets from our observations was calculated to be about 6–8 m, in agreement with direct visual observations and with measurements from the helicopter during monitoring activities. Nine out of twenty jets dip ∼ 7–13 ◦ towards the NW, regardless of wind direction. In the other cases, vertical explosions were observed. These were likely generated by gas slugs bursting on the surface of a magmatic column whose free surface is very close to the base of the vent. In this case, volcanic jets were not deflected by the conduit walls and could expand freely, generating vertical, fountain-like explosions, with pyroclasts falling symmetrically around the crater. Inclined jets could be generated by deep-seated slug bursts in an inclined conduit. In this case, the shape of these jets may be constrained by the conduit walls, and pyroclasts are ejected towards the steep slope of the Sciara del Fuoco. In the images taken by the optical monitoring camera located at Pizzo Sopra La Fossa, the same inclined explosions recorded by the IR camera appear to be vertical. However, although the FLIR thermal camera captures a lateral view of the eruptive vent, it is located in an almost orthogonal position with respect to the optical camera (Fig. 1a) and for this reason the recorded inclination of the eruptive jets is considered to represent the true value. On the other hand, the inclination of jets might also be due to the morphology of the shallow conduit that undergoes continuous remodelling after each explosion. The same inclination of the Strombolian jets is evident in some images published by Patrick et al. (2007). These features could be explained by the seaward inclination of the conduit in its upper segment, which might be linked to the sliding movement affecting the Sciara Unit (Fig. 6). Indeed, the volcanic material constituting the two youngest depositional units of Stromboli (Pre-Sciara and Sciara units) has undergone a series of differential displacements (Arrighi et al. 2004; Apuani et al. 2005) (Fig. 6). Starting from < 5.6 ± 3.3 ka (Gillot & Keller, 1993), the Pre-Sciara Unit, made up mainly of lava flows, probably underwent a small rotational slump in the area below the craters. Such a displacement might have caused the displacement of the feeding system with the formation of a sort of siphon, probably located 220– 260 m below the craters, along the sliding plane. The presence of this siphon, as suggested by Ballestracci (1982), hinders the normal ascent rate of gas pockets from depth, playing a role in the regularity of the summit activity of Stromboli. A second more feasible explanation envisages the inclination of the very shallow portion of the conduit (a few tens of metres) related to small-volume landslides and rockfalls into the crater terrace below Pizzo Sopra la Fossa (Fig. 6). These events are triggered by both the seismic activity that accompanies the explosive activity and by the collapse of the Northeast Crater walls due to the height variation of the magma column free surface inside the conduits. This process was highlighted during the 2007 eruption (Neri, Lanzafame & Acocella, 2008), when continuous obstruction of the crater forced ascending magma to find a way out where principal stresses offered the minimum resistance, that is, towards the NW. As a result, the uppermost segment of the conduit is characterized by asymmetric walls (Fig. 7) which can deviate and bow the volcanic jets generated by the explosions. Stromboli volcano is one of the main concerns of Italian Civil Protection, due to the proximity of urbanized areas to the craters and the numerous tourists who climb the volcano every day. Continuous monitoring of its activity is of fundamental importance for the safety of the inhabitants of the island and for the application of innovative methodologies to comprehend eruptive mechanisms. Installation of surveillance devices is one of the greatest efforts of the scientific community. After the violent explosions of 5 April 2003 and 15 March 2007, the damaged or destroyed monitoring network on the island was completely substituted with new equipment, which is providing new data of better quality. This study, based on images of a series of low-energy Strombolian explosions from the Northeast Crater, captured by the thermal monitoring camera, provides new information about the dynamics of Strombolian activity and the role of the conduit morphology and the summit instability. The careful observation of sequences of explosions has made it possible to outline some features of Strombolian explosions. In particular, it was evident that a perturbation is visible for a few fractions of a second after the generation of the volcanic jet. Similar perturbations had been captured in the past by high- speed shutter cameras during great explosive events (mostly Vulcanian) in other volcanoes, and interpreted as air waves. Their presence during Strombolian eruptions was only hypothesized after the application of other geophysical methodologies but they were never detected. By integrating these data with the results achieved by other authors, it was demonstrated that summit instability could play an important role in the present geometry of the conduit of the Northeast Crater and in the constant and rhythmic occurrence of explosions at Stromboli volcano since the fourth century BC . The morphology of the upper segment of the conduit appears to be affected by continuous remodelling, which could be the reason for the temporary seaward inclination by tens of metres of the shallow portion of the conduit itself. This inclination could be also related to the sliding movement of the Sciara del Fuoco and/or to the small-scale landslides (rockfalls) that involve the area located immediately below the Pizzo Sopra la Fossa and that subsequently fall into the ...
Context 2
... linked to the eruptive jet. The rapid release of pressurized gas during the burst of a gas slug prompts an air wave (e.g. Braun & Ripepe, 1993; Johnson, 2002), which propagates above the crater and precedes the development of the main volcanic jet. The thermal peak shown in Figure 4 may be explained by the sudden compression and decompression of the plume mixture crossed by the wave. This flashing air wave is evident in Figure 5, where a low-temperature halo, enveloping the eruptive jet and lasting only a few fractions of a second, is shown. When the air wave reaches the crater rim, it initiates a rapid lateral expansion, moving horizontally, at a velocity that is, on average, twice that of the volcanic jet. This phenomenon is thought to be always present, but it was evident only in the most powerful explosions showing a vertical blast. On the contrary, it was not observed in low fountain-like explosions. The presence of an air wave during explosive events has been reported during observations of Vulcanian explosions (e.g. Perret, 1912; Nairn, 1976; Livshits & Bolkhovitinov, 1977; Yokoo, Ichihara & Taniguchi, 2004; Yokoo & Taniguchi, 2004), but not previously reported from Strombolian-type explosions. A few ideas can be put forward as to the nature of the other peaks shown in Figures 3 and 4, in the absence of synchronized high-resolution visible images. To be cautious it may be noted that the flashing wave is always soon followed by the expansion of a dense cloud of magmatic gases and quite small ( < 138 cm) incandescent ejecta, which constitute most of the visible erupted material. The velocity values measured for the eruptive jets are in agreement with those reported by Chouet, Hamisevicz & McGetchin (1974); Blackburn, Wilson & Sparks (1976); Weill et al. (1992); Ripepe, Rossi & Saccorotti (1993), Ripepe, Ciliberto & Della Schiava (2001); Ripepe (1996); Hort & Seyfried (1998); Hort, Seyfried & Voge (2003); Patrick (2007) and Patrick et al. (2007). Ejecta of an appreciable size ( > 138 cm) become visible only after 1.5 s after the onset of the event (Fig. 2g–q). The ejection of this coarse material can last for a few seconds, forming something like a very small-volume lava fountain. The presence of large hot bombs cannot be excluded during the first development phases of an eruptive jet, due to the high density of the particles contained in it. Single clasts reach heights that are comparable with those of the mass erupted some instants before. The material hurled from the vent, considered as a whole, becomes buoyant at a height between 70 and 115 m, commonly 2 seconds (s) after emission from the conduit. Once the maximum height allowed by the kinetic energy of the explosion is reached, the volcanic mixture remains stable for a few fractions of a second, and then starts to expand laterally and to rise, because of the development of thermals above the crater (Fig. 4a). The minimum diameter of the base of the volcanic jets from our observations was calculated to be about 6–8 m, in agreement with direct visual observations and with measurements from the helicopter during monitoring activities. Nine out of twenty jets dip ∼ 7–13 ◦ towards the NW, regardless of wind direction. In the other cases, vertical explosions were observed. These were likely generated by gas slugs bursting on the surface of a magmatic column whose free surface is very close to the base of the vent. In this case, volcanic jets were not deflected by the conduit walls and could expand freely, generating vertical, fountain-like explosions, with pyroclasts falling symmetrically around the crater. Inclined jets could be generated by deep-seated slug bursts in an inclined conduit. In this case, the shape of these jets may be constrained by the conduit walls, and pyroclasts are ejected towards the steep slope of the Sciara del Fuoco. In the images taken by the optical monitoring camera located at Pizzo Sopra La Fossa, the same inclined explosions recorded by the IR camera appear to be vertical. However, although the FLIR thermal camera captures a lateral view of the eruptive vent, it is located in an almost orthogonal position with respect to the optical camera (Fig. 1a) and for this reason the recorded inclination of the eruptive jets is considered to represent the true value. On the other hand, the inclination of jets might also be due to the morphology of the shallow conduit that undergoes continuous remodelling after each explosion. The same inclination of the Strombolian jets is evident in some images published by Patrick et al. (2007). These features could be explained by the seaward inclination of the conduit in its upper segment, which might be linked to the sliding movement affecting the Sciara Unit (Fig. 6). Indeed, the volcanic material constituting the two youngest depositional units of Stromboli (Pre-Sciara and Sciara units) has undergone a series of differential displacements (Arrighi et al. 2004; Apuani et al. 2005) (Fig. 6). Starting from < 5.6 ± 3.3 ka (Gillot & Keller, 1993), the Pre-Sciara Unit, made up mainly of lava flows, probably underwent a small rotational slump in the area below the craters. Such a displacement might have caused the displacement of the feeding system with the formation of a sort of siphon, probably located 220– 260 m below the craters, along the sliding plane. The presence of this siphon, as suggested by Ballestracci (1982), hinders the normal ascent rate of gas pockets from depth, playing a role in the regularity of the summit activity of Stromboli. A second more feasible explanation envisages the inclination of the very shallow portion of the conduit (a few tens of metres) related to small-volume landslides and rockfalls into the crater terrace below Pizzo Sopra la Fossa (Fig. 6). These events are triggered by both the seismic activity that accompanies the explosive activity and by the collapse of the Northeast Crater walls due to the height variation of the magma column free surface inside the conduits. This process was highlighted during the 2007 eruption (Neri, Lanzafame & Acocella, 2008), when continuous obstruction of the crater forced ascending magma to find a way out where principal stresses offered the minimum resistance, that is, towards the NW. As a result, the uppermost segment of the conduit is characterized by asymmetric walls (Fig. 7) which can deviate and bow the volcanic jets generated by the explosions. Stromboli volcano is one of the main concerns of Italian Civil Protection, due to the proximity of urbanized areas to the craters and the numerous tourists who climb the volcano every day. Continuous monitoring of its activity is of fundamental importance for the safety of the inhabitants of the island and for the application of innovative methodologies to comprehend eruptive mechanisms. Installation of surveillance devices is one of the greatest efforts of the scientific community. After the violent explosions of 5 April 2003 and 15 March 2007, the damaged or destroyed monitoring network on the island was completely substituted with new equipment, which is providing new data of better quality. This study, based on images of a series of low-energy Strombolian explosions from the Northeast Crater, captured by the thermal monitoring camera, provides new information about the dynamics of Strombolian activity and the role of the conduit morphology and the summit instability. The careful observation of sequences of explosions has made it possible to outline some features of Strombolian explosions. In particular, it was evident that a perturbation is visible for a few fractions of a second after the generation of the volcanic jet. Similar perturbations had been captured in the past by high- speed shutter cameras during great explosive events (mostly Vulcanian) in other volcanoes, and interpreted as air waves. Their presence during Strombolian eruptions was only hypothesized after the application of other geophysical methodologies but they were never detected. By integrating these data with the results achieved by other authors, it was demonstrated that summit instability could play an important role in the present geometry of the conduit of the Northeast Crater and in the constant and rhythmic occurrence of explosions at Stromboli volcano since the fourth century BC . The morphology of the upper segment of the conduit appears to be affected by continuous remodelling, which could be the reason for the temporary seaward inclination by tens of metres of the shallow portion of the conduit itself. This inclination could be also related to the sliding movement of the Sciara del Fuoco and/or to the small-scale landslides (rockfalls) that involve the area located immediately below the Pizzo Sopra la Fossa and that subsequently fall into the ...
Context 3
... in the most powerful explosions showing a vertical blast. On the contrary, it was not observed in low fountain-like explosions. The presence of an air wave during explosive events has been reported during observations of Vulcanian explosions (e.g. Perret, 1912; Nairn, 1976; Livshits & Bolkhovitinov, 1977; Yokoo, Ichihara & Taniguchi, 2004; Yokoo & Taniguchi, 2004), but not previously reported from Strombolian-type explosions. A few ideas can be put forward as to the nature of the other peaks shown in Figures 3 and 4, in the absence of synchronized high-resolution visible images. To be cautious it may be noted that the flashing wave is always soon followed by the expansion of a dense cloud of magmatic gases and quite small ( < 138 cm) incandescent ejecta, which constitute most of the visible erupted material. The velocity values measured for the eruptive jets are in agreement with those reported by Chouet, Hamisevicz & McGetchin (1974); Blackburn, Wilson & Sparks (1976); Weill et al. (1992); Ripepe, Rossi & Saccorotti (1993), Ripepe, Ciliberto & Della Schiava (2001); Ripepe (1996); Hort & Seyfried (1998); Hort, Seyfried & Voge (2003); Patrick (2007) and Patrick et al. (2007). Ejecta of an appreciable size ( > 138 cm) become visible only after 1.5 s after the onset of the event (Fig. 2g–q). The ejection of this coarse material can last for a few seconds, forming something like a very small-volume lava fountain. The presence of large hot bombs cannot be excluded during the first development phases of an eruptive jet, due to the high density of the particles contained in it. Single clasts reach heights that are comparable with those of the mass erupted some instants before. The material hurled from the vent, considered as a whole, becomes buoyant at a height between 70 and 115 m, commonly 2 seconds (s) after emission from the conduit. Once the maximum height allowed by the kinetic energy of the explosion is reached, the volcanic mixture remains stable for a few fractions of a second, and then starts to expand laterally and to rise, because of the development of thermals above the crater (Fig. 4a). The minimum diameter of the base of the volcanic jets from our observations was calculated to be about 6–8 m, in agreement with direct visual observations and with measurements from the helicopter during monitoring activities. Nine out of twenty jets dip ∼ 7–13 ◦ towards the NW, regardless of wind direction. In the other cases, vertical explosions were observed. These were likely generated by gas slugs bursting on the surface of a magmatic column whose free surface is very close to the base of the vent. In this case, volcanic jets were not deflected by the conduit walls and could expand freely, generating vertical, fountain-like explosions, with pyroclasts falling symmetrically around the crater. Inclined jets could be generated by deep-seated slug bursts in an inclined conduit. In this case, the shape of these jets may be constrained by the conduit walls, and pyroclasts are ejected towards the steep slope of the Sciara del Fuoco. In the images taken by the optical monitoring camera located at Pizzo Sopra La Fossa, the same inclined explosions recorded by the IR camera appear to be vertical. However, although the FLIR thermal camera captures a lateral view of the eruptive vent, it is located in an almost orthogonal position with respect to the optical camera (Fig. 1a) and for this reason the recorded inclination of the eruptive jets is considered to represent the true value. On the other hand, the inclination of jets might also be due to the morphology of the shallow conduit that undergoes continuous remodelling after each explosion. The same inclination of the Strombolian jets is evident in some images published by Patrick et al. (2007). These features could be explained by the seaward inclination of the conduit in its upper segment, which might be linked to the sliding movement affecting the Sciara Unit (Fig. 6). Indeed, the volcanic material constituting the two youngest depositional units of Stromboli (Pre-Sciara and Sciara units) has undergone a series of differential displacements (Arrighi et al. 2004; Apuani et al. 2005) (Fig. 6). Starting from < 5.6 ± 3.3 ka (Gillot & Keller, 1993), the Pre-Sciara Unit, made up mainly of lava flows, probably underwent a small rotational slump in the area below the craters. Such a displacement might have caused the displacement of the feeding system with the formation of a sort of siphon, probably located 220– 260 m below the craters, along the sliding plane. The presence of this siphon, as suggested by Ballestracci (1982), hinders the normal ascent rate of gas pockets from depth, playing a role in the regularity of the summit activity of Stromboli. A second more feasible explanation envisages the inclination of the very shallow portion of the conduit (a few tens of metres) related to small-volume landslides and rockfalls into the crater terrace below Pizzo Sopra la Fossa (Fig. 6). These events are triggered by both the seismic activity that accompanies the explosive activity and by the collapse of the Northeast Crater walls due to the height variation of the magma column free surface inside the conduits. This process was highlighted during the 2007 eruption (Neri, Lanzafame & Acocella, 2008), when continuous obstruction of the crater forced ascending magma to find a way out where principal stresses offered the minimum resistance, that is, towards the NW. As a result, the uppermost segment of the conduit is characterized by asymmetric walls (Fig. 7) which can deviate and bow the volcanic jets generated by the explosions. Stromboli volcano is one of the main concerns of Italian Civil Protection, due to the proximity of urbanized areas to the craters and the numerous tourists who climb the volcano every day. Continuous monitoring of its activity is of fundamental importance for the safety of the inhabitants of the island and for the application of innovative methodologies to comprehend eruptive mechanisms. Installation of surveillance devices is one of the greatest efforts of the scientific community. After the violent explosions of 5 April 2003 and 15 March 2007, the damaged or destroyed monitoring network on the island was completely substituted with new equipment, which is providing new data of better quality. This study, based on images of a series of low-energy Strombolian explosions from the Northeast Crater, captured by the thermal monitoring camera, provides new information about the dynamics of Strombolian activity and the role of the conduit morphology and the summit instability. The careful observation of sequences of explosions has made it possible to outline some features of Strombolian explosions. In particular, it was evident that a perturbation is visible for a few fractions of a second after the generation of the volcanic jet. Similar perturbations had been captured in the past by high- speed shutter cameras during great explosive events (mostly Vulcanian) in other volcanoes, and interpreted as air waves. Their presence during Strombolian eruptions was only hypothesized after the application of other geophysical methodologies but they were never detected. By integrating these data with the results achieved by other authors, it was demonstrated that summit instability could play an important role in the present geometry of the conduit of the Northeast Crater and in the constant and rhythmic occurrence of explosions at Stromboli volcano since the fourth century BC . The morphology of the upper segment of the conduit appears to be affected by continuous remodelling, which could be the reason for the temporary seaward inclination by tens of metres of the shallow portion of the conduit itself. This inclination could be also related to the sliding movement of the Sciara del Fuoco and/or to the small-scale landslides (rockfalls) that involve the area located immediately below the Pizzo Sopra la Fossa and that subsequently fall into the ...
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Citations
... These were characterised by initial high velocities, stable ascent velocities, and buoyant phases. Zanon et al. (2009) used thermal images recorded from the INGV fixed stations, for characterising weak explosions from the NE summit crater of Stromboli. Through thermal image analysis, Zanon et al. (2009) derived jet heights, sizes, and durations. ...
... Zanon et al. (2009) used thermal images recorded from the INGV fixed stations, for characterising weak explosions from the NE summit crater of Stromboli. Through thermal image analysis, Zanon et al. (2009) derived jet heights, sizes, and durations. Moreover, the authors observed that eruptive jets were either vertical or inclined (7-13°) towards the NNW. ...
... Moreover, the authors observed that eruptive jets were either vertical or inclined (7-13°) towards the NNW. The inclination was interpreted to be a consequence either of (1) the partial obstruction of the uppermost segment of the NE crater conduit; or (2) of the displacement of the crater inner conduit induced by explosive activity; or (3) of the instability of the volcano summit area producing likely deformation of the conduit (Zanon et al., 2009). ...
... Whereas in nature, gas-particle ejection and jet inclination might be influenced by inclined conduits (Zanon et al. 2009), debris coverage (Capponi et al. 2016), variable explosion depth (Dürig et al. 2015;Salvatore et al. 2018), preexisting craters (Graettinger et al. 2015;Taddeucci et al. 2013) or an inhomogeneous high-viscosity layer (Kelfoun et al. 2020). In this study, we can exclude all of these factors and investigate the sole effect of subsurface initial conditions (pressure, particle size and density) and vent geometry. ...
Explosive volcanic eruptions eject a gas-particle mixture into the atmosphere. The characteristics of this mixture in the near-vent region are a direct consequence of the underlying initial conditions at fragmentation and the geometry of the shallow plumbing system. Yet, it is not possible to observe directly the sub-surface parameters that drive such eruptions. Here, we use scaled shock-tube experiments mimicking volcanic explosions in order to elucidate the effects of a number of initial conditions. As volcanic vents can be expected to possess an irregular geometry, we utilise three vent designs, two “complex” vents and a vent with a “real” volcanic geometry. The defining geometry elements of the “complex” vents are a bilateral symmetry with a slanted top plane. The “real” geometry is based on a photogrammetric 3D model of an active volcanic vent with a steep and a diverging vent side. Particle size and density as well as experimental pressure are varied. Our results reveal a strong influence of the vent geometry, on both the direction and the magnitude of particle spreading and the velocity of particles. The overpressure at the vent herby controls the direction of the asymmetry of the gas-particle jet. These findings have implications for the distribution of volcanic ejecta and resulting areas at risk.
... Eruptive activity is known to vary on several scales, including -but not limited to -eruption frequency and height, erupted volume, grain size distribution, pyroclast temperature and jet directionality [e.g. Andronico et al. 2009;Harris and Ripepe 2007;Taddeucci et al. 2013b;Zanon et al. 2009]. Changes in eruption characteristics may be influenced by observable topographic changes of active vents (shape, size, depth, open/closed, rim height) [e.g. ...
... These studies suggest the presence of a NE-SW trending structural weak zone that coincides with the direction of dykes exposed in the edifice. Zanon et al. [2009] analysed explosions from Instituto Nazionale di Geofisica e Vulcanologia's (INGV) monitoring webcams and observed inclined jets which they linked to conduit geometry or conduit inclination. Calvari et al. [2014] proposed that morphology changes between 2002 and 2007, together with a massive collapse of the summit crater, have modified the shallow feeder conduit, leading to changes in the eruptive style (i.e. ...
... This change occurred over few days and supports the assumption that the new direction of the explosions was a result of a modified exit geometry of the vent ( Figure 1E, F). Previous authors have linked inclined explosions at Stromboli to conduit inclination [Zanon et al. 2009]. We suggest that, for the two cases presented here (S1, May 2019; S2, July 2019), conduit incli-nation played no role in the change of direction of the explosions and that the inclined explosions were solely based on the asymmetric crater and/or vent geometry. ...
Active volcanoes are typically subject to frequent substantial topographic changes as well as variable eruption intensity,
style and/or directionality. Gravitational instabilities and local accumulation of pyroclasts affect conditions at the active vents, through which gas-particle jets are released. In turn, the vent geometry strongly impacts the eruption characteristics. Here, we compare five high-resolution topographic data sets (<4 cm/pixel) of volcanic
craters and vents from Stromboli volcano, Italy, that were acquired by unoccupied aerial vehicle (UAV) during five field campaigns between May 2019 and January 2020. This period includes two paroxysmal explosions (3 July and 28 August 2019) and exhibited significant changes on day-to-month timescales. Our results highlight changes
to vent geometry and their strong control on the directionality of explosions. Recurrent UAV surveys enable the monitoring of temporal morphologic changes and aid the interpretation of observed changes in eruption style. Ultimately, this may contribute to repeatedly revised risk areas on permanently active volcanoes, especially those that
are important tourist destinations.
... Conversely, paroxysms are those impacting the settled areas, located 1.5 km beyond the craters. The main problem with Barberi's et al. [12] classification is that major explosions may or not impact the summit area of Il Pizzo as a result not of the explosion magnitude and intensity, but of the vents shape [27,[62][63][64] and/or wind speed and direction (e.g., [57,58]). ...
Strombolian activity varies in magnitude and intensity and may evolve into a threat for the local populations living on volcanoes with persistent or semi-persistent activity. A key example comes from the activity of Stromboli volcano (Italy). The “ordinary” Strombolian activity, consisting in intermittent ejection of bombs and lapilli around the eruptive vents, is sometimes interrupted by high-energy explosive events (locally called major or paroxysmal explosions), which can affect very large areas. Recently, the 3 July 2019 explosive paroxysm at Stromboli volcano caused serious concerns in the local population and media, having killed one tourist while hiking on the volcano. Major explosions, albeit not endangering inhabited areas, often produce a fallout of bombs and lapilli in zones frequented by tourists. Despite this, the classification of Strombolian explosions on the basis of their intensity derives from measurements that are not always replicable (i.e., field surveys). Hence the need for a fast, objective and quantitative classification of explosive activity. Here, we use images of the monitoring camera network, seismicity and ground deformation data, to characterize and distinguish paroxysms, impacting the whole island, from major explosions, that affect the summit of the volcano above 500 m elevation, and from the persistent, mild explosive activity that normally has no impact on the local population. This analysis comprises 12 explosive events occurring at Stromboli after 25 June 2019 and is updated to 6 December 2020.
... Asymmetrical gas-particle jets and eruption plumes have been described for large, pyroclastic density current issuing eruptions (Cole et al. 2015;Lagmay et al. 1999;Major et al. 2013). Inclined jets have also been observed for less energetic eruptions for example, at Stromboli Volcano, Italy, where inclination of the shallow plumbing system beneath the active craters in February 2004 was proposed by Zanon et al. (2009). Nine out of twenty observed jets exhibited a dip of around 7-13°towards the northwest regardless of wind direction. ...
... They stated that the inclined jets could be generated by a combination of deep-seated slug bursts within an inclined conduit. However, it was also reported that the morphology of the Northeast Crater was characterised by a deep and wide opening in the north-western crater wall at the time of the survey (Zanon et al. 2009). This kind of crater asymmetry is equivalent to those reported by Lagmay et al. (1999) and might deserve some consideration in accounting for the jet inclination for supersonic jets at Stromboli Volcano. ...
Many explosive volcanic eruptions produce underexpanded starting gas-particle jets. The dynamics of the accompanying pyroclast ejection can be affected by several parameters, including magma texture, gas overpressure, erupted volume and geometry. With respect to the latter, volcanic craters and vents are often highly asymmetrical. Here, we experimentally evaluate the effect of vent asymmetry on gas expansion behaviour and gas jet dynamics directly above the vent. The vent geometries chosen for this study are based on field observations. The novel element of the vent geometry investigated herein is an inclined exit plane (5, 15, 30°slant angle) in combination with cylindrical and diverging inner geometries. In a vertical setup, these modifications yield both laterally variable spreading angles as well as a diversion of the jets, where inner geometry (cylindrical/ diverging) controls the direction of the inclination. Both the spreading angle and the inclination of the jet are highly sensitive to reservoir (conduit) pressure and slant angle. Increasing starting reservoir pressure and slant angle yield (1) a maximum spreading angle (up to 62°) and (2) a maximum jet inclination for cylindrical vents (up to 13°). Our experiments thus constrain geometric contributions to the mechanisms controlling eruption jet dynamics with implications for the generation of asymmetrical distributions of proximal hazards around volcanic vents.
... The July/August 2019 eruption fits exactly into this trend, but it was characterized by explosive intensities/frequencies never reached in recent decades. This evolution could be determined by a modification of the shape of the uppermost central conduit, which has become progressively wider and warmer due to the movements that occurred in the Sciara del Fuoco after the 2002 landslide events, and due to the graben-like collapses of the summit craters that occurred in 2007 [4][5][6]59]. Therefore, the present geometric configuration of the shallow portion of the feeder conduit seems to decrease the probability of lateral dike intrusions and to favor an increase of activity at summit craters [6]. ...
On July 3, 2019 a rapid sequence of paroxysmal explosions at the summit craters of Stromboli (Aeolian-Islands, Italy) occurred, followed by a period of intense Strombolian and effusive activity in July, and continuing until the end of August 2019. We present a joint analysis of multi-sensor infrared satellite imagery to investigate this eruption episode. Data from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) was used in combination with those from the Multispectral Instrument (MSI), the Operational Land Imager (OLI), the Advanced Very High Resolution Radiometer (AVHRR) and the Visible Infrared Imaging Radiometer Suite (VIIRS). The analysis of infrared SEVIRI data allowed us to detect eruption onset and to investigate short-term variations of thermal volcanic activity, providing information in agreement with that inferred by nighttime AVHRR observations. By using Sentinel-2 MSI and Landsat-8 OLI imagery we better localized the active lava flows. The latter were quantitatively characterized using infrared VIIRS data, estimating an erupted lava volume of 6.33∙10^6±3.17∙10^6 m³ and a mean output rate of 1.26 ± 0.63 m³/s for the July/August 2019 eruption period. The estimated mean output rate was higher than the ones in the 2002/03 and 2014 Stromboli effusive eruptions, but was lower than in the 2007 eruption. These results confirmed that a multi-sensor approach might provide a relevant contribution to investigate, monitor and characterize thermal volcanic activity in high-risk areas.
... It is known qualitatively that the Stromboli crater terrace is affected by migration and evolution of eruptive vents, and their location, size, and geometry may change over the years. These changes, also induced by occasional paroxysmal explosions, make it difficult to identify individual vents (e.g., Calvari et al. 2005Calvari et al. , 2010Capponi et al. 2016); thus, most authors refer to craters or vent areas, each grouping a few active vents (e.g., Ripepe et al. 2005Ripepe et al. , 2009Harris and Ripepe 2007a, b;Zanon et al. 2009;Taddeucci et al. 2013b;Gaudin et al. 2014). Historically, three main vent areas have been described at Stromboli, i.e., south-west, central, and north-east, hereafter SW, C, and NE, respectively (Washington 1917;Ripepe et al. 2005;Harris and Ripepe 2007a) (Fig. 1). ...
The crater terrace of Stromboli Volcano (Italy) hosts several active vents which have evolved and migrated through time within
three main vent areas: south-west (SW), central (C), and north-east (NE). Frequent, jet-like explosions typically take place,
episodically interrupted by larger-scale paroxysms, which can substantially modify the morphology of the crater terrace and vent
geometries. However, the link between the time-space evolution of vent activity and the shallow conduit system are still a matter
of debate. In this work, we analyze the vent position and explosion parameters (jet duration and geometry) of 4296 events at
Stromboli in five 72-h-long time-windows between 2005 and 2009, as recorded by an infrared surveillance camera. Vent
locations illustrate the resilience of the shallow conduit system, which controls explosive activity at different time scales and
depths. At the shallowest depth, where slugs burst, conduit branching and merging determines the evolution of simultaneous or
alternating twin vents, while vent shape and slug size control local explosion parameters. These processes show variability on an
hourly to daily time scale. Below the depth of the slug burst, the conduit system feeding each vent area controls which specific
vent will host the explosions and also, possibly, the size of the slugs. Several observations suggest that the C and SW vent areas
may be connected at this depth. The deeper conduit system, common to all vent areas, sets the overall explosion rate of the
volcano and maintains a balance of this rate between the NE and the combined SW and C vent areas.
... Complementary instruments that naturally combine are the thermal cameras of the video-surveillance system of INGV-OE, also comprising high definition visible cameras, as heat is mainly conveyed by particles and heat flux also depends on particle size and number. They also provide information on volume, height, durations and intensity (e.g., Zanon et al. (2009)). These could be related to radar-inferred parameters like tephra mass and mass flux during lava fountains. ...
Over the last twenty years Mount Etna has produced more than one hundred explosive events ranging from light ash emissions to violent sub-plinian eruptions. Significant hazards arise from tephra plumes which directly threaten air traffic, and generate fallout affecting surrounding towns and infrastructures. We describe the first radar system, named VOLDORAD 2B, fully integrated into a volcano instrumental network dedicated to the continuous near-source monitoring of tephra emissions from Etna's summit craters. This 23.5 cm wavelength pulsed Doppler radar is operated in collaboration between the Observatoire de Physique du Globe de Clermont-Ferrand (OPGC) and the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo (INGV-OE) since 2009. Probed volumes inside the fixed, northward-pointing conical beam total about 1.5 km in length, covering the summit craters which produced all recent tephra plumes. The backscattered power, related to the amount of particles crossing the beam, and particle along-beam velocities are recorded every 0.23 s, providing a proxy for the tephra mass eruption rate. Radar raw data are transmitted in real-time to the volcano monitoring center of INGV-OE in Catania and are used to automatically release alerts at onset and end of eruptive events. Processed radar parameters are also made available from the VOLDORAD database online (http://voldorad.opgc.fr/). In addition to eruptive crater discrimination by range gating, relative variations of eruption intensity can be tracked, including through overcast weather when other optical or infrared methods may fail to provide information. Short-lived dense ash emissions can be detected as illustrated for weak ash plumes from the Bocca Nuova and New South East craters in 2010. The comparison with thermal images suggests that the front mushroom of individual ash plumes holds the largest particles (coarse ash and small lapilli) and concentrations at least within the first hundred meters. For these short-lived ash plumes, the highest particle mass flux seems to occur typically within the first 10 s. We also analyze data from the first lava fountain generating an ash and lapilli plume on 12 January 2011 that initiated a series of 25 paroxysmal episodes of the New South East Crater until April 2012. We illustrate the pulsating behavior of the lava fountain and show that vertical velocities reached 250 m s-1 (with brief peaks exceeding 300 m s-1), leading to mean and maximum tephra fluxes (DRE) of 185 and 318 m3 s-1 (with peaks exceeding 380 m3 s-1) respectively, and a total volume of pyroclasts emitted during the lava fountain phase of 1.3 × 106 m3. Finally, we discuss capacities and limits of the instrument, along with future work aimed at providing source term inputs to tephra dispersal models in order to improve hazard assessment and risk mitigation at Etna.
... The source of explosions and tremor is located at depths shallower than 200 m beneath the summit crater (Chouet et al. 1997) and shifted NW towards the SdF (Martini et al. 2007), with most of the seismic energy radiated by a small volume at ∼100-m depth beneath the SdF (Chouet et al. 2003). Thus, the uppermost part of the volcanic conduit is close to the SdF surface and is prone to be intersected and/or structurally conditioned by shallow landslides, as happened during the 2002-2003 and 2007 flank eruptions (Bonaccorso et al. 2003;Calvari et al. 2005a, b;Martini et al. 2007;Neri and Lanzafame 2009;Zanon et al. 2009;Casagli et al. 2010;Di Traglia et al. 2014). ...
... This volcano is reckoned for being characterised by a remarkable steady supply (Calvari et al. 2011c;Francalanci et al. 2012), and we lack signs of a greater input from the source region, given that the supply detected between 2008 and 2009 is much less than that causing the 2007 eruption (Fig. 4c, d). Thus, we suggest that the increase of eruptive activity observed at Stromboli from April 2007 to December 2012 was not caused by a greater supply of gas-rich magma from the source, but instead resulted from a wider and hotter uppermost conduit, initiated by movements that occurred in the SdF after the 2002 landslide events (Acocella and Neri 2009;Falsaperla et al. 2008) and that followed the graben-like collapses that occurred during the 2007 eruption, which involved the entire summit crater zone Neri and Lanzafame 2009;Zanon et al. 2009;Di Traglia et al. 2013). This is also confirmed by more recent GBInSAR results that indicate an increased magmastatic pressure within the shallow plumbing system causing its lateral expansion (Di Traglia et al. 2014). ...
Stromboli is known for its mild, persistent explosive activity from the vents located within the summit crater depression at the uppermost part of the Sciara del Fuoco (SdF) depression. Effusive activity (lava flows) at this volcano normally occurs every 5-15 years, involving often the opening of eruptive fissures along the SdF, and more rarely overflows from the summit crater. Between the end of the 2007 effusive eruption and December 2012, the number of lava flows inside and outside the crater depression has increased significantly, reaching a total of 28, with an average of 4.8 episodes per year. An open question is why this activity has become so frequent during the last six years, and was quite rare before. In this paper, we describe this exceptional activity and propose an interpretation based on the structural state of the volcano, changed after the 2002-2003 and even more after the 2007 flank effusive eruption. We use images from the Stromboli fixed cameras network, as well as ground photos, plume SO2 and CO2 fluxes released by the summit crater, and continuous fumarole temperature recording, to unravel the interplay between magma supply, structural and morphology changes, and lava flow output. Our results might help forecast the future behaviour and hazard at Stromboli, and might be applicable to other open-conduit volcanoes.
... Thermal data also allows identification of different eruption styles and eruption phases and definition of emission components and trajectories (e.g. Patrick et al., 2007;Zanon et al., 2009;Calvari et al., 2012;Harris et al., 2012). Infrared monitoring has also been used to investigate thermal anomalies and morphological changes prior to, during and after explosions (e.g. ...
