Eruption columns consist of two components. The lower gas thrust component results from decompression of the gas phase, and decelerates rapidly to near zero velocity at heights of 1.5-4.5 km for initial gas velocities of 4oo-6oo m/s. The upper conzective thrust component is due to the column having a lower density than the atmosphere, and can transport the column to heights of 30-40 km. At the base, the effective density of a column is considerably greater than that of the atmosphere and is very sensitive to changes of gas content. Fall out ofclasts and incorporation and heating of air reduce the density substantially during the gas thrust part. It is shown that columns formed from magmas with high water contents (5%) are likely to show convective motion. Magmas with low water contents (1/2%) or high proportions of CO₂ will form a column with an effective density greater than the atmosphere, and gravitational column collapse can occur to generate ignimbriteo forming pyroclastic flows. In magmas with intermediate gas contents, the occurrence of convection (plinian case) or collapse (ignimbrite-forming) depends on vent radius, proportion of ash and gas content. The model presented here can explain: the sharp transition from plinian to ignimbriteforming activity; the increase of temperature with time shown by some ignimbrites; the common association of low temperature ignimbrites with preceding plinian eruptions, and the apparent mobility of pyroclastic flows.