Geological Survey Bulletin 1288 Surficial Geology of Mount Rainier National Park Washington

Landslides have played an unusually important role at Mount Rainier, both in determining the volcano's shape and in forming some of the park's surficial deposits. Although landslides are common features elsewhere, those at Mount Rainier are exceptionally large. The largest one destroyed much of the volcano's former summit about 5,700 years ago. The top of Mount Rainier today is the crest of a small lava cone that perhaps is only about 2,000 years old. The cone occupies a broad depression whose broken rim is represented by Gibraltar Rock, Point Success, and the ridge between Liberty Cap and Russell Cliff (fig. 1). The broad depression resulted from great landslides of rock from the former summit of the volcano, which had become decomposed to clay and weakened by hot volcanic gases and solutions over many centuries. These slides of wet clayey rock formed a truly spectacular mudflow that streamed downvalley from the vol cano at least 70 miles.

EAST SIDE OF MOUNT RAINIER. The summit cone lies within a broad depression whose rim is represented by Point Success, Gibraltar Rock, and the ridge between Liberty Cap and Russell Cliff. The depression was formed when the former top of the volcano, which consisted of rock that had been partly altered to clay, slid off to the east and descended Emmons Glacier in a tremendous avalanche that produced the Osceola Mudflow 5,700 years ago. The distance from Point Success to Liberty Cap is 1/4 miles, and the summit crater is about 1,300 feet across. (Fig. 1)

Similar but smaller slides of clayey rock have occurred repeatedly on other parts of the volcano during the last 10,000 years and have also resulted in clayey mudflows. These mudflows contain clays that originated from long-continued steaming and chemical breakdown of volcanic rocks. Decomposed and altered rocks of this kind at Sunset Amphitheater on the west side of Mount Rainier are part of a central plug of lava that cooled and solidified in a former throat of the volcano. Although its shape is similar to that of depressions carved wholly by glacial erosion, Sunset Amphitheater was probably formed in large part by repeated landslides and is analogous in origin to the former broad depression at the summit of the volcano. Several clayey mudflows in the valleys on the west side of the volcano originated in these slides at Sunset Amphitheater, and the landslide of yellowish-orange clay and altered rock from the same source that lies at the end of Tahoma Glacier (frontispiece) is no more than a few decades old.