Report on St. Helens (United States) — 27 October-2 November 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 27 October-2 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, 27 October-2 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 28 October to 1 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. Field work conducted on 27 October revealed several new observations. A new GPS (Global Positioning System) station on the southern part of the new lava dome moved downward and SE. A GPS station near the summit of the old lava dome had moved northward about 7 cm since 20 October. Thermal imaging showed an elongate band of elevated surface temperature, locally as great as 775° C along the W face of the new lava dome coincident with the area of exposed newly extruded lava. Gas-emission rates measured that day were similar to recent previous measurements (SO2 at about 250 tons per day, CO2 at about 300 tons per day, H2S at about 2 tons per day). In addition, samples of lava-dome rock similar in appearance to the rock of the older lava dome were collected from two localities in the vicinity of the exposed new lava. Overall, the results indicated that the character and rise of magma continued as it has over the past few weeks. CVO reported on 29 October that GPS, LIDAR (LIght Detection and Ranging), and photogrammetric measurements, in combination with visual observations over several days suggested that the lava-dome complex was spreading outward at its margins, similar to the expected behavior of a viscous lava flow. 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.