When we think of volcanos, most of us picture spewing lava or, as in the case of the great Mount St. Helens eruption of 1980, a raining cloud of rock and ash after the volcano blows its top off. But the most devastating result of a volcanic eruption can actually be the lahar: a flood of mud, debris, and water that flows from a volcano when the water stored in snowpack or glaciers (Mount Rainier has plenty of both) is suddenly released.
Mount Rainier feeds 11 different river valleys, including the Puyallup River valley where many cities and towns are built on top of lahar deposits that are only about 500 years old.
To cap Washington’s Volcano Preparedness Month, DNR’s Ear to the Ground takes a look at the nation’s most potentially-dangerous volcano.
Because of its 14,410-feet elevation, massive icecap, glacier-fed valleys, and proximity to Seattle and Tacoma suburbs, Mount Rainier is the most potentially dangerous volcano in the nation — it’s also ranked among the top ten most-most dangerous in the world.
Lahars present prime hazard
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) isn’t the primary concern. Mount Ranier 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 mapped the routes of past Mount Rainier lahars.
A devastating history
In the past 10,000 years more than 60 of these devastating lahars have traveled at least 70 miles downstream of Mount Rainier. These lahars have reached not only the Puyallup River valley but also reached Puget Sound through the Nisqually River Basin, Commencement Bay and Elliott Bay, including the ports of Seattle and Tacoma.
Mount Rainier feeds 11 different river valleys, including the Puyallup River valley where many cities and towns are built on top of lahar deposits that are only about 500 years old. The most destructive — and most likely — lahar routes are on the mountain’s north and west sides.
DNR estimates a moderately-large lahar in the Puyallup River valley would cause $6 billion or more in damages to structures and other property. A major lahar could cover schools, farms, businesses, and homes with about thirty feet of concrete-like mud. Those and other areas could also be affected by flooding after a major lahar.
Volcanos typically will produce warnings before an eruption. However, a lahar can also be caused by an avalanche unrelated to an eruption. Here in Washington we live with the risk of earthquakes, landslides, tsunamis, forest fires, volcanos, and lahars, which is why DNR works to assess and estimate these risks. We do so by producing maps, analysis and other information that landowners, residents, community leaders and emergency personnel can use.
Preparation is key
Whatever the disaster, make sure you know your evacuation route and have your family’s emergency preparedness plan in place.
Have you given much thought to the potential of a large lahar from Mount Rainier? Has it influenced your decisions on where to live or work? Join the discussion on DNR’s Facebook page.