CHAPTER 5 BALLISTICS SHOWER DURING PLINIAN SCENARIO AT VESUVIUS

Abstract

The distribution of pyroclasts from 79 A.D. eruption of Vesuvius is analysed to assess the ejection velocities of ballistic particles pertaining to the white and gray eruption phases. This distribution is related to the energy of the eruptive mixture and conditions of the atmosphere during the eruption. Ballistic debris is common in the deposits, and within the sampled area (3-14 km S-SE from the vent) the ejected blocks are scattered throughout the fine-grained pumice fall. We measured about 300 ballistic blocks with diameters between 0.07 and 1 m. Some fragments as large as 0.3 m are located at 9 km from the vent, which probably represents the ballistic limit of such fragments. By using a ballistic model for large blocks permitted an assessment of their initial velocities which range from 170 to 2300m/s, and since some of these velocities exceed the maximum observed velocities of plinian eruptions we conclude that the ballistic model is deficient. The trajectories of smaller blocks (0.1<d<0.3m) are not truly ballistic, because these can be sustained in the eruptive column and dispersed by means of the finger-like projections from the jet thrust region of the column from where they fall or produce gravity currents on the slopes of the volcano. The gas expansion in the column reduces the drag force on particulates and aids in their vertical and lateral transport. In modeling an explosive scenario at Vesuvius it is thus necessary to account for a wide variety of particulate sizes in the presence of local and stratospheric wind conditions and changing characteristics of magma as it is being evacuated from the volcanic system. © 2006 Elsevier B.V. All rights reserved.