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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Pictures not seen much around on the Internet (911 Pyroclastic flow) as the…

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Mayon Volcano, Philippines: Pyroclastic flow is a fast-moving current of hot gas and rock (collectively known as tephra), which reaches speeds moving away from a volcano of up to 450 mph. The gas can reach temperatures of about 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...

Pyroclastic flow, Philippines. "A pyroclastic flow is a fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcanic edifice. These awesome features are heavier-than-air emulsions that move much like a snow avalanche, except that they are fiercely hot, contain toxic gases, and move at phenomenal, hurricane-force speeds, often over 100 km/hour.

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2010.10.29 - Mount Merapi releases a pyroclastic flow during eruption as seen from Deles, Central Java, Indonesia. There have been no new reports of injuries or damage. (AP Photo)

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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…

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