Report on St. Helens (United States) — 8 December-14 December 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 8 December-14 December 2004
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2004. Report on St. Helens (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 8 December-14 December 2004. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
During 8-14 December, growth of the new lava dome inside the crater of St. Helens continued and was accompanied by intermittent emissions of steam and ash. Overall seismicity remained at low levels compared to that observed early in this unrest, consistent with a continuing, slow rise of magma driving uplift of the crater floor and feeding the extrusion of lava onto the surface, where it builds a dome. The overall low rates of seismicity and gas emission suggested that the lava reaching the surface was gas poor, thereby reducing the probability of highly explosive eruptions in the near term.
A reading on 8 December from a single GPS (global positioning system) station high on the outer SE flank of the volcano showed about 2 cm of progressive southeastward movement over the previous 3 weeks. According to CVO, this minimal movement was apparently in response to new lava impinging on the SE crater wall and is an expected consequence of the nature of the dome growth occurring in the crater. Aerial views of the crater on 11 December showed that the new lava dome was becoming increasingly fractured as a new, ambiguous, pattern of growth began to emerge. St. Helens remained at Volcano Advisory (Alert Level 2); aviation color code Orange.
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.