US Geological Survey volcanologists raised the alert levelat Mt. St. Helen's to the highest possible and warned that amajor eruption appeared to be "imminent."
The tremor this morning lasted 25 minutes, and is anindication that the volcano is moving toward an additionaleruption. There was a release of steam on Saturday thatattracted the largest crowd ever to the volcano's visitorcenter.
While scientists anticipate a major eruption, they expectthat it will be smaller than the May 18, 1980 explosion thatkilled 57 people and ejected millions of tons of ash intothe atmosphere, which rained down across the PacificNorthwest for weeks.
The volcanic tremor that took place on Sunday came after twoeruptions, one on Friday that ejected rocks into the air,and the smaller Saturday event, which involved only steam.
The effect of these two eruptions has been to weaken thelava dome, and experts expect that an explosion will takeplace within 24 hours of Sunday morning.
Visitors at the Johnson Ridge Visitor Center five miles fromthe volcano were asked to leave the area.
Interior Secretary Gale Norton overflew the volcano onSaturday, and said that the greatest concern was the ashplume that might spread from an explosion. Mt. St Helen's isnear a heavily traveled air flight path, and ash plumes areextremely hazardous to aircraft.
If Mt. St. Helens does send a plume of ash into thestratosphere, it will have the effect of warming thatatmospheric layer slightly, which might reduce the intensityof winter storms. It will also cause an eerie natural effectknown as a 'blue moon,' which occurs when gray ash in theatmosphere obscures moonlight.
There is no evidence, as yet, that the volcano presents athreat beyond its local region.
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