In the latest call for posts for the Accretionary Wedge, Ron Schott asked geoscience bloggers to relate "the story of the most memorable or significant geologic event that you've directly experienced".
For me that's easy, and yet also difficult. Easy because there's really only one significant geologic event that I've directly experienced that I'd call memorable & significant, but difficult because I had just turned 6 years old and don't recall a lot of it.
On the 18th day of the month of May, 1980, the lower 48 states of the U.S. experienced the eruption of Mt. St. Helens. Obviously when the mountain starts shakin' in a serious way, you don't live to tell about it from up close. Over 50 people died from the eruption, including one USGS geologist David Johnston who was monitoring the volcano at the time. He sent word via radio just as the eruption began "This is it!", and gave his life in the study of this mountain. I lived about 100 miles north of the volcano in a small town called Bremerton, WA. I don't recall a whole lot about the event, but I do remember watching some of the news reports on TV. Reports showed video of the ash-clogged & log-jammed streams, snow plows being used to remove the ash from roads, and pictures of entire forests flattened in one direction like matchsticks. It was amazing.
The mountain had been building up prior to this, with a large bulge on the north flank. The catastrophic blast of the mountain that day occurred after the bulge over-steepened the hillside and a huge landslide removed material down the mountain, lowering the pressure on the magma below and releasing the main blast. The blast mainly came out of one side of the summit, the north face of the mountain.
The experiences of geologists from the USGS and the University of Washington monitoring the mountain at the time are documented very well in a CNN video on youtube that unfortunately I can't embed here, but here's the link: http://www.youtube.com/watch?v=3XYfBxdVDJE The video is about 7 minutes long and well worth viewing to get a bigger idea of the impact of this eruption. Also for more info on the blast itself, check out this USGS eruption fact sheet.