...since Mount St. Helens's massive eruption of May 18, 1980.
We actually lived in Hillsboro, OR at the time of the 1980 eruptions (there were quite a few, starting earlier than that, but May 18 was the Mother of 'em all). I remember volcanic ash covering our driveway and seeing the enormous ash clouds dominating the sky, even though the mountain was sixty miles away.
Here's an infographic that explains the geology of the event (via):
The most amazing factoid to me about Mt. St. Helens is that Spirit Lake, on the mountain's northern side, was elevated to such a degree by the 1980 eruptions that its current bottom is now higher than its pre-1980 surface level. That's the power of Earth, folks!