A pyroclastic flow is a fast avalanche of hot gas, ash and pieces of rock, from grains of dust to boulders, which rushes down the slopes of a volcano during an explosive eruption. (Pyro means fire and clastic means made of broken pieces.)
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Example 19.11 (Racing a pyroclastic flow)
A pyroclastic flow moving at heads for a village down the valley. It takes hour, that is minutes. Three minutes: nobody can run, or even drive, out of the valley in time. People are only safe if they have left before the eruption.