Deposit‐Derived Block‐and‐Ash Flows: The Hazard Posed by Perched Temporary Tephra Accumulations on Volcanoes; 2018 Fuego Disaster, Guatemala

Abstract

Pyroclastic density currents (PDCs) are flowing mixtures of hot gas and volcanic particles. They are the dominant single cause of fatalities around volcanoes (S. K. Brown et al., 2017) and can be generated by the fountaining of eruption columns, lateral blasts, and growing lava domes (Branney et al., 2021;Druitt, 1998). They transport large volumes of hot, ash-rich pyroclastic debris rapidly across the landscape, and span a wide range of scales, particle concentrations and grain-sizes (Branney & Kokelaar, 2002). A key factor in the hazard they present is the distance they travel (the "runout distance") over a given topography, and this is significantly affected by the current's mass flux at the source (e.g., Bursik & Woods, 1996; Shimuzu et al., 2019;Williams et al., 2014). In many cases, it can be assumed that the mass flux of the pyroclastic current derives directly from the vent discharge rate (mass flux) of the eruption (e.g., Roche et al., 2021;Sparks et al., 1997). However, PDCs are known to entrain loose substrate

The fragmentation-induced fluidisation of pyroclastic density currents

“…Fatalities from volcanic eruptions in the past decades have been largely related to pyroclastic density currents (PDCs) 1 . The most common PDCs are known as block-and-ash flows 2 – 4 (BAFs), which form by gravitational collapse of a hot lava dome 5 , 6 lava lakes or flows 7 or perched pyroclastic debris 8 , and have been the deadliest volcanic processes in the past century 1 , 9 . Observations and depositional evidence suggest these flows are characterised by a concentrated basal avalanche (underflow) overlain by a dilute turbulent ash-cloud (ash-cloud surge) 10 – 13 .…”

“…However, the source of the fluidisation mechanism in BAFs remains elusive because, unlike column collapse PDC, they do not form by impact of a column collapse or rapid-sedimentation, and are coarse (permeable) mixtures in proximal reaches 30 . For example, the 2018 Fuego dome-less BAFs that originated from the collapse (>70% volume 31 ) of existing “perched” proximal tephra, lava, and spatter were some of the most mobile and deadly events of the past century 8 , 32 , and yet cannot be explained by our existing knowledge of auto-fluidisation 26 , 33 .…”

Forecasting and communicating the dispersion and fallout of ash during volcanic eruptions: lessons from the September 20, 2020 eruptive pulse at Sangay volcano, Ecuador

“…In contrast, the March 2021 eruptive pulses produced two high-altitude (~15 km asl) eruptive columns without longrunout PDCs (IG-EPN, 2021). Therefore, we postulate that the September 20, 2020 PDCs also correspond to collapses of lava Frontiers in Earth Science frontiersin.org flows and other perched deposits emplaced in the southeast drainage, as occurred just after the June 3, 2018 Fuego eruption (Risica et al, 2022). Accordingly, based on the eruptive column height and the fallout volume, the September 20, 2020 eruptive pulse can be classified as a VEI two event with a magnitude of 2.2-2.3 and an intensity of 8.5-8.6.…”

A new hazard scenario at Vesuvius: deadly thermal impact of detached ash cloud surges in 79CE at Herculaneum

“…The recent volcanic episodes occurred in 1991 at Unzen 7,8,26,59 , in 2010 at Merapi Volcano 9,12 , in 2018 in Guatemala 60,61 , and in 2019 in New Zealand 62,63 highlight the need to deepen our understanding of diluted PDCs hazard in terms of their thermal impact 11 .…”