## 1 Introduction  
Rapid changes of the surface morphology often occur in open-conduit basaltic volcanoes that frequently erupt, such as Etna and Stromboli (Italy), Piton de la Fournaise (La Reunion Island), or Kilauea (Hawaii). Cinder-cones \(\sim\)60 m high can form in just one or a few weeks [1; 2], large lava flows can spread over roads or villages [3; 4; 5; 6], and summit collapses following major lava withdrawal can involve large areas and result in new calderas [7; 8; 9; 6]. However, as they are rapid in building new reliefs, the often loose and unstable mixture of breccia, ash, and spatter, which accumulates quickly and gets possibly destabilized by dike intrusion or overloading, can also suddenly collapse, affecting small portions of the summit cone [10; 11; 12; 13; 14] or even the volcano flank [15; 16; 17; 18]. Magma drainage from the feeder conduit can cause significant summit collapses, such as those experienced at Stromboli in 2003 and 2007 [19; 20; 21], at Piton de la Fournaise in 2007 [8], and at Kilauea in 2018 [9; 6]. Regardless of the cause, instability as well as the formation of pyroclastic density currents (PDC) at active and inhabited volcanoes can cause injuries and even loss of lives. This happened, for example, at Stromboli in 1930 and 2019 [22; 23; 24; 25], at Unzen (Japan) in 1991 [26], at Merapi (Java) in 1994 and 2010 [27]. For this reason, PDCs are identified among the primary sources of fatalities at active volcanoes [28]. Stromboli volcano is the easternmost island of the Aeolian Archipelago (Italy; Figure 1a,b). It is known as the \"Lighthouse of the Mediterranean\", because of its persistent explosive activity from the summit craters, with bursts occurring every few minutes [29; 30; 31]. The summit crater of the volcano is a depression \(\sim\)300 m long in a NE-SW direction (Figure 1c), \(\sim\)50 m wide, and \(\sim\)50 m deep, located at \(\sim\)750 m elevation [14; 19]. Three crater areas are located within the summit depression: the NE crater zone (NEC), the Central crater zone (CC), and the SW crater zone (SWC), each of them comprising a variable number of active vents (Figure 1c). The capacity of the uppermost feeder conduit increased after the 2002-2003 and 2007 flank eruptions [20; 32], but sudden changes in the magma level may result in a greater magmatic pressure, which could cause the destabilization and collapse of portions of the summit cone [14; 33]. The 2019 eruption at Stromboli has twice shown how dangerous PDCs can be to those tourists approaching the slopes of an active volcano, even from the seaside [34]. Being less dense than water, the PDC formed by 4-5 km high explosive column collapse managed to expand on the sea surface for several hundred meters, running towards a tourist boat sailing nearby ([[https://www.youtube.com/watch?v=RPKg53](https://www.youtube.com/watch?v=RPKg53) sPP1Y]([https://www.youtube.com/watch?v=RPKg53](https://www.youtube.com/watch?v=RPKg53) sPP1Y)). PDCs can also trigger tsunamis that can devastate the coast, as happened at Stromboli in 1930 and 1944 [22; 23]. More recently, PDCs spreading along the Sciara del Fuoco (SdF) slope formed as a consequence of high magma level within the conduit, instability, and collapse of the summit cone [14; 35]. PDCs are common at Stromboli especially during the initial phases of flank eruptions, when the opening of an eruptive fissure breaches the summit craters causing the spread of a mixture of hot debris made of lava blocks, lithics and pyroclastics running down the slope and towards the sea [36; 37; 38; 39; 40]. More recent examples of this phenomenon occurred in 2013 and 2014 [14; 40; 41], caused, respectively, by the erosion of the crater rim due to overflows and the opening of an eruptive fissure. PDCs can result from the collapse of eruptive columns during paroxysmal explosive eruptions [13; 19; 23; 39; 42] or from gravitational instability (gravity-induced PDCs); e.g., [10; 12; 13; 23; 25; 43; 44].