Report on St. Helens (United States) — July 1998
Bulletin of the Global Volcanism Network, vol. 23, no. 7 (July 1998)
Managing Editor: Richard Wunderman.
St. Helens (United States) Earthquakes, but CO2 flux returns to normal
Please cite this report as:
Global Volcanism Program, 1998. Report on St. Helens (United States). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 23:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199807-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
The rate of earthquake activity, which accelerated markedly from May through mid-July, (BGVN 23:05 and 23:06) returned in August to a level similar to that of last winter. The number of well-located earthquakes in July was 445, compared to 318 in June, but most of the earthquakes that took place during July occurred during the first three weeks of the month. The average rate for the first two weeks of August was only about four well-located earthquakes per day. Several temporary increases in earthquake activity have occurred since the last dome-building eruption in October 1986. This recent episode was the most intense.
Airborne gas surveys revealed that magmatic carbon dioxide (CO2) decreased since June. However escaping CO2 was still measurable. The CO2 was probably being released from magma that entered the magma reservoir during the past few months. The reservoir's top was estimated to be about 7 km below the crater. Because CO2 is heavier than air, it can concentrate in surface depressions on the dome or crater floor, especially under calm conditions, and pose an asphyxiation hazard. Poorly ventilated cavities, such as caves in the mass of snow and ice behind the dome, could be hazardous.
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.
Information Contacts: Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: http://volcanoes.usgs.gov/); Geophysics Program, University of Washington, Seattle, WA 98195 USA (URL: http://www.geophys.washington.edu/SEIS/PNSN/HELENS/).