Pyroclastic materials are classified according to their size, measured in milli meters: dust (less than 0.6 mm [0.02 inch]), ash (fragments between 0.6 and 2 mm [0.02 to 0.08 inch]), cinders (fragments between 2 and 64 mm [0.08 and 2.5 inches], also known as lapilli), blocks (angular fragments greater than 64 mm), and bombs (rounded fragments greater than 64 mm).

The fluid nature of a pyroclastic flow is maintained by the turbulence of its internal gases. Both the incandescent pyroclastic particles and the rolling clouds of dust that rise above them actively liberate more gas. The expansion of these gases accounts for the nearly frictionless character of the flow as well as its great mobility and destructive power.

Pyroclastic flow, in a volcanic eruption, a fluidized mixture of hot rock fragments, hot gases, and entrapped air that moves at high speed in thick, gray-to-black, turbulent clouds that hug the ground. The temperature of the volcanic gases can reach about 600 to 700 °C (1,100 to 1,300 °F). The velocity of a flow often exceeds 100 km (60 miles) per hour and may attain speeds as great as 160 km (100 miles) per hour.

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