The summer 2019 basaltic Vulcanian eruptions (paroxysms) of Stromboli

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.

Variable Magnitude and Intensity of Strombolian Explosions: Focus on the Eruptive Processes for a First Classification Scheme for Stromboli Volcano (Italy)

“…The end is difficult to estimate due to fallout and the ash cloud spreading for several minutes and limiting visibility from all cameras. Giordano and De Astis [41] estimated a maximum height of the eruptive column to 8.4 km above the crater (Table 4).…”

“…The maximum velocity of the jet was~71.11 m s −1 (Table 4), and the duration of the event is almost 2 min, with the end difficult to estimate due to fallout and ash cloud spreading for several minutes and obscuring sight from all cameras ( Table 4). The eruptive column rose up to 4 km above the craters (INGV report, Table 4), or as much as 6.4 km ( [41]; Table 4), and the fallout from the collapsing column produced two PDCs (Figure 7e) that spread along the SdF and over the sea surface for at least 540 m (Figure 7h). The explosion deeply modified the morphology of the crater area, widening the NEC towards NW.…”

“…During paroxysms, and to a lower extent also during major explosions, a deep-seated, gas-rich and low porphyritic (LP) magma is erupted together with the gas-poor, high porphyritic (HP) magma residing in the upper conduit [28,[36][37][38][39]. A common result of paroxysms is a deep modification of the crater area, with cinder cones and hornitos around vents being blown out and leaving a much wider and deeper crater depression [12,[28][29][30][31]40,41]. Some paroxysms are associated with the emission of lava flows, and may occur both during [29,30,42] or at the start of [32,33] effusive eruptions, whereas others are not associated with lava effusion [12].…”

“…More rarely, PDCs may affect the other slopes of the island, such as occurred after the 1930 and 1944 paroxysms [12,46]. PDCs are caused by the opening of flank fissures [42,47,48], by the collapse of eruptive columns during paroxysms [41], by the collapse of small portions of the summit cone [49], by flank failure [50], or by the brecciation of lava flow fronts along the steep and incoherent SdF slope [29][30][31]42,44].…”

Characterising vent and crater shape changes at Stromboli: implications for risk areas

“…Stromboli, Italy; Villarica, Chile; Whakaari/White Island, New Zealand; Yasur, Vanuatu) are famous for the accessibility of observational points but infamous for unheralded strongly explosive events [e.g. Dempsey et al 2020;Giordano and De Astis 2021;Viccaro et al 2021]. Depending on eruption frequency, recurrent up-close surveys of the active crater area can reveal high(er) temporal resolution information on changes.…”