What will happen when Mount Rainier erupts?

When geologist Carolyn Driedger talks about Mount Rainier, she feels like she’s trash-talking.

“Everybody has all these positive vibes from the volcano,” Driedger says. “Sometimes I feel like I’m gossiping about the prom queen when I tell people that I’m here to talk about the hazards of the volcano. And not everybody wants to hear that.”

She’s talking about a possible eruption, which could happen in our lifetime. Statistics show there’s a volcanic eruption in the Cascades two to three times every century; Mount Rainier is the tallest mountain in that range.

“This is just an impermanent piece on our landscape. Our lifetimes are just too short to appreciate how much change can happen,” Driedger says.

But we may see some change in our lifetime. The last big one was Mount St. Helens in 1980; Lassen Peak in Northern California erupted in 1915.

The mountain, as locals call it, defines us as a region. It’s on our license plates and our quarters. And when it comes into view on a sunny day, grand and gorgeous, as we’re barreling down Interstate 5, we interrupt each other to point it out. “Look at the mountain,” we say in wonder.

What could happen to Seattle and surrounding areas when Mount Rainier erupts? Listener Russ Keldorph asked KUOW.

“You can envision that there would be blocks half the size of the visitor’s center here at Paradise or the size of Volkswagens and fine grain material being blasted into the atmosphere and then falling back on the snow’s surface,” Driedger says.

It would be hot, and it would melt the ice and snow. And tumble over cliffs.

“The lava flows encounter those very steep slopes and make avalanches of hot rocks and gas that are hurtling down the mountain maybe 100 miles per hour or so,” Driedger says.

The lava would stop flowing near the boundaries of the national park.

But the water it would melt would create a much bigger hazard: A flash flood that would look like concrete and chew up everything in its path.

It would pull down trees. Giant boulders would bounce on its surface, cracking as they collide with each other.

This nightmare mudflow is called a lahar.

And it would sound like “a rocket launching. Or maybe a train barreling down a track where no railroad tracks exist.”

We follow the path of a historic lahar down the mountain, to a river bottom far below.

“We should be able to find a trail down here somewhere,” Driedger says.

This is where that lahar, which was called the National lahar, dropped boulders and trees about 2,200 years ago. A river has cut through the plain, revealing a cross-section of the lahar deposits.

“On our left, we’re seeing this wall of rocks,” she says. “Big boulders the size of basketballs and it looks like something that a human being has constructed.”

It looks like an ancient ruin.

The town of Orting is built on lahar deposits, which tells us a lahar could surge through the area again.

Scientists say Orting will probably have plenty of warning before an eruption. But just in case, there’s a backup plan, a siren that gives people in Orting roughly 40 minutes warning before the lahar hits.

Scott Heinze, with the Pierce County department of emergency management, recommends moving to higher ground – fast.

“Get off of the valley floor in a hustle,” he says.

He and I walked through a neighborhood full of kids, cul-de-sacs and basketball hoops. Most of the houses are two stories, but Heinze says that’s not high enough for safety.

A lahar at Mount St. Helens proved that.

“There’s a famous picture of a house coming down the Toutle River and smashing into a bridge and demolishing the house.”

He continues, “It’s why we want people to preplan and to have an escape route and to know how they’re going to evacuate. Because chaos during the evacuation is going to lead to more fatalities.”

No one knows how big a lahar would be for Mount Rainier – or how far it would travel.

But Heinze wants to show me how far it could come.

He takes me to Fireman’s Park, high on a bluff above the port of Tacoma.

“You can see the cranes over there,” he says. “You can see the rail yard, you can see 167 and I-5, a steady stream of semis that are coming in and out of port.”

Everything we see, he told me, could be drowned by a lahar.

Routes connecting Tacoma to Seattle could be buried.

Tacoma could face shortages of food and supplies. Many of its hydroelectric dams and water sources also lie in lahar zones.

Then, there are long-term problems: Decades of flooding and erosion as rivers cut through fresh lahar deposits. Sediment settling in the ports of Tacoma and Seattle could cost billions of dollars to dredge.

Driedger, the geologist, looks at the majestic vistas that line our Northwest cities and she sees remnants of volcanoes past. The Cowlitz Chimneys, to the east. They’re made of cooled magma. These are the skeletons of volcanoes that have eroded away. And someday, after it’s done erupting, the same fate could await Mount Rainier.

Safety tips if you live in a lahar zone around Mount Rainier.

Sign up for Pierce County's alert system. You can opt to receive emergency alerts by phone, by text, and via several email addresses.

Purchase an emergency radio with a NOAA weather channel and keep it plugged in or recharged. In the event of a lahar, the radio should wake up and send out an alert message.

Carry an emergency kit with water for four days in your car. Rescue workers may not be able to reach you immediately.

Have an emergency evacuation route in mind that brings you quickly to higher ground. Escaping by road may be an option, but an evacuation route could be as simple as a foot trail that climbs a few hundred feet up the side of the valley.

Have a backup evacuation route in mind in case the first route becomes inaccessible. Learn to recognize volcano evacuation route signs.

Have a reunification plan in place: A safe place to reunite with family in the event that you become separated.

This story was originally published on May 24, 2016.

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