<P> A pyroclastic flow (also known as a pyroclastic density current) is a fast - moving current of hot gas and volcanic matter (collectively known as tephra) that moves away from a volcano reaching speeds of up to 700 km / h (430 mph). The gases can reach temperatures of about 1,000 ° C (1,830 ° F). </P> <P> Pyroclastic flows are a common and devastating result of certain explosive eruptions; they normally touch the ground and hurtle downhill, or spread laterally under gravity . Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope . </P>

How fast was the pyroclastic flow in pompeii