Any of various processes and phenomena associated with the surface discharge of molten rock or hot water and steam, including volcanoes, geysers, and fumaroles. Most active volcanoes occur where two plates converge and one overrides the other (see plate tectonics). Volcanism can also occur along the axis of an oceanic ridge, where the plates move apart and magma wells up from the mantle. A few volcanoes occur within plates, far from margins. Some of these are thought to occur as a plate moves over a “hot spot” from which magma can penetrate to the surface; others appear to result from an extremely slow form of plate spreading.
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On September 23, 2004, around 2:00 a.m. 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 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. They also closed some trails in the area because of the risk of debris flows from the possible melting of the Crater Glacier in the volcanic crater.
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, 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. 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 a magnitude 3.3.
At 12:02 p.m. PDT on October 1, 2004, the mountain threw 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. 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 12:14 p.m. PDT, which was stronger than Friday's steam release. A low-frequency harmonic tremor followed the steam release, which led 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 (14 km) away, while tourists moved to various locations for several miles along State Route 504.
On October 3 there was low-frequency harmonic tremor activity begin at around 3:00 a.m. PDT and lasting for up to 90 minutes, which may have indicated the movement of magma beneath the mountain. The tremors were followed by a steam release at around 10:40 a.m.
Mount St. Helens eruptive activity continued over the following days, with steam releases occurring on October 4 at 9:47 a.m., 2:12 p.m., and at 5:40 p.m.; then again on the morning of October 5 at around 9:03 a.m., with an ash plume that dusted Randle, Morton, and Packwood, Washington, towns on or near U.S. Route 12 about 30 miles (48 km) from the volcano. Between steam releases, elevated seismic activity on the mountain continued with the strongest tremors remaining near magnitude 3.0.
On October 6, the U.S. Geological Survey announced that the alert level was being lowered, saying "We no longer think that an eruption is imminent in the sense of minutes or hours."
Magma reached the surface of the volcano around October 11, resulting in the building of a new lava dome on the existing dome's south side. Dome building continued, with the USGS reporting in early November 2004 that magma was extruding onto the surface at a rate of 7 to 10 cubic meters per second. Had magma continued to extrude at this rate uninterrupted, the crater would have been completely filled and Mount St. Helens regained its former height in about eleven years. The new dome became visible behind and to the right of the old dome, when viewed from the Johnston Ridge Observatory. The Mount St. Helens VolcanoCam located at Johnston Ridge is able to view the new dome especially at night when the glow of new magma is visible via the camera's infrared capabilities.
Included in the new dome was a feature dubbed the "whaleback" (named such because of its close resemblance to the back of a whale), which was a long shaft of solidified magma being exuded by pressure of magma underneath it. This feature was very hot but fragile. The edges of it began crumbling rapidly, forming loose material around the new dome. The rate of crumbling soon matched the rate of growth, with the size of the whaleback remaining fairly constant.
Mount St. Helens had major activity again on March 8, 2005 at about 5:30 p.m. local time, 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. The plume was visible from the metropolitan areas of Seattle, Washington, to Salem, Oregon, but only lasted for about 20-30 minutes. Ash was reported falling from the sky in Yakima, Washington, and surrounding areas. This activity was not considered a large eruption like those that may still be possible but merely a minor release of pressure consistent with the nature of ongoing dome building. Scientists think it was triggered by a partial collapse of the lava dome. There was no increase in quake activity before the volcanic event.
As of May 5, 2005, the highest point on the new dome was 7,675 feet (2,339 m), 688 feet (210 m) below the highest point of the volcano. It contained approximately 58 million cubic yards (45 million cubic meters) of material. Growth of the new dome continues steadily and has not abated, and small earthquakes continue to be observed every few minutes. The whaleback feature is disintegrating steadily but continues to be extruded as solidified lava pushes upward from underneath it. If the growth of the new dome continues at its current pace, the new dome could replace the amount of material lost in the 1980 eruption (estimated at 3.7 billion cubic yards, or 2.8 cubic kilometers) within the next 40-50 years.
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 had in early October for years to come, without ever causing a major, dramatic eruption.
On December 19, an eruption sent a steam plume billowing over Washington.