Report on St. Helens (United States) — 20 October-26 October 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 20 October-26 October 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, 20 October-26 October 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 21-26 October, the new lava dome inside the emerging dome of St Helens continued to grow, and seismicity remained at low levels compared to early in the unrest. Geological and thermal-imaging observations on 20 October confirmed that both the area of uplift, and the new lava extrusion increased in size noticeably since last seen on 14 October. In addition, the area of uplift and intense deformation continued to move southward and was nearing the crater wall. About 0.3 m of new snow with a light dusting of ash covered much of the uplift, except for the new lava extrusion, which was steaming heavily. The new lava extrusion, which occupies the western part of the uplift, was about 275 m long by 75 m wide, 70 m high, and had a volume of almost 1.5 million cubic meters. Its maximum temperature was about 600 degrees C. Low levels of carbon dioxide, sulfur dioxide, and hydrogen sulfide were recorded on the 20th, an observation which did not rule out the continued rise of magma from depth. On 21 October a new protrusion that had a maximum temperature of about 650 degrees C.
Analyses of samples from the new lava dome collected on 20 October suggested that since lava first appeared on 11 October it has been rising more rapidly from depth and not spending much more than a few days at shallow levels before being extruded onto the surface. Reviews of several lines of evidence confirmed that the average rate of volume change for the deformed area and new lava dome between late September and mid-October was about 8 cubic meters (a typical dump-truck full) per second. According to CVO, a substantial part of that change must be magma, which suggests a rate similar to that for many other lava-dome-building eruptions.
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.