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South Sound Escapes Fallout

by Rob Carson
The News Tribune, October 2, 2004

The western flank of Mount St. Helens is dark after Friday's 24-minute eruption of steam and ash. Mount Adams is in the background. -- Right: A photograph taken Monday from the same angle shows streaks of snow on the west side of the mountain. It is unclear whether Friday's activity melted the snow or covered it with ash. From the perspective of the South Sound, Mount St. Helens' mini-eruption Friday was not a whole lot different than the disaster of May 18, 1980.

We pretty much missed them both.

Only about 70 miles separate Tacoma from the volcano, but thanks to a combination of geography and weather patterns, the city experienced almost nothing of either eruption.

Almost all of the ash from the 1980 blast - between 1.7 and 2.4 billion cubic yards of it - headed east, to the other side of the Cascades.

The giant mudflow created when St. Helens collapsed on itself flowed west and south. It tore out bridges and clogged river drainages all the way to the Columbia River, stranding freighters in the Port of Portland.

But here in the South Sound, there was no sign of it.

Residents were treated to some faraway views as the eruptive column shot 15 miles into the air, then continued for nine hours.

But they avoided effects that elsewhere added up to disaster - 57 dead; at least 200 homes either washed away, buried in mud or otherwise damaged; 4.7 billion board feet of trees mown down like tall grass.

One aspect of the May 18 eruption, however, was more intense in Pierce County than it was in the blast zone itself: sound.

Because of a counterintuitive characteristic of physics, the sound of the eruption, as powerful as hundreds of bombs the size of the ones that leveled Hiroshima and Nagasaki, made no noise within a 60-mile ring around the volcano.

U.S. Geological Survey geologist Don Swanson, in a helicopter hovering next to the eruptive column, compared the experience to watching a silent movie.

"All that was missing was the tinkling piano," he said.

But in Tacoma, many people reported hearing the sound of the eruption, which they described variously as a single "foomp," an artillerylike barrage, a distant roar and a series of loud bangs.

The sound of the initial eruption, curiously erratic, reportedly was heard as far away as Saskatchewan, Canada, but not in the town of Toutle, near the mountain's base.

Scientists explained the phenomenon this way: Sound waves travel faster in warmer air and tend to be refracted, or bent, toward lower temperatures. The sound that radiated out of the volcano was bent upward, toward cooler air at higher altitudes.

At a height of about 15 miles, where air temperatures were sufficiently warmed by the radiant energy of the sun, the sound waves were refracted back down toward the surface of the earth in a spreading, doughnut-shaped ring.

The sound apparently bounced back and forth between the earth and the upper atmosphere several times, resulting in alternating zones of loudness and quiet at various distances from the volcano.

This part of Washington did not entirely escape the ash, either.

The spectacular eruption of May 18 was the one that went down in history, but few people remember that the volcano produced several spectacular outbursts of steam and ash in the weeks and months after that.

One eruption in particular, on May 25 - one week to the day after the big one - turned out to be especially memorable for people on this side of the Cascades.

That day, variable winds brought most of ash this way, giving Western Washington a taste of what the eastern side had suffered the previous week.

South of Olympia on Interstate 5, ash the consistency of flour drifted down onto car, sticking to windshields like gumbo. Traffic slowed to a crawl, narrowing to a single lane in each direction, and drivers following each other's taillights.

Rain made matters worse. Windshield wipers smeared the mess. Drivers whose vision was suddenly blocked veered crazily out of line, then braked to a stop. Others drove leaning their heads outside the driver's window, or frantically swabbing at their windshields with rags.

Rain flowed under the ash deposited on the pavement, turning the freeway into a slippery mess of jackknifed trucks and stalled cars.

The rain quickly washed the ash away, however, leaving no trace of it a week later.

The ash cloud from the minor burst of steam Friday also headed away from the South Sound, this time toward southwestern Washington and Portland.

Lucky again.

But, as geologists have been warning for decades: Just wait.

Mount Rainier is certain to erupt again, they say, and when it does, "Tacoma's mountain" could make what happened on Mount St. Helens look like child's play.

Rob Carson
South Sound Escapes Fallout
The News Tribune, October 2, 2004

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