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

Mount Rainier, Washington (Photo: USGS)

When the smell of rotten eggs began wafting through Alaska's Lake Clark National Park in late summer of 2008, scientists with the Alaska Volcano Observatory knew it meant trouble. And they weren't taking any chances with nearby Mount Redoubt, a 10,120-foot volcano that only gave 24 hours' notice the last time it erupted in 1989, its only previous eruption in recorded history.

The scientists were right to be concerned. Although Redoubt's buildup was slower this time around, it finally erupted on the night of March 22, sending a plume of ash 60,000 feet above sea level.

It's the latest reminder of the threats posed not only by Alaska's 50 historically active volcanoes, but also by more than 100 other active U.S. volcanoes in the Cascade and Hawaiian ranges, nearly all of which inhabit the Pacific Ring of Fire.

With increased seismic activity around the Pacific Rim in 2009, you may be wondering if a volcano is about to blow its top next. The Mother Nature Network scans the country for the volcanoes most likely to erupt in the near future.

Volcanic eruptions are notoriously difficult to predict in the long term, so this list isn't comprehensive. These six volcanoes are, however, considered to pose serious threats whenever they do eventually erupt again.

Mount Rainier, Washington

The highest peak in the Cascade Range is also a volcano loaded with the most glacier ice of any mountain in the contiguous United States, which will complicate things whenever it does erupt because erupting through ice tends to create lahars (volcanic mud flows that form when hot gas, rocks, and lava melt ice and churn up a superheated slurry).

The U.S. Geological Survey calls Mount Rainier "potentially the most dangerous" mainland U.S. volcano because in addition to all that ice, it looms over the Seattle-Tacoma metro area and its 3.2 million inhabitants.

Mount Rainier's potential volatility and its proximity to large cities helped make it one of two U.S. Decade Volcanoes, a group of 16 volcanoes worldwide that U.N. delegates deemed especially dangerous to human populations. It last erupted in the 1840s, and larger eruptions occurred as recently as 1,000 and 2,300 years ago. It's now considered active but dormant.

Still, it's one of the most intensely monitored volcanoes in the United States due to the havoc it could wreak.

Mauna Loa, Hawaii

The other U.S. Decade Volcano is also the largest volcano on the planet: Hawaii's Mauna Loa. Its last eruption was in 1984, when the lava flow reached to within four miles of Hilo, a city of more than 40,000.

It's an especially active volcano, having erupted 33 times in recorded history -- the two largest were in 1950 and 1859, and one in 1880-81 covered land now in Hilo's city limits. Like Mount Rainier, it's also closely monitored, and one theory suggests it's currently at the end of a 2,000-year cycle, with its summit lava flows poised to increase toward the northwest and southeast.

Mount Saint Helens, Washington

About 50 miles south of Mount Rainier sits the shell of Mount St. Helens, the scene of one of the worst volcanic eruptions in U.S. history, which took place on May 18, 1980. Fifty-seven people and thousands of animals were killed in all, and damages topped $1 billion. 

Mount St. Helens reawakened in 2004, when four explosions blasted steam and ash 10,000 feet above the crater. Lava continued gurgling out and forming a dome on the crater floor until late January 2008.

Although it's calmed down now, this remains an "active and dangerous" volcano, according to the USGS, and history shows it's been relatively active since the Middle Ages, including a blast in 1480 that was five times stronger than the 1980 eruption.

Mount Baker, Washington

After Mount Rainier, Mount Baker is the most glaciated mountain in the Cascades, supporting more ice than all the range's other peaks combined, aside from Rainier. This means it presents many of the same mudslide dangers as Rainier, although 14,000 years of sediments show Baker to be less explosive and less active than some other Cascade mountains.

Baker gave locals a scare in 1975 when it began emitting large amounts of volcanic gases, and heat flows around the mountain increased tenfold, but the feared eruption never happened. The fumarolic activity still continues, but there's no evidence it's tied to the movement of magma, which signals an eruption may be imminent.

Lassen Peak, California

The southernmost active volcano in the Cascades, Lassen Peak has one of the most massive lava domes on earth, totaling half a cubic mile. It's the largest of more than 30 volcanic domes in Lassen Volcanic National Park that have erupted in the last 300,000 years, and it's part of a region that's been volcanically active for more than 3 million years.

Lassen Peak is now dormant but remains active, posing a distant threat to some nearby cities such as Redding and Chico.

Mount Hood, Oregon

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.

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.

The USGS identifies two past eruptions at Mount Hood that offer perspective on future eruptions. Mount Hood dominates the Cascade skyline from Portland, OR, and while it's probably not close enough to douse Portland with a volcanic mudflow, it could dust it with tephra or ash, as Mount St. Helens did in 1980.

Russell McLendon is an associate editor at the Mother Nature Network, where a version of this post originally appeared.

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