Pyroclastic Flow - Flying Rocks
If a large volume of volcanic debris is erupted quickly from a volcano, the eruption column can collapse, like pointing a garden hose directly up in the sky. As the eruption column collapses it can transform into an outwardly expanding flood of hot solid ejecta in a fluidizing gas cloud. This is known as a pyroclastic flow, or surge if the flow is dilute and turbulent.
The flow direction may be topographically controlled. Flows and surges often travel at speeds up to 200 km/h, and cause total destruction of the areas they cover.
Flows and surges maybe very hot (several hundred oC) and can start fires. Some pyroclastic surges are cooler (usually less than 300oC) and often deposit sticky wet mud.
Pyroclastic flows and surges represent the most destructive manifestations of volcanic activity.
People caught in the direct path of a flow or surge are most unlikely to survive. Those who are lucky enough to survive will receive severe injuries. Buildings offer some protection at marginal areas of flows and surges but they are often destroyed. The best protection is to evacuate prior to the event.