The 2004–2008 volcanic activity of Mount St. Helens in Washington, United States has been documented as a continuous eruption in the form of gradual extrusion of magma.

Activity increased, and on September 26, the U.S. Geological Survey (USGS) and Pacific Northwest Seismograph Network issued a "notice of volcanic unrest", stating that a "hazardous event" was possible, and the U.S. Forest Service closed the mountain to all climbing.

Seismic activity continued to accelerate following the USGS advisory, with earthquakes approaching magnitude 2.5 occurring at a rate of about four per minute on September 29, prompting the USGS and Pacific Northwest Seismograph Network to issue a second advisory, increasing the "alert level" to the second of three levels, and warn of an increased likelihood of a steam explosion or eruption from the lava dome within the next few days.

At 12:02 p.m. PDT on October 1, 2004, the mountain erupted a plume of steam and volcanic ash about 9,700 feet (about 3 kilometers) into the air (according to pilot reports), from a vent in the then-unnamed Crater Glacier just southwest of the lava dome.

On October 3, low-frequency harmonic tremor activity began at around 3:00 a.m. PDT and lasted for up to 90 minutes, which may have indicated the movement of magma beneath the mountain.

Had magma continued to extrude at this rate uninterrupted, the crater would have been completely filled and Mount St. Helens would have regained its former height in about eleven years.

The Mount St. Helens VolcanoCam located at Johnston Ridge was able to view the new dome especially at night when the glow of new magma was visible via the camera's infrared capabilities.

Mount St. Helens experienced major activity again on March 8, 2005, at about 5:30 p.m. PST, when a 36,000-foot (11 km) plume of steam and presumably ash was witnessed emerging from the volcano, accompanied by a tremor that measured about magnitude 2.5.

Approximately the size of a football field, the large, cooled volcanic rock was being forced upward as quickly as 6 feet (1.8 m) per day, though the top part of the slab would usually collapse on a daily basis, under its own weight.

[5] After a relatively quiet 2007, on January 16, 2008, USGS geologist John S. Pallister spotted steam seeping from the lava dome in Mount St. Helens' crater.