Which U.S. volcanoes are likely to erupt next?

With more than 160 active volcanoes, the United States is at greater risk from eruptions than many realize.

 

At more than 500,000 years old, Mount Hood is a moody volcano, following centuries of frequent eruptions with quiet periods that have lasted a few centuries to more than 10,000 years. It last erupted in the 1790s, a few years before Lewis and Clark reached the Pacific Northwest — the remaining lahars in 1806 led them to call the Columbia River "Quicksand River" — but its eruptions tend to be less explosive than nearby Mount St. Helens. Oregon's tallest peak has produced many debris avalanches of various sizes throughout its history, the largest of which removed the mountain's summit and big chunks of its flanks, forming huge lahars that flowed down the Columbia River.

 

The USGS identifies two past eruptions at Mount Hood that offer perspective on future eruptions. During one that occurred about 100,000 years ago, the volcano's summit and north flank collapsed, forming a lahar that swept down the Hood River valley and was still 400 feet deep at the river's mouth, where the town of Hood River now stands. The lahar continued north, crossing the Columbia River and surging up the White Salmon River valley in Washington state.

 

About 1,500 years ago, another eruption one-tenth the size of the outburst 100,000 years ago sent out a lahar that traveled the length of the Sandy River valley, pushing boulders as large as eight feet wide 30 feet above the river's normal level. The lahar spread across the delta at the Sandy River's mouth, pushing the Columbia River north. The USGS says Mount Hood's next eruption is likely to mimic this one, although a re-enactment of the blast 100,000 years ago is also still possible. Mount Hood dominates the Cascade skyline from Portland, Ore., and while it's probably not close enough to douse Portland with a lahar, it could dust it with tephra or ash, as Mount St. Helens did in 1980.