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Pyroclastic Flows Are Rare at Mount Rainier

Spirit Lake and the devastated forest area surrounding Mount St. He...
Spirit Lake and the devastated forest area surrounding Mount St. Helens, Mount Rainier on skyline.This lake was once surrounded by lush forest and is within the area devastated by the directed blast, covered by ash and tephra, then inundated by pyroclastic flows. Remnants of the forest float on the surface of the lake. Mount Rainier (4,392 m or 14,410 ft) is in the distance. The view is from the south. (Topinka, Lyn. Public domain.)

Pyroclastic flow deposits are not as common at Mount Rainier than at some other Cascades volcanoes, such as Mount St Helens. This is probably because fewer eruptions produced pyroclastic flows, but Mount Rainier's glaciers also have an effect. As pyroclastic flow descend glaciers, they incorporates snow and ice. The snow and ice melts, cooling the pyroclastic flow and transforming it to a lahar that can continue far beyond the foot of the volcano. This has happened many times at Mount Rainier.

Deposits of small pyroclastic flows also often come to rest as thin, unconsolidated rubble and ash that erode readily in high alpine settings. A small pyroclastic flow that descended the valley of the South Puyallup River about 2,400 years ago left a dark, sandy and bouldery rubble deposit exposed along the West Side Road. Rarely at Mount Rainier, pyroclastic flows accumulated to thicknesses sufficient to retain heat, giving the debris time to cool slowly and compress as rock fragments and ash weld together into hard rock. Deposits from such a welded pyroclastic flow flank the headwaters of Kautz Creek, supporting Mildred Point, Pearl Falls, and Basaltic Cliff.