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Lava Flows, Domes, and Pyroclastic Flows Build Cascade Volcanoes

Cascade Range stratovolcanoes are built as repeated eruptions accumulate hundreds of overlapping lava flows, lava domes, and layers of rock rubble from pyroclastic flows.

Pyroclastic flow during August 7, 1980 Mount St. Helens eruption. T...
Pyroclastic flow during August 7, 1980, Mount St. Helens eruption. The view is from Johnston Ridge, located 8 km (5 mi) north of Mount St. Helens. (Credit: Lipman, Peter. Public domain.)

Collapsing lava flows, domes, and large ash columns create pyroclastic flows and lahars.

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.

Ricksecker Point lava flow (the ridge in the center of the photo) erupted 40,000 years ago on the flanks of Mount Rainier along a high ridge adjacent to valley filling glaciers. (Credit: Sisson, Tom. Public domain.)

To learn more about general pyroclastic flow hazards, visit the Volcano Hazards Program webpage about their effects.

Hot lava meets snow and ice at ice-clad Cascade volcanoes.

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.

Ice-chilled lava columns at the end of a Glacier Peak lava flow, Wa...
Impressive columns like these form along the margins of ice-confined lava flows but usually are eroded off the sides due to continued glacial movement after an eruption ceases. Columns are better preserved at the ends of the flows (this photo) where they are protected somewhat from the ice. The columns have irregular orientations due to the irregularly shaped ends of the flows against the ice, with lots of meltwater sluicing over and around the lava. (Credit: Bleick, Heather. Public domain.)