Large avalanches have also occurred many times during the last 3,000 years on the west side of the volcano. Sunset Amphitheater (fig. 11) is part of the large scar left by them. About 2,800 years ago one of these avalanches created a mudflow in the valleys of the South Puyallup River and Tahoma Creek that was temporarily deep enough to submerge Round Pass (on the West Side Road) to a depth of nearly 400 feet. This is especially remarkable when we see that Round Pass itself is 600-700 feet above the nearby valley floors. Another deep mudflow, started by an avalanche at Sunset Amphitheater, moved down the Puyallup River valley about 600 years ago and buried the site of the present town of Orting in the Puget Sound lowland under 15 feet of mud and rock.
Table 2.—_Summary of important geologic events in the history of Mount Rainier National Park_
Geologic Years ago Geologic events in the area of the park time scale
“Postglacial” Present summit cone of Mount Rainier probably was built about 2,000 years ago. The last known pumice eruption occurred between 1820 and 1854. Glaciers started to grow and advance about 3,000 years ago. Maximum extents were reached about 1850 A.D. From then until about 1955, glaciers were receding; now they are in balance or advancing. Huge masses of rock have slid from the volcano repeatedly during the last 10,000 years. One of these destroyed the summit of Mount Rainier and formed the Osceola Mudflow about 5,800 years ago. 10,000 Pleistocene Last major glaciation. (Ice Age) 25,000 Birth and growth of Mount Rainier volcano, and repeated glaciation. 2-3 million Pliocene Uplift and erosion of the Cascade Range. 12 million Miocene Intrusion of granodiorite. Folding of older rocks. Deposition of Fifes Peak and Stevens Ridge Formations. 26 million Oligocene Deposition of Ohanapecosh Formation. 37-38 million Eocene Deposition of Puget Group. 53-54 million
Avalanches and mudflows like those described are normal events at Mount Rainier and are expected to happen again in the future. Almost any cliff on the volcano can produce a large rockfall, but which cliff will collapse next, or when, cannot be predicted. Should the volcano again become active, earthquakes and volcanic explosions would trigger avalanches and mudflows that would rush down the mountain. Molten rock would melt snow and ice at the volcano’s summit and send floods of water down the volcano’s flanks. These indirect effects of an eruption would be much more hazardous than lava flows and pumice, if eruptions are on a scale similar to that of the past 10,000 years.