Report on St. Helens (United States) — 24 May-30 May 2006
Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 24 May-30 May 2006
Managing Editor: Sally Sennert.
Please cite this report as:
Global Volcanism Program, 2006. Report on St. Helens (United States) (Sennert, S, ed.). Weekly Volcanic Activity Report, 24 May-30 May 2006. Smithsonian Institution and US Geological Survey.
St. Helens
United States
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.
Geological Summary. Prior to 1980, Mount St. Helens was a conical volcano sometimes known as the Fujisan 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 breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it has been the most active volcano in the Cascade Range during the Holocene. Prior to 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of basaltic as well as andesitic and dacitic products from summit and flank vents. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.
Sources: US Geological Survey Cascades Volcano Observatory (CVO), Washington Volcanic Ash Advisory Center (VAAC)