Some of the most effective means of erosion at Mount Rainier are landslides and mudflows. Erosion of this kind is sometimes spectacular. Within an interval of only minutes or a few hours huge masses of rock may fall, slide, or flow off the volcano and move far downvalley.
Large landslides have occurred at many other places in the park—one in the area northeast of Mount Rainier is so conspicuous that its source has been named Slide Mountain. The ragged scar left by another slide near Grand Park is aptly called Scarface. You cross a slide on the Mather Memorial Parkway (U.S. Highway 410) just north of Cayuse Pass. Broken and jumbled rock debris of many sizes borders both sides of the highway there. This landslide broke loose in rocks of the Ohanapecosh Formation, slid downslope to the bottom of the valley, and dammed Klickitat Creek to form Ghost Lake. Rocks have also slid downslope on the west side of Backbone Ridge and on the east side of the Ohanapecosh River valley a short distance north of Ohanapecosh campground. The slide on Backbone Ridge is still slowly moving today. Another slide moves a few inches each year on the west side of the Nisqually River valley about 1 mile northwest of the visitor center at Paradise Park. You can recognize the slide by a jagged horizontal crack 1,000 feet long at its top.
A far more spectacular variety of landslide occurs when a mass of rock drops from a cliff to form a rockfall. The largest rockfalls on Mount Rainier in historic time occurred in December 1963 on the east flank. Masses of rock hundreds of feet across fell repeatedly from the steep north face of Little Tahoma Peak onto Emmons Glacier. The rock masses shattered into dust and countless fragments, fanned out across the glacier, then avalanched down the steep ice surface at tremendous speed. When the avalanches reached the end of the glacier they shot out into space as sheets of rock debris. As these hurtling sheets settled toward the valley floor, a cushion of compressed air formed beneath them, comparable to the air cushion that momentarily buoys up a sheet of plywood that is dropped onto a flat surface. Air that was trapped beneath these speeding avalanches reduced friction and permitted one of the avalanches to move almost 2 miles beyond the end of the glacier. This avalanche passed completely over a small wooden gage house about 5 feet high on the valley floor without damaging it, then ran headlong into the north base of Goat Island Mountain where it scraped away trees and bushes. A later avalanche stopped just short of the gage house, and wind that was expelled from beneath the rock debris blasted the still-undamaged house several hundred feet forward. It now rests in the scar left by the earlier avalanche on the side of Goat Island Mountain.
At least seven rockfalls and avalanches descended from Little Tahoma Peak, separated perhaps by only minutes or hours. The reddish-gray masses of broken and pulverized rock—some spread helter-skelter, some piled in long sharp-crested ridges—now lie on the valley floor between the White River campground and Emmons Glacier ([fig. 20]).
The rockfalls might have been caused by a steam explosion near the base of Little Tahoma Peak. Steam jets and small explosions are not unusual phenomena at Mount Rainier, although they never have had such dramatic effects in historic time.
Incredibly larger avalanches of rock fell repeatedly from the sides of Mount Rainier during prehistoric time. One such avalanche originated near the summit of the volcano and blanketed Paradise Park and Paradise Valley with a yellowish-orange mixture of clay and rocks sometime between 5,800 and 6,600 years ago. You can see this avalanche deposit in shallow cuts along trails and roads in the Paradise area. Huge blocks of rock that came down with the avalanche are scattered in the meadows of Paradise Park between the visitor center and Panorama Point. Although the deposit is now less than 15 feet thick in most places, the mass that flowed down Paradise Valley must have been 600 feet thick, because we can find remnants of it on top of Mazama Ridge. A tongue of the wet mass flowed through a low saddle near the south end of Mazama Ridge and extended into the basin now occupied by the Reflection Lakes. You can see the yellowish-orange deposit in the first roadcut west of the lakes, where it lies on top of gray glacial deposits.
Avalanche deposits in the White River valley. The rockfalls and avalanches from Little Tahoma Peak formed a mass of reddish-gray rock debris that contrasts with the darker gray glacial debris deposited by Emmons Glacier within the last century. The avalanche deposits are about 1,500 feet across at their widest point. (Fig. 20)
The avalanche probably was wet when it crossed the Paradise area, and the moisture in it may have already been present in the rocks in which the avalanche originated. The mass was fluid enough to move down the Paradise and Nisqually River valleys as a mudflow hundreds of feet thick, and the resulting deposits extend at least 18 miles downstream from the volcano. The volume of rock that slid off to produce the mudflow may have been as much as 100 million cubic yards—or roughly enough to cover a 1-mile-square area to a depth of 100 feet.
At about the same time as the Paradise avalanche and mudflow occurred, a tremendous rock mass also slid off the east side of the volcano in the area between Steamboat Prow and Little Tahoma Peak. This slide formed a mudflow on the floor of the White River valley that was several hundred feet deep at the north boundary of the park and that extended at least 30 miles beyond the base of the volcano. The most remarkable feature of the deposit left by this mudflow is its surface, which is dotted with scores of mounds 5-35 feet high and as much as several hundred feet in diameter. These mounds have cores of huge rocks which are similar in size to those scattered on the surface of the avalanche deposits from Little Tahoma Peak. You can see the mounds best in an area which is a few miles north of the park boundary, west of the White River, and which can be reached by the Huckleberry Creek Forest Road. The total volume of this mudflow deposit may be as much as one-fifth of a cubic mile.