Tephra dispersal

Abstract

Our understanding of the spread of volcanic ejecta from eruption columns has evolved significantly since the first quantitative studies. Given recent advances, it is possible to describe much of the physics of tephra dispersal, and to construct a relatively robust model for the spread of tephra in the proximal region of a volcanic eruption plume. Results from such modelling compare well with data on tephra fall deposits. More distal dispersal, driven as it is by atmospheric motions, has also been treated extensively, by atmospheric scientists as well as by volcanologists. Results from these studies have been compared with satellite data and have been found to be accurate in terms of predicting ash cloud trajectories. Comparisons of the various models with data point out several areas where research could be productively directed in future. The transition from tephra transport by the plume to that by the atmosphere is poorly understood: processes related to transport between plume and ground are poorly known: the layering of the stratosphere needs to be considered more thoroughly to understand transport processes over periods of days to months; and the total grain size distributions of fall deposits and eruption plumes have probably been incompletely characterized in the past.

Dynamics of wind‐affected volcanic plumes: The example of the 2011 Cordón Caulle eruption, Chile

“…Crosswind spreading of the clouds associated with the transitional plumes developed on 4 and 6 June could be best described by the linear combination of gravitational spreading and turbulent diffusion with values of diffusion coefficients similar for both days (i.e., 9000 m 2 s −1 ) that are significantly higher than those observed for low‐energy bent‐over plumes advected as lenses of aerosol and gas with nearly constant width (e.g., ~10 m 2 s −1 for Mount Augustine eruption, 3 April 1986 [ Sparks et al ., ; Bursik , ]) but are in the range of observed horizontal diffusivity over brief time intervals (10–10 4 m 2 s −1 [ Heffter , ; Pasquill , ]). Contrary to the 1996 Ruapehu eruption for which the deposit was wider than the cloud, both cloud spreading and deposit during the first couple of days of the Cordón Caulle eruption seem to be characterized by similar crosswind dispersal (Figure ).…”

“…In contrast, plumes that are strongly affected by wind maintain the vorticity structure characteristic of the convective column also when they reach their maximum height and start spreading horizontally [ Sparks et al ., ]. Their crosswind spreading at the neutral buoyancy level is typically described by turbulent diffusion (i.e., Fickian diffusion) such that [ Bursik , ] where K is the horizontal eddy diffusivity, i.e., diffusion coefficient (m 2 s −1 ). We have already shown how the plumes developed on 4 and 6 June were transitional between weak and strong plumes.…”

“…The best fit is given by the linear combination of gravity spreading and turbulent diffusion following Bursik [], with a diffusion coefficient of 9000 m 2 s −1 for both days (Figure a). The observed crosswind width lays in between the linear combination of gravitational spreading and turbulent diffusion for the minimum and maximum observations of volumetric flow rate (solid and dashed lines in Figure a).…”

Fragmentation in kimberlite: products and intensity of explosive eruption

“…size, gas density, drag coefficient; Wilson and Parfitt 2008), the density (2,700-3,400 kg m −3 ) of kimberlitic pyroclasts Gernon et al 2008;Moss et al 2008) would lead to significant settling velocities relative to the partly expanded gas-dominated flux of fragmented magma. We use a sequence of equations as outlined by Bursik (1998) to estimate settling velocities for kimberlite pyroclasts within the expanding eruption column. The drag coefficient is 2 Kelvin-Helmholz shear instabilities form asymmetric waves at the boundary between gas and liquid of wavelength; l ¼ d…”

A Framework for Probabilistic Multi-Hazard Assessment of Rain-Triggered Lahars Using Bayesian Belief Networks

“…A substantial proportion of the material erupted during these eruptions is deposited over the areas surrounding the volcano as tephra fallout (e.g., Bursik, 1998) or transported in Pyroclastic Density Currents (PDCs; e.g., Sulpizio et al, 2014). The deposits from tephra fallout typically affect areas of hundreds to thousands of square kilometers, while PDC deposits usually have areal extents of tens to hundreds of square kilometers (e.g., Orsi et al, 2004).…”