Deformation of the composite lava dome, seismicity, and rates of SO₂ emission remained at background levels through early January. The maximum rate of outward movement on the N and S sides of the dome in December and early January was 5 mm/day. The rate of SO₂ emission, 10 t/d or less on 3 and 5 December, had increased slightly to 50 ± 10 t/d by the next measurement on 2 January and 25 ± 4 t/d 5 January. No tephra emissions have been observed by geologists working in the crater, nor have fresh tephra layers been found in the snowpack accumulating in the crater. Unusually good weather during the early winter has allowed the emplacement of two telemetering tiltmeters on the dome itself (tiltmeters had previously been installed on the crater floor near the dome), a telemetering strainmeter on a crack near the dome's summit, and a small trilateration network consisting of two benchmarks and five targets in the dome's summit area.

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.