Volcanic Landslides at Mount Rainier volcano, Washington
Illustrations modified from J. Vigil
A mass of sliding rocks, snow, and ice swept down the northeast side of Mount Rainier volcano about 5,600 years ago. The landslide quickly transformed into an enormous lahar, raced down the west and main forks of the White River, and spilled into Puget Sound more than 50 km downstream (see map). The landslide removed the summit area of the volcano and created a deep horseshoe-shaped crater. A relatively small explosion accompanied the landslide. This event is known as the Osceola Mudflow. The original images appear in a video program, Perilous Beauty--the hidden dangers of Mount Rainier.
Aerial view of the east flank and summit of Mount Rainier volcano. Eruptions of thick stubby lava flows quickly rebuilt part of Mount Rainier's cone that was removed by the Osceola landslide. Parts of the old crater rim, however, are still visible today.
Landslides trigger largest lahars at Mount Rainier
Geologists have identified the deposits of at least 55 lahars in the valleys draining Mount Rainier younger than 10,000 years in age. The largest lahars were generated by landslides of water-saturated, clay-rich debris from the volcano. In the past 5,600 years, at least 6 and possibly as many as 13 clay-rich landslide-generated lahars inundated the lower reaches of valleys that head on Mount Rainier. These lahars have an approximate recurrence interval of 500 to 1,000 years.
Most recent large landslide--Electron Mudflow
The youngest of the large landslide-generated lahars inundated most of the Puyallup River valley about 500 years ago (see map). The landslide broke loose from Sunset Amphitheater high on the volcano's west flank and raced down the narrow upper reaches of the Puyallup River, reaching depths of at least 50 m. Below the town of Electron, the lahar spread across the broad valley floor (2-3 km wide) at depths as great as 10m. Today, several communities and many homes and farms are built atop the Electron lahar deposit.
View toward the northeast of Sunset Amphitheater high on the west flank of Mount Rainier. Sunset Amphitheater and Point Success (high point in upper right) both expose thick layers of lava and pyroclastic debris erupted by the volcano. The South Tahoma Glacier descends from the base of Point Success and Puyallup Glacier descends from Sunset Amphitheater. The white band cutting across the middle of the amphitheater is a layer of pumice about 35 m thick. Sunset Amphitheater is the source for at least two large landslides in the past 3,000 years, including that which generated the Electron lahar about 500 years ago.
This Douglas Fir tree stump was encased in deposits of the Electron lahar and excavated in 1993 during construction of a housing subdivision in the Puyallup River valley. Clearly, an old growth forest grew on the valley floor when the lahar swept from Mount Rainier abtou 500 years ago. The lahar deposit is about 5 m thick at this location, a few kilometers downstream from Orting, Washington. Mount Rainier is visible in the distance. Like most lahars generated by large landslides, the Electron deposit is rich in clay-sized particles (6-11 percent). The most distinctive rock in the deposit is a scoriaceous, black andesite lava, which is as large as 1.5 m in diameter. The largest boulders, however, are a reddish-brown breccia as large as about 10 m in diameter. Both rock types were derived from Mount Rainier's cone. Photograph by P. Pringle, Washington Department of Natural Resources, Division of Geology