Questions and Answers About the 2004-2008 Eruption
How large is the lava dome, and how does it compare to the 1980–1986 dome?
In July 2007, the volume of the new dome was 93 million cubic meters (121 million cubic yards). This is slightly larger than the volume of the 1980–1986 dome. The new dome grew a small additional amount from August 2007 to January 2008, but a thick winter snowpack in the crater this winter prevents us from measuring that value accurately.
Dome growth during 2004-2008 lasted half as long (3 years) as the 1980-1986 eruption (6 years) while adding approximately the same amount of lava to the crater. The combined volume of the 1980-86 and 2004-08 lava domes is about 7 percent of the volume that was removed by the landslide on May 18, 1980, and 11 percent of the volume of the present horseshoe-shaped crater. The most obvious difference between the two periods is that dome growth during 1980–1986 was episodic, with growth spurts that lasted from a few days to nearly a year, interspersed with periods of no growth that lasted from several weeks to almost a year. In contrast, dome growth during 2004–2008 was continuous from October 2004 to January 2008.
What is the evidence that the lava dome stopped growing in 2008?
All of the indicators of dome growth have gradually declined to pre-eruption levels, where they have remained since late January 2008. These indicators include seismicity, ground deformation, and volcanic gas emissions. The most direct evidence is that, since late January, repeat photographs from cameras on the crater rim and floor show little or no change in the shape of the new dome, except for a small amount of sagging that would be expected even if lava extrusion has stopped. That observation was confirmed recently by placing a GPS spider on the part of the new dome that had been growing for the past year. Data from the spider indicate that it has moved downwards and away from the center of the dome by less than an inch per day since February 15, motion that is consistent with gravitational settling of the dome and strong evidence that dome growth has stopped or slowed dramatically.
Why is this considered to be a pause? Why is the Alert Level being lowered to Advisory (Color Code Yellow) instead of Alert Level Normal (Color Code Green)?
Our experiences with lava-dome-building eruptions at Mount St. Helens and other volcanoes around the world have taught us that lava-dome extrusion can start anew after pauses of weeks, months, or even years. During such pauses, levels of seismicity, deformation, and gas emission sometimes decline to pre-eruption levels, then increase again prior to renewed eruptive activity. It is too soon for us to say whether the current Mount St. Helens eruption has ended. Because lava- dome growth has stopped, we are no longer in alert level Watch and aviation color code Orange, as that combination signifies an eruption underway that poses limited hazards and has no to minor ash emissions. Our experience has shown us that many lava-dome eruptions are episodic, and we are continuing to monitor the volcano closely. We have reduced the alert level to Advisory, and the aviation color code to Yellow, to signify that volcanic activity has decreased significantly, but continues to be closely monitored for signs of renewed activity. For information about the alert levels read the U.S. Geological Survey’s Alert Notification System for Volcanic Activity.
If the eruption resumes, what kinds of precursors are likely to provide warning?
If dome growth were to resume, we would expect it to be preceded by several days or more of seismicity, ground deformation, and perhaps increased volcanic gas emissions as magma again rises up the conduit toward the surface. However, small explosions of steam or volcanic gas, capable of ejecting rocks inside the crater and minor amounts of ash above the crater rim, could occur without warning. Six such explosions occurred between 1989-1991 at St. Helens, a period when the volcano was not erupting. Other types of non-eruptive activity that may occur include rockfalls from the dome or crater walls and small mudflows triggered by rapid snow melt or heavy rainfall.
What are some of the lessons that scientists learned from the 2004-2008 eruption?There have been many lessons learned from the eruption already, and the learning process is still ongoing. Lessons include:
- We now know that dome growth can resume, even after a long hiatus, without a long period of obvious precursory activity. The earthquake swarm that signaled the start of renewed eruptive activity in 2004 began on September 23, just eight days before the first small explosion on October 1 and eighteen days before lava extrusion began on October 11. For the first 2 days, the swarm resembled several others that had occurred beneath the volcano in preceding years. There is no evidence that the volcano was inflating when the September 2004 earthquake swarm began, as might be expected if magma was accumulating at shallow depth.
- Once extrusion started, we learned that dome growth can occur continuously for more than 3 years with relatively little seismicity, ground deformation, and volcanic gas emission. The persistence of the eruption was all the more remarkable given that the erupted lava was almost entirely solid. These observations will serve as fodder for models that attempt to explain mechanics of this and similar eruptions.
- The nature of the eruption prompted scientists to develop new monitoring tools and techniques, including helicopter-deployable sensor packages (spiders).
- This was an unusual opportunity to observe the extrusion of a dacite lava dome through glacier ice, and the glacier’s response to the severe disruption it experienced as the dome grew.