Originally published July 14, 2008
Sept. 23, 2004: Two or three miles beneath Skamania County, for reasons nobody yet understands, a swollen reservoir of molten rock at least six times the size of Lake Merwin began to tremble with 200 little shudders.
Volcanologists tensed. For two weeks, FOX and CNN would cut away from the presidential election to the steaming crater of Mount St. Helens.
Then the cooling lava hit the autumn air — easy as toothpaste from a tube. The earth held still. The Toutle Valley sighed with relief. The satellite trucks drifted off. And the lava just kept flowing.
Now that the volcano’s 40-month encore is over — a conclusion declared in July 2008, after five months of silence at the crater — what did we learn?
More than anything, scientists say, Mount St. Helens showed the world how gentle a killer can become.
The calmest eruption
It came in like a lamb. It went out like a lamb. It hardly even burped.
“There was very little volume of gas,” said Dan Dzurisin, a geologist for the Cascades Volcano Observatory in Vancouver who’s devoted his career to St. Helens. “Yet it continued to go on, which was a surprise, because gas is the driving force.”
In many eruptions, gas that dissolved in magma during the melting process rushes out of the liquid like carbonation from an open soda can, shooting ash or rocks into the air.
That’s what happened at St. Helens in the six years after its catastrophic eruption on May 18, 1980: episodic bursts of pressure, escaping from a lava dome that grew larger than 100 Rose Garden Arenas.
This time around, the mountain grew a new, very similar cone: a series of cooled-lava spines and swells that scientists dubbed “whalebacks” because that’s what they looked like. But it was a startlingly calm process.
“These earthquakes, which came and went throughout the 18-year period (from 1986 to 2004) — they quit,” said Steve Malone, a seismologist for the University of Washington in Seattle. “They absolutely stopped dead when this eruption started.”
It wasn’t as if volcano geeks weren’t looking. This was the eruption they had been waiting for.
They hired a helicopter to scoop still-warm rocks from the peak with a contraption they called “Jaws.” Atop the whaleback, they dropped a GPS device named “AHAB” to track the mountain’s every move.
Mysteries continue
They found new surprises. For one, even though the lava burst out right beneath a glacier, its quiet emergence meant that mud flows never poured down the Toutle River, as some had feared.
“I think we were all surprised that you could punch a new lava dome through the glacier, which in places was 400 feet thick, and not cause steam explosions,” Dzurisin said. “Ninety percent of the ice that was there in October 2004 is still there; it just got shoved out of the way.”
Another surprise: Despite the mysterious lack of magmatic fizz, the rock seemed to be almost exactly the same as the stuff that emerged 25 years before — crumbled from similar caverns, pushed through similar channels.
Was it sent up because new magma entered the vast reservoir? Because the reservoir contracted somehow?
Nobody’s sure, Dzurisin said. Part of the problem: The reservoir is so far down and so diffuse that scientists can’t figure out how big it is.
There are two ways to assess a magma reservoir:
• Capture seismic waves traveling through the crust from far-off earthquakes. A big-enough lake of liquid will dampen the waves, leaving its dark outline in seismic records.
• Look for the epicenters of many tiny earthquakes. If you find an area where earthquakes never start, that’s the reservoir. Maybe.
The first method rarely works with St. Helens, Dzurisin said, meaning the reservoir is probably more like a cave — full of solid nooks and crannies — than like a lake.
Using method two, estimates put the St. Helens reservoir anywhere from 0.72 of a cubic mile to 4.8 cubic miles, almost the volume of Mount St. Helens itself.
A new assignment
Now that the mountain is silent, scientists will again have a while — whether it’s months or millennia, no one can say for sure — to crunch new data and look for patterns.
When everything was changing, Malone said, people like him didn’t tend to have time for big new theories.
“It’s really difficult from our perspective of having our nose right down in it,” Malone said. “It’s the sort of thing you want to talk about over a beer.”
Dzurisin does have the beginnings of a theory, or at least the beginnings of a new assignment.
“The volcano must have been primed for this eruption,” he said. “It didn’t take a lot to get to get going, and once it got going, it just kept going. … It looks in hindsight like the volcano was ready to go for a long time.”
Other volcanoes, he said, surely go through similar things.
“We need to come up with ways to identify when a volcano might be primed,” he said.
