The basis of the term comes from "pyro" which means fire, and "clast" which refers to broken pieces of rock. Therefore, pyroclastic is a term used for broken pieces of rock associated with volcanic eruptions. It is a general term which covers a variety of sizes, ranging from ash to boulders. Pyroclastic flows are also referred to as pyroclastic clouds, and as "nuee ardentes" - a French term which means "glowing cloud." They are hot and fast, moving at speeds up to several hundred miles per hour and at temperatures of several hundred degrees Celsius. There's a pretty good chance that you couldn't outrun one of these events, which is one of the reasons why most people try to stay out of their way.
http://physics911.net/thermite
http://www.911-strike.com/demolition_explosive.htm
911 Eyewitness - Pyroclastic Flow was a telltale sign of con
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