After the 12 June explosion, a lava dome began to grow in the crater, accompanied by occasional weak bursts of harmonic tremor. No significant explosions have taken place since the start of dome emplacement, nor has the dome growth generated any nuées ardentes.

Richard Janda calculated that about 2 km³ of debris was emplaced in the N fork of the Toutle River and Spirit Lake by the 18 May eruption. Richard Waitt estimated the total 18 May airfall component as 1.1 km³ of unconsolidated material, with a mean bulk density of 0.5 g/cm³. Poor weather has prevented the aerial photography necessary for the production of a topographic map of the post-18 May volcano. After a map is produced, the amount of material removed from the edifice on 18 May can be accurately determined.

After the 25 May eruption most of the few individual earthquakes recorded on local seismographs were centered 10-15 km N of Mt. St. Helens and were thought likely to be related to regional crustal stresses rather than volcanic activity (figure 11). Harmonic tremor continued, increasing in amplitude early 3 June, but dropping to a very low level by 5 June before ending shortly thereafter. A persistent steam column rose from the crater to altitudes of 3-4.5 km in early June. A small amount of ash was occasionally included in the lower portion of the plume. Improved visibility on 10 June revealed that a roughly crescent-shaped lake, about 300 m long and 100 m wide, had formed on a portion of the [S] part of the crater floor within the past 5 days. Evidence of recent rock avalanches from the crater walls was also visible.

At about 1300 on 12 June, Steven Malone noted the onset of weak harmonic tremor and an increase in small discrete events on seismographs operated by the University of Washington USGS. A marked increase in harmonic tremor amplitude occurred at 1905 and an eruption plume rising to 4 km altitude was observed 5 minutes later. Portland airport radar tracked the eruption column to 11 km altitude shortly thereafter. Eruptive activity fluctuated in intensity for the next 2 hours. Harmonic tremor continued at roughly the same amplitude until about 2000, when it declined to a low level. A dramatic increase in tremor amplitude at 2111 was accompanied by an eruption column that quickly reached almost 16 km altitude, producing pulsating echoes on Portland airport radar. An explosion was heard more than 200 km away in Toledo, Oregon. Activity remained vigorous until about midnight, when cloud height and tremor amplitude began to decline. By 0200, the altitude of the top of the plume had dropped to about 4.5 km.

Infrared images returned by NOAA's GOES West weather satellite show the eruption cloud drifting SW. Marble-sized pumice and ash fell on Cougar accompanied by a strong sulfur odor. Ashfall in Portland began about 2300, reaching accumulations of around 0.5 cm. Infrared imagery shows that the cloud reached the Pacific coast of N Oregon by 2330 and continued to drift SW for several more hours, before becoming impossible to distinguish from heavy weather clouds in the area. Ash fell on more than 11,000 km² of NW Oregon and SW Washington, reaching Lake Oswego, Oregon, 110 km SW of Mt. St. Helens.

About 1,500 people were evacuated from a zone within about 30 km of Mt. St. Helens. No deaths or injuries were reported near the volcano. Portland airport was closed for several hours during the night, but few flights were affected. Rain before and after the ashfall in the Portland area turned the ash into a slippery mud, as it did to much of the 25 May ash. However, blowing ash continued to cause occasional disruptions to travel more than a week after the eruption.

Several pumice and ash flows were emplaced on the N flank by the 12 June eruption, one stopping roughly 20 m from the shore of Spirit Lake, which was still partially covered by floating debris. The deposits ranged from 2-10 m in thickness, and internal temperatures as high as 600°C were measured.

The explosions of 25 May and 12 June were of similar size. Both sent most of their ash toward the heavily populated areas W of the Cascade Range, although analysis of wind directions at various altitudes indicates that prevailing winds blow toward the E half of the compass almost 90% of the time (Crandell and Mullineaux, 1978).

On 15 June, USGS personnel confirmed the presence of an active lava dome, roughly 200 m in diameter and 40 m high, at the bottom of the crater. Water vapor and other gases rose from a shallow lake surrounding the dome. Red glow was visible at night through cracks in the dome's surface. Poor weather often made observations of the dome and its rate of growth difficult. Clear weather on 18 and 19 June revealed that the dome was growing upward about 6 m/day, reaching a height of 65 m by the 19th. Harmonic tremor had stopped by 15 June and did not resume until two episodes of very weak tremor, lasting 30 and 65 minutes, were recorded during the night of 24-25 June. Sporadic tremor continued through late June, but very few discrete earthquakes were recorded. Clear visibility 28-29 June indicated that no vertical dome growth had occurred since 19 June. The dome was about 200 m in diameter and was crossed by a 12 m-long fissure in which cherry red incandescence could be seen.

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: R. Christiansen, D. Peterson, R. Waitt, USGS, Menlo Park, CA; R. Tilling, USGS, Reston, VA; D. Mullineaux, D. Crandell, USGS, Denver, CO; R. Janda, USGS, Tacoma, WA; A. Krueger, NOAA/NESS; S. Malone, R. Crosson, E. Endo, Univ. of Washington; UPI; AP; New York Times.