The dynamics of explosive volcanic eruptions

Abstract

Explosive volcanic eruptions involve the ejection of dense mixtures of ash and gas from a volcanic vent at high speed and pressure. This mixture is generated as liquid magma rises from a crustal magma chamber and decompresses, exsolving water vapor. As gas is exsolved, the mixture inflates, accelerates, and becomes foam‐like. Once the liquid films around the bubbles are unable to spread as rapidly as the bubbles are expanding through decompression, the films rupture, and a fragmented mixture of ash and volatiles ascends to the volcanic vent. On eruption from the vent, the material decompresses, either into a volcanic crater or directly into the atmosphere. In the case of free decompression, the mixture typically has a high speed, while decompression in a crater can lead to either very low or very high eruption speeds. After decompression, the hot, dense mixture begins to entrain and heat ambient air, thereby lowering the mixture density, but it also decelerates under gravity. If the eruption velocity is sufficiently high, then the material can become buoyant and will generate a buoyant ash plume, called a Plinian eruption column, which rises above the vent. In contrast, if the eruption velocity is small or the mass flux is very large, then the material will typically collapse back toward the Earth and form a dense, laterally spreading flow. Buoyant eruption columns are able to transport the material high into the atmosphere, since they provide an efficient means of converting the initial thermal energy of the mixture into potential energy through entrainment and heating of ambient air. The height of rise of such eruption columns depends upon the eruption rate, the stratification of the atmosphere, the degree of thermal disequilibrium between the particles and the air, and the amount of water vapor in the atmosphere. Dense, hot ash flows, generated by collapsing fountains, transport ash and clasts laterally from the vent, sedimenting many of the larger clasts and entraining air en route. As a result, the density of the mixture may fall below that of the atmosphere, and the finer‐grained solid material may thereby become buoyant and rise from the flow. The distance it travels increases with both the cloud mass and the mean particle size. The ensuing buoyant ash plume, called a coignimbrite eruption column, may have a source several kilometers from the original volcanic vent. Once the thermal energy of an eruption column has become exhausted, the ash intrudes laterally into the atmosphere. Ultimately, the cloud is swept downwind, where sedimentation of ash leads to fall deposits over hundreds of kilometers from the volcano.

The size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions

“…Second, it is remarkable that the histogram of D values for fall deposits has a sharp cutoff at the value of 3.0 (Figure 7). With exponent values smaller than 3.0, the amount of gas released at fragmentation is a small fraction of the total available (Figure 10), and such conditions are not in favor of a sustained Plinian regime which requires a minimum amount of continuous gas [ Woods, 1995]. A rigorous test requires a calculation involving the mass discharge rate and the initial amount of volatiles present in the melt, and must be done for specific case histories.…”

A three‐dimensional numerical simulation of spreading umbrella clouds

“…[36] In Figure Normally, the value of the entrainment coefficient is assumed to be 0.10 in the 1-D plume model [e.g., Woods, 1995]. Horizontal line and shaded zone in Figure 10a are the altitude of spreading umbrella cloud and the range of the total height of eruption column observed in the Pinatubo 1991 eruption, respectively [Tanaka et al, 1991;Koyaguchi and Tokuno, 1993;Holasek et al, 1996a].…”

“…[8] The 1-D plume model of eruption column [e.g., Sparks, 1986;Woods, 1988Woods, , 1995 is based on the fluid dynamics and thermodynamics model of Morton et al [1956]. In this model, the eruption column is assigned horizontally averaged properties at each height and it is assumed that the pressure of the eruption cloud is equal to the atmospheric pressure at all heights.…”

“…The total height of eruption column, the altitude of NBL and the volumetric flow rate at the NBL are calculated as a function of the mass discharge rate at the vent on the basis of a steady 1-D plume model [Sparks, 1986;Woods, 1988Woods, , 1995. The spreading rate (the increase rate of radius) of umbrella cloud is also calculated from the volumetric flow rate at the NBL on the basis of axisymmetrical gravity current model [Holasek et al, 1996b;Sparks et al, 1997].…”

On the development of highly‐viscous skins of liquid around bubbles during magmatic degassing

“…Several models have been developed with the aim of predicting the dynamics of magma flow in volcanic conduits [Papale and Dobran, 1994;Woods, 1995;Toramaru, 1995;Jaupart, 1996; Proussevitch and Sahagian, 1998; Melnile and Sparles, 1999]. These models incorporate controversial assumptions about bubble nucleation kinetics [Jaupart, 1996; Mourtada-Bonnefoi and Laporte, 1999], equilibrium versus disequilibrium bubble growth [Gardner et al, 1999], and magma fragmentation mechanism [Melnile, 1999; Papale, 1999; Sahagian, 1999;Zhang, 1999].…”