Report on St. Helens (United States) — 24 November-30 November 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 24 November-30 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, 24 November-30 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)
During 24-30 November, growth of the new lava dome inside the crater of St. Helens continued and was accompanied by intermittent emissions of steam and ash. Overall seismicity remained at low levels compared to that observed early in this unrest, consistent with a continuing, slow rise of magma driving uplift of the crater floor and feeding the extrusion of lava onto the surface, where it builds a dome. The overall low rates of seismicity and gas emission suggest that the lava reaching the surface is gas poor, thereby reducing the probability of highly explosive eruptions in the near term.
A shallow M 3.1 earthquake that occurred in the volcano's crater on 27 November around 0500 was the first earthquake greater than M 3 that had been recorded since the new lava dome emerged in mid-October. This and other similar-sized earthquakes during the report period represented nothing unusual in the expected sequence of events accompanying lava-dome growth.
Good weather conditions on 29 November allowed photographs to be taken of the new lava dome in the S part of the crater with a welt, or broad area of uplift. The dome was smooth and elongated due to ongoing extrusion from a vent on its N end, which lies at the S margin of the 1980-86 lava dome. The lava emerged from the vent with enough strength that it pushed earlier-extruded lava S toward the crater wall. The leading edge of the extruded lava reached the crater wall. 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.