Volcanoes can be surprisingly easy to overlook. They often seem deceptively serene on the surface, and even volcanoes that are already erupting can catch us off-guard. Hawaii's Kilauea, for example, has been continuously erupting for three decades, but it still occasionally throws a curveball — as it did in the spring of 2018, when residential neighborhoods were suddenly ripped open by a series of vents, forcing hundreds to evacuate from lava fountains and dangerous volcanic gases.
There are three main sections of the U.S. that tend to experience volcanic activity, and scientists believe some of the volcanoes there may be roughly due for an eruption. Below is a brief look at these three regions, followed by a list of several specific U.S. volcanoes that pose some of the highest risks:
Alaska: Mostly located along the Aleutian Islands, volcanoes in Alaska generally make a big production out of erupting thanks to their gas-rich magma. While the region's human populations are sparse, its volcanoes still emit tall ash plumes that plague airplanes flying overhead, since the particles can clog their engines, disrupt sensitive temperature gauges, reduce visibility, and damage windows and other surfaces. Alaska has more than 40 volcanoes that are currently considered active.
Cascades: The vast majority of active volcanoes in the Lower 48 states are in the Cascade Range, a mountain chain stretching from southern British Columbia to Northern California. Like the Aleutian volcanoes, they tend to erupt explosively due to high concentrations of gas in their magma. They aren't as active as Alaska's volcanoes, but they've caused the most powerful eruptions on U.S. soil, and they're dangerously close to population hubs in California and the Pacific Northwest.
Hawaii: The third major patch of volcanoes on U.S. territory is in Hawaii — or, rather, it is Hawaii, since the islands were all formed by slow, undersea lava flows building up over millions of years. Whereas volcanoes in the Aleutians and the Cascades erupt explosively, Hawaiian volcanoes tend to ooze out lava slowly because of their lower gas content. That's on display at Kilauea, which has been erupting continuously since 1983 and makes Hawaii the only U.S. state that's still growing.
With dozens of U.S. volcanoes capable of abrupt outbursts in the near future, here is a list of some suspects to keep an eye on, courtesy of the U.S. Geological Survey's Volcano Hazards Program. Volcanic eruptions are notoriously difficult to predict in the long term, so this list isn't comprehensive. These volcanoes are, however, considered to pose serious threats whenever they do erupt:
Unlike the other volcanoes in this list, Kilauea is currently erupting — and has been doing so almost nonstop since 1983. Its slow-moving lava has been relatively harmless for much of that period, creating spectacular scenery as it gradually expands the Island of Hawaii. But Kilauea also sometimes sends lava through new vents with little warning. In 1990, for instance, it destroyed much of the town of Kalapana, and also raised concerns with a northward lava flow in 2014.
And in a more recent reminder of Kilauea's potential danger, the volcano began invading residential neighborhoods near Pahoa in the spring of 2018. A series of new eruptive vents began spewing lava into the Leilani Estates and Lanipuna Gardens subdivisions, along with dangerous sulfur gas, destroying dozens of buildings and forcing more than 1,700 people to evacuate.
"This eruption is still evolving and additional outbreaks of lava are possible," the USGS wrote in an update on May 14. "Ground deformation continues and seismicity remains elevated in the area. The location of future outbreaks could include areas both uprift (southwest) and downrift (northeast) of the existing fissures, or, existing fissures can be reactivated. Communities downslope of these fissures could be at risk from lava inundation. Activity can change rapidly."
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 U.S., which will complicate things whenever it does erupt, as Mount Rainier looms over the Seattle-Tacoma metro area and its roughly 3.7 million inhabitants.
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. Two giant lahars made it all the way to Puget Sound following a catastrophic eruption about 5,600 years ago, pushing out its shoreline by several miles. 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.
Mount Rainier 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. History is usually the best tool for predicting future volcanic eruptions — aside from the clues they give hours to months beforehand. Still, it's one of the most intensely monitored volcanoes in the U.S. due to the havoc it could wreak.
Mauna Loa, Hawaii
Mauna Loa tends to erupt at a slow, oozy pace, which has formed a wide dome. (Photo: Larry Johnson/Flickr)
The other U.S. Decade Volcano is also one of largest volcanoes on Earth: Hawaii's Mauna Loa. It may not look so huge from ground level, but if you count its long submarine flanks that depress the sea floor, its summit is actually more than 10.5 miles above its base. Like Kilauea and other Hawaiian volcanoes, it erupts at a slow, oozy pace, which has formed a wide dome. But it's near cities on the island of Hawaii like Hilo and Holualoa, which helped push it onto the Decade Volcanoes list.
Mauna Loa's 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 Rainier, it's closely monitored, and one theory suggests it's near the end of a 2,000-year cycle, with its summit lava flows poised to increase toward the northwest and southeast.
Mount St. 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. On May 18, 1980, an earthquake caused a major chunk of the volcano to cleave and slide off, forming a landslide that traveled about 14 miles down the North Fork Toutle River. The earthquake also released a blast of pressurized gas and vaporized groundwater, which exploded laterally at about 300 mph, shooting a tower of ash up 30,000 feet and knocking down trees across 230 square miles. Subsequent eruptions sent pyroclastic flows — ground-hugging avalanches of hot ash, pumice, rocks and gas — screaming down the slopes at 50 to 80 mph. Prevailing winds carried 520 million tons of ash eastward across the U.S., and Spokane was cast in "complete darkness" 250 miles away.
Following the blast came several lahars, some not until later that afternoon as the glaciers melted. 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. About 125 million cubic yards of lava erupted during that time, filling 7 percent of the 1980 crater. Although it's calmed down now, it remains an "active and dangerous" volcano, according to the U.S. Geological Survey (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.
Glacier Peak, Washington
Glacier Peak is the most remote of Washington state's five active volcanoes. (Photo: Walter Siegmund/Wikimedia Commons)
Glacier Peak is one of the most active volcanoes in the Cascades, having produced some of the area's largest eruptions, but it's thankfully also the most remote of Washington state's five active volcanoes. It and Mount St. Helens are the only volcanoes in Washington that have generated big, explosive eruptions in the last 15,000 years. Because their magma is too viscous to flow normally from the eruptive vent, it must instead be blasted out at high pressure. Expanding gas bubbles in the magma burst and break it into fragments as it rushes to the surface. These fragments are called tephra, and the smallest tephra is volcanic ash.
About 13,000 years ago, a sequence of nine tephra eruptions shot out of Glacier Peak within a few hundred years, the largest of which ejected more than five times as much tephra as the 1980 Mount St. Helens eruption. As its name suggests, Glacier Peak is also heavily ice-covered, and it has produced severe lahars and pyroclastic flows during its history as well. The volcano last erupted about 300 years ago, and since its eruptions are generally separated by several hundred to a few thousand years, the USGS says it's "unlikely that we will see an eruption within our lifetimes." Still, it keeps close tabs on Glacier Peak, which could feasibly send ash, lahars or other debris as far as Seattle, about 70 miles away.
Mount Baker, Washington
Mount Baker has proven to be less explosive and less active than some other Cascade mountains. (Photo: Steve Voght/Wikimedia Commons)
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. It erupted several times in the 1800s and has also produced dangerous pyroclastic flows in modern times — which, like lahars, don't necessarily require a full-scale eruption.
Baker gave locals a scare in 1975 when it began emitting large amounts of volcanic gases, and its heat flows 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 to erupt in the last 300,000 years, and is part of a region that's been volcanically active for 3 million years.
On May 30, 1914, Lassen Peak rumbled back to life from a 27,000-year-long siesta. It began spitting steam and lava during the next 12 months, leading to several explosions, avalanches and lahars in May 1915. Then, after two quiet days, it released its show-stopper on May 22, a climactic eruption that pumped a column of ash 30,000 feet into the air and unleashed pyroclastic flows, which devastated an area of three square miles now known simply as the Devastated Area. Pumice from the explosion extended 25 miles to the northeast, and fine volcanic ash reached as far away as Winnemucca, Nevada — about 200 miles away. The outbursts continued through 1917, and steam vents were still detectable as recently as the 1950s.
Lassen Peak is now once again dormant but remains active, posing a distant threat to some nearby cities such as Redding and Chico.
Mount Hood, Oregon
Mount Hood dominates the Cascade skyline from Portland, Oregon. (Photo: Robert DuVernet/Wikimedia Commons)
At more than 500,000 years old, Mount Hood is a moody volcano, following centuries of frequent eruptions with quiet periods that last a few centuries to millennia. 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 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.
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 a lahar that traveled the length of the Sandy River valley, pushing boulders as large as 8 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 will likely mimic this one, but a re-enactment of the blast 100,000 years ago is still possible, too. Mount Hood dominates the Cascade skyline from Portland, and while it may be too far away to hit Portland with a lahar, it could dust it with tephra or ash, as Mount St. Helens did in 1980.
Mount Shasta, California
Mount Shasta towers over the town of Weed, California. (Photo: Mark Stensaas/USGS Volcano Hazards Program)
Located just south of the Oregon-California border, Mount Shasta is a huge stratovolcano and one of the tallest peaks in the Cascades, rising 14,162 feet.
On average, Shasta has erupted at least once every 800 years during the past 10,000 years, about once every 300 years during the past 3,500 years, and about once every 250 years during the past 750 years. The last known eruption is thought to have occurred roughly 230 years ago, according to the USGS.
Future eruptions like those of the last 10,000 years will probably produce deposits of ash, lava flows, domes and pyroclastic flows, the USGS adds, and could endanger local infrastructure. "Lava flows and pyroclastic flows may affect low areas within about 15-20 km (9 to 13 mi) of the summit of Mount Shasta or any satellite vent that might become active," the agency explains. "Lahars could affect valley floors and other low areas as much as several tens of kilometers from Mount Shasta."
Three Sisters, Oregon
A labeled view of the Three Sisters, as seen from south to north from Broken Top. (Photo: USGS Volcano Hazards Program)
Oregon's Three Sisters volcanoes are commonly grouped together as one unit, but each was formed at a different time by a different type of magma. Neither the North or Middle Sister has erupted in about 14,000 years, but South Sister last erupted about 2,000 years ago, and is considered the most likely of the three to do so again.
South and Middle Sister are both recurrently active over thousands to tens of thousands of years, according to the USGS, and may erupt explosively or produce lava domes that could collapse into pyroclastic flows. Less explosive eruptions "could occur almost anywhere in the surrounding area," the agency says.
The most recent eruptions from South Sister produced tephra that fell more than 7 feet (2 meters) thick within 1 mile (2 kilometers), and spread a coating of ash as far as 25 miles (40 km) away from the vents. A new eruption could endanger nearby communities within minutes, research suggests, with a hazard zone stretching about 12 miles (20 km) in diameter.
Mount Redoubt, Alaska
Redoubt is located in Alaska's Lake Clark National Park and Preserve, where the nearly 11,000-foot-tall stratovolcano forms the tallest peak in the Aleutian Range. It began erupting about 900,000 years ago, and its present-day cone began to form about 200,000 years ago, according to the Alaska Volcano Observatory (AVO).
Redoubt has erupted at least 30 times in the last 10,000 years, including modern eruptions in 1902, 1966, 1989 and 2009. The mountain's summit crater is filled with ice that feeds a north-flowing glacier, and during the 1966 eruption, melting ice produced a type of glacial outburst flood known as a "jokulhlaup." In 2009, the volcano lurched to life for several months, at times sending ash clouds as high as 65,000 feet above sea level. Before it erupted, Redoubt triggered up to 30 earthquakes per second, blurring into a scream-like "harmonic tremor."
Crater Lake, Oregon
Oregon's Crater Lake, held by the collapsed caldera of Mount Mazama, was formed when a series of explosive eruptions rocked the volcano about 7,000 years ago. These events were some of the largest known eruptions during the Holocene, the current geological epoch that began about 11,500 years ago. They ejected tephra as far away as Canada, according to the Smithsonian Institution's Global Volcanism Program, and produced pyroclastic flows that traveled 25 miles (40 km) from the volcano.
The most recent eruption here was about 6,600 years ago, according to the USGS, which anticipates a "very high" threat potential from a future eruption at Crater Lake. Although only about 50 people are estimated to live within 6.2 miles (10 km) of the volcano, the population within 62 miles (100 km) is roughly 273,000.
Yellowstone Caldera, Wyoming
A view of Excelsior Geyser at Midway Geyser Basin in Yellowstone National Park. (Photo: Bryan Ungard/Flickr)
There is a supervolcano beneath Yellowstone National Park, capable of spewing 1,000 cubic kilometers of magma in a single eruption. That hasn't happened in more than 600,000 years, but the supervolcano is still active.
Another eruption like that is "very unlikely in the next thousand or even 10,000 years," according to the U.S. National Park Service. Still, it's unwise to ignore the risk; NASA has even considered a plan to defuse the supervolcano by cooling it with water. Beyond the immediate destruction near Yellowstone, another big eruption could release a vast blanket of ash with far-ranging effects.
First, however, there should be detectable movements of magma below the surface, a process many scientists have expected to unfold over thousands of years. Recent research suggests supervolcanoes aren't always so sluggish, however, with ancient eruptions at some calderas possibly occurring as quickly as 500 years after the earliest signs. And according to recent research of past Yellowstone eruptions, magma has sometimes moved into position only decades before an outburst.
For more information about these and other high-priority volcanoes across the U.S., check out the USGS' Volcano Hazards Program.
Editor's note: This article has been updated since it was originally published in April 2009.