Pyroclastic flow

fast-moving 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 1,000ยบC and is from 450-700 miles per hour.

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

They are extremely dangerous and can be deadly.