Beneath an icy coating bubbles pressurized liquid rock.
As part of Washington’s Volcano Preparedness Month, DNR is taking a look at the interaction between the hot lava beneath our iconic stratovolcanoes and the icy glaciers that cover them.
Nowhere is this more evident than in our most iconic volcano, Mount Rainier. The mountain that graces our skyline and our license plates is covered by some 88 square kilometers (34 square miles) of glaciers, and has been throughout its half-a-million-year life span.
And those glaciers influenced volcanic processes to form Mount Rainier into the unique shape we see today.
Home to the Washington Geological Survey, DNR houses vital information about volcanoes and a new web page we just created about glaciers, how they shaped our state in the past and how they continue to influence our communities today.
Mount Rainier erupted repeatedly during past ice ages, when glaciers as thick as 1,000 meters coated the valleys at its base. As lava poured out of the summit, it would melt glaciers, but not totally. Glaciers can push back on lava flows and influence their directions. Ice age glaciers cooled lava flows, making the mountain’s lava ridges like Paradise, Mazama and Emerald ridges.
Glaciers as sculptors
Glaciers also helped form much of the area surrounding Mount Rainier.
As lava melts glaciers, the hot rock and melting snow and ice form floods of water and debris known as lahars. Lahars are volcanic debris flows, or rapidly flowing mixtures of rock debris that are mobilized by water and that originate on the slopes of a volcano.
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.
It’s this and Mount Rainier’s proximity to Seattle and Tacoma suburbs, that makes it the most potentially dangerous volcano in the nation — among the ten most dangerous in the world.
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.
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.
Heat from the volcano can melt snow or ice and cause dangerous mudslides called lahars. Molten rock called lava can erupt and flow downhill, destroying everything in its path.