A laboratory study of ash flows

Hot ash flows propagate from volcanic vents during some explosive volcanic eruptions. As a flow entrains and heats ambient air and simultaneously deposits pyroclasts, part of the flow may become less dense than the air and ascend from the flow. If entrainment of air is more important than sedimentation in controlling the flow behaviour, then a large, buoyant ash cloud may form above the flow. Conversely, if sedimentation of particles dominates, then a much smaller fraction of the particles are able to ascend in the buoyant cloud. A series of analog laboratory experiment have been carried out to simulate the motion and buoyancy generation in dilute ash flows propagating along the ground. The experiments suggest that flow separation and hydraulic jumps resulting from sudden changes in slope angle can enhance entrainment or sedimentation and increase buoyancy. -from Authors