Role of regional slope in controlling the runout distance of PDCs generated by catastrophic explosive eruptions: Implications for Volcanic Hazard Assessment

Pyroclastic density currents (PDCs) are gravity-driven mixtures of pyroclasts, lithic fragments and gas produced by the partial or total collapse of pyroclastic material or volcanic domes (e.g. Druitt, 1998; Dufek et al., 2015). PDCs represent one of the most hazardous processes associated with volcanic eruptions (e.g. Cole et al., 2015; Neri et al., 2015a), and thus the analysis of their mobility and the expected runout distance is of paramount importance for volcanic hazard assessment. PDC mobility is controlled by the collapsing material properties (e.g. solid particle concentration, volume, and temperature), by the interaction with the surrounding atmosphere (e.g., air entrainment), and also by the interplay between the flow base and the substrate, where different processes occur or may occur, such as erosion (Roche, 2015), self-channelization (Brand et al., 2014; Gase et al., 2017), self-fluidization (Breard et al., 2018; Chédeville and Roche, 2018), and pyroclast deposition (Branney et al., 2002).


In particular, within the pyroclastic mixture and especially at the impact zone of a collapsing fountain (e.g. Valentine and Sweeney, 2018), the differential motion between the interstitial gas (flowing relatively upwards) and the solid particles (moving relatively downward) is able to increase pore pressure, to reduce friction and thus to increase flow mobility and runout distance (Iverson, 1997; Roche, 2012). The effective influence of pore pressure is controlled by the gas diffusion rate (Roche, 2012), which is in turn influenced by the properties of the PDC material. Roche (2012) studied the factors controlling gas retention capacity, as termed by Druitt et al. (2007), showing that slow gas pressure diffusion is favored by thick pyroclastic flows and by grain size distributions dominated by fine particles (Druitt et al., 2007; Roche, 2012). Moreover, different authors have argued that the conditions that promote a long-lasting effect of pore pressure in reducing frictional forces are probably associated with low to moderate flow velocities (Cas et al., 2011; Roche et al., 2016). On the other hand, Shimizu et al. (2019) showed that the extremely high runout distances observed in nature are probably associated with the transport dynamics of the dense portion of PDCs. However, numerical constraints are still lacking to define the eruptive conditions that determine whether pore pressure plays a significant role on PDC dynamics. This limits our understanding of the mobility of large-volume PDCs, as well as the interpretation of the characteristics of ignimbrites. Our current knowledge of the influence of pore pressure on runout distance is mainly based on scaled analog experiments and the analysis of pyroclastic deposits, while the systematic application of numerical models at experimental and flow scale is lacking. Moreover, the influence of regional slope on pore pressure diffusion is unclear, although some authors have mentioned its role on promoting particularly high runout distances in large-volume PDCs.

Figure 1. Simplified scheme of the experimental setup used to address the effect of pore pressure in run-out distance (Aravena et al., 2024).


Accordingly, with the objective of determining the factors controlling flow runout distance in large volume PDCs, this topic is proposed to be addressed using an approach based on numerical modeling and the analysis of laboratory experiments, aimed at giving numerical constraints for simulations and also at validating their results. This will allow to define the eruptive conditions where pore pressure is relevant in the context of large explosive eruptions, with enormous implications for volcanic hazard assessment, as well as its interplay with surface slope in controlling the run-out distance of PDCs.


Alvaro ARAVENA – Facultad de Ciencias Básicas, Universidad Católica del Maule (UCM), Talca, Chile