Report on St. Helens (United States) — 12 January-18 January 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 12 January-18 January 2005
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2005. Report on St. Helens (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 12 January-18 January 2005. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Lava-dome growth continued at St. Helens during 12-18 January. Seismicity continued at a very low level. A large slab on the west side of the dome collapsed and generated a small rock avalanche and ash cloud that drifted over the south crater rim. A bright glow on the VolcanoCam seen the night of 13 January was likely caused by this event.
New instrumentation packages installed on and near the new lava dome on 14 January, including a video camera, gas sensor, GPS, and seismometer, stopped transmitting data early on 16 January. Analysis of seismic and other data from about 0300 on 16 January, when two instruments on and near the new dome ceased functioning, suggests that a steam and ash emission occurred, perhaps accompanied by ejection of ballistic fragments. The event lasted about 18 minutes. During that time radio-telemetry signals from a few other instruments in the crater were interrupted temporarily, probably as the result of ash in the air. In the 24 hours prior to the event, the GPS on the north end of the new dome moved southward and upward more than 8 m, showing that dome extrusion continues at a vigorous pace. 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.