Mt. St. Helens remained quiet as of 10 March, as it has since the end of the lava extrusion episode of 5-7 February. The February lava approximately doubled the volume of the composite dome in the crater, adding about 5 x 10⁶ m³ of new material to the 1.5 x 10⁶ m³ extruded 18-19 October and the 3.5 x 10⁶ m³ extruded 27 December-4 January. All of the pre-existing dome, except for a portion of the December-January SE lobe, was covered by the February lava. Between 8 and 21 February, the February lobe spread 12 m while sagging 3 m, resulting in dimensions for the new lava of 281 m in E-W direction and 119 m in maximum height above the crater floor.

Low-frequency volcanic earthquakes associated with the February lava extrusion ended 9 February. Occasional bursts of seismicity continued to be recorded. One, on 10 February at 0915, coincided with the emission of a steam cloud, containing a minor amount of ash, that rose to 4 km altitude. Field crews reported hearing a boom prior to this event. Some rock-avalanche events were also recorded after dome emplacement ended. A M 5.5 tectonic earthquake occurred late 13 February about 12 km N of Mt. St. Helens. As of 28 February, about 175 aftershocks stronger than M 1 had been recorded. Through the end of February, seismographs continued to record a few rock-avalanche events and bursts of seismicity of the type that has sometimes been associated with steam explosions. Clouds prevented observations of the crater for much of February, but clear weather on the 26th revealed evidence of numerous minor steam explosions on the N side of the lava dome.

Geodetic measurements showed a few centimeters of horizontal contraction of the Mt. St. Helens edifice between 4 February and 5 March. No significant movement of the N crater rampart occurred after the early February dome emplacement, nor has there been any measurable deformation of the crater floor during this period.

The following, from W. G. Melson, is based on microprobe analyses performed on the 1980-81 eruptives. "The SiO₂ content of the essential ejecta underwent a slight increase in the 7 August eruption, peaked in the 17-19 October eruption, but remained lower than for the 18 May tephra. This temporarily reversed a prior trend toward more basic compositions, which resumed with the December-January and February dome enlargements. CaO, FeO, and MgO show an inverse relationship to SiO₂ (figure 15), an expected relationship in a 'normal' fractionation sequence."

Figure 15. Change of SiO2, CaO, FeO, and MgO as a function of time of eruption at Mt. St. Helens. Electron microprobe analyses of fused powders were performed at SI by W. Melson, J. Nelen, and T. O'Hearn. Each data point value is the average of the following number of individual analyses of essential ejecta: 18 May, 9d; 25 May, 11; 12 June, 9; 22 July, 7; 7 August, 10; 17-18 October, 11; December-January dome enlargement, 6; February dome enlargement, 1 (sample from D. Swanson, USGS). Analytical precision for each analysis is about a 2 of: SiO2=0.62, FeO* (all Fe as FeO)=0.43; MgO=0.33, CaO-0.17.

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, C. Newhall, USGS, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA; W. Melson, SI.