Mount Rainier, a snowcapped giant towering over Washington state at 2.7 miles above sea level, hasn’t had a significant eruption in over a thousand years.
However, it worries scientists more than any other U.S. volcano, even the well-known Hawaii volcanoes or the Yellowstone supervolcano.
This is mainly because of its location and potential for creating massive lahars — dangerous mudflows made of water, volcanic rock, and debris.
“Mount Rainier keeps me up at night because it poses such a great threat to the surrounding communities,” says volcanologist Jess Phoenix.
The cities of Tacoma and parts of Seattle are actually built on ancient mudflows from past Mount Rainier eruptions, making the area extremely vulnerable.
Why Lahars Are the Real Concern
If Mount Rainier were to erupt, a lava flow wouldn’t likely be the main problem; the lava would probably only travel a few miles.
Volcanic ash would blow eastward, away from major cities.
The real danger comes from the lahars, which are fast-moving slurries of mud, water, and rock triggered by melting snow and ice.
Since Mount Rainier is covered in ice and snow, even a small eruption could cause a significant amount of water to rush downhill, carrying debris and creating dangerous mudflows.
These lahars can quickly sweep through valleys and river channels, reaching populated areas.
Seth Moran, a research seismologist at the US Geological Survey (USGS), puts it this way: “There are tens, if not hundreds of thousands of people who live in areas that could be impacted by a large lahar, and it could happen quite quickly.”
What Happens During a Lahar
A lahar is like a river of thick mud, traveling fast and picking up everything in its path.
The deadliest lahar in recent history occurred in 1985 when Colombia’s Nevado del Ruiz volcano erupted, killing over 23,000 people.
To understand the scale of a potential Mount Rainier lahar, Moran points out that the volcano has around eight times the glaciers and snow as the Nevado del Ruiz had at the time of its deadly eruption.
“There’s the potential to have a much more catastrophic mudflow,” says volcanologist Bradley Pitcher.
Mount Rainier’s Volcanic History
According to the USGS, Mount Rainier is ranked as the third most hazardous volcano in the U.S., following Hawaii’s Kīlauea and Mount St. Helens.
While lahars often occur during eruptions, they can also be caused by landslides and earthquakes.
Over the last 6,000 years, Mount Rainier has produced at least 11 large lahars that have reached the surrounding Puget Lowlands.
One of these lahars happened roughly 500 years ago and was likely caused by a landslide, not an eruption.
Loose, weak rock remains in this area today, making it possible for a landslide-triggered lahar to occur without any warning.
A large lahar could reach nearby communities in as little as 10 minutes and larger towns within an hour, giving little time for evacuation.
What If a Lahar Happens?
A 2022 study modeled a worst-case scenario: a massive lahar that could reach the densely populated town of Orting, Washington, within an hour, traveling at 13 feet per second.
Another risk area is the Nisqually River Valley, where a large mudflow could spill over the 330-foot-tall Alder Dam, potentially causing even more flooding.
Survival Stories and Preparedness
In 1980, Mount St. Helens, a cousin of Mount Rainier, erupted and triggered a lahar.
Though it didn’t reach densely populated areas, a couple on a camping trip got caught in the mudflow.
“I tried to hang on as we were being swept downstream,” recalls survivor Venus Dergan, who eventually escaped but took two years to recover from her injuries.
Evacuation and Safety Measures
To keep people safe, the USGS has set up a lahar detection system on Mount Rainier.
Since 2017, this system has been upgraded to include real-time data from seismometers, trip wires, infrasound sensors, web cameras, and GPS receivers.
These sensors can quickly identify a lahar and alert authorities, but they aim to be effective even if a lahar is triggered by a landslide rather than an eruption.
In March 2024, some 45,000 students in surrounding towns like Puyallup and Orting participated in a lahar evacuation drill, the largest ever held.
Around 13,000 students walked up to two miles to designated safe areas, while others practiced sheltering in place.
The goal is to evacuate everyone within the 45-minute window that authorities have to react once a lahar is detected.
Staying Prepared
Lahars are unique in their ability to travel far from the volcano and have widespread impact, making it crucial to stay prepared.
With Mount Rainier, the main aim is to ensure the people in potentially affected areas are aware and ready to evacuate quickly if the worst were to happen.