Report on St. Helens (United States) — 16 February-22 February 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 16 February-22 February 2005
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2005. Report on St. Helens (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 16 February-22 February 2005. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
During 16-22 February, growth of the new lava dome inside the crater of St. Helens continued, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. A GPS unit deployed on 16 February on the E arm of the glacier within the volcano's crater moved northward about 1.2 m per day. This rapid rate of flow was consistent with the thickening of the glacier that resulted from its compression between the growing lava dome and E crater wall. A thermal imaging flight on the 16th suggested that a longitudinal crack was developing along the top of the new lava dome.
On the afternoon of 18 February, a rockfall off of the lava dome produced an ash plume that rose several hundred meters above the crater rim. CVO reported that extensive cracking on the long, smooth, whaleback-shaped lava dome suggested that increased rockfall activity and similar small plumes could occur in the coming weeks. Analysis of recent airphotos showed that as of 1 February the high point on the whaleback-shaped extrusion was 2,330 m (nearly 425 m above the 1980 crater floor and 150 m above the top of the old lava dome). The extrusion was about 470 m long, and 150 m wide. The new lava dome, uplifted area of crater floor, and deformed glacier ice grew to a combined volume of about 38 million cubic meters, almost one-half the volume of the old lava dome. St Helens remained at Volcano Advisory (Alert Level 2); aviation 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.