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Pyroclastic Flows at Lassen Volcanic Center

Explosive eruptions that produce volcanic ash can also form pyroclastic density currents—both pyroclastic flows and surges. 

Pyroclastic flow (dense darker grey billowing mass) travels down slope of Mayon volcano, Philippines while ash column rises from the vent. Dilute, lighter-colored ash billows off leading edge. (Credit: Newhall, Chris. Public domain.)

If gas-charged dacite magma erupts explosively, it will not only produce a high eruption column, but as the rising column cools it loses its ability to convect the ash and tephra in the plume upward. When the column collapses, it may generate highly mobile pyroclastic flows that can rush several kilometers down a volcano’s slopes and adjacent valleys.

A different type of pyroclastic flow may occur as unstable, steep-sided lava domes collapse while they grow. The resulting block and ash flows can travel several kilometers down slope. Such a sequence of events is recorded by the deposits related to the emplacement of Chaos Crags domes between 1,100 and 1,000 years ago.