User:Hutvh/sandbox/Pyroclastic Density Currents
Fast-moving current of hot gas and volcanic matter that moves away from a volcano / From Wikipedia, the free encyclopedia
A pyroclastic density current or PDCs ( the umbrella term for pyroclastic flow or pyroclastic surge)[1] is a fast-moving mixture of hot gas and volcanic matter (collectively known as tephra) that flows along the ground. Pyroclastic density currents have caused more fatalities than any other volcanic hazard [2]. They can move at velocities of 100 km/h (62 mph) on average but is capable of reaching speeds up to 700 km/h (430 mph).[3] The gases and tephra can reach temperatures of between 200°C and 700°C (390-1300°F). [4]
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PDCs form as byproducts of certain explosive eruptions. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope. They are extremely hazardous due to their high mobility. PDCs are able to flow over large topographic obstacles[5][6] and travel for tens to hundreds of kilometers[1].
A pyroclastic density current is a type of gravity current.[7][8] It flows due to negative buoyancy (it is more dense than the surrounding air) until it reaches buoyancy reversal where it become less dense than the surrounding air and floats away as a pyroclastic cloud or phoenix cloud.
Currently, over 100 million people world wide live in areas with a risk of PDCs.[9]