Ash/Tephra Fall (including Volcanic Ballistic Projectiles)
Tephra is a collective term for volcanic fragments (pyroclasts) generated by the fragmentation of fresh magma and old (i.e., pre- existing) rocks ejected into the atmosphere during an explosive eruption, irrespective of size, composition and shape. The term 'volcanic ash' refers to the finest particles of tephra (<2 mm in diameter)(cf. Pistolesi et al. 2025).
Tephra also include relatively large bombs (fragments of fresh magma) and blocks (fragments of pre-existing rocks) that are ejected during an explosive eruption at variable velocity and angle on cannon ball-like trajectories (Volcanic Ballistic Projectiles); they are not entrained within the volcanic plume, are not significantly affected by the wind field, and are dispersed in proximity to the vent (typically <5 km) (adapted from Biass et al., 2016 and Bonadonna et al., 2021).
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.
Annotations
Additional scientific description
Although 'volcanic ash' refers to particles less than 2 mm in diameter, the term is often used loosely to include larger fragments, more correctly termed 'lapilli' (2 to 64 mm in diameter). The largest tephra clasts (>64 mm) are called blocks and bombs. 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.
When blocks and bombs are not entrained within volcanic plumes and follow cannon ball-like trajectories, they are defined as volcanic ballistic projectiles (VBP). VBP may be a few centimetres to several metres in diameter and, in most cases, they sediment between a few hundred metres to 5 km from the vent. In the most powerful explosions, they can be thrown to distances over 10 km. Some large lapilli and small blocks and bombs can also be entrained within the volcanic plume and sedimented at larger distances than ballistics (Osman et al., 2019).
Along with emissions of gas, tephra is the most frequent and widespread volcanic hazard. It is ejected into the atmosphere and coupled to the gas phase remains aloft if updraft velocities exceed the gravitational settling of individual clasts. Lateral transport by wind and/or lateral gravitational spreading of umbrella clouds allow the finest particles to travel hundreds to thousands of kilometres, before eventually falling out under gravity (Pistolesi et al. 2025). Fine tephra (mainly volcanic ash) also rises convectively above pyroclastic density currents and lava fountains. Tephra can thus affect very large areas for long periods since volcanic ash can remain airborne for days causing damage to infrastructure and disruption to air traffic. Blocks and bombs mostly follow a ballistic trajectory and are not affected by wind; nonetheless, the smallest bombs can also be entrained within convective plumes impacting a larger area than ballistic clasts.
Lightning may be generated by friction between the fine airborne particles, which can be localised above the volcano or accompany large ash plumes as they move downwind.
Metrics and numeric limits
Various analytical and numerical models have been developed to forecast tephra dispersal (e.g. Folch 2012). The International Civil Aviation Organization (ICAO) leads operational forecasting of ash cloud transport for the benefit of the aviation sector (ICAO, 2012; Lechner et al., 2017).
Approximate tephra thicknesses (hazard intensities) that relate to key damage and functionality states for a range of building types, critical infrastructure and agricultural categories are given by Jenkins et al. (2015).
To assess severity at specific sites, tephra falls are most commonly described (e.g., eyewitness accounts) or measured in the field according to their thickness. Increasingly though, loading (tephra mass per unit area; kg/m2) is more informative for assessing impact on structures and agriculture, and enables consideration of water saturation (Jenkins et al., 2015). For respiratory health exposure and hazard assessment, monitoring of airborne concentrations of fine particulates is preferable, alongside physicochemical and toxicological characterisation of the tephra particles (e.g., Horwell et al., 2013).
Various analytical and numerical models have been developed that forecast sedimentation of large lapilli and small blocks and bombs (Osman et al. 2019) and ballistic dispersal (e.g., Biass et al., 2016). Volcanic Ballistic Projectiles may be ejected at over 300 m/s but slow down during flight, with terminal velocities typically <150 m/s. Impact energy (kinetic energy at the moment of impact) is strongly controlled by the size of a VBP because this limits both its terminal velocity and mass. Alatorre-Ibargüengoitia et al. (2012) modelled impact energies of VBPs 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).
Key relevant UN convention / multilateral treaty
Sendai Framework for Disaster Risk Reduction 2015–2030 (UNDRR, 2015).
Drivers
Ash/tephra fall is driven by explosive volcanic activity, dependent on factors including the viscosity of the magma, volcanic gases, and conduit geometry. These control the ascent speed, decompression and degassing of the magma as it rises to the surface (Cassidy et al., 2018). Tephra fall hazard can be exacerbated by rainfall, and wind following tephra fall can result in volcanic ash resuspension, which can prolong exposure and impacts (Dominguez et al. 2025).
Associated hazards include volcanic gases and aerosols (GH0205), ground gases (GH0407), toxic gases (CH300) and asphyxiant gases (CH0400) which occur simultaneously but independently to tephra fall during explosive eruptions.
Impacts
Tephra can cause fatalities directly, owing to ballistic impact, and indirectly due to collapse of buildings (mostly roofs) and trees due to tephra load. There were 52 recorded fatal incidents as a result of tephra (not including ballistics) between 1500 AD and 2017 resulting in 4315 fatalities and these occurred between 0.5 and 170 km from the source volcano at a median distance of 10 km (Brown et al., 2017). Over the same period, there were 57 fatal incidents due to ballistics, with 367 recorded fatalities 0 to 7 km from the volcanic source (Brown et al., 2017). Fatalities have also occurred due to falling off roofs during clean-up.
In addition, public health threats, clean-up and disruption to critical infrastructure services, aviation and primary production can lead to substantial societal impacts and costs, even at thicknesses on the ground of a few millimetres. The impacts can be experienced across wide areas and can be long-lived, since eruptions can last from hours to years (IVHHN, 2021). Volcanic ash in the atmosphere can be a hazard to aviation.
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. 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 (Tsunematsu et al., 2016).
Large lapilli and blocks and bombs retaining sufficient thermal energy after landing can also trigger fires if they fall on ignitable material (e.g., dry vegetation, wooden structures). Intense tephra fall reduces visibility and may cause complete darkness during daylight hours, creating significant hazards for driving (tephra can also affect traction on roads and obscure road markings).
There were 57 fatal incidents due to Volcanic Ballistic Projectiles between 1500 AD and 2017, resulting in 367 recorded fatalities 0–7 km from the volcanic source (Brown et al., 2017). Many more people have been injured due to VBP impacts, frequently suffering from blunt force trauma (broken bones), lacerations, burns, abrasions and bruising (Baxter and Gresham, 1997). Large lapilli clasts falling from the margins of volcanic plumes can also result in injury and fatalities.
Tephra loading can damage agriculture crops, and particles can have acid coatings which may react with rain to damage vegetation and cause corrosion. The acid coating is rapidly removed by rain, which may then pollute local water supplies. Tephra can also increase river turbidity leading to environmental problems.
In most eruptions, volcanic ash causes relatively few health problems but generates much anxiety. Fine ash particles may irritate the lungs and eyes (humans and animals) and exacerbate the symptoms of existing respiratory conditions (e.g., asthma and bronchitis) (IVHHN, 2021). However, there is insufficient evidence to be certain whether ash can trigger chronic diseases such as lung cancer and silicosis (if crystalline silica is a major component) (IVHHN, 2021), and all fine particulate matter (e.g., PM2.5) is considered to negatively impact mortality and morbidity, particularly for respiratory and cardiovascular diseases (WHO, 2013).
Medical services can expect an increase in the number of patients with respiratory and eye symptoms during and after a tephra-fall event, which can be measured by existing syndromic surveillance or by application of the International Volcanic Health Hazard Network standardised epidemiological protocols (IVHHN, 2021).
Multi-hazard context
The figure below summarises common interactions between ash and tephra fall and ballistics and other hazards. This information should be used with caution and not be solely relied upon in Disaster Risk Management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.
Multi-hazard diagram
Risk Management
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.
A combination of field and experimental data (e.g. impact energy thresholds; Biass et al., 2016; Williams et al., 2017) are enabling building design recommendations for emergency situations, but reducing exposure to ballistics is the best risk reduction measure (Williams et al., 2017).
The fertility of the soils around many volcanoes is due to the weathering of old ash deposits, and the addition of thin tephra falls to soil can be beneficial in the long term. In many cases though, volcanic ash needs to be removed from urban and agricultural areas to prevent remobilisation and repeated impacts, as well as to prevent it from washing into drainage networks. Therefore, sites need to be identified, preferably before an eruption, to dispose of the ash. Cleaning tephra from roofs, roads, agricultural land, and critical infrastructure may require significant volumes of water, trucks, diggers, etc., and can have significant associated costs (Hayes et al., 2017).
Livestock should ideally be under cover during tephra falls and veterinary services may be needed for respiratory, ingestion, eye and dental problems.
Monitoring
The section and the table below offer an overview of monitoring ash and tephra falls and ballistics. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.
| Which institution(s) produce(s) Disaster Risk Data/Information? | Volcano Observatories monitor precursory activity that could lead to hazards associated with volcanic unrest or eruption (including but not limited to ground shaking, ground fracturing, subsidence and uplift, and volcanic gas emissions,). Volcano Observatories issue warnings and, in association with civil protection and emergency management organizations, recommendations. Volcano Observatories use probabilistic ash dispersion models to forecast ash dispersal direction and anticipate ash/tephra fall. Warnings are issued. The International Civil Aviation Organization (ICAO) leads operational forecasting of ash cloud transport for the benefit of the aviation sector. |
| How is the Hazard Observed/Monitored/Forecast? | Accumulation and movement of magma in the subsurface that may lead to an explosive eruption can be monitored using a variety of instruments including seismometers, GPS, satellite imagery such as InSAR to detect deformation, thermal imaging and gas sensors (ground and satellite based). Various analytical and numerical models are used to forecast tephra dispersal. |
References
Alatorre-Ibargüengoitia, M.A., H. Delgado-Granados and D.B. Dingwell, 2012. Hazard map for volcanic ballistic impacts at Popocatepetl volcano (Mexico). Bulletin of Volcanology, 74:2155-2169.
Baxter, P. and A. Gresham, 1997. Deaths and injuries in the eruption of Galeras Volcano, Colombia, 14 January 1993. Journal of Volcanology and Geothermal Research, 77:325-338.
Brown, S., S. Jenkins, R.S.J. Sparks, H. Odbet and M.R. Auker, 2017. Volcanic fatalities database: analysis of volcanic threat with distance and victim classification. Journal of Applied Volcanology, 6:15. doi.org/10.1186/s13617-017-0067-4.
Cassidy, M., Manga, M., Cashman, K. et al. Controls on explosive-effusive volcanic eruption styles. Nat Commun 9, 2839 (2018).
Dominguez et al. Aeolian Remobilization of Volcanic Ash, Editor(s): C. Bonadonna et al., The Encyclopedia of Volcanoes (Third Edition) [Manuscript in preparation], Academic Press, 2025
Hayes, J., Wilson, T.M., Deligne, N.I. et al. A model to assess tephra clean-up requirements in urban environments. J Appl. Volcanol. 6, 1 (2017). https://doi.org/10.1186/
ICAO, 2012. Flight Safety and Volcanic Ash. 1st Ed. Doc 9974. International Civil Aviation Organisation (ICAO). Accessed 15 October 2020.
IVHHN, 2021. Health impacts of volcanic ash. International Volcanic Health Hazard Network (IVHHN). Accessed 22 April 2021.
Jenkins, S.F., R.J.S. Spence, J.F.B.D. Fonseca, R.U. Solidum and T.M. Wilson, 2014. Volcanic risk assessment: quantifying physical vulnerability in the built environment. Journal of Volcanology and Geothermal Research, 276:105-120.
Jenkins, S.F., T.M. Wilson, C. Magill et al., 2015. Volcanic ash fall hazard and risk. In: Loughlin, S.C., S. Sparks, S.K. Brown et al. (eds.), Global Volcanic Hazards and Risk. Cambridge University Press, pp. 173-222.
Lechner, P., A. Tupper, M. Guffanti, S. Loughlin and T. Casadvell, 2017. Volcanic ash and aviation: The challenges of real-time, global communication of a natural hazard. In: Fearnley, C.J., D.K. Bird, K. Haynes et al. (eds.), Observing the Volcano World: Advances in Volcanology. Springer, pp. 51-64.
Osman, S., E. Rossi, C. Bonadonna, C. Frischknecht, D. Andronico, R. Cioni and S. Scollo, 2019. Exposure-based risk assessment and emergency management associated with the fallout of large clasts at Mount Etna. Natural Hazards and Earth System Sciences, 19:589-610.
Pistolesi et al. Tephra fallout and associated deposits, Editor(s): C. Bonadonna et al., The Encyclopedia of Volcanoes (Third Edition) [Manuscript in preparation], Academic Press, 2025
Tsunematsu, K., Y. Ishimine, T. Kaneko, M. Yoshimoto, T. Fujii and K. Yamaoka, 2016. Estimation of ballistic block landing energy during 2014 Mount Ontake eruption. Earth Planets and Space, 68:88. doi.org/10.1186/s40623-016-0463-8.
UNDRR, 2015. Sendai Framework for Disaster Risk Reduction 2015-2030. United Nations Office for Disaster Risk Reduction (UNDRR). Accessed 12 October 2020.
WHO, 2013. Review of evidence on health aspects of air pollution – REVIHAAP project. Technical Report. World Health Organisation (WHO) Regional Office for Europe. Accessed 29 November 2019.
Williams, G.T., B.M. Kennedy, T.M. Wilson, R.H. Fitzgerald, K. Tsunematsu and A. Teissier, 2017. Building vs Ballistics: Quantifying the vulnerability of buildings to volcanic ballistic impacts using field studies and pneumatic cannon experiments. Journal of Volcanology and Geothermal Research, 343:171-180.