Transport capacity of pyroclastic density currents: Experiments and models of substrate‐flow interaction

Abstract

[1] One of the more distinctive features of many ignimbrites is the presence of large lithics (some greater than meter scale) and pumices that have been transported great distances (>10 km) from the eruptive vent, sometimes over steep terrain and expanses of water. In many cases, these particles have been transported much further than can be explained by aerodynamic forces and ballistic trajectories. We examine the forces responsible for transport of large clasts and examine in detail the momentum transfer occurring when particles interact with their boundaries. We performed a suite of experiments and numerical simulations to quantify the mass and momentum transfer that occurs when particles interact with a pumice bed substrate and with water substrate, two geologically motivated flow end-members. We find that clasts transported in dilute currents are particularly sensitive to the nature of the boundary, and while large particles can skip several times on a water substrate, they travel less far than particles that impact pumice bed substrates. All else being equal, large particles in dense pyroclastic density currents are themselves relatively insensitive to the details of their boundaries; however, one of the most important ways boundary conditions influence large particles is not through direct interaction but by changing the local concentration of fine particles. Momentum transfer from fine particles to large particles appears to be required to transport large clasts great distances. If initially dense flows become dilute during transport, then the transport capacity of large particles in the flow is substantially decreased.

The flow dynamics of an extremely large volume pyroclastic flow, the 2.08-Ma Cerro Galán Ignimbrite, NW Argentina, and comparison with other flow types

“…Valentine and Wohletz 1989;Giordano and Dobran 1994;Bursik and Woods 1996 and others) or have tried to simulate grain behaviour in granular flow parts of pyroclastic flows (e.g. Dufek and Bergantz 2007;Dufek et al 2009). In simulations of whole flows, the volumetrically dominant low particle concentration ash cloud overwhelms the simulations, and little seems to be gleaned about the behaviour of the denser body under-flow, except that it exists and is susceptible to the effects of topographic impediments.…”

“…Dufek et al (2009) furthermore emphasise that in poly-disperse granular flow systems, for the larger grains to be transported significant distances, momentum transfer from the smaller grain population to the larger grain population must occur.…”

“…Relevance of experimental and modelling studies to understanding the flow behaviour of very large volume pyroclastic flows Many experimental studies and numerical simulations have been undertaken in recent years on the flow behaviour of high particle concentration granular flows (e.g. Roche et al 2005;Dufek and Bergantz 2007;Dellino et al 2007;Druitt et al 2007;Dufek and Manga 2008;Dufek et al 2009;Giordano and Dobran 1994;Girolami et al 2008Girolami et al , 2010Valentine and Wohletz 1989;Bursik and Woods 1996). The experiments all involve instantaneous lock release of small volumes of artificial beads or natural fine-ash mixtures from ignimbrite in small experimental tanks or down ramps.…”

“…Hildreth 1983;Freundt and Schmincke 1986;Pittari et al 2006), as well as experiments and modelling of pyroclastic flow behaviour (e.g. Roche et al 2005Roche et al , 2008Roche et al , 2010Dufek and Bergantz 2007;Dellino et al 2007;Druitt et al 2007;Dufek and Manga 2008;Dufek et al 2009;Giordano and Dobran 1994;Valentine and Wohletz 1989;Bursik and Woods 1996), have contributed greatly to the understanding of the flow behaviour of pyroclastic flows. However, extremely large volume (≥VEI 8) pyroclastic flow forming eruptions have not occurred in historic times, and there are few studies of the detailed facies architecture of ignimbrites of this magnitude (e.g.…”

Rounding of pumice clasts during transport: field measurements and laboratory studies

“…The with-ash pumice loses mass about 30% more slowly compared to the same pumice without ash. In addition, the presence of ash in real flows will affect comminution because ash influences transport of clasts (Dufek et al 2009) and flow mechanics (e.g., Roche et al 2005).…”