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The eruptions of May 25, June 12, and October 16, which were accompanied by some pyroclastic flows, left 1/8 to 1/2 inch of ash on Vancouver, Southwestern Washington, and Portland, Oregon.
The first dome of crusty, volcanic lava was observed after the June 12 eruption. This dome was destroyed by the July 22 eruption. A second dome, observed on August 8, was destroyed by the October 16 eruption and a third was observed forming on October 18. A non-explosive event occurred December 27, 1980 - January 4, 1981, adding two additional lobes to the October dome. Non-explosive eruptions beginning February 5, April 10, June 18, September 6, and October 30, 1981, added new extrusions to the pre-existing composite dome. The next eruption began March 19, 1982, with moderate explosive activity, accompanied by mudflows, followed by the extrusion of two additional lobes of lava on the dome and further small explosive events. Two subsequent non-explosive events, May 14 and August 17, added new lobes to the existing dome. The mountain remains calm with minor steam plumes and low-level seismic activity.
Missing People: 21
March 20, 1980
First earthquake of magnitude 4 reported. The number of earthquakes gradually increased during the succeeding week suggesting impending volcanic activity.
March 27, 1980
First eruption of steam and ash. Similar explosions continued until about April 22.
May 7, 1980
Eruptions of steam and ash resumed and continued until May 14.
May 18, 1980
Huge landslides and associated violent explosive eruption, accompanied by mudflows, pyroclastic flows, flooding, and extensive ash deposits.
May 25 through October 16
Series of 5 separate ash eruptions accompanied by pyroclastic flows. Lava dome destroyed by eruption on October 16.
October 18 & 19
A new lava dome grew in crater.
December 27, 1980, February 5, 1981, April 10, 1981, June 18, 1981, September 6, 1981, October 30, 1981
Last major ash eruption occurred October 16, 1980, but since that time there have been several non-explosive (minor ash) eruptions, each adding a portion to the pre-existing composite dome formed during the October eruption.
March 19, 1982 Dome-growth eruptive phase with minor explosive events and small mudflows, adding two new lobes to the pre-existing dome. Continued until April 12, 1982.
May 14, 1982, August 17, 1982
Non-explosive (one minor and one no-ash) eruptive phases, May 14-20 and August 17-23, added new lobes on the northwest and south-southwest sides of the composite dome.
February 1983 to March 1984
This period was marked by a slow, continuous addition of lava to the dome. Size of the dome, March 1984: 800 feet high, by approximately 2500 feet across. (Check with Monument Headquarters for latest dimensions.)