Theoretical modeling of the generation, movement, and emplacement of pyroclastic flows by column collapse

Abstract

Models are presented for the physical processes that occur in the formation of pyroclastic flows generated by the gravitational collapse of a vertical eruption column. The main controlling parameters are considered to be the vent radius (R), gas content (N), and initial velocity (W) of the gas. For the ranges R = 50 to 600 m, N = 0.5 to 5% gas, and W = 200 to 600 m/s, column collapse occurs at heights between 0.6 and 9.0 km above the vent, and the initialvelocities of the flows range from 60 to 310 m/s. The eruption column collapse is modeled as an inverted turbulent jet, modified by including expressions for the efficiency of heat transfer between air and small pyroclasts. Entrainment of air during the collapse can result in initial cooling of the flow by up to 350°C. The variation in the amount of cooling is considered toaccount for the considerable ranges of emplacement temperatures and the widely differing degrees of welding observed in different ignimbrites. High emplacement temperatures are favored by eruption columns with low gas contents and low gas velocities, whereas low emplacement temperatures are favored by eruption columns with high gas velocities and high gas contents. The initial stages of flow are modeled as a highly turbulent, low‐particle‐concentration density current. Numerical solutions of the turbulent stages of flow are presented assuming uniform radial spreading from a central source (the vent). Flows from large eruptions may still have velocities of up to 100 m/s at distances of tens of kilometers from the source. The methods have also been applied to uphill flow and demonstrate that flows produced at high volume rates of eruption can surmount topographic barriers of several hundred meters at distances of several tens ofkilometers from the vent and explain the spectacular mobility of some large pyroclastic flows.Turbulent suspension in a gas flow with a low concentration of particles is not a viable mechanism of particle transport, as many of the clasts (about 1 mm) found in the deposits have terminal velocities well above the shearing stress velocities of even a rapidly moving gas flow. (v* = 1 to 12 m/s). The flows are deduced to segregate into a high‐concentration basal zone within a few kilometers of the vent, as larger clasts settle to the base of the flows. Fines are thought to be generated by crushing within the high‐concentration basal zone and are fluidized by exsolving gases to produce a pyroclastic flow with high concentrations of fluidized particles. The upper dilute part of the flow and the fine ash elutriated by fluidization contribute to the formation of widely dispersed ash fall deposits which are as voluminous as theassociated ignimbrite. The flows are capable of transporting clasts of several centimeters or tens of centimeters to tens of kilometers distance. The motion of the dense lower part (the pyroclastic flow) disassociates itself from the upper turbulent cloud of fine ash and gas, whicheventually mixes with the atmosphere sufficiently to form a convective plume.

Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits

“…In contrast, pyroclastic flows are high particle concentration currents with characteristics akin to debris flows or debris avalanches, and transport is influenced by fluidization. Walker (1983) and Walker and McBroome (1984) had argued that turbulent transport could be effective only relatively near the source of a pyroclastic current, citing settling velocities from a column-collapse mixture (Sparks et al 1978). However, field evidence shows clearly that the blast involved a highly turbulent suspension current Hoblitt and Miller 1984;Kieffer and Sturtevant 1988;Fisher 1990;Druitt 1992;Bursik et al 1998).…”

“…At SHV currents of ∼30×10 6 m 3 were sustained over <12 min, yielding an average flux of order 10 8 kg s −1 ; but given the pulsations in the seismic signal, higher peak fluxes are certain . The appropriate conditions are provided by exposed cryptodomes or lava domes, which can be many hundreds of meters wide, noting the correspondence between vent diameter and magma discharge rate (Wilson et al 1980;Sparks et al 1978). The large 'effective conduit' size and discharge rate promotes instantaneous collapse of the fountain and generation of an overloaded current.…”

Chapter 10 The effect of topography on ash-cloud surge generation and propagation

“…The total ashcloud surge area is thus the area covered by the dense basal avalanche plus the detached pyroclastic surge area. A number of processes have been proposed to contribute to the generation of ash-cloud surges and these include: (1) elutriation of fine particles and gas from the basal avalanche (Fisher 1979;Wilson 1980); (2) entrainment of ambient air into the flow and subsequent thermal expansion of the mixture (McTaggart 1960;Sparks 1976;Wilson & Walker 1982); (3) particle collision and disintegration (comminution and release of gas from pressurized clasts) (Sparks et al 1978;Fujii & Nakada 1999;Dufek & Manga 2008); (4) turbulent diffusion across a boundary layer (Denlinger 1987;Burgisser & Bergantz 2002); and (5) explosive decompression of the pressurized dome (Woods et al 2002). Conversely, Doyle et al (2008) modelled dense basal avalanches as developing out of dilute currents through mass transfer by sedimentation to the base of the current.…”

A combined field and numerical approach to understanding dilute pyroclastic density current dynamics and hazard potential: Auckland Volcanic Field, New Zealand

“…The second term on the right-hand side, containing sin, accounts for gravitational forces. The third term is a basal friction term where f, the friction coefficient, accounts for surface roughness (f = 0.001-0.02; Sparks et al, 1978;Bursik and Woods, 1996), and the fourth term accounts for loss of momentum due to sedimentation of solid particles (Equ. 6).…”