Pyroclastic flows may also result from a laterally directed explosion , or the fall of hot rock debris from a dome or thick lava flow . Rock fragments in pyroclastic flows range widely in grain size and consist of dense rock, pumice, or both. Ash may fall from the cloud over a wide area downwind from the basal flow. Pyroclastic flows generally follow valleys or other depressions, but can have enough momentum to overtop hills or ridges in their paths. The larger the mass of a flow and the faster it travels, the higher it will rise onto obstacles in its path.
Pyroclastic flows are extremely hazardous because of their high speeds and temperatures.
Pyroclastic flows have been erupted repeatedly at many volcanic centers in the Cascade Range during Holocene time. Such eruptions can produce pyroclastic flows which could travel more than 50 km from a vent and could be extremely destructive over wide areas.