Volcano Hazards: | Types and Effects | Location | | Gas | Lahars | Landslides | Lava Flows | Pyroclastic Flows | Tephra |

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.

East flank of Mt. 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 of Sunset Amphitheater high on the west flank of Mount Rainier, the source area of the Electron landslide and lahar. A much smaller landslide broke from this area in the early 1900's.
	During construction of new housing, this huge stump of a Douglas Fir tree was excavated from the Electron lahar deposit in the Puyallup River valley. The lahar swept through and buried an old-growth forest growing on the valley floor about 500 years ago.

Map of Osceola and Electron lahar deposits

Map prepared by L. Faust

More information about Mount Rainier

• Mount Rainier Volcano Hazard Assessment, revised in 1998.
• Fact Sheet, Mount Rainier - Living With Perilous Beauty.
• Overview of a USGS Lahar Warning Pilot Project at Mount Rainier.
• Mount Rainier Information (including photos, maps, and publications), from the USGS Cascades Volcano Observatory.