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Report on St. Helens (United States) — January 1987

St. Helens

Scientific Event Alert Network Bulletin, vol. 12, no. 1 (January 1987)
Managing Editor: Lindsay McClelland.

St. Helens (United States) Background seismicity, deformation, SO2 emission

Please cite this report as:

Global Volcanism Program, 1987. Report on St. Helens (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 12:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198701-321050

St. Helens

United States

46.2°N, 122.18°W; summit elev. 2549 m

All times are local (unless otherwise noted)

Seismicity, rates of ground deformation in the crater, and gas discharge remained at background levels throughout January. SO2 discharge was 35 plus or minus 10 t/d on 8 January, 30 plus or minus 5 t/d on 15 January, and 15 plus or minus 5 t/d on 21 January. There were no ash emissions or explosions from the lava dome.

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.

Information Contacts: D. Swanson and J.Sutton, CVO.