Mount St. Helens is poised for its biggest eruption since it blew its top in 1980, killing 57 people. Here are some common questions and answers about the volcano located about 100 miles south of Seattle.
Q: Why is the volcano active now?
A. An active volcano like Mount St. Helens always experiences minor rumblings. Now, hot magma is rising higher inside the mountain and filling its interior chamber. This triggers swarms of earthquakes. The gases dissolved in the magma build up pressure and are very explosive.
Q: What will happen when the volcano erupts?
A: Scientists aren't sure. So far, it has emitted steam and a little ash. A large eruption throws rocks and vents off poisonous gases. Volcanic lava flows downhill and melts anything in its path, although at Mount St. Helens scientists say it is most likely to remain inside the crater's steep walls. However, an eruption also could produce a pyroclastic flow of superheated rocks and ash that shatters and burns anything in its path, as the 1980 cataclysm did.
Mostly, people near the volcano must contend with large clouds of gritty ash that spew 16,000 feet high, threatening aircraft.
Q: Is the ash fallout similar to that from a forest fire?
A: No. Wood ash is fluffy. Volcanic ash is a gritty mixture of pulverized rock and natural glass. It can scratch and ruin the painted finish of cars and homes. It clogs engines, machinery, irrigation and ventilation systems. It can kill plants by coating their leaves and preventing photosynthesis. People should wear masks and not breathe in the ash.
Q: How can scientists predict eruptions?
A: Scientists don't know exactly what will happen or when, but they look for clues in the changes registered by seismic and other monitoring devices. Mount St. Helens is one of the world's most heavily instrumented mountains, and researchers are reconnecting instruments that were initially knocked out by tremors. Plus, federal agencies have dispatched airplanes to make air sampling flights. GPS satellites measure bulges and other changes to the mountain's lava dome and steep flanks that signal an impending eruption. Weather stations forecast wind conditions that steer ash clouds.