In the Pacific Northwest, a rumbling from middle earth

The Pacific Northwest has always been the scene of geologic rock 'n' roll. Slip- sliding plates beneath Earth's surface cause frequent earthquakes.

Every so often, one of the mountains in the Cascade Range stretching from northern California to British Columbia blows its top. As Mt. Mazama did some 7,000 years ago creating Crater Lake. Or as Mt. St. Helens did in 1980, killing 57 people in the largest landslide in recorded history, mowing down forests like matchsticks, and blocking the sun with eerie ash clouds that climbed to the stratosphere and traveled around the globe.

Over the weekend, Mt. St. Helens showed again that it may be 1,300 feet shorter but no less ready to rattle and sputter back to life. Spouting steam and ash several thousand feet into the air, it tossed boulders around and cracked the glacier inside its crater, promising more excitement to come.

As long as they're not an immediate threat to life and limb, volcanoes seem to be fascinating in a way that other forces of nature are not. Crowds don't race to see a 150-m.p.h. hurricane or building-tipping earthquake. If anything, they head in the other direction. Come the tornado, we burrow underground.

But from the time they're kids building junior high school science projects or piling up fireworks out in the driveway, Americans want to get close to nature's Roman candles. They're a reminder that there's something major, something primal going on down there in middle earth - that the terra beneath our feet is not necessarily firma. Until officials shooed them off the mountain, hundreds of SUVs and family vans headed up Mt. St. Helens to the best view points.

But by Saturday afternoon, officials said seismic activity had changed from rapidly recurring slight earthquakes (principally "rock breakage events") to a continuous low-frequency tremor, which indicates that magma (molten lava below ground) had begun to move.

"This is the highest energy level since 1980," said geologist Dan Dzurisin at the US Geological Survey's Cascades Volcanic Observatory in Vancouver, Wash., 50 miles south of Mt. St. Helens.

Taking no chances, officials declared "alert level 3" (highest level) meaning that an imminent eruption could threaten people and property. The several hundred people who had gathered at a lookout several miles away to watch what promised to be mother nature's drama were told to leave.

"Explosions from the vent could occur suddenly and without further warning," federal and state government agencies coordinating the volcano watch warned. At press time, seismic activity had returned to shallow earthquakes every one to two minutes. Still, volcano alert level 3 remained in force.

Mt. St. Helens' geologic dance comes as no surprise. Active volcanoes are just that - active. They're constantly building and reforming. And with equipment not in wide use when the mountain blew 24 years ago, scientists now can detect earth movements as small as one millimeter. Such equipment includes global positioning systems (GPS) and radar inferometry using space-based sensors.

Among the signs that the Cascades are far from asleep: The Middle Sister and South Sister volcanoes in central Oregon have tilted slightly as the earth swells beneath their western flanks. Why? Because molten rock has been rising in recent months, lifting an area the size of Portland, Ore., (10-12 miles in diameter) by as much as a foot.

Still, geologists, hydrologists, and other earth scientists can offer only informed guesses about future volcanic activity.

"We don't know what's going to happen and when it's going to happen," says Dan Miller, a scientist with the US Geological Survey. "But the things we've been seeing in the past 10 days typically precede an explosive eruption."

What they're observing is evidence that molten magma is moving up underneath the volcano. The bursts of steam and ash so far may be the result of melting glacier and rainwater seeping down toward the hot magma. And if that substance in fact is "new magma," scientists say, it has a relatively high amount of gas. This, in turn, could cause larger eruptions. Helicopter pilots flying in the area have noticed the smell of rotten eggs - probably hydrogen sulfide gas.

"This magma is very much like a bottle of Coca-Cola," says Tom Pierson, another USGS scientist. "It's got a lot of gas dissolved in it, and as it comes up the pressure gets released. That's like popping the top off a bottle Coke, and that energy drives a lot of the explosions. My colleagues expect this to go on from days to weeks."

Around the world there are more than 600 active volcanoes (by definition, those that have erupted within the past 200 years). There are 14 active volcanoes in the Cascade Range, including such well-known mountains as Lassen, Shasta, Hood, and Rainier.

In all, the US has 50 active volcanoes, most of them in Alaska's Aleutian Islands chain. They're all part of the "ring of fire," the necklace of volcanoes that stretches from the tip of South America up through Alaska, Japan and the Kamchatka Peninsula of Russia, down through the Philippines and Indonesia into New Zealand, including about three-fourths of the world's volcanoes.

Meanwhile, scientists do not expect the kind of widespread damage and loss of life that occurred when Mt. St. Helens blew in 1980. The mountain is in the middle of a large national forest, and the nearest population center - the small community of Toutle, Wash. - is 30 miles away.

"The greatest concern at this point is an ash plume and the spread of ash itself that might come from an explosion," says US Interior Secretary Gail Norton, who visited the area Saturday. "This is a concern for aircraft travel."

In 1989, a commercial jumbo jet flew through what the pilots thought was haze but in fact was ash from an eruption of Alaska's Redoubt volcano. Though the aircraft lost power, it landed safely in Anchorage - with $80 million in damage to its engines.