Lassen Peak erupted in '15

Answer: Until Mount St. Helens awoke in 1980, Lassen Peak in California was the most recent volcano to have a significant eruption in the contiguous 48 states.

Lassen Peak, also known as Mount Lassen, is the southernmost volcano in the Cascade Range, the same string of mountains that includes St. Helens. Lassen is about 100 miles south of the Oregon-California line in the Lassen Volcanic National Park, the site of volcanic activity for millions of years. It's believed to be the Earth's largest "plug dome volcano." In this kind of volcano, the lava is too thick and gooey to spread much from the vent, and thus builds up a dome.

In May 1914, when Woodrow Wilson was president and Europe was about to experience a political explosion of its own — World War I — Lassen began a series of eruptions.

The most spectacular eruption came in 1915, when Lassen shot a cloud of debris several miles high. Smaller outbursts continued at intervals until 1921, when the volcano became quiet again. Many geologists, though, believe it will erupt again someday.

For more information about Lassen Peak and its eruptions, go to this Web site: vulcan.wr.usgs.gov/Volcanoes/Lassen/Locale/framework.html.