Tephra dispersal

This paper is included in the Special Publication entitled 'The physics of explosive volcanic eruptions', edited by J.S. Gilbert and R.S.J. Sparks. 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.