A pyroclastic flow (also known as a pyroclastic ensity current) is a common and devastating result of some volcanic eruptions. The flows are fast-moving currents of hot gas and rock (collectively known as tephra), which travel away from the volcano at speeds generally greater than 80 km/h (50mph). The gas can reach temperatures of about 1,000 degrees Celsius (1,800 F). The 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.