In every cartoon I've ever watched, when a volcano erupts, it spews out waves of thick orange lava. So when I watch footage of Mount St. Helens nearing eruption again, I'm a little disappointed that all I see is white clouds of steam. Where's all the lava? Where's the wall of flaming-red liquid, destroying everything in its path? It happens in Hawaii-why isn't it happening in Washington?
Volcanic eruptions can vary in the way they are manifested, said Clifford Thurber, UW-Madison geophysics professor. The determining factor is how much gas, generally carbon dioxide and steam, is dissolved in the lava.
\Think of a soda bottle, with its dissolved carbon dioxide,"" he said. ""Shake it up and you build up pressure, so when you pop the top, it explodes. But if you open a bottle without shaking it, there's a small release of a gas, but the soda pours out normally.""
A volcano works the same way. In Hawaii, the magma, which is molten rock below the Earth's surface, has lost most of its gas, so it just oozes out ""like toothpaste,"" according to UW-Madison associate professor of geology Brad Singer. ""But when the magma contains gas that's under pressure, when it's released, you get that explosion.""
As the magma is discharged, the gas bubbles expand, breaking the magma into tiny glass particles that cool as soon as they hit the air. That forms solid ash that, combined with the water vapor released as steam, is what we're seeing from Mount St. Helens now.
So how do researchers know if a mountain is also a volcano? Did people ever think Mount St. Helens was just any other mountain until one day it rumbled to life with a plume of smoke and ash?
""No, there are telltale signs,"" said Thurber. ""You can look at the record of the rock in the area to find evidence of volcanic rock. But mountains that are made of granite won't ever erupt.""
Singer said that's how scientists know there was once volcanic activity here in Wisconsin.
""There's evidence in the Wausau area of volcanic activity 1.4 to 1.5 billion years ago,"" he said, noting the rocks have eroded so much an observer wouldn't recognize the signs anymore. There is no more magma under Wisconsin, so we don't have to worry about an eruption here.
In Washington, however, geologists, thrill seekers and tourists alike are fascinated by what's happening with Mount St. Helens, although Singer said this won't be a catastrophic blast.
""This eruption may look spectacular, but it's at the very small end of the geologic spectrum. It will be magnitudes smaller than the 1980 eruption,"" he said.
How do scientists know that?
""There's a pattern of activity,"" Thurber said. ""There have already been earthquakes caused by the magma moving and the gases trying to escape."" The earthquakes will be sporadic at first, then more regular, and then there will be a harmonic tremor-a continuous vibration that people can't feel, though it can be measured.
""Like the Earth singing, humming with the flow of magma,"" as Thurber mused. As the vibrations build, there may be a bulge in the ground, from centimeters to meters high, visually observable. And then, boom.
Thurber said the greatest danger will be to commercial aircraft in the area, from the airborne ash. But he noted flights have been diverted from that area since last week. The 57 deaths in 1980 were caused when the erupting material mixed with both soil and water from a melted glacier, producing a lethal mudslide. But so much of the molten material has already been expended that he says the chances of that happening again are tiny.
You can watch a live Webcam of Mount St. Helens at www.fs.fed.us/gpnf/volcanocams/msh/.
Dinesh Ramde is a graduate student in journalism. Contact him at dramde@wisc.edu.
