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Pyroclastic Flow 

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Pyroclastic flow USGS

Pyroclastic means" fire of broken rocks".Nuee Ardente is the French version. The flow appears as a billowing grey cloud moving at great speed down the slopes of a volcano. They consist of hot ash,rocks,dust and gas at a temperature of 1000C and can move at speeds of up to 100km/hour.

The grey cloud is the cooled exterior of the cloud and thermam imaging equipment would show them to be very hot. Dome collapse and eruption colomn collapse are the main causes. They can even travel across water. Vesuvius, Soufriere Hills (Montserrat), Mt.Pelee and Mt.St.Helens all produce Pyroclastic flows.