Pyroclastic flows

Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds. They may result from the explosive eruption of molten or solid rock fragments, or both. They may also result from the nonexplosive eruption of lava when parts of dome or a thick lava flow collapses down a steep slope. Most pyroclastic flows consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.

Pyroclastic flows can reach speeds moving away from a volcano of up to 700 km/h (450 mph).  The gas can reach temperatures of about 1,000 °C (1,830 °F). Pyroclastic flows normally hug the ground and travel 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. They are a common and devastating result of certain explosive volcanic eruptions.

A pyroclastic flow will destroy nearly everything in its path. With rock fragments ranging in size from ash to boulders traveling across the ground at speeds typically greater than 80 km per hour, pyroclastic flows knock down, shatter, bury or carry away nearly all objects and structures in their way. The extreme temperatures of rocks and gas inside pyroclastic flows can cause combustible material to burn, especially petroleum products, wood, vegetation, and houses.

Testimonial evidence from the 1883 eruption of Krakatoa, supported by experimental evidence, shows that pyroclastic flows can cross significant bodies of water. One flow reached the Sumatran coast as much as 48 km away.

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

The towns of Pompeii and Herculaneum, Italy, for example, were engulfed by pyroclastic surges in 79 AD with many lives lost.

"Garden of the Fugitives". Plaster casts of victims still in situ; many casts are in the Archaeological Museum of Naples.

For more information and some video clips visit:-
http://www.geo.mtu.edu/volcanoes/hazards/primer/pyro.html
http://en.wikipedia.org/wiki/Pyroclastic_flow
http://volcanoes.usgs.gov/hazards/pyroclasticflow/