A laboratory simulation of pyroclastic flows down slopes

Abstract

Huppert, H.E., Turner, J.S., Carey, S.N., Sparks, R.S.J. and Hallworth, M.A. 19~6. A laboratory simulation of pyroclastic flows down slopes. J. Volcanol. Geotherm. Res., Laboratory experiments are described which explore the dynamical consequences of buoyant convective upflow observed above hot pyroclastic flows. In nature, the convection is produced by the hot ash particles exchanging heat with air mixed into the front and top of the pyroclastic flow. This effect on the buoyancy due to the mixing of air and ash has been modelled in the laboratory using mixtures of methanol and ethylene glycol (MEG), which have a nonlinear density behaviour when mixed with water. Intermediate mixtures of these fluids can be denser than either initial component, and so the laboratory experiments were inverted models of the natural situation. We studied MEG flowing up under a sloping roof in a tank filled with water. The experiments were performed both in a narrow channel and on a laterally unconfined slope. The flow patterns were also compared with those of conventional gravity currents formed using fresh and salt water. The presence of the region of reversed buoyancy outside the layer flowing along the slope had two significant effects. First, it periodically protected the flow from direct mixing with the environment, resulting in pulses of relatively undiluted fluid moving out intermittently ahead of the main flow. Second, it produced a lateral inflow towards the axis of the current which kept the current confined to a narrow tongue, even on a wide slope.In pyroclastic flows the basal avalanche portion has a much larger density contrast with its surroundings than the laboratory flows. Calculations show that mixing of air into the dense part of a pyroclastic flow cannot generate a mixture that is buoyant in the atmosphere. However, the overlying dilute ash cloud can behave as a gravity current comparable in density contrast to the laboratory flows and can become buoyant, depending on the temperature and ash content. In the August 7th pyroclastic flow of Mount St. Helens, Hoblitt (1986) describes pulsations in the flow front, which are reminiscent of those observed in the experiments. As proposed by Hoblitt, the pulsations are caused by the ash cloud accelerating away from the front of the dense avalanche as a density 0377-0273/86/$03.50

Palaeotemperature determinations for the 1.8-ka Taupo ignimbrite, New Zealand, and implications for the emplacement history of a high-velocity pyroclastic flow

“…Bursik and Woods 1996;Dade and Huppert 1996;Todesco et al 2002) and reproduced in laboratory analogue experiments (e.g. Huppert et al 1986;Woods and Bursik 1994;Choux and Druitt 2002).…”

Contribution of Fine Ash to the Atmosphere From Plumes Associated With Pyroclastic Density Currents

“…Ash injection and dispersion in the atmosphere during explosive eruptions is 28 primarily associated with the formation of buoyant convective columns above volcanic vents 29 (e.g. Bonadonna and Phillips 2003;Bursik 2001;Carey and Sparks 1986;Sparks 1986). In 30 this context, a mixture of particles and gas is ejected from the conduit in a high velocity jet 31 following fragmentation of magma ascending from depth in the conduit ( Figure 1A).…”

Effect of layered heterogeneity of a porous medium on density-driven natural convection

“…二酸化炭素気泡には界面張力に伴うトラップが働き，多孔質内部にトラップされる Suekane et al, 2008)． さらに， 二酸化炭素は徐々に地下水へと溶解する (Gilfillan et al, 2009;Iglauer, 2011;Lindeberg and Wessel-Berg, 1997;Lindeberg and Bergmo, 2003;Riaz and Cinar, 2014 Ghesmat et al, 2010;Hewitt et al, 2014;Hidalgo and Carrera, 2009;Pau et al, 2010;Xie et al, 2011)や実験 (Backhaus et al, 2011;Huppert et al, 1986;Neufeld et al, 2010) Fig. 2 Evolution of three-dimensional finger structure associated with the Rayleigh-Taylor convection in a porous medium with the layered structure.…”