Abstract
Stromboli is an active, open conduit mafic volcano, whose persistent mild Strombolian activity is occasionally punctuated by much stronger explosions, known as paroxysms. During summer 2019, the volcano unexpectedly produced one such paroxysm on July 3, followed by intense explosive and intermittent effusive activity culminating in a second paroxysm on August 28. Visual observations and the analysis of the fall deposits associated with the two paroxysms allowed us to reconstruct ballistic exit velocities of up to 160 m s−1. Plume heights of ~ 8.4 km and 6.4 km estimated for the two events correspond to mass eruption rates of 1.1 × 106 kg s−1 and 3.6 × 105 kg s−1, respectively. This is certainly an underestimate as directional pyroclastic flows into which mass was partitioned immediately formed, triggering small tsunamis at the sea entrance. The mass of ballistic spatters and blocks erupted during the July 3 event formed a continuous cover at the summit of the volcano, with a mass calculated at ~ 1.4 × 108 kg. The distribution of fall deposits of both the July 3 and August 28 events suggests that pyroclasts characterized by terminal fall velocities < 10–20 m s−1 remained fully suspended within the convective region of the plume and did not fall at distances closer than ca 1700 m to the vent. Based on the impulsive, blast-like phenomenology of paroxysms as well as the deposit distribution and type, paroxysms are classified as basaltic Vulcanian in style. The evolution of the summer 2019 eruptive events was not properly captured within the framework of the alert level system which is focused on tsunamigenic processes, and this is discussed so as to provide elements for the implementation of the reference scenarios and an upgrade of the system to take into account such events. In particular we find that, although still largely unpredictable, at least at operational time scales, and not necessarily tsunamigenic, Vulcanian eruptions and the subsequent evolution of the eruptive phenomena should be considered for the alert level system. This serves as a warning to the implementation of alert systems where the unexpected needs to be taken into account, even at systems that are believed to be relatively “predictable” as is the case at many persistently active, open vent mafic systems.
Highlights
Open conduit mafic volcanoes, such as Etna, Kilauea and Yasur, usually show weak and very small volume explosive activity, rarely interrupted by poorly understood and largelyEditorial responsibility: L
The violent Strombolian activity describes prolonged of mild explosivity usually lasting for days, to build scoria cones plus relatively thick and scoria fall deposits, dispersed well away from the cone itself (e.g. Walker 1973; Calvari and Pinkerton 2004; Pioli et al 2008)
The 2019 summer eruptive period at Stromboli involved different phases: (i) the progressive rise of the intensity and frequency of the ordinary Strombolian activity during the second half of June, with a major explosion occurring on June 25; (ii) an unexpected paroxysm occurring on July 3; (iii) a period of intermittent effusive and high intensity Strombolian activity from July 8 to August 28; (iv) a second paroxysm on August 28; and (v) a major explosion and effusive activity on August 29–30
Summary
Open conduit mafic volcanoes, such as Etna, Kilauea and Yasur, usually show weak and very small volume explosive activity, rarely interrupted by poorly understood and largelyEditorial responsibility: L. Walker 1973; Harris and Ripepe 2007a; Houghton et al 2013; Bonadonna et al 2016) Among such eruptive styles, paroxysms lack of a clear definition and have sometimes been described within the family of violent Strombolian activity Walker 1973; Calvari and Pinkerton 2004; Pioli et al 2008) Such activity displays a substantially different eruption style and deposit characteristic with respect to the short-lived, impulsive style of paroxysms that have occurred at Stromboli (Barberi et al 1993; Rosi et al 2013). Andronico and Pistolesi (2010) used “small-“ to “large-scale” paroxysms to qualitatively illustrate the large variations in intensity and dispersal that higher than “normal” explosive activity (cf. Barberi et al 1993) may produce at Stromboli (see Bertagnini et al 2008). Calvari et al (2006) and D'Auria et al (2006) have alternatively described such paroxysmal events as Vulcanian, as for the 2003 paroxysm at Stromboli, as it impulsively cleared a vent that had been blocked for 3 months prior to the event
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