Report on St. Helens (United States) — 7 November-13 November 2001
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 7 November-13 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, 7 November-13 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)
The swarm of tiny earthquakes that began at Mount St. Helens on the evening of 2 November ended by noon of 4 November. During this period ~2,000 earthquakes occurred that may have been related to increasing ground water levels due to autumn rain. Most of the earthquakes had magnitudes less than 0. The largest event, M 1.9, occurred shortly before noon on the 4th. All located events were shallow (< 2 km) and in or below the lava dome or crater floor near the dome. Most of the events were too small to locate accurately. After the main swarm, during 4 November to at least 10 November, about 10 small, shallow earthquakes occurred per day.
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.