Although most volcanologists felt that Mount St. Helens was still probably the most likely volcano to erupt next in the Cascades, the timing of the 2004-2005 eruption of Mount St. Helens was totally unexpected.  Seismicity for the previous year had been low, there was no inflation or deflation of the  outer flanks, nor was there a hint of magmatic gases. When the seismic swarm began on September 23, 2004, it did not, at first glance, look much different than the many previous swarms, none of which had culminated in an eruption, which had occurred beneath the volcano since the end of the 1980-86 eruption. Three days into the swarm, however, a burst of higher magnitude earthquakes indicated that this swarm was not like previous ones.

From then on, events accelerated quickly.  The latest seismicity originated at depths less than 3 km. Gradual increase in seismic activity, together with occasional explosions led to official hazard warnings followed by a volcano alert. In less than a week later the first explosive steam and ash emission and a little over two weeks later the first lava would reach the surface. 

 Cascade Volcano Observatory scientists monitored the volcano every two hours around the clock, and thier finding indicated no significant potential for explosive eruptions. Phreatic (i.e. steam) eruptions early on were confirmed by thermal infra-red radiometer which showed temperatures of the plume to be equal to or less than the boiling point. The October 1, 2004 eruption had destroyed one of the more important seismic stations, and the response was to use helicopters to deploy portable accelerometer and GPS units. This enabled CVO to increase the effectiveness of monitoring with minimal risk to personnel. Gas emissions were low in comparison to those of the eruptions of the 1980s. 

The appearance and growth of the current lava dome is unlike that of 1980-86.  Of the many remarkable aspects about this eruption, one of the most intriguing is the interaction of the lava dome and the glacier into which it intruded. On October 11, 2004 a hot lava spine  was extruded from the cold dome, accompanied by ash which slid down the slope onto the glacier. By late October 2004, a large volcanic welt had lifted the within-crater glacier 100 meters. In the almost continuous eruption since then, the magma cools to a solid before reaching the surface.  Lava is extruded as spines which move north, shift to the east and collapse. Dome growth is in the direction where there is room to expand. The spines behave as if the glacier, which is up to 140 meters thick, is not there. The ejecta is dacite, without glass, vesicles, or  pumice.

For more than a year now [as of late September 2005], lava has been continuously extruding  [at a rate of about 2-12 cubic meters per second] to build a new lava dome that is almost the size of the 1980 - 86 one. With a volume estimated at 60,000,000 cubic meters,  at the present rate of formation, the new dome would fill the caldera in about 20 years. 

Seismicity has remained constant in frequency but there has been decreasing magnitude since July 2005. There are no clues to the future behavior of the volcano.  It is a mystery why no seismic signals below 3 km are detected, although it is believed that the ultimate magma chamber should be 8 km below the surface.

   Adapted from summary by  Cynthia Gardner  and Jim Vallance, CVO.  Oct. 2005