Insights into Coignimbrite Plume Dynamics from Numerical Models

Abstract:

Great advances have been made in recent years to better understand and model the processes that occur in Plinian plumes. However, comparatively little work has been conducted on modeling coignimbrite plumes, which form as fine-grained material is lofted from the top of pyroclastic density currents, rising into the atmosphere due to buoyancy. This fundamental difference in source condition (gas thrust vs. buoyancy) means that the parameters used to describe Plinian plumes, for example initial source radius, upwards velocity, temperature, gas mass fraction and grain-size distribution are not appropriate for modeling coignimbrite events.

In this study, the ash flow model of Bursik and Woods (1996) is coupled with a plume model (Bursik 2001, Barsotti et al. 2008) to investigate the controls on coignimbrite plume formation, and once formed, the height and dynamics of the plume. Sensitivity analysis was conducted using DAKOTA software and results show that source temperature and gas mass fraction play a key role in controlling when ‘lift-off’ occurs. Once formed, maximum plume height is controlled by the source radius, the temperature at ‘liftoff’ and the entrainment assumption.

Finally, we use the May 18^th 1980 Mount St. Helens co-blast eruption to test the application of an ash dispersion model, specifically VOLCALPUFF (Barsotti et al., 2008), to the coignimbrite problem, with the specific aim of distinguishing differences in application for coignimbrite and Plinian events. The results highlight the importance of coignimbrite events when considering ash fall hazard and a requirement to treat such events separately to Plinian events.

Andrews, B.J. and Manga, M.. JVGR , 225-226, 30-44, 2012

Barsotti, S, Neri, A, and Scire, JS. The vol-calpuff model for atmospheric ash dispersal: 1. Approach and physical formulation. JGR, 113(B03208), 2008

Bursik, M. Effect of wind on the rise height of volcanic plumes. GRL, 28(18), 3621–3624, 2001.

Bursik, M and Woods, A. W. The dynamics and thermodynamics of large ash flows. Bull. Volc, 58(2-3), 175–193, 1996

Esposti Ongario, T, Clarke, A.B., Voight, B, Neri, A., and Widiwijayanti, W. Multiphase flow dynamics of pyroclastic density currents during the May 18, 1980 lateral blast of Mount St. Helens. JGR, 117(B06208), 2012.