Pyroclastic flow

A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud)[1] is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of 100 km/h (62 mph) but is capable of reaching speeds up to 700 km/h (430 mph).[2] The gases and tephra can reach temperatures of about 1,000 °C (1,830 °F).

Pyroclastic flows sweep down the flanks of Mayon Volcano, Philippines, in 1984

Pyroclastic flows are the most deadly of all volcanic hazards[3] and are produced as a result of certain explosive eruptions; they 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.

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