March 27 to April 18, 1980From here, and also mentioned here:
Earthquakes and steam-driven explosions occur off and on during this period.
Seven days after the initial earthquake, March 27, 1980, a loud boom was widely heard by many residents of Southwest Washington and aerial observers noted a dark dense column of volcanic ash rising through the clouds, eventually reaching a height of 6,000 feet above the volcano.I had arrived in Corvallis shortly before this first venting, and was riveted. For that spring term, I was living in a dorm on campus. There was a newspaper box outside the front door, and most days I dropped the coinage to get a copy of the Oregonian, just so I could go through and look for news on The Volcano.
A quick search on teh googler turns up a bit more information that confirms what I had remembered: these first blasts did not involve any juvenile rock, but were steam blasts created as magma encountered groundwater. Ashy material was actually pulverized pre-existing rock, not freshly extruded lava.
In early afternoon on March 27, a loud explosion was heard from the direction of Mt. St. Helens. Although the volcano was shrouded in clouds, a summit eruption was verified by a news team from the Vancouver Columbian. As they circled the summit in an airplane, they spotted a dense column of ash rising through the clouds to a height of about 2000 m. As the weather cleared later in the day, a new crater was visible, with a diameter of about 70 m, and snow on the summit area was covered by a thin veneer of dark ash. The summit eruption on March 27 was typical of several small eruptions that would occur through April and early May. None of these eruptions were magmatic in character, but instead they were steam eruptions generated by the heating of groundwater above a rising plug of magma that had invaded the central conduit of the volcano.In the photo above, taken from the north side, you can see the fissure and the summit crater. Again, relying on ancient memories, I don't believe the bulge was identified (or at least announced) until some days later. But that crater, crack and ashy material were very visible. (below, looking east, both from March 27, 1980) In the days to come, the fissure would open wider; the summit depression eventually dropped into it completely. And the bulge on the north face would grow ominously larger...
The March 27 eruption generated a huge east-trending fissure high on the north side of the summit. It extended down both sides of the volcano over a distance of about 1500 m. Another, less extensive fracture system had developed farther down the north flank of the volcano, parallel to the higher fracture. Measurements showed that the region between the two fractures had expanded outward to produce a huge north-flank bulge. The bulge was verified by a US Forest Service aerial spotter who reported seeing both fractures open and close as the north flank bulged upward during the hours immediately following the steam eruption.
We had been served notice that the beast was alive.
(USGS photo archive- larger sizes available)
Followup: I just checked; 3/27/80 was a Thursday. This sounds about right. I had arrived at OSU by Greyhound bus the previous Sunday, which would have been 3/23. I don't think I found out about this blast until the next day, though, when it was on the front page of every PNW newspaper.