Pyroclastic flow:
A pyroclastic flow (also known scientifically as a pyroclastic density current[1]) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra), which reaches speeds moving away from a volcano of up to 700 km/h (450 mph).[2] The gases can reach temperatures of about 1,000 °C (1,830 °F). Pyroclastic flows 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. They are a common and devastating result of certain explosive eruptions. [Wikipedia]
I’ve read this phrase so many times, and yet have never been entirely clear on its meaning. Noted in “Indonesia’s Agung Raised to Highest Alert for Eruption,” Erik Klemetti, Rocky Planet:
The 1963 eruption of Agung was big. It ranked as a VEI 5, which is on the same scale as the 1980 eruption of Mount St. Helens. The volcano has a history of eruptions of this scale, with another VEI 5 eruption that produced pyroclastic flows, lots of ash fall and some lava flows in 1843. The 1963 eruption also emitted lots of sulfur dioxide and chlorine into the atmosphere, which caused a brief global cooling over the next year.
