Ballistics (Volcanic)

Primary reference(s)

Biass, S., J.-L. Falcone, C. Bonadonna, F. Di Traglia, M. Pistolesi, M. Riso and P. Lestuzzi, 2016. Great Balls of Fire: A probabilistic approach to quantify the hazard related to ballistics – A case study at La Foss volcano, Vulcano Island, Italy. Journal of Volcanology and Geothermal Research, 325:1-14.

Bonadonna, C., S. Biass, S. Menoni and C.E. Gregg, 2021. Assessment of risk associated with tephra-related hazards. In: Papale, P. (ed), Forecasting and Planning for Volcanic Hazards, Risks, and Disasters. Chapter 8.

Additional scientific description

Ballistics may be a few centimetres to several metres in diameter. In most cases, the range of ballistics is a few hundred metres to 5 km, but they can be thrown to distances over 10 km in the most powerful explosions (Blong, 1984). Some blocks and bombs (i.e., tephra clasts >64 mm) can also be entrained within the volcanic plume and sedimented at larger distances than ballistics (Osman et al., 2019).

Fragments of all sizes generated during fragmentation of magma and lava are also known as ‘pyroclasts’ whether they travel through the atmosphere or are directly entrained in lateral moving flows.

Various analytical and numerical models have been developed that forecast ballistic dispersal (e.g., Fitzgerald et al., 2014; Biass et al., 2016).

Primary hazards. The high kinetic energies of ballistics when they land makes them hazardous to people, buildings, infrastructure and other assets. Ballistics may be ejected at over 300 m/s but slow down during flight, with terminal velocities typically <150 m/s (Walker et al., 1971). Impact energy (kinetic energy at the moment of impact) is strongly controlled by the size of a ballistic because this limits both its terminal velocity and mass (Williams et al., 2017). Alatorre-Ibargüengoitia et al. (2012) modelled impact energies of ballistics 0.2–0.6 m in diameter during small explosive eruptions (VEI 2–3) to be up to 106 J, well over the threshold required to penetrate reinforced concrete slabs (Jenkins et al., 2014).

Fragments of lava can be over 1100°C so, although they cool during flight, they may retain sufficient thermal energy on landing to burn certain building materials or other flammable materials (Vanderkluysen et al., 2012).

Secondary hazards. Ballistics may cause indirect fatalities and damage owing to the collapse of buildings (mostly roofs) or damage to infrastructure (power, roads). Hot ballistics can start fires if falling on ignitable material (e.g., dry vegetation, wooden structures).

Intense volcanic explosions that generate ballistics may cause shock and infrasonic waves in the atmosphere, which can shatter windows and damage delicate equipment (e.g., electronic doors) at distances of several kilometres from the volcano.

Ballistics and other loose fragmentary material may be remobilised in lahars or landslides.

Metrics and numeric limits

Not applicable.

Examples of drivers, outcomes and risk management

Ballistics are associated with all types of explosive volcanic eruption (Fitzgerald et al., 2017). Explosions may be sudden with no precursory signs, especially if triggered by steam interaction with hot rocks or magma. Tourists and scientists have proven particularly vulnerable to unexpected explosive eruptions, as they tend to get close to volcanic vents. The sudden explosion of Mount Ontake, Japan, on 27 September 2014, resulted in the deaths of 58 hikers, 56 of whom were killed by ballistic rocks (Oikawa et al., 2016; Tsunematsu et al., 2016).

There were 57 fatal incidents due to ballistics between 1500 AD and 2017, with 367 recorded fatalities 0–7 km from the volcanic source (Brown et al., 2017). Many more people have been injured due to ballistic impacts, frequently suffering from blunt force trauma (broken bones), lacerations, burns, abrasions and bruising (Blong, 1984; Baxter and Gresham, 1997).

The high kinetic and thermal energy of ballistics can cause damage to buildings, infrastructure, agriculture and the environment through knock down, puncturing, crushing, burning and melting (Fitzgerald et al., 2017).

There have been studies of impact energy thresholds to perforate buildings (Blong et al., 1981; Pomonis et al., 1999) and the first fragility functions were presented by Biass et al. (2016). A combination of field data and experiments are enabling building design recommendations for emergency situations, but reducing exposure to ballistics is the best risk reduction measure (Williams et al., 2017).

As with other volcanic hazards, a combination of probabilistic volcanic hazard assessment and risk assessment combined with effective communication among scientists, emergency managers, local communities and other stakeholders can lead to effective management of risk (Fitzgerald et al., 2017).