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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