Landslides & Rockfalls Can Trigger Lahars at Mount Rainier
The combination of weak, hydrothermally altered rocks, large quantities of snow and ice, and a seismically active volcano make landslides and rockfalls potential hazards at Mount Rainier. Approximately 5,600 years ago, volcanism or volcanic precursors triggered a massive avalanche of already weak, hydrothermally altered rocks on the northeast flank and summit, which created a 1.8 km-wide (1 mi-wide) horseshoe-shaped crater opening to the northeast. As the landslide moved downstream it transformed into the largest lahar of the past 10,000 years, the Osceola Mudflow. In 1963 a large rockfall avalanche swept seven kilometers off Little Tahoma Peak, crossed the Emmons Glacier, and came within a kilometer of White River campground. More information on this process is found in the lahar hazards section.
Small landslides and rockfalls are common at Mount Rainier, with rockfalls typically occurring each year. Several large masses of ice fell off a headwall above the Nisqually glacier in June 2011, and picked up additional ice, snow, and rock to form a spectacular avalanche. This video, ffilmed by climbers not affiliated with the USGS, nicely illustrates a moderately a moderate-size ice-and-snow avalanche at Mount Rainier.