BGS, no date. Tephra/ash fall. British Geological Survey (BGS). Accessed 22 April 2021.
" }, "editorialNote": { "en": [ "source:UNDRR HIPS" ] }, "scopeNote": { "en": [ "The term ‘volcanic ash’ 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. Along with emissions of gas, tephra is the most frequent and widespread volcanic hazard. It is ejected into the atmosphere and transported laterally by wind and/or lateral gravitational spreading of umbrella clouds before falling out under gravity. Fine tephra (mainly volcanic ash) also rises convectively above pyroclastic density currents and lava fountains (Bonadonna et al., 2015, 2021; Jenkins, 2015). Tephra can affect very large areas; volcanic ash can remain airborne for days and can be transported for thousands of kilometres and may disrupt air traffic. Blocks and bombs mostly follow a ballistic trajectory, and so are not strongly affected by wind; nonetheless, the smallest blocks can also be entrained within convective plumes impacting a larger area than ballistic clasts. Tephra can cause fatalities directly, owing to ballistic impact, and indirectly due to collapse of buildings (mostly roofs) and trees due to tephra load. 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. Hot tephra (e.g., large lapilli and blocks and bombs) can also trigger fires if falling 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, for example (USGS, no date). 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. The impacts can be experienced across wide areas and can be long-lived, since eruptions can last from hours to years (IVHHN, 2021). Tephra-fall deposits may also be the source of secondary hazards (e.g., lahars) and can be remobilised into the atmosphere by wind, traffic and human activities, prolonging the impacts. Tephra varies in appearance depending upon the composition of the magma and the style of the eruption (Bonadonna et al., 2015). Various analytical and numerical models have been developed that forecast tephra dispersal and deposition from the finest fractions to ballistic blocks (e.g., Folch, 2012; Bonadonna et al., 2015; Biass et al., 2016; Osman et al., 2019). 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). To assess severity at a site, tephra falls are most commonly described (e.g., eyewitness accounts) or measured according to their thickness. Increasingly though, loading (mass per unit area; kg/m2) is more informative for assessing impact to 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 ash particles (e.g., Horwell et al., 2013). 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). 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. 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