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Explosive volcanic eruptions — V. Observations of plume dynamics during the 1979 Soufrière eruption, St Vincent

Summary. During the 1979 eruption of the Soufritre of St Vincent, West Indies, a 14min period of explosive activity occurred on April 22. The first three minutes of this activity were filmed. Five individual explosions formed distinct plumes which fed an eruption column which eventually ascended to a height of over 18 km. The ascent velocities of the fronts of these plumes ranged from 8.5 to 61.7m s-'. The half-angle of spreading of the plume fronts ranged from 21.5 to 24". One of the plumes was observed to 8 km height and decelerated steadily from 5 1.5 to 23 m s-l. The main column fed by individual explosions was, however, observed to accelerate from 8.5 to 58 m s-l in the same height interval. A theoretical treatment of volcanic plume motion is presented. Measurements of dimensions, velocities and spreading rates from the film are used to estimate plume parameters such as temperature, particle content and volume discharge rate of magma from the theoretical relationships. These calculations show that the individual plumes became hotter and richer in juvenile ash with time. The acceleration of the main eruption column was the result of being fed by increasingly hotter and more ash-rich explosions. An average volume discharge rate of 12600m3s-' is estimated from the analysis of the plume motions. This value agrees closely with an estimate of discharge rate based on the heat flux required to form an 18 km high column. This agreement suggests that theories of convective motions in plumes can be successfully adapted to the volcanic case, as long as the effect of hot entrained ash particles is considered.