Report on St. Helens (United States) — 18 January-24 January 2006
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 18 January-24 January 2006
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2006. Report on St. Helens (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 18 January-24 January 2006. 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 18-24 January 2006, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. The dome-building eruption has proceeded at a slow and steady pace, quietly extruding dacitic lava. Seismometers, GPS receivers, and tiltmeters show patterns basically unchanged since the first of January. With the first clear weather in over a month on 23 January, crews were in the field observing the new dome, repairing instruments, replacing batteries, and exhuming cameras from ice and snow. The new dome is noticeably taller and broader than when last viewed in December. Rockfalls from its summit generated small ash plumes that slowly rose above the crater rim and dissipated as they drifted E.
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.