Pyroclastic flow

fast-moving concentrated current of hot gas and volcanic matter that moves away from a volcano

A pyroclastic flow, pronounced pie-roh-class-tick, (also known as a pyroclastic density current) is a flow of "lava", as commonly associated with volcanic eruptions. The flows are currents of hot gas, or tephra, during an eruption that sometimes causes rock falls. A cloud forms over the river of hot rock. This boiling column of superheated gas, sand, tephra, obsidian, and pumice can reach soaring temperatures of 1,000ºC and is from 450 to 700 miles per hour.

Pyroclastic flows going down Mayon Volcano, Philippines, in 1984.

They are extremely dangerous and can be deadly. Remember you cannot outrun a pyroclastic flow!