Explosive eruption of magma and solid-rock fragments or the collapse of a vertical eruption column of ash and larger rock fragments may generate pyroclastic flows. A series of photographs of an eruption at Mount St. Helens on July 22, 1980, shows the development of a pyroclastic flow.

The fall of fresh lava and hot rock debris from a lava dome or thick lava flow can generate scores of pyroclastic flows. The repeated collapse of a growing lava dome atop Unzen Volcano caused thousands of small but dangerous pyroclastic flows between 1991 and 1995.

Avalanching of hot, thick pyroclastic flow deposits in valleys downslope from Mount Pinatubo, Philippines, triggered many small pyroclastic flows for more than two years after its climactic eruption on June 15, 1991. Some of these flows formed deposits as long as 10 km, 1 km wide, and 10 m thick. Such secondary pyroclastic flows present post-eruption hazards that were not recognized until they were observed at Pinatubo.