Mount St. Helens’ Lava Baffles Scientists
SEATTLE — Roughly every three seconds, the equivalent of a large dump truck load of lava – 10 cubic yards – oozes into the crater of Mount St. Helens, and with the molten rock comes a steady drumfire of small earthquakes.
The unremitting pace, going on for 15 months now, is uncommon, said U.S. Geological Survey geologist Dave Sherrod. Experts say it is unclear what the activity signifies or how much longer it will continue.
“One view of this eruption is that we’re at the end of the eruption that began in 1980,” Sherrod said. “If it hadn’t been so cataclysmic … it might instead have gone through 30 or 40 years of domebuilding and small explosions.”
St. Helens’ violent May 18, 1980, eruption blasted 3.7 billion cubic yards of ash and debris off the top of the mountain. Fifty-seven people died in the blast, which left a gaping crater in place of the perfect, snowclad cone that had marked the original 9,677-foot peak known as “America’s Mount Fuji.”
St. Helens – now 8,325 feet – rumbled for another six years, extruding 97 million cubic yards of lava onto the crater floor in a series of 22 eruptions that built a 876-foot dome.
The volcano, about 100 miles south of Seattle, fell silent in 1986.
Then, in September 2004, the low-level quakes began – occasionally spiking above magnitude 3. Since then, the mountain has squeezed out about 102 million cubic yards of lava, more in 15 months than in the six years after the eruption.
Sherrod describes the movement of lava up through the volcano as being “like a sticky piston trying to rise in a rusty cylinder. These quakes are very small – we think they’re associated with that sticking and slipping as the ground is deformed and relaxes.”
The dome collapses and grows and collapses and grows, he said. “It changes its location … it can’t seem to maintain its height at much more than it is now ” – about 1,300 feet. “Then it kind of shoves the sandpile aside and starts over.”
It’s not entirely clear where the lava is coming from. If it were being generated by the mountain, scientists would expect to see changes in the mountain’s shape, its sides compressing as lava is spewed out.
At the current rate, “three or four months would have been enough time to exhaust what was standing in the conduit. … The volume is greater than anything that could be standing in a narrow 3-mile pipe,” Sherrod said.
That suggests resupply from greater depths, which normally would generate certain gases and deep earthquakes. Neither is being detected.
“That’s one of the headscratchers, I guess,” Sherrod said.
All the recent activity has remained within the crater, though scientists – keenly aware of the potential damage that silica-laced ash can pose to jet engines – monitor St. Helens closely for plumes of smoke and ash. Some have gone as high as 30,000 feet.
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