Mount Rainier’s last eruption occurred around 1100 AD, and the volcano that sits south of Auburn and Kent has remained dormant since.

But the mountain’s activity, combined with its proximity to Tacoma, Seattle and Yakima, has earned it a label as one of the most dangerous in the world in the event of an eruption, according to the International Association of Volcanology and Chemistry of the Earth’s Interior’s Decade Volcano list.

Decade volcanos, of which there are 16, were identified in the 1990s as part of a United Nations effort to reduce the damage caused by natural disasters through education.

While the volcano has been dormant for the last several hundred years, it is still due for an eruption of varying degree in the near future. Because of the unpredictability of volcanos, there’s no telling exactly what will happen until shortly before it actually happens, said Tim Walsh, chief hazards geologist with the Washington State Department of Natural Resources.

“The tricky thing about volcanos,” said Walsh, “is they can show signs of activity that build up to a moderate level, stay there for years and disappear.”

Walsh explained that as activity occurs on the mountain, it trips notifications for additional monitoring to predict exactly what’s going on. For example, if a magma pocket known as a harmonic tremor – a sub-surface energy burst that pulses – were detected, it could indicate magma build up and warrant additional monitoring tools deployed on the mountain.

Should the mountain erupt, people will feel tremors and see steam vents coming off of the mountain, but Walsh says that outside of that don’t expect to have magma explosions or cascades of lava seen in movies. “Every volcano has its personality, and Mount Rainier’s personality is it’s more of a lahar (debris flow) former than an ash former,” he said.

Walsh says that there’s some possibility that an eruption would generate an ash cloud, it won’t be nearly as damaging or destructive as the mudslides resulting from melting snow and ice.

“Rainier is a lot different from St. Helens in that St. Helens doesn’t have a lot of snow and ice on it anymore, while Rainier has more than the rest of the cascades combined,” he said.

Threat of mudslides

The greatest threat to the area from Mount Rainier won’t be from traditional hazards like soot or flaming meteors from the volcano. It will be from avalanches of mud, rock and other debris carried by rapidly melting ice and snow as the mountain discharges lava.

Heat from an eruption will melt ice and glaciers on the mountain and turn them into mudslides moving up to 50 mph, with the potential to be more than 400 feet deep in nearby valleys. Rainier has had a history of lahars, ranging from more than 5,600 years ago to only 500 years ago.

The largest of these debris flows – the Osceola Mudflow – occurred 5,600 years ago, covered 212 square miles of land from Rainier to Kent and was hundreds of feet deep. Thankfully, Walsh says, the chances of a similar flow happening again are very low.

“In geology, the size of an event is inversely proportional to its frequency,” he said.

What can be expected instead is a flow that could reach south Auburn, completely destroying the communities to the south and causing severe damage to infrastructure along the way. According to Walsh, even if a building were able to stand up to the mudslide, it’s complete envelopment would make it “functionally unusable.”

These lahars are a greater threat to Buckley, Orting and Enumclaw, but they’ll still have the capacity to cause damage as they flow into south Kent and Auburn. As the debris flows move north down the valleys, they’ll eventually fill up riverbeds, and that displaced water has to go somewhere. The flooding will inevitably move into Kent and Auburn. Despite the speed at which a lahar will travel and abruptness with which it will occur, residents of the area should have plenty of time to evacuate, according to Auburn and Kent emergency management officials.

“Our understanding is that the volcano wouldn’t have an eruption like Mount St. Helens,” said Auburn Emergency Management Director Sarah Miller. Miller, who literally wrote the book on lahar emergencies for Auburn, said that the greater threat will be from the post eruption debris flows.

“We would have a considerable amount of advance warning,” said Miller.

Warnings, then quick mobilization

While the U.S. Geological Survey would know an eruption was going to occur, it might not know the exact time. Miller says warning signs will come likely months in advance, allowing people time to prepare. But when the eruption hits, they’d better be able to move.

Miller said that city residents would have approximately 90 minutes to evacuate from the time an eruption occurred until a lahar flow would reach their areas. During this time, she says residents would be evacuated along the predetermined routes to the western and eastern hills, out of the Auburn and Kent valleys. Fortunately, she says that the routes and safe areas available shouldn’t require anyone to move more than a mile if an evacuation were sounded.

One thing Miller stresses is that a lahar can occur without serious volcanic activity, such as a steam vent or other volcanic heat source causing glacial melting on the mountain, so it’s possible to have a lahar warning go off without any volcanic activity.

Between the volcano and possibility of an earthquake, there’s no telling what will come first or when.

“For us it’s just a matter of being prepared and planning for these eventualities,” says Dominic Marzano, Kent’s Emergency Management director.

Marzano’s team has planned for what will happen if a ash cloud disrupts communication, or flooding cuts off road access. In the case of the former, he says they look at satellite phones and amateur radio instead of traditional cellphones. In the case of transportation, he says he focuses on making sure that emergency coordinators know alternate routes to safe sites.

Marzano says that the biggest priority for emergency management is alerting citizens to the ash cloud and lahar hazards. The next will be managing traffic as some evacuees from southern cities come north to escape the debris flows.

“You’re talking about people, and you gotta have some place for them to go,” he says.

To combat these problems, he says the city has developed plans to sustain communications, mobility and shelters, but he admits that “for emergency management, a lot of our work is speculation.”

They won’t really know what to face until they actually face it, and until then it’s simply planning and preparing as best as possible.

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Volcano danger

For more information about lahar (debris flow) dangers, check out the emergency management pages for Kent and Auburn at www.auburnwa.gov/services/emergency_preparedness/links.htm and kentwa.gov/emergencymanagement/

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A Mount Rainier damage assessment map: