In autumn 2004, Mount St. Helens became active again.
On September 23, 2004, around 2:00am PDT, Mount St. Helens experienced an earthquake swarm, with about 200 small (less than magnitude 1) earthquakes occurring less than one-half mile (one kilometer) below the lava dome. Activity continued increasing, and on September 26, 2004, 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, as well as closing some trails in the area due to the risk of debris flows.
Seismic activity continued accelerating following the USGS advisory, with earthquakes approaching magnitude 2.5 occurring at a rate of about four per minute on September 29, 2004, prompting the USGS and Pacific Northwest Seismograph Network to issue a second advisory, increasing the "alert level" to the second of three levels and warning of an increased likelihood of steam explosion or eruption from the lava dome within the next few days. Such an event was expected to be relatively small, and not pose a threat beyond the immediate area of the mountain itself. However, the earthquake intensities and occurrences continued to rise. In the near vicinity to the volcano, earthquakes were felt as much as four in one minute. The largest earthquake recorded was 3.3 on the Richter Scale.
At 12:02 PM PDT on October 1, 2004, the mountain threw a plume of steam and ash about 9700 feet (about 3 kilometers) into the air (according to pilot reports), from a vent in the unnamed glacier just southwest of the lava dome. The resulting ash plume was reported to have drifted south to Vancouver, Washington and Wood Village, Oregon, dusting cars with a fine layer of black, sooty ash.
Mount St. Helens vented another burst of steam the next day at 12:14pm PDT, which was stronger than Friday's steam release. A low-frequency volcanic tremor followed the steam release, which led Government seismologists to raise the "alert level" to the third of three levels, indicating a potential threat to life and property. Accordingly, the Johnston Ridge Observatory overlooking Mount St. Helens was evacuated; television media established their bases at Castle Lake Viewpoint about nine miles away, while tourists moved to various locations for several miles along Washington State Route 504.
October 3 saw low-frequency harmonic tremor activity begin at around 3:00am PDT and lasting for up to 90 minutes, which may indicate the movement of magma beneath the mountain. The tremors were followed by a steam release at around 10:40am PDT.
Mount St. Helens eruptive activity continued over the following days, with steam releases occurring on October 4 at 9:47am, 2:12pm, and at 5:40pm PDT; then again on the morning of October 5 at around 9:03 AM PDT, with an ash plume that dusted Randle, Morton, and Packwood, Washington , towns on or near U.S. Highway 12 about 30 miles from the volcano. Between steam releases, elevated seismic activity on the mountain continues with the strongest tremors remaining near magnitude 3.0.
On October 11, a new lava dome formed just behind the existing one in the volcano's crater. The top of the new dome is almost level with the old one.
Scientists closely monitoring the volcano have scaled back their predictions of a bigger eruption. Some are now saying that Mt. St. Helens could continue to experience volcanic events similar to what the mountain saw in early October for years to come, without ever seeing a major, dramatic eruption.
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