During historical eruptions of Mount St. Helens, pyroclastic flows have originated from collapsing eruption columns and from gravitational or explosive disruption of growing lava domes.
Driven by gravity, pyroclastic flows seek topographically low areas and can travel down valleys at high velocities beyond the flanks of the volcano. They pose lethal hazard from incineration, asphyxiation, burial, and impact. They are difficult or impossible to escape; therefore evacuation of likely hazardous areas must take place before such events occur.
Pyroclastic flows from the May 18, 1980, eruption ran out no farther than 8 km (5 mi) from the vent. During the past 4,000 years, numerous pyroclastic flows are known to have traveled at least as far as 10 to 15 km (6 to 9 mi) and one older flow reached 20 km (12 mi) from source. The present orientation of the volcano, with the open crater to the north, favors distribution of pyroclastic flows into the North Fork Toutle River valley; however, all flanks of the volcano are subject to pyroclastic- flow hazard during a large explosive eruption.