Report on St. Helens (United States) — 13 October-19 October 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 13 October-19 October 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, 13 October-19 October 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 13-18 October, seismicity was at low levels at St. Helens and growth of the new lava dome inside the volcano's crater continued. Gas-sensing flights on 13 October detected low levels of sulfur dioxide and hydrogen sulfide, but no carbon dioxide. Measurements of flow rate and temperatures in streams draining the crater showed no significant change from late September values. On 14 October, visual observations and thermal imaging of the crater showed enlargement of both the section of intense deformation and uplift on the S side of the 1980-86 lava dome, and the new lobe of lava in the W part of that area. Temperatures of almost 700 degrees C were measured in parts of the new lobe from which ash-rich jets rose tens of meters. Abundant steam continued to rise from the area of lava extrusion to the crater rim, where it dispersed southwestward in strong winds. On the 14th only sulfur-dioxide flux was detected, and there was no detectable carbon dioxide or hydrogen sulfide. Thermal imaging showed slow extrusion of lava continuing on the 14th. Global Positioning System (GPS) measurements on 15 October continued to indicate only minor deformation of the northern part of the 1980-86 dome and no deformation of the volcano's outer flanks. Parts of the area of uplift and new lava dome were higher than when previously seen on 14 October. On the evening of 16 October, rainfall triggered a small debris flow that traveled N from the crater and changed rapidly into a muddy stream flow within 5 km of the volcano. Through 18 October, the level and character of seismicity was consistent with a continuing rise of magma driving uplift of the crater floor and feeding the surface extrusion of lava. According to CVO, low rates of seismicity and gas emission suggested that the lava reaching the surface was gas poor.
CVO stated on 18 October that as long as this eruption is in progress, episodic changes in the level of activity can occur over days, weeks, or even months. Increase in the intensity of eruption could occur suddenly or with very little warning and may include explosive events that produce hazardous conditions within several kilometers of the volcano. Small lahars (volcanic debris flows) could suddenly descend the Toutle River valley if triggered by heavy rain or by interaction of hot rocks with snow or glacier ice. These lahars pose a negligible hazard below the Sediment Retention Structure (SRS), but could pose a hazard to people along the river channel upstream of the SRS. At this time of year, it is not unusual for rivers draining the volcano to contain high concentrations of sediment that turn the water murky. Although considered less likely at this time, the current eruptive activity could evolve into a more explosive phase that affects areas farther from the volcano and sends significant ash thousands of feet above the crater where it could be a hazard to aircraft and to downwind communities.
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.