Report on St. Helens (United States) — 31 October-6 November 2001
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 31 October-6 November 2001
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2001. Report on St. Helens (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 31 October-6 November 2001. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Beginning on 2 November at about 1800 a swarm of approximately 200 very small, shallow earthquakes was detected for at least 24 hours beneath Mount St. Helens. The earthquakes had magnitudes less than 0, occurred at depths less than 1 km, and were mostly in or under the lava dome's N flank. Such earthquakes are common at Mount St. Helens, but a swarm with so many earthquakes had not been recorded for several years. Cascades Volcano Observatory (CVO) scientists were uncertain what caused the earthquakes, but they suggested that increasing ground water levels due to autumn rain could have caused slippage on fractures in and below the lava dome and crater floor. CVO stated that the probability of small landslides, debris flows in the crater, and steam explosions is enhanced during these periods. Larger-scale eruptions are unlikely without significant additional precursory activity.
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.