|
The 2004-2008 volcanic activity of Mount St. Helens, Washington, United States has been documented as a continuous eruption in the form of gradual extrusion of magma. Starting in October 2004 and ceasing in January 2008, a new lava dome was built up. The new dome did not rise above the caldera created by the 1980 eruption of Mount St. Helens. ==2004 activity== 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 1980's 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 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. Such an event was expected to be relatively small and not pose a threat to regions beyond the immediate area of the mountain. However, the intensities and occurrences of the earthquakes continued to rise. The largest earthquake recorded was a magnitude 3.3. 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. 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 plume of steam the next day at 12:14 P.M. PDT, which was stronger than the previous 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, low-frequency harmonic tremor activity begin 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. 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 being extruded 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 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. 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 extruded 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「2004–08 volcanic activity of Mount St. Helens」の詳細全文を読む スポンサード リンク
|