Is Washington’s Mount Rainier Volcano a Potential Ticking Time Bomb?

The aftermath of mudslides could be more deadly than an eruption—volcanologists model Rainier's risk to Alder Dam and surrounding areas.

Erik Klemmeti Volcanologist, who writes for, is often asked what he considers is the most dangerous volcano in the United States. He believes that this question, in more ways than one, is a loaded question. Even defining the term “dangerous” is loaded with ambiguity. Is it riskier for a volcano to erupt regularly but with lesser eruptions? Or perhaps it only makes massive blasts on rare occasions that occur near a populated area? Is it possible for a volcano to pose a threat even when it is not erupting?

When assessing how dangerous a volcano Klemmeti says you must consider the following:

  • When was the last time it erupted?
  • What was the size of these eruptions?
  • What type of eruptions did the volcano have? Flows of lava, explosions, or pyroclastic flows?
  • What is the distance between the volcano and populated areas?
  • How far away is the volcano from major air routes?

Threat assessment of volcanoes last carried out in 2018

According to the US Geological Survey (USGS) rankings, which consider the combination of people living near the volcano and how frequently it erupts.


Hawaii’s Kilauea is regarded as the most dangerous. Mount St. Helens is ranked second because it is the only Cascade volcano to erupt in the last century.

But the volcano that concerns Klemmeti the most is Rainier, which is ranked third.

Not erupted since 1450, Rainier still poses a significant threat

Rainier is the largest Cascade volcano, located in the Seattle-Tacoma-Olympia area, and one that scientists have demonstrated can have a major impact on this urban area. Although there may have been minor, unconfirmed puffs during the 1800s, Rainier hasn’t experienced a confirmed eruption since 1450 CE. Regardless of whether Rainier is erupting or not, it poses a threat.


Rainier’s muddy threat

Rainier is known for creating lahars, or volcanic mudflows. These flows are made up of volcanic material, water, and debris from the environment. In full flow, slurry from lahars resembles a river of streaming cement rather than water.

Because of the thick consistency of lahars, they can destroy almost everything in their path, including buildings and bridges. It also means they can dump tens or hundreds of feet of material into a valley. What’s left is a landscape that appears to have been paved.

The four forms of lahars

Lahars often take one of four forms.

  • First, during a volcano eruption with snow and ice, the frozen material might thaw and mix with volcanic debris, creating a flow that rushes down the volcano’s river basins.
A lahar on the east side of Pinatubo volcano
A lahar on the east side of Pinatubo volcano by Chris Newhall
  • Second, as additional water is mixed in during or after an eruption, heavy rains may cause loose volcanic debris (such as ash) to flow.
  • Third, a crater lake at the volcano’s summit could burst, dumping water down the slopes and mixing it with volcanic debris.
  • Finally, a volcano landslide might dump a mountain of volcanic material into river valleys. These last two could occur during an eruption, be triggered by an earthquake, or happen unexpectedly.

The most deadly lahar killed twenty-thousand people

Armero aftermath Marso
Armero aftermath Marso by Jeffrey Marso

If individuals aren’t alerted, lahars can be quite dangerous. The most notable example is the 1985 eruption of Colombia’s Nevado del Ruiz. A modest eruption triggered a lahar, which washed down the volcano and obliterated the settlement of Armero. The mudslide likely killed over 20,000 individuals.

Computer modeling lahars may save lives

At volcanoes all around the world, lahars constitute a constant menace. Because of the risk of lahars from Rainier and other volcanoes, it’s critical to understand what might happen if one occurs. The US Geological Survey employed a new set of computer models (dubbed D-Claw) to produce simulations of prospective lahars from the west side of Rainier, the most likely location for the next lahars, and discovered that they might devastate settlements far below the volcano’s summit.

Lahars can create floods that could bury a city in 10 feet of volcanic ash

Lahars have a lengthy history of Rainier. Hundreds of millions of feet of cubic debris have been displaced by at least nine large lahars over the last 6,000 years.


The Osceola Mudflow was the largest of these floods, which occurred 5,600 years ago, and transported 130 billion cubic feet of material, enough to bury Manhattan under 130 feet of it. What is the size of that? That much volcanic ash would cover Manhattan in 1-10 feet of murky volcanic ash.

The Electron flow, which occurred barely 500 years ago, was Rainier’s most recent lahar. With a volume of “just” 9 billion cubic feet, this lahar descended the volcano’s west slope and traveled almost 30 miles to reach Orting. Unlike the Osceola lahar, the Electron lahar was formed by a landslide induced by weak rock and gravity rather than an eruption.

The new D-Claw simulations look at fluxes as small as the Electron flow to far smaller, potentially more prevalent phenomena. The USGS was curious about how quickly the flows could travel, how far they could travel, and how deeply they could bury communities along the route.


Lahars can travel at heights of over 12 feet

Using simulations, the USGS discovered that an Electron-sized discharge from Rainier’s Sunset Amphitheatre would rush down the slopes at over 10 miles per hour, with a flow front towering over 12 feet. This flood would arrive in Orting in approximately an hour and eventually settle in Sumner and Puyallup near Tacoma.

A huge lahar from Mt Rainier could cause the Alder Dam to breach

Lahars that traveled down the Nisqually River valley were studied in other simulations. These would impact Lake Alder and certainly trigger the Alder Dam to breach, posing a severe threat to those further downstream.

Adler dam by Canticle under CC BY-SA 4.0

The volume of material moving down the Nisqually and Puyallup River valleys down big lahars is incredible. According to calculations, the lahars would be pushing 8-10 million cubic feet per second down the valleys. That’s more than 300-400 times the flow of Niagara Falls! This volume and mass of water pouring downstream into populated areas would cause billions of dollars in damage.


Disaster relief planning

The USGS models are not predictions or projections. They’re only models to help hazard planners figure out what’s at stake. The new simulation, according to Brian Terbush, the Earthquake/Volcano Program Coordinator for the Washington State Emergency Management Division, reinforces the necessity for preparedness.

The report revealed that the amount of warning time people have been practicing is correct, which is great news! Especially because 14,500 children and instructors demonstrated on April 29th that they could move out of hazard zones in that timeframe. However, this research underlines the need for communities in the path of Mt. Rainier lahars, particularly those in the Puyallup and Nisqually River Valleys, to understand their evacuation routes and how they would be notified of an approaching lahar.

Mt Rainier’s early warning system and evacuation plan

Lahars are a worldwide threat, but people can avoid them with enough warning. The Pierce County government and the USGS have a lahar early warning system atop Mount Rainier, which uses sensors in river valleys to transmit a signal to cities below the volcano if a lahar is detected. The system is being upgraded to include more sensors that will respond in real-time to any lahar. Some of the new sensors will be deployed in the Nisqually River valley to supplement the Puyallup River sensors.


Finally, these models will aid in better preparing residents in the Rainier area to evacuate during the next lahar.

If you live near a volcano that could produce lahars, check with local emergency management to learn about evacuation plans and preparedness, it could save your life.

While volcanoes can be incredibly destructive, they are also a prerequisite for the formation of all life.