Why Mount Rainier is the US volcano keeping scientists up at night

(CNN) — The snowcapped peak of Mount Rainier, which towers 4.3 kilometers (2.7 miles) above sea level in Washington state, has not produced a significant volcanic eruption in the past 1,000 years. Yet, more than Hawaii’s bubbling lava fields or Yellowstone’s sprawling supervolcano, it’s Mount Rainier that has many US volcanologists worried.

“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,” Jess Phoenix, a volcanologist and ambassador for the Union of Concerned Scientists, said on an episode of “Violent Earth With Liev Schreiber,” a CNN Original Series.

The sleeping giant’s destructive potential lies not with fiery flows of lava, which, in the event of an eruption, would be unlikely to extend more than a few miles beyond the boundary of Mount Rainier National Park in the Pacific Northwest. And the majority of volcanic ash would likely dissipate downwind to the east away from population centers, according to the US Geological Survey.

Instead, many scientists fear the prospect of a lahar — a swiftly moving slurry of water and volcanic rock originating from ice or snow rapidly melted by an eruption that picks up debris as it flows 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, and so if there is any kind of eruptive activity, hot stuff … will melt the cold stuff and a lot of water will start coming down,” said Seth Moran, a research seismologist at USGS Cascades Volcano Observatory in Vancouver, Washington.

“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.”

A lahar is a fast-moving debris flow

The deadliest lahar in recent memory was in November 1985 when Colombia’s Nevado del Ruiz volcano erupted. Just a couple hours after the eruption started, a river of mud, rocks, lava and icy water swept over the town of Armero, killing over 23,000 people in a matter of minutes.

“When it comes to rest … you’ve got this hardened almost, like, concrete substance that can be like quicksand when people are trying to get out of it,” Bradley Pitcher, a volcanologist and lecturer in Earth and environmental sciences at Columbia University, said in an episode of CNN’s “Violent Earth.”

Pitcher said that Mount Rainier has about eight times the amount of glaciers and snow as Nevado del Ruiz had when it erupted. “There’s the potential to have a much more catastrophic mudflow.”

In the the US Geological Survey’s most recent threat assessment from 2018, the federal agency considered Hawaii’s Kīlauea the most hazardous US volcano — no surprise given how many people live near it and how frequently it erupts. Mount St. Helens, which cataclysmically erupted in May 1980, ranked as second most hazardous before Mount Rainier in third place.

Past eruptions reveal multiple mudflows

Lahars typically occur during volcanic eruptions but also can be caused by landslides and earthquakes. Geologists have found evidence that at least 11 large lahars from Mount Rainier have reached into the surrounding area, known as the Puget Lowlands, in the past 6,000 years, Moran said.

Scientists have not connected the most recent of these lahars, which occurred about 500 years ago, with any kind of volcanic activity. A large landslide on the mountain’s west flank may have caused the flow event, according to researchers.

Loose, weak rock remains in that spot, and it’s the threat of a similar, spontaneous landslide-triggered lahar that particularly troubles Moran and other volcanologists.

“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.

“Should it be the same size, then it’s 10 minutes to the nearest places where people are living, and 60 minutes to the nearest large communities. And those are really short time frames,” he added.

A 2022 study modeled two worst-case scenarios. In the first simulation, a 260 million-cubic-meter, 4-meter deep (9.2 billion-cubic-foot, 13-foot deep) lahar would originate on the west side of Mount Rainier. The debris flow would be equivalent to 104,000 Olympic-size pools, according to Moran, and could reach the densely populated lowlands of Orting, Washington, about one hour after an eruption, where it would travel at the speed of 13 feet (4 meters) per second.

A second area of “pronounced hazard” is the Nisqually River Valley, where a massive lahar could displace enough water from Alder Lake to cause the 100-meter-tall (330-foot-tall) Alder Dam to spill over, according to the simulation.

Surviving a lahar

Mount Rainier’s cousin Mount St. Helens, farther south in the Cascade Range, triggered a devastating lahar when it erupted four decades ago, although it did not reach any densely populated areas.

Venus Dergan and her then-boyfriend, Roald Reitan, were caught in the Mount St. Helens lahar during a camping trip and are two of only a few people known to have survived being swept up in a debris flow.

“I tried to hang on as we were being swept downstream, the bark on the trees were just scraping. … I could feel it on my legs, on my arms,” she recalled during an interview for CNN’s “Violent Earth.”

“And at one point, I went under the logs and the mud, and I just resigned myself that this was it. I wasn’t going to get out of this and that I was going to die.”

Reitan managed to lift her out of the mudflow, and they rode on a huge log down the river. When the log came to a stop, they jumped on to an embankment and crawled up a hillside, where they were rescued. It took Dergan two years to recover fully from her injuries.

World’s largest lahar evacuation drill

In the wake of the Mount St. Helens eruption, the US Geological Survey set up an lahar detection system at Mount Rainier in 1998, which since 2017 has been upgraded and expanded.

About 20 sites on the volcano’s slopes and the two paths identified as most at risk of a lahar now feature broadband seismometers that transmit real-time data and other sensors including trip wires, infrasound sensors, web cameras and GPS receivers.

The system is geared toward both detecting a lahar should the volcano wake up in the future and the specific scenario of a lahar triggered by a landslide, Moran said.

The original system had low bandwidth and low power requirements due to the limitations of 1990s-era technology, which meant that data was only transmitted every two minutes.

There is a lack of historical reference data since there aren’t many lahars around the world that monitoring stations have recorded, so a wider range of instruments will help determine whether a seismic signal received from one of the stations is actually from a debris flow, and not from an eruption or an earthquake, Moran said.

Infrasound instruments, for instance, would tell the researchers that there was a disturbance at ground surface rather than deeper in the earth.

In March, some 45,000 students from Puyallup, Sumner-Bonney Lake, Orting, White River and Carbonado, Washington, participated in a lahar evacuation drill. It was the first time that multiple school districts practiced on the same day, making it the world’s largest lahar drill, according to the USGS.

Around 13,000 students walked up to 3.2 kilometers (2 miles) to designated locations outside of the mapped lahar zone, while the remainder in schools situated outside the lahar zone practiced sheltering in place.

Moran said that the fail-safe parts of the lahar detection system are located about 45 minutes from the nearest large community, making that the time frame with which communities had to work.

“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.”