Report on St. Helens (United States) — 3 November-9 November 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 3 November-9 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, 3 November-9 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)
According to CVO, during 3-8 November, seismicity at St. Helens remained at a low level compared to early in the unrest. The seismicity during the report period was consistent with a continuing, slow rise of magma driving uplift of the crater floor and feeding a surface extrusion of lava. The overall low rates of seismicity and gas emission suggested that the lava reaching the surface was gas poor, thereby reducing the probability of highly explosive eruptions in the near term.
Fieldwork conducted on 4 November revealed that the elongated new lava dome, which extends S from the 1980-1986 lava dome, had undergone substantial vertical growth since 27 October. A new mass of dacite extruded upward as much as 100 m. Exposed rock faces had temperatures between 400 and 500 degrees Celsius, creating incandescence that could be seen from the N on clear nights. Samples of the dome were similar to those collected on 27 October and to lava erupted in the 1980s. Most dome growth was vertical, with only about 30 m of outward growth in some directions. Hot rockfalls and avalanches occurred from the steep new faces on the dome. The finer particulates from these deposits roiled upward within the steam plume to about 800 m above the crater rim. Consequently, the near S and SW flanks of the volcano received a notable dusting of ash. The thick glacial ice that forms a buttress on the S and E sides of the dome remained largely intact. All dome growth was contained within the St. Helens crater. A continuous GPS (Global Positioning System) station N of the volcano had moved to the S by about 2 cm since late September or early October. Aerial observations on 7 November revealed that the new lava dome continued to expand and move upward. Small aprons of rockfall debris accumulated at several sites around the new dome. CVO stated that some ash emissions may be caused by these rockfalls as collapsing hot dome lava disintegrates into smaller fragments. 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.