Abstract Beds of solid particles under high pressure have been suddenly decompressed. The resulting high-speed two-phase flows develop a highly inhomogeneous character with interpenetrating regions of high and low particle concentration. The flow is highly turbulent with large velocity and density fluctuations. The flows approach homogeneity only when highly expanded with low particle concentrations (< 1% by volume). Graded beds in which either particle size or density increases upwards rapidly enough are highly stable and move upwards as dense slugs. Beds with particle size or density constant or decreasing upwards are highly unstable and develop into inhomogeneous complex flows. These observations show that assumption of a uniform density for explosive volcanic two-phase flows is not a good one. In Plinian eruptions close to the transition to column collapse, pyroclastic flows could be generated by fall-back of high-concentration regions and by turbulent bursts at the vent forming intra-plinian flow and surge deposits. In fully collapsing columns high-concentration regions would immediately form dense pyroclastic flows whereas dilute regions would segregate quickly to form co-ignimbrite clouds. Variations in discharge rate could result from grain size variations in the ejecta and vent-wall collapse leading to slugging effects. The experimental studies indicate that violent high-velocity flows such as the Mount St. Helens blast in 1980 can be highly inhomogeneous. Such inhomogeneous flows are fully turbulent with large fluctuations in both velocity and density. Much of the mass would initially be carried in high-concentration regions giving the deposits a ‘flow-like’ character in proximal regions, whereas the flow would become more dilute with distance progressing to a more ‘surge-like’ character.