The hazards of unconfined pyroclastic density currents: a new synthesis and classification according to their deposits, dynamics, and thermal and impact characteristics

Abstract

Pyroclastic density currents (PDCs) that escape their confining channels are among the most dangerous of volcanic hazards. These unconfined PDCs are capable of inundating inhabited areas that may be unprepared for these hazards, resulting in significant loss of life and damage to infrastructure. Despite their ability to cause serious impacts, unconfined PDCs have previously only been described for a limited number of specific case studies. Here, we carry out a broader comparative study that reviews the different types of unconfined PDCs, their deposits, dynamics and impacts, as well as the relationships between each element. Unconfined PDCs exist within a range of concentration, velocity and temperature: characteristics that are important in determining their impact. We define four end-member unconfined PDCs: 1. fast overspill flows, 2. slow overspill flows, 3. high-energy surges, and 4. low-energy detached surges (LEDS), and review characteristics and incidents of each from historical eruptions. These four end-members were all observed within the 2010 eruptive sequence of Merapi, Indonesia. We use this well-studied eruption as a case study, in particular the villages of Bakalan, 13 km south, and Bronggang 14 km south of the volcano, which were impacted by slow overspill flows and LEDS, respectively. These two unconfined PDC types are the least described from previous eruptions, but during the Merapi eruption the overspill flow resulted in building destruction and the LEDS in significant loss of life. We discuss the dynamics and deposits of these unconfined PDCs, and the resultant impacts. We then use the lessons learned from the 2010 Merapi eruption to assess some of the impacts associated with the deadly 2018 Fuego, Guatemala eruption. Satellite imagery and media images supplementing fieldwork were used to determine the presence of both overspill flows and LEDS, which resulted in the loss of hundreds of lives and the destruction of hundreds of buildings in inundated areas within 9 km of the summit. By cataloguing unconfined PDC characteristics, dynamics and impacts, we aim to highlight the importance and value of accounting for such phenomena in emergency management and planning at active volcanoes.

Evaluating and ranking Southeast Asia's exposure to explosive volcanic hazards

“…For instance, our method considers a binary impact from PDCs in which inundation implies impact. Recent studies have demonstrated that this assumption has been disproved by field observations (Jenkins et al, 2013;Lerner et al, 2022). Shifting from probabilistically estimating exposure to impact for flows requires advances in two direc- tions.…”

“…A common mechanism of PDC generation is the gravitational collapse of a lava dome (Cole et al, 2015). These PDCs are typically valleyconfined, but the possible detachment of the dilute component can overspill and inundate populated areas (Lerner et al, 2022). We simulated the likely flow paths of dome collapse PDCs using a recalibrated version of the LAHARZ model (Iverson et al, 1998;Schilling, 1998), with empirical coeffi- (Thouret et al, 2007;Solikhin et al, 2012); and (d) Soputan: several dome collapse PDCs on 1 August and 25 October 2007 (Pallister et al, 2012).…”

“…Buffer distances were chosen based on extents observed in previous unconfined PDCs (e.g. Merapi 2010, Fuego 2018: Lerner et al, 2022). For each simulated volume, we output the 10 %, 50 % and 90 % probabilities for each of the buffer extents.…”

Curie Temperatures and Emplacement Conditions of Pyroclastic Deposits From Popocatépetl Volcano, Mexico

“…Given the great destructiveness posed by PDCs, the understanding of their formation mechanism and behavior is important for hazard prediction. PDCs can be characterized by multiphase flow transport regimes and gas‐particle interactions, spanning from fast and high‐energy surges to slow and dense flows, which produce a wide variety of deposit characteristics (e.g., Lerner et al., 2022). The Plinian eruption, for example, occurs in pulses that mostly start with minor ash falls and ash flows and reach their peak with the deposition of pumice falls, the emplacement of hot ash flows, and finally extensive mudflows (e.g., Siebe et al., 1996).…”

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

“…The main factors that cause casualties and injuries from ash cloud surges arise from a combination of i) burnings due to their high temperatures [11][12][13][14][15][16][17] , ii) acidic gases injuries 18 , and iii) asphyxia from ash inhalation 14,19 .…”