Report on St. Helens (United States) — 6 April-12 April 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 6 April-12 April 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, 6 April-12 April 2005. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
During 6-11 April, growth of the new lava dome inside the crater of Mount St. Helens continued, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. Observations on 6 April revealed that the smooth whaleback-shaped portion of the growing lava dome was broken by numerous fractures, and the edges had crumbled greatly. Several deep gashes on the E, N, and W sides frequently produced rockfalls and accompanying ash clouds. On 10 April the new dome continued to fracture and spread laterally. As a consequence, the dome's summit has dropped by a few tens of meters during the previous 2-3 weeks, except for isolated high-standing remnants.
Earthquakes steadily decreased in magnitude and number over the week. A GPS receiver 200 m N of the new dome crept steadily NNW about 10 cm per day. The combination of the GPS measurements adjacent to the lava dome and the qualitative estimate of lateral spreading suggested that extrusion of new lava continued during the report week. 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.