The snowcapped peak of Mount Rainier, standing majestically at 4.3 kilometers (2.7 miles) above sea level in Washington state, has been a sleeping giant for over a millennium. Despite its dormancy, Mount Rainier poses a significant threat to the surrounding communities, more so than even Hawaii’s bubbling lava fields or Yellowstone’s sprawling supervolcano.
Jess Phoenix, a volcanologist and ambassador for the Union of Concerned Scientists, voiced her concerns on an episode of “Violent Earth With Liv Schreiber,” a CNN Original Series. “Mount Rainier keeps me up at night because it poses such a great threat to the surrounding communities. Tacoma and South Seattle are built on 100-foot-thick (30.5-meter) ancient mudflows from eruptions of Mount Rainier,” Phoenix explained.
The primary danger of Mount Rainier lies not in fiery lava flows but in the potential for lahars—swiftly moving slurries of water and volcanic rock formed by rapidly melting ice or snow during an eruption. These lahars can pick up debris and cause massive destruction as they rush through valleys and drainage channels.
“The thing that makes Mount Rainier tough is that it is so tall, and it’s covered with ice and snow,” said Seth Moran, a research seismologist at USGS Cascades Volcano Observatory. “If there is any kind of eruptive activity, hot stuff will melt the cold stuff and a lot of water will start coming down. And there are tens, if not hundreds of thousands of people who live in areas that potentially could be impacted by a large lahar, and it could happen quite quickly.”
One of the deadliest lahars in recent history occurred in November 1985 when Colombia’s Nevado del Ruiz volcano erupted, causing a river of mud, rocks, lava, and icy water to sweep over the town of Armero, killing over 23,000 people. Mount Rainier, with about eight times the amount of glaciers and snow as Nevado del Ruiz had at the time, poses a risk for an even more catastrophic mudflow.
In the US Geological Survey’s most recent threat assessment from 2018, Mount Rainier ranked as the third most hazardous US volcano, following Hawaii’s Kīlauea and Mount St. Helens. Lahars typically occur during volcanic eruptions but can also be triggered by landslides and earthquakes. Evidence shows that at least 11 large lahars from Mount Rainier have reached the Puget Lowlands in the past 6,000 years, with the most recent occurring about 500 years ago, likely caused by a landslide rather than volcanic activity.
Loose, weak rock remains in the spot of the last large lahar, and it is the threat of another spontaneous landslide-triggered lahar that troubles volcanologists like Moran. “There’s the knowledge now that the volcano is potentially capable of doing it again. And then we’re in this world of it could happen at any time,” Moran said.
A 2022 study modeled two worst-case scenarios. In the first, a lahar equivalent to 104,000 Olympic-size pools could reach the densely populated lowlands of Orting, Washington, within an hour, traveling at a speed of 13 feet (4 meters) per second. The second scenario involves the Nisqually River Valley, where a massive lahar could displace enough water from Alder Lake to cause the 100-meter-tall Alder Dam to spill over.
Mount Rainier’s cousin, Mount St. Helens, triggered a devastating lahar during its eruption four decades ago. Venus Dergan and Roald Reitan, caught in the lahar during a camping trip, are among the few survivors of such an event. Their harrowing experience underscores the deadly nature of these mudflows.
In response to the risks, the US Geological Survey set up a lahar detection system at Mount Rainier in 1998, which has since been upgraded. About 20 sites on the volcano’s slopes and paths most at risk of lahars now feature advanced sensors, including broadband seismometers, trip wires, infrasound sensors, web cameras, and GPS receivers.
In March, approximately 45,000 students from five Washington school districts participated in a lahar evacuation drill, marking the world’s largest lahar drill. Around 13,000 students walked up to 3.2 kilometers (2 miles) to designated safe locations, while others practiced sheltering in place.
Moran emphasized the importance of preparedness: “Most of what happens at volcanoes is close by, and that’s why you try to keep people away because things happen fast, but lahars can travel a long way from the volcano and have a big impact.”
The real-time data from the detection system, along with community preparedness drills, aims to mitigate the impact of a potential lahar, ensuring swift evacuation and safety for the tens of thousands of people living in the shadow of Mount Rainier.
