The research, led by Andrea Bevilacqua and colleagues at the Istituto Nazionale di Geofisica e Vulcanologia in Pisa, analyzed a total of 67 explosive events, comprising 43 major explosions and 24 paroxysms. These two categories have long been used by volcanologists to distinguish the more intense interruptions of Stromboli’s ordinary activity, but the boundary between them has always been somewhat fuzzy. The team’s new dataset suggests that the fuzziness is real: when the maximum distances reached by ballistic projectiles and the total areas affected are plotted across all events, the distributions are remarkably continuous, with no clean separation between major explosions and paroxysms in terms of how far their projectiles fly.

The numbers are striking. Between 12 and 14 percent of major explosions produce ballistics that travel beyond 1,000 meters from the vents, while 29 percent of paroxysms extend projectiles over 2,000 meters. At the other end of the scale, between 23 and 37 percent of major explosions never surpass 500 meters, and between 17 and 42 percent of paroxysms fail to reach 1,500 meters. The average affected area was about 69,000 square meters for major explosions and roughly five times larger, about 360,000 square meters, for paroxysms. The largest distance recorded for any major explosion in the dataset, around 1,200 meters plus uncertainty, occurred on 19 July 2020, an event that other studies have placed right at the boundary between the two categories. The farthest paroxysm projectile, reaching approximately 2,500 meters, was thrown during the 22 May 1919 eruption, which damaged buildings in the island’s village.

What makes this study methodologically interesting is how the team dealt with the messy, incomplete nature of the historical record. Many of the most vivid accounts of Stromboli’s explosions date from the late nineteenth and early twentieth centuries, written mostly in Italian or German, and they describe affected places using place names that have evolved over time or distances rounded to multiples of 100 meters. Rather than trying to pinpoint the coordinates of individual clasts, the researchers developed a simplified mapping scheme in which each event is represented by a circular proximal area around the Crater Terrace, where all active vents lie, plus up to three circular sectors with variable radius and width that envelope the areas reached by projectiles.

This geometry is not arbitrary. Circular sectors mirror, to first order, the envelope of trajectories of projectiles ejected radially from a central vent, and most pre-existing hazard maps were drawn as hand-drawn lobes or open half-ovals that approximate this shape anyway. Each event was assigned to one of three uncertainty classes depending on the quality of the available information: low uncertainty for well-mapped major explosions, moderate uncertainty for events fully described around the source, and high uncertainty for historical paroxysms with weak directional constraints. The uncertainty ranges, typically representing a mean enlargement of 50 to 100 meters on radial distances, were treated as independent uniformly distributed variables and propagated through the entire analysis using Monte Carlo simulations that randomly perturbed the simplified maps 1,000 times.

The directional results reveal a strong asymmetry that differs significantly between the two event types. About 87 percent of the sector bisectors for major explosions point into the eastern half-plane, with a pronounced probability peak of 77 percent, plus or minus 2 percent, toward the southeast at an azimuth of 140 degrees. Paroxysms, by contrast, show 64 percent of their bisectors directed into the northern half-plane, with a maximum probability of 70 percent, plus or minus 9 percent, toward the north. The researchers note that the most frequented trails and inhabited areas are also the easiest places to survey after an eruption, so these peak directions may partly reflect observation bias, a problem the team flags as needing dedicated mitigation methods in hazard map production.

The physical reasons for these directional differences likely lie in the complex interplay between the volcano’s conduit and crater architecture and the very different scales of the two eruption types. Paroxysms appear capable of reshaping the shallow conduits and craters more deeply than major explosions, and laboratory detonation experiments have shown that explosion depth and pre-existing crater morphology can strongly influence the resulting jets and projectile dispersal. Prevailing winds, which at Stromboli blow mainly toward the east and southeast, appear insufficient to explain the observed patterns on their own; a typical 5-meter-per-second crosswind would displace a 10-centimeter lithic clast by only about 60 to 125 meters during a major explosion. Bouncing and rolling of projectiles on the volcano’s steep slopes may also extend their final resting positions beyond the initial impact points.

The stakes of this work are high because ballistics are the most common cause of fatal accidents within 5 kilometers of active volcanoes worldwide, causing tens of deaths and hundreds of injuries in recent decades. At Stromboli specifically, the current early-warning systems can detect precursor signals only a few minutes before major explosions and paroxysms, far too little time for anyone on the summit trails to reach safety. Quantitative assessments of ballistic hazard at different sites of the island are therefore a crucial input for risk mitigation and emergency planning, particularly for a volcano that attracts large numbers of tourists to its slopes every year.

The researchers are careful to note the limitations of their reconstructions. They focused exclusively on the most dangerous projectiles, lithic and scoria clasts larger than about 5 to 10 centimeters in diameter, and did not quantify the areal density of projectiles, which can vary by up to two orders of magnitude with distance from the crater. Under-recording of projectiles landing in the inaccessible Sciara del Fuoco, the horseshoe-shaped depression on the volcano’s northwest flank, is likely significant for both event types, since any ballistics falling more than about 1,200 meters to the northwest would land in the sea and go undocumented unless directly observed. Despite these challenges, the team found that the influence of the mapping uncertainty on the reconstructed dispersal areas was limited, and the resulting statistics were robust.

This paper is the first part of a two-part contribution. The dataset of distances, directions, and affected areas, complete with quantified uncertainty, provides the essential foundation for the companion study, which presents the first probabilistic hazard maps of ballistic fallout at Stromboli. Together, the two papers transform a century and a half of scattered observations, from a 1915 conglomerate boulder photographed by early volcanologists to impact craters documented by drones after the 2022 major explosion, into a rigorous statistical framework that could ultimately help decide where it is safe to walk on one of the world’s most persistently active and most closely watched volcanoes.

Subject of Research: Ballistic projectile hazard mapping from major explosions and paroxysms at Stromboli volcano

Article Title: Ballistic projectile hazard of major explosions and paroxysms at Stromboli (Italy) with uncertainty quantification – Part 1: Mapping method and data analysis

Article References: Bevilacqua, A., Landi, P., Del Carlo, P., Neri, A., & Pompilio, M. (2026). Ballistic projectile hazard of major explosions and paroxysms at Stromboli (Italy) with uncertainty quantification – Part 1: Mapping method and data analysis. Natural Hazards and Earth System Sciences, 26(9), 4479-4502. https://doi.org/10.5194/nhess-26-4479-2026

Image Credits: AI Generated

DOI: 10.5194/nhess-26-4479-2026

Keywords: Stromboli, volcanic ballistics, paroxysms, major explosions, hazard mapping, uncertainty quantification, Monte Carlo simulation, volcanic hazard, Italy, INGV, Natural Hazards and Earth System Sciences, risk mitigation

The size of the particles an eruption produces is not a trivial detail. It is the single most important factor controlling how far pyroclasts travel, how long they remain aloft, and what hazards they pose once they land. Coarse bombs and lapilli fall close to the vent, while fine ash can drift for hundreds or thousands of kilometers, fouling jet engines, coating crops, and settling into water supplies. Yet despite decades of study, volcanologists have lacked a complete picture of exactly how and where these particles form inside and around an erupting volcano. A new study published in Geology, led by Jacopo Taddeucci, a volcanologist at Italy’s Istituto Nazionale di Geofisica e Vulcanologia, now offers the most detailed microtextural evidence yet that the answer is far more complicated than simplified eruption models have assumed.

The stakes are easy to underestimate. As Taddeucci explains, volcanic ash and larger pyroclasts interfere with nearly every aspect of life downwind of an eruption. They disrupt air traffic and road traffic, and once the eruption is over the work has only begun: the material must be removed, and its effects on crops, plants, vegetation in general, and wildlife must be assessed. Understanding how these particles are formed, he argues, is essential precisely because the formation process controls their features—their size, shape, density, and ultimately their behavior in the atmosphere and on the ground.

To interrogate that process, the research team turned to techniques more often associated with materials science than with volcanology. Using scanning electron microscopy and X-ray microtomography, they analyzed pyroclasts collected from three mafic, or basaltic, volcanoes: Etna and Stromboli in Italy, and Cumbre Vieja in Spain. These instruments allowed the researchers to peer inside individual particles at microscopic scales, resolving features far too small to see with the naked eye but large enough to record the physical history of the magma that produced them. Scanning electron microscopy reveals surface textures and fine-scale fractures, while X-ray microtomography builds three-dimensional images of a particle’s interior without cutting it open, preserving the spatial relationships between crystals, voids, and welded fragments.

What the team found inside these particles was a record of violence and repair. By identifying and quantifying characteristic microtextures—including broken crystals, sutures, and incorporated clasts—the researchers demonstrated that fragmentation does not occur at a single depth and a single moment, as many simplified eruption models assume. Instead, the process is protracted, unfolding across several distinct stages. As Taddeucci puts it, this is not a process occurring at a single point in space and time; it covers a span of time and a span of spaces that reaches from inside the volcano to outside it. The particles that rain down on cities like Catania are, in effect, archives of a prolonged sequence of breaking and healing events that began deep within the plumbing system.

Some of this protracted behavior had already been documented in the open air. Volcanologists have long observed bombs and lapilli breaking apart in flight, shattering into smaller fragments as they arc away from the vent. What has remained invisible is everything that happens before the magma exits the volcano, where direct observation is impossible. The new study provides the first microtextural evidence that fragmentation is also protracted inside a volcano, extending the known sequence of breaking events from the observable exterior into the hidden interior of the edifice. The microscopic scars preserved in pyroclasts—fractured crystals, sutured contacts, embedded fragments of earlier generations of magma—now serve as a proxy for processes no camera can reach.

Perhaps the most consequential finding concerns the opposite of breaking. The team showed that crack healing and the welding together of pyroclasts can be just as important as fragmentation in controlling the final particle size distribution. Taddeucci notes that researchers are accumulating more and more evidence that not only the breaking of the magma controls the size of the particles, but also the fact that different particles can stick together after they are broken. In other words, an eruption’s ash output is not simply the residue of how thoroughly the magma shattered; it is the net result of a dynamic competition between fracturing and reassembly. Cracks can close, and fragments that separate can fuse back together, particularly in hot, low-viscosity basaltic systems where freshly exposed surfaces remain plastic enough to weld.

This reframing has direct implications for hazard modeling. Particle size governs how pyroclasts are dispersed through the atmosphere, and computer models of eruptions rely on assumptions about that size distribution to forecast ash cloud trajectories, ground deposition, and aviation impacts. If fragmentation is protracted and healing is significant, then models that treat fragmentation as a single instantaneous event at a fixed depth may systematically misestimate the grain size of the erupted material—and with it, the reach of the hazard. Incorporating the full break-and-heal history into eruption models could therefore sharpen forecasts of how far ash will travel and how long airports, roads, and agricultural areas will remain affected.

The microtextures identified in the study are not unique to the three volcanoes examined. Asked whether the same features would appear in material from Etna’s eruption this summer, Taddeucci did not hesitate: yes, of course, he says, adding that most basaltic eruptions that are explosive in nature will produce these kinds of features. That generality is what makes the finding powerful. Rather than a quirk of one volcano or one eruption, the break-and-heal signature appears to be a common feature of explosive basaltic volcanism worldwide, from the pulsing fountains of Stromboli to the sustained activity of Cumbre Vieja on La Palma.

There is also a retrospective dimension to the work. Because these microtextures can be preserved in rocks from past eruptions, researchers can use them to reconstruct the fragmentation conditions of events that occurred centuries or millennia ago, extending the observational record far beyond the era of modern monitoring. Taddeucci highlights this as a significant advantage volcanology holds over other geophysical hazards: unlike earthquakes, which leave no comparable physical archive of their internal dynamics, eruptions preserve a durable record of the conditions under which their products formed. Reading that record could improve models of future eruptions at well-studied volcanoes and at sites whose behavior is known only from ancient deposits. Taddeucci also emphasizes that the research depended on collaboration, comparing eruptions across different volcanoes and countries—a scope possible only through cooperation with colleagues at observatories and institutions, including researchers beyond the paper’s official list of co-authors. As ash clouds continue to test the resilience of aviation and communities, the microscopic scars inside volcanic particles are proving to be one of the most informative records science has of what really happens when magma comes apart.

Subject of Research: Microtextural evidence for protracted magma fragmentation and healing during basaltic explosive eruptions

Article Title: Scientists find microscopic evidence that magma repeatedly breaks and heals during eruptions

Article References: Scientists find microscopic evidence that magma repeatedly breaks and heals during eruptions. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: volcanology, pyroclasts, magma fragmentation, Mount Etna, Stromboli, Cumbre Vieja, scanning electron microscopy, X-ray microtomography, volcanic ash, basaltic eruptions, Geology journal, eruption modeling