After downslope movement of lava extruded onto the N flank of the composite dome ended about 19 May, activity was limited to ejection of vapor plumes that sometimes contained tephra. Vigorous gas emission from the top of the dome produced plumes that lasted 5-45 minutes. Old material from the walls of the vent was carried upward by the gas, and blocks 20-30 cm across fell about 125 m from the dome. On 9 June at 1316, the largest of these rose to 6 km altitude, dropping ash a few kilometers to the N and larger tephra in the crater. The accompanying high-amplitude burst of seismicity saturated nearby seismographs but appeared to consist of a series of individual events. Before 9 June, most of the plume emissions were associated with lower amplitude seismic signals, but about half of those after 9 June were accompanied by bursts of stronger seismicity. Plumes were ejected about once a day until 22 June, but none has been observed since then. Before the May extrusion episode, similar gas and tephra ejections were typically preceded by 1-3 minutes of felt earthquakes, and electronic distance measurements showed that they were accompanied by several-centimeter expansions of the dome. However, neither precursory earthquakes nor deformation were associated with the post-extrusion plume emissions. The rate of SO₂ emission had remained high for several weeks after the May lava extrusion (SEAN 07:05), but returned to the normal inter-eruption background level of about 100 t/d by early June.

Electronic distance measurements to targets on the dome and crater floor began to show [swelling] in mid to late June. The rate of [swelling] was a few millimeters per day in early July and was not accelerating significantly. Tilt stations on the crater floor also showed uplift, at a rate about twice as high 7 June-6 July as mid May-early June. The 7 June-6 July rate was similar to that observed several weeks before the May lava extrusion.

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: T. Casadevall, W. Chadwick, D. Dzurisin, USGS CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington.