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.