Fifteen months after Mount St. Helens reawakened, the volcano is continuing to release massive amounts of lava in an unusual geologic display that in some respects confounds scientists.
Roughly every three seconds, a large dump truck-load of lava – 10 cubic metres – oozes into the mountain’s crater. And with the sticky molten rock comes a steady drumfire of small earthquakes.
The unremitting, monthslong pace is not common, said U.S. Geological Survey geologist Dave Sherrod. Experts said it is unclear what the activity signifies or how long 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 about 3.5 billion cubic metres 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 2,950-metre peak known as “America’s Mount Fuji.”
St. Helens – now 2,538 metres – rumbled for another six years, extruding about 100 million cubic metres of lava onto the crater floor in a series of 22 eruptions that built a 267-metre dome. The volcano, located about 160 kilometres south of Seattle, fell silent in 1986.
Then in September 2004, low-level quakes began – occasionally spiking above magnitude 3 but generally ranging between magnitude 1 and 2. Since then, the mountain has squeezed out about 100 million cubic metres of lava.
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 that can go as high as 9,000 metres.
“We haven’t had that kind of plume since March 8, which is either a blessing or it leads us into complacency,” Sherrod said, adding quickly: “We avoid complacency.”
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.
But at the current rate of extrusion, “three or four months would have been enough time to exhaust what was standing in the conduit,” 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.