Report on St. Helens (United States) — 24 May-30 May 2006
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 24 May-30 May 2006
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2006. Report on St. Helens (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 24 May-30 May 2006. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
During 24-25 May, seismicity remained at levels typical of the continuing lava-dome extrusion at Mount St. Helens. On 29 May, a M 3.1 earthquake and simultaneous large rockfall occurred. An ash plume was produced at 0810 that reached an altitude of 4.9 km - 6.1 km (16,000-20,000 ft) a.s.l. according to ground observations and pilot reports. An additional pilot report suggested the plume reached an altitude of 7.3 km (24,000 ft) a.s.l. By 1308, ash from the event was no longer visible on satellite imagery. On 30 May, the rockfall was confirmed to predominantly originate from the N side of the growing fin. The volcano 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.