Pyroclastic flows can be formed in a couple of different ways. If lava flows and domes break apart, gravity may cause the material to flow rapidly downhill to form these avalanches of hot rock and gas. Also, during highly explosive eruptions that produce large vertical columns of ash and pumice, a portion of the columns can collapse to form pyroclastic flows that sweep down the flanks of volcanoes. Regardless of their source, pyroclastic flows on snow and ice are hot and turbulent, and as they flow they erode and melt snow and ice in their path, which can create enough melt water to mobilize loose volcanic rock into muddy slurries called lahars. This has occurred at all of the principal Cascade volcanoes including Mount St. Helens and Mount Rainier.To learn more about general pyroclastic flow hazards, visit the Volcano Hazards Program webpage about their effects.
An estimated two cubic miles of glacier ice and perennial snow cover the Cascade Range volcanoes today, and more than half of that amount is on Mount Rainier. During past Ice Ages, glaciers were more extensive and enveloped many Cascade Range volcanoes. Ice–age glaciers played an important role in shaping some volcanoes by influencing the placement of lava flows. At many locations, glacier ice slowed or halted the movement of lava flows. Elsewhere, glacier ice channeled lava flows and allowed them to travel several miles from the base of the volcano. At Mount Rainier, the 40,000 year-old Ricksecker Point lava flow pooled at the juncture of the ancestral Nisqually and Paradise Glaciers.To learn more about general lava flow hazards, visit the Volcano Hazards Program webpage about their effects.