Report on St. Helens (United States) — 19 January-25 January 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 19 January-25 January 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, 19 January-25 January 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 19-25 January, 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. On 19 January, crews investigated the effects of an eruption that occurred on 16 January around 0300. During a 17-minute period, there were explosive emissions of ash and volcanic blocks from the vent area at the N end of the growing lava dome. A shower of ballistic fragments pockmarked a snow-covered area up to several hundred meters NE of the lava dome with craters up to 1 m in diameter. Ash fell thickly in E and W parts of the crater and drifted eastward over the rim depositing a thin layer of gray ash on the E flank outward for at least 3 kilometers. The scale and impact of the explosion was similar to that of 1 October 2004.
Analysis of a digital-elevation model made from photographs taken on 3 January provided new information about the size of the lava dome. Since last measured on 11 December 2004, the lava dome had maintained its 475-meter length, which was constrained by the old lava dome and crater wall, but widened from 310 to 410 m. Its highest point was 7 m higher. The entire lava dome increased in volume from 30 to 34 million cubic meters, an average rate of about 2 cubic meters per second. Based on these results CVO suggested that the rate of lava extrusion had decreased from autumn 2004 rates. 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.