Pyroclastic flows are a common and devastating result of some volcanic eruptions. They are an extremely fast moving body of gas, ash and rock which travels at over 100km/h.

There are potential causes of pyroclastic flows:

• Collapse of the ash column from a plinian eruption (e.gMount Vesuvius Pliny the Younger).
• Collapse of a volcanic dome (e.g.Montserrat).
• The directional blast when part of a volcano explodes outwards (e.g.Mount St Helens[?])