Pierce County, Washington
Mount Rainier is the king of the snow and ice-covered volcanoes that tower over the surrounding peaks of Washington’s Cascade Range. Rainier, along with Mount Baker, Glacier Peak, Mount Adams, and Mount Saint Helens, owes its existence to subduction of the Juan de Fuca crustal plate beneath the North American plate off Washington’s coast. As the Juan de Fuca plate moves deeper into the hot interior of the Earth, its rocks melt, producing magma that rises through the crust and erupts as a volcano at the surface.
A predecessor to Mount Rainier occupied the same location between about 1 and 2 million years ago, but the stratovolcano that grew to become Washington’s highest peak began erupting around 500,000 years ago. The volcanic cone is built mostly of andesitic lava flows with some dacite and minor pyroclastic flows. Truncated lava flows in the upper part of the mountain indicate that the volcano once rose about 1,000 feet (300 m) higher than today’s summit elevation. However, Mount Rainier’s rocks are notoriously “rotten” as the result of being chemically altered by sulfurous gases and hot waters that circulate through the mountain’s plumbing. About 5,600 years ago, the decomposed rocks in the upper part of the volcano collapsed and flowed down the east side of the mountain as a gigantic landslide (lahar) called the Osceola mudflow. Part of the mudflow, which had a total volume of half a cubic mile (2 km3; over 270 million dump-truck loads), traveled some 70 miles (110 km) all the way to the Puget Sound. The broad depression at the top of the mountain, whose rim is represented by Liberty Cap, Point Success, and Gibraltar Rock, gradually filled with a new cone that forms the modern summit (Columbia Crest). Eruptions of the summit lava flows ended around 2,200 years ago.
Mount Rainier’s height and maritime climate have resulted in two major glacial episodes as well as numerous smaller glaciations during the past 170,000 years. Today, Rainier’s 25 officially named glaciers comprise the most extensive glacial system in the conterminous United States. The largest glaciers, which reach thicknesses of hundreds of feet (~100 m+), have eroded deeply into Rainier’s volcanic cone. Little Tahoma (11,117 feet; 3,390 m), Rainier’s satellite peak, is a part of the volcano’s formerly continuous eastern slope which is slowly being eaten away by glacial erosion. Glaciers have cut most deeply into the shadowed north face of the mountain, creating the spectacular Willis Wall and Liberty Wall.
- LAT./LONG. 46.853 N / 121.760 W
- Land Status/Administration
Mount Rainier National Park
- Physiographic Province
- Representative Rocks:
- Class: Igneous
- Type: Andesite
- Age: Pleistocene
- Landform: Glaciated stratovolcano