Summary

The mechanisms controlling the transport of pyroclasts in a plinian-type eruption column are discussed. An estimate is made of the density of the magmatic gas in the vent, and initial (‘muzzle’) velocities are deduced for 18 eruptions using the areal distribution of pyroclasts in the resulting air-fall deposits. A model of the physical properties of the lower part of an eruption column is presented, and used to deduce the heights to which plinian eruption columns should commonly extend. It is demonstrated that column collapse to form ignimbrites may be a common result of the change, with time, of the volatile content of the erupted material as progressively deeper levels in a magma chamber are tapped. Vent radii are estimated for those eruptions for which the duration of the eruption is known. 1