The dome of Unzen, Japan Volcano collapsed and created a pyroclastic flow in '91. The people here escaped when the flow stopped before reaching them, but the deadly and terrible speed of it, is astonishing. Pyroclastic flows are fluidized masses of rock fragments and gases that move rapidly in response to gravity. They can form in several different ways, such as when an eruption column collapses, as the result of gravitational collapse or from an explosion on a lava dome.
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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 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, between 200°C and 700°C, can cause combustible material to burn, especially petroleum products, wood, vegetation, and houses. -- Samuel…
Pyroclastic flow sweeps down the side of Mayon Volcano, Philippines, during an explosive eruption on 15 September 1984. Note the ground-hugging cloud of ash (lower left) that is billowing from the pyroclastic flow and the eruption column rising from the top of the volcano. Credit: Photograph by C. Newhall on 15 September 1984.