Report on St. Helens (United States) — 17 November-23 November 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 17 November-23 November 2004
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2004. Report on St. Helens (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 17 November-23 November 2004. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Growth of the new lava dome inside the crater of St. Helens continued and was accompanied by intermittent emissions of steam and ash. When weather permitted during the report period, a plume was observed rising passively and drifting out of the crater. The plume occasionally contained minor ash, which fell out in the crater and on the flanks of the volcano, darkening the snow.
Seismicity remained at low levels compared to that observed early in this unrest. The current seismicity is consistent with a continuing, slow rise of magma driving uplift of the crater floor and feeding the extrusion of lava onto the surface, where it builds a dome. The overall low rates of seismicity and gas emission suggest that the lava reaching the surface is gas poor, thereby reducing the probability of highly explosive eruptions in the near term.
A new GPS site installed 20 November near the top of the new lava dome showed that the highest point on the new lava dome is at an altitude of 2256 m a.s.l, or about 76 m higher than the summit of the 1980-86 lava dome. In its first 24 hours near the top of the new lava dome, this GPS site moved about 10 m to the SE and 2 m vertically, confirming visual and photographic observations that the new lava dome is moving at an impressive rate.
Gas measurements taken on 20 November show that daily gas emissions remain at a more or less constant rate of around 180 metric tons of sulfur dioxide, about 900 metric tons of carbon dioxide, and several metric tons of hydrogen sulfide.
St. Helens remained at Volcano Advisory (Alert Level 2); aviation color code Orange.
Geologic Background. Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fuji-san of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km horseshoe-shaped crater now partially filled by a lava dome. Mount St. Helens was formed during nine eruptive periods beginning about 40-50,000 years ago and has been the most active volcano in the Cascade Range during the Holocene. Prior to 2200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older St. Helens edifice, but few lava flows extended beyond the base of the volcano. The modern edifice was constructed during the last 2200 years, when the volcano produced basaltic as well as andesitic and dacitic products from summit and flank vents. Historical eruptions in the 19th century originated from the Goat Rocks area on the north flank, and were witnessed by early settlers.