Report on St. Helens (United States) — 10 November-16 November 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 10 November-16 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, 10 November-16 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 on both the crater and flanks, darkening the snow.
Seismicity remained low compared to that observed early in this episode of unrest, consistent with continued slow uplift of the crater floor and surface extrusion of lava. Overnight on 16 November, three earthquakes in the M 2.5-2.8 range shook the crater floor, behavior considered typical during an episode of dome growth. The overall low rates of seismicity and gas emission suggested that the lava reaching the surface was gas-poor and unlikely to generate highly explosive eruptions in the near term.
Recent extrusion-rate estimates of ~4 cubic meters per second were slightly lower than those in mid-October. These rates, which are based on the volume of deformed crater floor and the amount of airborne sulfur dioxide measured during gas-monitoring flights, have uncertainties associated with their calculation. Volcanoes like St. Helens are expected to undergo slight variations in their extrusion rates during eruptive cycles. This change is small and does not indicate a notable change in the eruptive process.
Good viewing conditions on 10 November revealed continued growth of the lava dome. Current estimates are that the welt, the broad area of deformation, is about 600 m in diameter. The new lava dome, which occupies the central and western parts of the welt, is about 400 by 180 m. The highest point on the new lava dome is about 250 m above the former surface of the glacier that occupied that point in mid-September. Maximum surface temperatures on the new dome remain at about 700 degrees Celsius. GPS instruments on the welt show rates of movement of up to several meters per day, while GPS instruments on the 1980-86 lava dome show movements of up to 1-2 cm per day northward, away from the growing welt and new dome.
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.