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What are pyroclastic flows?

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.

Pyroclastics are common to intermediate volcanoes such as are associated with subduction of the crust. Therefore, expect to find them along the Pacific Ring of Fire (conversely, you are probably pretty safe in Kansas). Several examples come to mind. The 1980 eruption of Mt. St. Helens resulted in a relatively small pyroclastic event, which deposited several hundred feet of material around the northeastern base of the mountain, with finer ash material actually circling the globe for several weeks before it finally all settled out. Mt. Pinatubo in the Philippines was a larger eruption, so the resulting pyroclastic cloud was also larger. In addition, the final eruptive scene in the movie Dante's Peak gives Hollywood's version of the event. This is actually very well done, and if you can kinda/sorta overlook the success the heroes had in outrunning it (in a nearly dead government vehicle with no tires) the reality of the process is fairly accurately portrayed.


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