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A pyroclastic flow (also known scientifically as a pyroclastic density current) is a fast-moving current of hot 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 1000 °C (1830 °F). Pyroclastic flows normally hug the ground and travel downhill or spread laterally under gravity.
