A pyroclastic flow (also known scientifically as a pyroclastic density current) is a fast-moving current of superheated gas and rock (collectively known as tephra), which reaches speeds moving away from a volcano of up to 700 km/h (450 mph). The gas can reach temperatures of about 1,000 °C (1,830 °F). Pyroclastic flows normally hug the ground and travel 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 volcanic eruptions.
Read more about Pyroclastic Flow: Origin of Term, Causes, Size and Effects
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