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Pyroclastic flow from volcanoes

Pyroclastic flows are a common and devastating result of some volcanic eruptions.

They are fast-moving fluidized bodies of hot gas, ash and rock (collectively known as tephra) which can travel away from the vent at up to 150 km/h.

The gas is usually at a temperature of 100-800 degrees Celsius.

The flows normally hug the ground and travel downhill under gravity, their speed depending upon the gradient of the slope and the size of the flow.

Volumes range from a few hundred cubic metres to more than a thousand cubic kilometres, and the larger ones can travel for hundreds of kilometres although none on that scale have occurred for several hundred thousand years.

Most flows are around one to ten cubic kilometres and travel for several kilometres.

Flows usually consist of two parts: the basal flow hugs the ground and contains larger, coarse boulders and rock fragments, while an ash cloud rises above it because of the turbulence between the flow and the overlying air.

Note:   The above text is excerpted from the Wikipedia article "Pyroclastic flow from volcanoes", which has been released under the GNU Free Documentation License.
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