Pyroclastic flows Substantial sections of the dome were now failing and collapsing to generate larger avalanches known as pyroclastic flows. A pyroclastic flow is a rapidly moving avalanche of hot volcanic debris. As a section of the dome fails, the debris disintegrates to form a mixture of lava blocks and ash. The lava is probably pre-stressed and can contain pressurised gases. Consequently the avalanche quickly generates a large amount of fine grain material.Pyroclastic flows consist of three parts; the main flow is a dense, concentrated avalanche of blocks and ash; above the flow a dense, turbulent cloud of ash, volcanic gas and hot air develops; the overlying cloud forms both by mixing with the surrounding air and by expansion of pressurised gases from the disintegration of lava in the flow. One part of the overlying cloud remains denser than air and is known as a pyroclastic surge. This moves like a density current. While the dense avalanche is confined to the base of the valley, the overlying surge can sweep up valley sides and can detach from the parent avalanche to inundate different areas. Another part of the overlying cloud has mixed with and heated so much air, that it becomes lighter than air, a buoyant plume of fine ash and hot air rises above the flow, often to heights of a few kilometres.
