Report on St. Helens (United States) — 6 October-12 October 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 6 October-12 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, 6 October-12 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)
Following a steam-and-ash emission at St. Helens on 5 October, seismicity dropped to low levels and CVO reduced the warning from Volcano Alert (Alert Level 3: aviation color code Red), to Volcano Advisory (Alert Level 2: aviation color code Orange). During 6-12 October, seismicity was at low-to-moderate levels, with the highest magnitude earthquakes (M 2.4) occurring at a rate of one every 2 minutes on 8 October.
On 6 October only weak steam puffs were emitted from the volcano and CVO scientists confirmed that the top of the area of intense uplift was at or slightly above the highest point on the lava dome, which suggested that some uplift occurred during the period of low seismicity. Scientists also confirmed that small lahars spilled out of the crater and onto the Pumice Plain during a rainstorm the evening of 5 October. Lahars traveled a short distance toward Spirit Lake and the North Fork Toutle River. CVO received reports that a light dusting of ash from the emission on 5 October affected the eastern part of Mount Rainier National Park, ~110 km NNE of St. Helens. A new steam vent opened during the evening of 6 October, joining two that had been present for several days.
Measurements from recent photographs and LIDAR (LIght Detection And Ranging) showed that as of 7 October the intensely deformed and uplifted area on the S side of the 1980-86 lava dome was ~400 m (N-S) by ~490 m (E-W) with a maximum uplift of about 90-120 m. Additional analysis revealed that the total volume change represented by the deformation between late September and 6 October was ~16 million cubic meters. The average rate of change was ~2 million cubic meters per day. On 11 October, thermal imaging of the western part of the uplifting area revealed temperatures of 500 to 600 degrees C on a large pinkish-gray fin of rock and in nearby fumaroles and cracks. These observations are consistent with new lava having reached the surface of the uplift. A gas-sensing flight on 11 October measured fluxes of sulfur dioxide and carbon dioxide that were similar to, or slightly smaller than, those measured on 7 October.
CVO reported on 13 October, "As a result of the intense unrest of the past two and one-half weeks and recent observations, we infer that magma is at a very shallow level and is extruding onto the surface. Incandescence from hot rock or gases reflects off steam clouds and is visible from north of the volcano. During times of unrest, Mount St. Helens and similar volcanoes elsewhere typically go through episodic changes in level of unrest over periods of days to weeks, or even months. Such changes are in part driven by variations in the rate of magma movement. We expect fluctuations in the level of unrest to continue during coming days. Escalation in the degree of unrest could occur suddenly or with very little warning. There may be little time to raise the alert level before a hazardous event occurs."
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.