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Rock Doc: Hopefully, Rainier stays silent

Mount Rainier is a stunning site. At more than 14,000 feet, its summit is worthy of respect from even serious hikers. There’s no wonder it’s a national park.

Mount 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 when she blows, great may be the calamity that ensues.

Some volcanoes erupt fairly gently. When lava comes up to the surface of the Big Island of Hawaii, the gases in the lava tend to separate steadily from the molten rock – like bubbles forming and rising in a soda-pop bottle after it has been opened. That’s because the lava is pretty runny – it’s not 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 for us Northwesterners, the Cascades are quite different from Hawaii. The molten material in the volcanoes in the Northwest is viscous or stiff. When a major eruption occurs, gases that were once at high pressure make their way to the air, and they do so 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.” 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 breathe.)

When Mount Rainier next erupts, the great heat of the lava and ash will melt snow and ice quickly. The meltwater, mixed with ash, will start moving downhill in a slurry that’s called a “lahar,” or volcanic mudflow. Lahars move much faster than people can run and destroy everything in their path. On the good side, the Cascades Volcano Observatory will try to give us as much warning as it can about what’s happening – but nevertheless, events may hit hard and fast.

Mount 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.

Unfortunately, many people are at risk from lahars flowing down from Rainier. And as more people move into the Puget Sound lowlands, more souls will be in danger. 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 Mount Rainier. But when it comes to geologic hazards, living well to the east of the Cascades ain’t all bad.

E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.


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