It’s not just the lava

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Mt. Rainier is only sleeping ... it will erupt again

By Kirsten Peters

Mt. Rainier in my native Washington State is a stunning site. It’s a beautiful mountain, covered in snow and ice in both winter and summer.
At over 14,000 feet, its summit is worthy of respect from even serious hikers. There’s no wonder it’s a National Park.
Like most all of the other beautiful peaks in the Cascades, Mt. Rainier is also a deadly volcano. It hasn’t erupted since 1894, but that’s not long ago to a geologist – we are sure it’s only sleeping and will be heard from again.
And it’s not simply lava that’s most likely to create a loss of life when the mountain next blows. That’s partly due to how volcanic gas separates from lava, and also due to an eruption’s effects on ice, soil and something we rock-heads call “ash.”
Here’s the story.
Some volcanoes erupt fairly gently. The Big Island of Hawaii is characterized by that kind of eruption.
When lava comes up to the surface of the Big Island, the gases in the lava tend to separate pretty gently from the molten rock – like bubbles forming and rising in a soda-pop bottle.  
That’s because the lava is pretty “runny,” or not very viscous as we geologists would say.
If you are a sane person (by that I mean, if you are not a geologist), you’ll likely stay at least a few feet away from the stream of lava – and you’ll be fine.
Unfortunately, the Cascades are quite different from Hawaii.
The molten material in the volcanoes in the Northwest is quite viscous and stiff.
When a major eruption occurs, gases that were once at high pressure are close to being released to the atmosphere – and they make their way to the air explosively.  
That’s exactly what happened at Mount St. Helens in 1980 when a catastrophic eruption launched tiny bits of lava into the sky.
The tiny particles are what geologists call ash. It’s not the same as ash in an old campfire, which is the remains of burned wood. Volcanic ash is just finely divided rock.
(And as someone who was downwind of St. Helens when she blew, I’m here to testify that tiny bits of rock in the air make it quite difficult to breath, even when you hold your sleeve over your face.)
Being scalded to death or enveloped in an ash cloud are serious issues.
But there’s another problem, too, with this style of volcanic eruption and the threat it poses for people.
Let’s get back to Mt. Rainier, covered in snow and glacial ice.
When it next erupts, the great heat of the lava and ash will melt snow and ice quickly.
The water, mixed with ash, will start moving downhill in a slurry that’s called a “lahar” or volcanic mudflow.
That’s the greatest hazard of all for people who live near this type of volcano, because lahars move much faster than people can run and destroy everything in their path.
On the good side, the Cascades Volcanic Observatory will try to give us as much warning as it can about what’s happening – but nevertheless, events may hit us hard and fast.
Mt. Rainier is so high and near the sea that it has more glaciers than any other mountain in the lower 48 – meaning it will have a lot of water available to create lahars.
And because it’s so tall, the flows will come screaming down the mountain with a lot of speed and run for a long way in the valleys of the lowlands.
Maps of lahar risks around Mt. Rainier are coded yellow, orange and red. More than 100,000 people are at some degree of risk from lahars streaming down from Rainier.
As more and more people move into the warmer-colored zones of the Puget Sound lowlands, more will be at risk from future lahars.
In a few places, public warning systems have been set up and schools and other organizations practice what they would do in the event of a lahar emergency from Mt. Rainier.
In much of the Northwest people live on top of volcanic rock or on old volcanic debris flows.
Whether I live to see it or not, the day will come that residents of this beautiful corner of the country will find things more exciting than anyone will like.
Dr. E. Kirsten Peters
Washington State