The correct answer is "a pyroclastic flow, also called a Nuée Ardent"
A Pyroclastic Flow is is a quick moving current of hot gas and volcanic issue that moves from a fountain of liquid magma achieving rates of up to 700 km/h. The gases can achieve temperatures of around 1,000 °C. Pyroclastic streams are a typical and obliterating aftereffect of certain hazardous ejections; they regularly touch the ground and tear downhill, or spread along the side under gravity.