Growth of the new lava dome inside the crater of Mount St. Helens has continued since the last report (BGVN 29:10), accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. During such eruptions, episodic changes in the level of activity can occur over days to months. The eruption can also intensify suddenly or with little warning and produce explosions that may cause hazardous conditions within several kilometers of the crater and farther downwind. The current status is Volcano Advisory (Alert Level 2), and the aviation color code is orange.

A small, short-lived explosive event at St. Helens volcano began at approximately 1725 hours on 8 March 2005. Airplane pilot reports indicated that the resulting steam-and-ash plume reached an altitude of about 11 km above sea level within a few minutes and drifted NE.

Results from analysis of imagery by the U.S. Geological Survey of 21 February 2005 showed that the highest part of the new lava dome stands at an altitude of 2.3 km, 160 m higher than the old lava dome, and only 28 m below Shoestring Notch, a low point on the SE crater rim. Further analysis of recent aerial photos revealed that as of 1 February, the whaleback-shaped dome extrusion was about 470 m long and 150 m wide. The new dome and uplifted welt of crater floor and deformed glacier ice have grown to a combined volume of about 38 million m³, almost one-half the volume of the old 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: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Pacific Northwest Seismograph Network (PNSN), Seismology Lab, University of Washington, Department of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310, USA (URL: http://www.pnsn.org/).