It's that day again, Mount St. Helens day, when the Washington volcano blew out its northern side and killed 57 people. Every year it gets a little more remote in my memory. But let me think: it's been 31 years since the 1980 eruption. The next previous eruption in the lower 48 states was 65 years before that, in California at Mount Lassen. But the 1800s were full of eruptions in the Pacific Northwest (the U.S. Geological Survey has a long timeline of them).
We really should not regard the 48 states as a safe zone for volcanism. Well, east of the Rockies it's a safe assumption, although large western eruptions can blanket vast areas downwind with ash. But travel to any state in the western cordillera and you'll see lava flows and ash beds and volcanoes themselves. The region has been volcano-infested for more than 100 million years, and the tectonics haven't changed in millions of years. The hazard and the spectacle are not restricted to Hawaii and Alaska. The need for geologic mapping and disaster planning and yes, volcano monitoring is as crucial here as it is in Hawaii, where the acid mist called vog sweeps down from Kilauea over Hilo, and in Alaska, where dozens of active volcanoes like Redoubt threaten trans-Pacific air routes.
In the lower 48 states, the 20th century has been a breather in terms of volcanism. Whether it was exceptionally calm or the 19th century was an exceptional spasm is not well determined. But I can pretty much guarantee that the 21st century will include a significant eruption in the "mainland" United States.
From the Geoblogs:
History of Geology - "Tah-one-lat-clah or St. Helens"
Clastic Detritus - "A Photo for Mount St. Helens Day"
Eruptions - "31st Anniversary"
Outside the Interzone - "31 Years Later"
En Tequila es Verdad - "The Big Ba-Boom"
Uncovered Earth - "I Think Something Happened Here..."
Hudson Valley Geologist - "Mount St Helens"
Geologic Froth - "It Was 31 Years Ago Today."