Story Photos
 Photo courtesy of USGS Mount St. Helens' ever-increasing lava dome is now large enough to fill the Portland Rose Garden Arena 163 times. |
Mount St. Helens sluggishly churns out molten rock
Monday, October 8, 2007 7:09 AM PDT
By Barbara LaBoe
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E-mail this articleIs Mount St. Helens winding down its three-year-long eruption or just getting ready for a quick cat nap?
Scientists at the U.S. Geologic Survey in Vancouver are debating that very issue as the eruption's steady production of molten rock continues to slow, geologist Dave Ramsey said last week.
No one expected the eruption to last three years when the mountain rumbled back to life in late September and early October of 2004 after an 18-year snooze. The post-eruption dome building of the 1980s lasted six years, but it was occurred in fits and starts, not one continuous event, Ramsey said.
"(In the 1980-86 dome building) you would often see a lot rock produced very quickly, which would add lobes inside the crater ... And then there would be explosions and great steam emissions and then things would shut down for awhile," Ramsey said. "This one has just been continuous and we haven't had any explosions since March 8, 2005, and that's been a surprise."
"This has just been a continuos extrusion of rock," Ramsey said. "We haven't seen this kind of cycle."
One theory for the difference is that the pool of molten rock driving this eruption is low in gas levels, making it less explosive, Ramsey said. Another is that the slowdown is simply the start of the first "break" in the eruption, similar to periods of no activity during the 1980-86 dome-building events.
"Any change at this point in time is going to be very interesting after three years of continuous eruption," Ramsey said. "If it were to slow down or stop, everyone would take a look and say 'Okay, is that the end of this or the first break? ... Are we about to get into the second phase?' "
There definitely are signs of the eruption slowing down.
When the eruption first started in October of 2004, 7.8 cubic yards of lava oozed out of the peak every second. The rate has declined to about 0.35 cubic yards.
The slow but steady growth has allowed the volcano to easily eclipse the amount of material pumped into the crater during the 1980s -- and in half the time.
The 1980-86 dome building produced a lava dome of 97 million cubic yards. The current three-year eruption has added 121 million cubic yards to the crater. That's enough material to fill the Rose Garden Arena in Portland 163 times.
The mountain has replaced about 7 percent of the bulk blown away in the May 18, 1980 eruption. That blast decapitated the volcano, which formerly towered 9,677 feet, to 8,363 feet and gouged out a crater 2,000 feet deep and about a mile wide.
When the latest eruption first began in 2004, scientists predicted it might be less than 200 years before the mountain was restored -- an incredibly short amount of time by geologic standards. Now, with the slowdown, Ramsey said the new estimate is between 200 and 300 years.
The volcano also has shifted where it's producing new material during the past three years, Ramsey said. At first new material appeared in the east end of the crater and then shifted to the south. Now, most of the new material is emerging from the southwest part of the crater.
In addition, the dramatic spines and whale backs shapes of the early eruption have given way to just a gradual overall increase in the size of the lava dome. Scientists theorize that's because there's so much material already in the crater that new volcanic rock just pushes the entire dome up from the bottom rather than spurting out to create distinct new lobes, Ramsey said.
The USGS has greatly beefed up the number of cameras and other monitoring equipment in the past three years, making this eruption the most carefully studied ever, Ramsey said.
"Because it's been three years, the mountain really is much better monitored than it was before," Ramsey said. "Just the application of newer technologies has been able to give us a better idea of what's going on. There's going to be a lot of papers and reports just from the amount of time the eruption has been going on an the ability to have all this data."
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