The 1980 Mount St. Helens Eruption

Only two significant Plinian volcanic eruptions occurred in the contiguous 48 states during the 20th Century. The second and larger of that pair happened on May 18, 1980, when Mount St. Helens in southwest Washington roared to life after 123 dormant years. You can read more about that eruption here:
1980 eruption of Mount St. Helens

For the record, the other Plinian eruption mentioned in the first paragraph was the 1915 eruption of Lassen Peak, further south in Cascades from the northern ultras of British Columbia, Washington, and Oregon.

The 1980 eruption was a bad event for the Pacific northwest, but in retrospect it could've been much worse. On March 18 an earthquake swarm began, which reached a crescendo on the 27th with an earthquake measuring 5.1 on the Richter scale topping off 174 shocks noticeable to human feet. Later that day the first eruption leading to the big event of May began, sending ash as far as Bend, Oregon and Spokane. Clearly something wicked was coming this way, and Washington governor Dixy Lee Ray ordered the zone immediately around the mountain evacuated of residents and commercial logging activity on April 30.

As magma moved within the mountain, the north face began to bulge outward. By mid-May the northern cryptodome extended 400 feet beyond the volcano's original contours and contained about 0.13 cubic kilometers of magma. It kept growing at five to six feet per day, but visible eruptions ceased on May 16. Those evacuated from the red zone grew impatient, and the next day 50 cars'-worth of residents were allowed to return to gather their belongings. A second trip was scheduled for 10 AM the next morning, a Sunday in 1980, and limited logging operations were to resume on the following Monday.

"Lucky" would be an odd way to describe the timing of an eruption that killed 57 people, but given that a two hour delay in the eruption timing would probably have tripled the death toll and that a 24 hour delay would've put 300 additional loggers in the blast zone, the geological scheduling seems vaguely miraculous. Western Washington's weather can be dodgy in the month of May, but the 18th was a beautiful day 33 years ago and the photography captured of the eruption was some of the century's most spectacular imagery.

Mount St. Helens's eruption wasn't the largest of the 1900s, but it still an event of terrific magnitude. The energy released was larger than that of the largest thermonuclear weapon ever tested by the United States. The blast removed over a thousand feet from the peak of the mountain, sent pyroclastic flows into the Toutle River valley faster than the speed of sound, and sent ash 12 miles into the air that fell as far away as Oklahoma. As comforting as the notion is that humans are really in control of our own affairs on Earth, clearly that's just not how it works.