ROCK DOC - Ash and 'lahars' a double threat from Mount Rainier

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Mount Rainier is a stunning sight - beautiful, covered in snow and ice in winter and summer.

Like most other Cascade peaks, Rainier also is a deadly volcano. It hasn't erupted since 1894, but that's not long ago to a geologist - we are sure it will be heard from again. And it's not simply lava that's most likely to create a loss of life. 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. Hawaii's big island is like that. When lava comes up to the surface of the island, gases tend to separate pretty gently from the molten rock - like bubbles rising in a soda-pop bottle. That's because the lava is pretty "runny," or not very viscous.

The Cascades volcanoes are quite different. The molten material in Pacific Northwest volcanoes is quite viscous and stiff. When a major eruption occurs, gases that were once at high pressure make their way to the air explosively.

That's exactly what happened at Mount St. Helens in 1980. The tiny particles of lava it spewed are what geologists call ash. It's finely divided rock, and very difficult to breathe.

Being scalded to death or enveloped in an ash cloud are serious issues. But there's another threat.

Let's get back to Mount Rainier. When it next erupts, the heat will melt its snow and ice quickly. The water, mixed with ash, will move downhill in a slurry called a "lahar," or volcanic mudflow. That's the greatest hazard of all, because lahars destroy everything in their path and move much faster than people can run.

Rainier has more glaciers than any other mountain in the lower 48 states - meaning it will have a lot of water to create lahars. And because it's so tall, flows will scream down with a lot of speed and run for a long way in valleys of increasingly populated lowlands.

E. Kirsten Peters holds a doctorate in geology. Her column is a service Washington State University. Follow her at rockdoc.wsu.edu

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