After the extrusion of a new lobe and endogenous growth of the composite lava dome in late May-early June, seismicity, deformation, and SO₂ emission returned to background levels. Displacement rates began to decline on 10 June and had reached background levels by late June. As of early July the dome was stable, with maximum displacement rates of a few millimeters per day. After reaching a high of 220 ± 10 t/d on 8 June, SO₂ emission had decreased to a background level of 60 ± 5 t/d by the 12th, and remained between 25 ± 5 and 45 ± 5 t/d through early July.

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, J. Sutton USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.