Enhanced Mobility in Concentrated Pyroclastic Density Currents: An Examination of a Self‐Fluidization Mechanism

Abstract

Pyroclastic density currents (PDCs) are a significant volcanic hazard. However, their dominant transport mechanisms remain poorly understood, in part because of the large variability of PDC types and deposits. Here we combine field data with experimental and numerical simulations to illuminate the twofold fate of particles settling from an ash cloud to form the dense PDC basal flow. At solid fractions >1 vol %, heterogeneous drag leads to formation of mesoscale particle clusters that favor rapid particle settling and result in a mobile dense layer with significant bed weight support. Conversely, at lower concentrations the absence of particle clusters typically leads to formation of poorly mobile dense beds that deposit massive layers. Based on this transport dichotomy, we present a numerical dense‐dilute parameter that allows a PDC's dominant transport mechanism to be determined directly from the deposit geometry and grainsize characteristics.

Continuum Modeling of Pressure‐Balanced and Fluidized Granular Flows in 2‐D: Comparison With Glass Bead Experiments and Implications for Concentrated Pyroclastic Density Currents

“…This suggests that pore pressure in pyroclastic flows with high amount of fine particles can arise through relative gas‐particle motion. In particular, generation of positive pore pressure caused by rapid settling of material (e.g., Breard et al, ) is expected to occur at impact zones of collapsing pyroclastic fountains and/or in deflating pyroclastic density currents. Though internal or external sources of gas will further enhance pore pressure, these are not required to create fluidized pyroclastic mixtures.…”

“…(Iverson, ), where P G is the pore fluid pressure, α is the slope angle, σ is the normal stress, and N is the fractional normal stress support. A similar approach has been used to simulate dense gas‐particle flows with a depth‐average approach (Gueugneau et al, ) or to predict the flow runout (Breard et al, ). The product (1 − N ) μ s is often referred to as the effective friction coefficient, and can take very small values as N increases.…”

“…However, in concentrated pyroclastic density currents with high amounts of fine particles (down to microns size), gas most likely plays an important role in controlling the flow dynamics, meaning that drag forces are nonnegligible. Additionally, elevated pore pressure may arise because of gas entrapment during their generation mechanism (i.e., column collapse, (Breard et al, ; Rowley et al, ; Sweeney & Valentine, ; Valentine & Sweeney, ), gas exsolution during transport (Sparks, , ), and ongoing incorporation of air (e.g., Benage et al, ). No internal measurements of pore pressure have hitherto been achieved; thus, the presence of elevated pore pressure in PDCs remains the hypothesis that we favor.…”

The Permeability of Volcanic Mixtures—Implications for Pyroclastic Currents

“…For instance, there is evidence that mobile pyroclastic flows have no continuous gas supply and that the grain size must have a large control on pore pressure decay, which will affect the mobility of pyroclastic flows (Calder et al, 1999). Generation of high pore pressure in pyroclastic flows has been attributed to upward fluidization by particle degassing (Sparks, 1976) and to hindered settling of a collapsing column (Valentine & Sweeney, 2018) or through rapid sedimentation of mesoscale clusters at the base of PCs (Breard et al, 2017). From the point where no continuous gas source exists, the intergranular pore-fluid pressure in pyroclastic flows wanes over time (Druitt et al, 2007).…”

The contribution of experimental volcanology to the study of the physics of eruptive processes, and related scaling issues: A review

“…In addition to these works, the effect of fluidization on the dynamics of granular flows was addressed in various experimental configurations. The key physical parameter of fluidization is the interstitial pore fluid pressure that arises as a consequence of (i) vertical gas-particle differential motion and associated drag, which occurs when a gas of internal or external sources percolates upwards and/or when a granular mixture deflates and expels the interstitial gas upwards (Bareschino et al 2008, Chédeville and Roche 2014, Breard et al 2018, or (ii) sustained gas-particle relative oscillations that cause steady rotational fluid currents across a boundary layer around the particles, a phenomenon called acoustic streaming Soria-Hoyo 2015, Soria-Hoyo et al 2019). Pore pressure reduces particle interactions and thus favors propagation of dense gas-particle mixtures.…”