Although Mount Rainier is an active volcano, as of 2010 there was no evidence of an imminent eruption. However, an eruption could be devastating for all areas surrounding the volcano.
Mount Rainier is currently listed as a Decade Volcano, or one of the 16 volcanoes with the greatest likelihood of causing great loss of life and property if eruptive activity resumes.
If Mt. Rainier were to erupt as powerfully as Mount St. Helens did in its May 18, 1980, eruption, the effect would be cumulatively greater, because of the far more massive amounts of glacial ice locked on the volcano compared to Mount St. Helens and the vastly more heavily populated areas surrounding Rainier. Lahars from Rainier pose the most risk to life and property, as many communities lie atop older lahar deposits.
Poor Seattle can’t catch a break. We’ve already covered how the mild-mannered city is particularly prone to giant earthquakes, but it gets even worse: the entire Greater Seattle is at the risk of being buried under a sea of mud.
The area lies downstream from Mount Rainier, which carries the questionable honor of being one of the most dangerous volcanoes in existence. However, this particular danger doesn’t come from soot and magma — sure, there would be some if it was to erupt, but that would be just the icing on the horror cake.
The true killer would be a lahar, whose nerdy name betrays its potential for destruction. Lahars are giant flows of hot mud, trees and water, rolling forward with the consistency of a zillion tons of wet cement and at speeds up to 60mph.
And they can get big: Urban Seattle could be facing a Lahar as tall 600 freaking feet. How do we know? Because it’s happened before!
Around 5,000 years ago, a giant lahar called the Osceola Mudflow filled a part of Puget Sound with three cubic kilometers of hot, steamy, gooey mud. What was once a pristine sea was, in a matter of hours, suddenly 200 square miles of new land. For comparison, the disastrous 1985 Nevado del Ruiz lahar that killed 25,000 people in Colombia only had 2.5 percent of the volume of the Osceola Mudflow.
So, What Can Be Done?
A lahar detection system was installed in 1998, but it remains loose and incomprehensive. To make matters worse, these mud tsunamis (mudnamis!) are a right bastard to detect: a lahar doesn’t need a volcanic eruption as an excuse to kick in: A sector collapse or some magma leakage could be enough to send a mudnami the size of Godzilla into Seattle.
If just the Puyallup Valley lahar (the purple one in the above picture) sparks off, material damages alone could be as high as $13 billion. Also, a non-volcanic lahar could easily spread from one to several of the six (six!) Mount Rainer lahar systems, multiplying the destruction.
According to USGS, about 150,000 people live on top of old lahar deposits of Rainier. Not only is there much ice atop the volcano, the volcano is also slowly being weakened by hydrothermal activity.
According to Geoff Clayton, a geologist with a Washington State Geology firm, RH2 Engineering, a repeat of the Osceola mudflow would destroy Enumclaw, Orting, Kent, Auburn, Puyallup, Sumner and all of Renton.
Such a mudflow might also reach down the Duwamish estuary and destroy parts of downtown Seattle, and cause tsunamis in Puget Sound and Lake Washington. Another Cascade Arc volcano with similar hazards is Mount Meager in southwestern British Columbia, Canada, which has produced several large landslides in the past 8,000 years, as well as an eruption 2,350 years ago that was similar in size to Mount St. Helens’ 1980 eruption. Rainier is also capable of producing pyroclastic flows and expelling lava.
According to K. Scott, a scientist with the USGS:
A home built in any of the probabilistically defined inundation areas on the new maps is more likely to be damaged or destroyed by a lahar than by fire For example, a home built in an area that would be inundated every 100 years, on the average, is 27 times more likely to be damaged or destroyed by a flow than by fire.
People know the danger of fire, so they buy fire insurance and they have smoke alarms, but most people are not aware of the risks of lahars, and few have applicable flood insurance.
The volcanic risk is somewhat mitigated by lahar warning sirens and escape route signs in Pierce County. The more populous King County is also in the Lahar area, but currently has no zoning restrictions due to volcanic hazard.
More recently (since 2001) funding from the federal government for lahar protection in the area has dried up, leading local authorities in at-risk cities like Orting to fear a disaster similar to the Armero tragedy.