Report on St. Helens (United States) — 13 July-19 July 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 13 July-19 July 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, 13 July-19 July 2005. 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 Mount St. Helens continued during 13-18 July, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. On 16 July at 1308, an M 3.2 earthquake occurred. The earthquake was followed by a rockfall that produced an ash plume that rose above the volcano's crater. Another rockfall at 2110 produced a small ash plume that was visible from the city of Vancouver, Washington ~70 km SW. CVO reported that frequent rockfalls are to be expected due to the steep-sided dome rising hundreds of meters above the crater floor. After the earthquake, seismicity returned to normal levels. 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.