Mount St. Helens eruption slowing, may halt

Mount St. Helens, erupting steadily for more than three years, may be showing signs that it’s pooping out.

Scientists who have been closely examining a sophisticated set of aerial photographs calculate that the lava dome is growing by 0.27 cubic meters per second as of the last measurement in April – roughly a pickup truck load every few seconds. That may sound like a lot, but it’s less than the 0.45 cubic meters calculated in two previous observations dating to the fall of 2006.

And it’s way below the rate when the eruption began in October 2004, which was characterized as a dump truck load per second.

Geologist Dan Dzurisin isn’t shook up about the change.

“It’s still possible, and still consistent with the data, that we could be in a long-term steady state eruption,” said Dzurisin, with the U.S. Geological Survey in Vancouver.

He noted that deformation of the volcano’s flanks has been minimal, suggesting the underlying magma chamber is recharging itself with a deep supply.

On the other hand, Dzurisin said, at least one other indicator points to a gradual slowing and possible end to the eruption: The unchanging chemical composition of fresh material scooped from the crater.

“The weight of the evidence might favor a continuing decline and eventual end to the eruption,” he said.

Taking the really long view, the current growth of the lava dome isn’t so remarkable.

Mount St. Helens is considered the youngest and most active volcano in the Cascade Range, erupting and rebuilding itself numerous times over the past 4,000 years. Of course, those eruptions are interspersed with centuries of quiescence.

Dzurisin said his colleague, Richard Iverson, calculated the volume of material erupted by Mount St. Helens over that period of time and divided it by the number of seconds in 4,000 years.

Iverson’s calculation: Mount St. Helens erupts at a rate of 0.2 cubic meters per second, averaged evenly across the past four millennia.

Scientists can’t say whether this eruption will continue for decades or end tomorrow.

“To me, we haven’t answered that question,” Dzurisin said. “It’s going to be hard to answer it until the eruption stops.”