Although deformation of the crater floor has typically begun a few weeks after past extrusion episodes, USGS monitoring had shown few changes within the crater by early February. The small crater-floor thrust faults produced by past expansions of the dome had shown no significant movement as of 10 February, nor had there been any outward movement of the N-crater rampart or inflation of the edifice as a whole. However, telemetry from a crater-floor tiltmeter 1 km N of the dome resumed 4 February after a 3-month hiatus and showed 17 µrad of inflation in 4 days. The dome continued to spread very slowly outward through December and January, at 1-3 mm/day. The rate of SO₂ emission remained low in January, usually ranging from 50 to 100 t/d, but increased steadily during the first 8 days of February from about 60 to 130 t/d.

During the first half of January, there were only three seismic events large enough to be recorded on more than one seismograph of the Mt. St. Helens net. However, during intermittent periods of increased seismicity in the last 2 weeks of January, several events per day were recorded. These active periods lasted as long as 3-4 days before seismicity declined to background levels. A total of 27 earthquakes in the Mt. St. Helens area, a few as large as M 1, were recorded during the second half of the month. Similar activity continued into early February.

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. Dzurisin, J. Ewert, D. Swanson, USGS CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.