A lava outcrop juts from the rim of Oregon's Crater Lake. Born of a blast that expelled more than 50 times the volume of magma as the Mount St. Helens eruption 30 years ago, this watery caldera is also the United States' tenth most dangerous volcano, according to the U.S. Geological Survey (USGS).
Of the 169 geologically active volcanoes in the U.S, 54 volcanoes have USGS threat levels of "high" or worse, based on perceived explosiveness and what's at risk near the volcano.
Mother Nature, though, can reshuffle the ranking at any time. "A volcano can be quiet for a long time, and we would give it a low threat level," said John Eichelberger, coordinator of the USGS Volcano Hazards Program. "But it can surprise us."
For instance the long-gone Mount Mazama volcano cluster staged quite a surprise when it exploded 7,700 years ago—the largest Cascade Range eruption of the last hundred thousand years. Water eventually filled the resulting three-mile-wide (eight-kilometer-wide) wide crater, forming Crater Lake.
"You could look at that as a system that exhausted itself," said William Scott, a geologist at the USGS Cascades Volcano Observatory in Vancouver, Washington. "It’s been quiet for the last 5,000 years." (See Crater Lake National Park travel guide).
(Related: "Mount St. Helens Still Highly Dangerous, 30 Years Later.")
—Anne Casselman in Vancouver, British Columbia