Region’s landmark mountain also is nation’s most dangerous volcano

The community of Orting, Washington, is built on top of 500-year-old lahar debris from Mount Rainier. Photo: USGS.
May is Volcano Awareness Month in Washington State. As this Sunday’s anniversary of the May 18, 1980, eruption of Mount St. Helens, approaches DNR Ear to the Ground is featuring one of our state’s five active volcanoes each day this week. Today, the spotlight is on Mount Rainier.

Because of its elevation (14,410 feet), massive icecap, glacier-fed valleys, and proximity to the Seattle-Tacoma metropolitan area, Mount Rainier is considered the most potentially dangerous volcano in the nation — it’s also ranked among the top ten most-most dangerous in the world.

According to the US Geological Survey’s Volcano Hazards Program, there hasn’t been a major eruption on Mount Rainier in 1,000 years, but an explosive eruption (a la Mount St. Helens in 1980) isn’t the primary concern. Mount Rainier can generate huge lahars — rapidly flowing slurries of mud and boulders — even without an eruption. Avalanches caused by heated rock or volcanic gases can swiftly melt snow and ice and produce torrents of meltwater that pick up loose rock and become a lahar.

In its role as the state’s geological survey, DNR has mapped the routes of past Mount Rainier lahars. The most destructive — and most likely — lahar routes are on the mountain’s north and west sides. A lahar here could feed into the Puyallup River valley where cities, towns, and housing developments have been built on top of lahar deposits from as recently as 500 years ago.

DNR estimates that a moderately large lahar in the Puyallup River valley would cause $6 billion or more in damages to structures and other property. Large lahars of the past have reached Puget Sound via the Nisqually River Basin, Commencement Bay and Elliott Bay, including the ports of Seattle and Tacoma.

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