Volcanic ballistic projectiles are centimetre to metric fragments of molten lava or solid rock launched from a vent during explosive eruptions and are particularly abundant during sudden strombolian and vulcanian explosions. These ballistics, travelling up to ten kilometres from the vent with exit velocities ranging from tens to hundreds of m/s1–4 represent one of the main volcanic hazards within 5 km from the vent5–6. At Stromboli, the ballistic projectiles affect mainly the summit area involving the tourists pathways and can reach inhabited centres generating heavy damages and trigger fires able to destroy the landscape burning vegetation and cultures7–8. One of the main actions to reduce the risk from ballistic fallout is the production of hazard maps. This requires, as basic data, the spatial distribution of the ballistic clasts emplaced during different explosive events and eruptions. Nevertheless, few studies are addressed to reproduce the spatial dispersion of ballistics and, moreover, are lacking of 2D digital geometry of the ballistic emplaced on the ground. Most of the published works map the clasts distribution through isopleths reconstructed from the counting of clasts directly observed on the field, or individuated in photos taken on the ground or in images acquired by a photographic camera mounted on aerial platforms9–15.
For Stromboli, the most recent studies reconstruct the areal dispersion of ballistic fallout for the major explosions and paroxysms8,16−18 by field surveys carried out within a few days after the eruption, when the newly erupted material was still well recognizable. However, it is very time consuming and forces operators to stay in dangerous areas of the volcano for a long time and, if some sectors of the volcano are inaccessible, this prevents a complete reconstruction of the deposit distribution. All this makes it difficult to collect a sufficient number of measurements to develop robust probabilistic hazard maps.
Our work reconstructs a high resolution geo-spatial map of spatter clasts emplaced on the east summit flank of Stromboli during the 3rd July 2019 paroxysm, elaborating drone images through GIS methodology. The resulting map allowed to derive the areal dimension of each spatter clast, their total extent and the relative volume as well as the dispersion parameters useful for the understanding of ballistics dynamics.
Stromboli And The 2019 Volcanic Crisis
Stromboli, the northernmost volcanic island of the Aeolian Archipelago (Southern Italy), has an elongated shape of ~ 3 x 4 km with maximum altitude of 924 m a.s.l. (Fig. 1). It presents an inaccessible horseshoe-shaped depression on the N flank (Sciara del Fuoco), resulting from several flank collapses19. The activity takes place in a flattened area (crater terrace, Fig. 1) located in the upper part of the Sciara del Fuoco (~ 750 m a.s.l), where several active vents are always present8,20−21. The normal activity of the volcano consists of mild strombolian explosions with a frequency from 5 to 30 explosions per hour, ejecting lapilli and bombs to heights of few hundreds of meters above the vent and up to few tens of meters beyond the edges of the crater terrace22–24. This mild activity is occasionally interrupted by energetic, impulsive explosions, lasting tens of seconds to few minutes, named major explosions and paroxysms7–8,18. During the major explosions, ballistic juvenile and lithic bombs fall in the upper portion of the volcano, while paroxysms produce fallout of heavy material that often affect Stromboli and Ginostra villages, located on the NE and SW coast of the island, respectively.
The last 2 paroxysms, occurred during the volcanic crisis of 2019, are dated on 3rd July and 28th August. The 3rd July paroxysm started with sudden explosions forming a “fireball” above the crater terrace that produced the fallout of decametric to metric molten ballistic projectiles and minor quantities of ballistic lava blocks17. Ballistics were launched down the volcano’s slope as low as 500 m a.s.l. along the N flank above Stromboli village, and below 200 m a.s.l. towards Ginostra village. The molten projectiles flattened on impact with the ground, forming a continuous spatter coverage within 500 m from the vent in the SW direction and 350 m in the NE direction. At greater distances, the areas were affected by fallout of scattered flattened bombs (hereafter spatter clasts). The fallout of spatter clasts was followed by an eruptive column 6–8 km high17–18,23 and, then, by the deposition of ash and lapilli in the SW sector of the island. Two pyroclastic flows descended into the Sciara del Fuoco, triggering small tsunamis at sea entrance, a gravity induced flow was generated by the accumulation of the abundant spatter clasts on the E flank23 and a glowing avalanche affected the N flank17. Finally, fires triggered by the ballistic fallout involved large areas of the island generating heavy environmental impacts concerning endemic vegetation loss, damages to agricultural heritage, and transformations to landscape patterns 25.