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 these volcanoes may be roughly due for an eruption. In an October 2018 update to its National Volcanic Threat Assessment, the U.S. Geological Survey ranked 18 volcanoes as "very high" threats, based on a combination of their eruptive history, recent activity and proximity to people.

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:

Kilauea, Hawaii

lava flow from Kilauea A Kilauea lava flow emerges from a fissure near Pahoa on May 13, 2018. (Photo: USGS/Getty Images)

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.

Mount St. Helens, Washington

Mount St. Helens in Washington When Mount St. Helens erupted in 1980, 57 people and thousands of animals died. (Photo: U.S. Geological Survey)

About 50 miles northeast of Portland, Oregon, sits the shell of Mount St. Helens, home to 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. Winds carried 520 million tons of ash east across the U.S., and Spokane was cast in "complete darkness" 250 miles away.

Following the blast came several lahars — volcanic mud flows that form when hot gas, rocks and lava melt ice and churn up a superheated slurry. Some didn't occur until later that afternoon, as the mountain's 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.

Mount Rainier, Washington

Mount Rainier volcano overlooking Seattle Mount Rainier in Washington looms over the Seattle-Tacoma metro area. (Photo: f11photo/Shutterstock)

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.

As Mount St. Helens demonstrated in 1980, volcanoes that erupt through ice tend to create lahars. Two giant lahars from Mount Rainier 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.

Mount Redoubt, Alaska

Redoubt Volcano An aerial view of Redoubt during its eruption in April 2009. (Photo: R.G. McGimsey/AVO/USGS)

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."

Mount Shasta, California

Mount Shasta volcano, 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."

Mount Hood, Oregon

Mount Hood dominates the Cascade skyline from Portland, 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.

Three Sisters, Oregon

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.

Akutan Peak, Alaska

Mount Akutan volcano in Alaska A blanket of snow covers Akutan Peak, also known as Mount Akutan, during winter. (Photo: Delta Whiskey/Flickr)

Akutan Island, part of Alaska's Aleutian Arc in the Bering Sea, is home to several coastal villages and a large fish-processing facility, according to the Smithsonian Institution's Global Volcanism Program (GVP). It's also home to Akutan Peak, a stratovolcano that rises 4,274 feet (1,303 meters) above the island.

Akutan is one of the most active volcanoes in the Aleutians and Alaska in general, with more than 20 eruptions recorded since 1790. It erupted 11 times between 1980 and 1992, according to the AVO, and although no new eruptions have occurred since, there are ongoing hints of activity. A seismic swarm took place in 1996, for example, causing minor damage and prompting some residents and employees of the fish-processing plant to evacuate the island. There are still active fumaroles and hot springs at Akutan, and the AVO has reported "noteworthy seismicity" multiple times this century, including more than 100 seismic events in 2008.

Makushin Volcano, Alaska

Makushin Volcano, Unalaska Island, Alaska This satellite image shows Unalaska Island, with Makushin forming a prominent snowy area near the top. (Photo: NASA)

Just southwest of Akutan is the much larger Unalaska Island, whose ice-covered Makushin Volcano is also one of the most active in Alaska. Although it stands roughly 6,000 feet (1,800 meters) tall, its broad, dome-like structure "contrasts with the steep-sided profiles of most other Aleutian stratovolcanoes," the GVP notes. The volcano shares Unalaska Island with the town of Unalaska, a main population center in the Aleutian Islands inhabited by more than 4,000 people.

Makushin has erupted explosively many times in the last several thousand years, sometimes generating pyroclastic flows and surges. One eruption roughly 8,000 years ago, for instance, had an estimated Volcanic Explosivity Index (VEI) score of 5 — the same score given to Mount St. Helens in 1980 and to Italy's Mount Vesuvius in the year 79. There have been many small-to-moderate eruptions at Makushin since 1786, most recently in 1995 with a VEI score of 1, and the volcano still features high-temperature geothermal areas on its summit caldera and eastern flanks.

Mount Spurr, Alaska

Mount Spurr volcano, Alaska An eruption from Mount Spurr could threaten Anchorage and disrupt air travel. (Photo: USGS)

Mount Spurr is the highest volcano in the Aleutians, standing more than 11,000 feet (3,350 meters) tall. It's located about 80 miles (130 km) west of Anchorage — the most populous city in Alaska, and part of a metro area with roughly 400,000 people.

The volcano is known to have erupted several times in the last 8,000 years, according to the GVP, including modern eruptions in 1953 and 1992, both with VEI scores of 4. Both of those eruptions came from the youngest vent of Mount Spurr, known as Crater Peak, and both deposited ash on the city of Anchorage. On top of the threat it poses to Anchorage, Mount Spurr also shares many Alaskan volcanoes' potential to disrupt air travel by spewing tall ash clouds into major trans-Pacific aviation routes.

Lassen Peak, California

Lassen Peak in California This photo of Lassen Peak in California shows the northeast side of the peak and the area devastated by mudflows and a lateral blast in 1915. (Photo: U.S. Geological Survey/Wikimedia Commons)

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

Augustine Volcano, Alaska

Augustine Volcano, Alaska, erupting in 2006 A gas plume rises from Augustine Volcano during its most recent eruption in January 2006. (Photo: Cyrus Read/AVO/USGS)

With nearly two dozen known eruptions during the current Holocene Epoch, Alaska's Augustine Volcano is the most historically active volcano in the eastern Aleutian Arc. It forms the uninhabited Augustine Island in the southwestern Cook Inlet, which is composed almost entirely of deposits from past eruptions.

Augustine has erupted several times in the past century alone, including confirmed eruptions in 1908, 1935, 1963-'64, 1971, 1976-'77, 1986 and 2005-'06. The most recent eruption began in December 2005, starting "with a series of 13 short-lived blasts over 20 days that sent pyroclastic flows; snow, rock and ice avalanches; and lahars down the volcano's snow-clad flanks," according to a USGS report, which also notes that ash clouds drifted hundreds of kilometers downwind. This explosive activity eventually gave way to lava flows that continued into February 2006, until the volcano finally quieted down in April. "The eruption resulted in ash fall on many south-central Alaskan communities," the USGS points out, "and disrupted air traffic in the region."

Newberry Volcano, Oregon

Newberry Volcano caldera, Oregon A view of Newberry Caldera from Paulina Peak, the volcano's highest point. (Photo: Kojihirano/Shutterstock)

Oregon's Newberry Volcano covers about 617 square miles (1,600 square km) in the eastern Cascades, roughly the size of Rhode Island, making it one of the largest volcanoes in the contiguous United States. The shield-shaped volcano has a large summit caldera spanning 17 square miles (44 square km), which contains two lakes, Paulina Lake and East Lake. The area is protected as Newberry National Volcanic Monument, located within Deschutes National Forest.

The volcano dates back at least 500,000 years, and has erupted at least 11 times since the early Holocene Epoch, according to the GVP. Although it hasn't erupted for centuries, the USGS considers it an active volcano with a "very high" threat level, ranking it No. 13 in its most recent National Volcanic Threat Assessment. Newberry is located about 20 miles south of Bend, Oregon, and any repeat of its historical eruptions could send lava flows through many inhabited areas.

Mount Baker, Washington

Mount Baker in Washington Mount Baker is less active than some Cascade volcanoes, but it's still a threat. (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.

Glacier Peak, Washington

Glacier Peak in 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.

Mauna Loa, Hawaii

Mauna Loa in Hawaii Mauna Loa tends to erupt at a slow, oozy pace, which has formed a wide dome. (Photo: Larry Johnson/Flickr)

Aside from Mount Rainier, the other U.S. Decade Volcano is also one of largest volcanoes on Earth: Hawaii's Mauna Loa. It may not look so big from ground level, but if you count its long submarine flanks that depress the sea floor, its summit is 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.

Crater Lake, Oregon

Crater Lake was filled up by rain. Crater Lake isn't fed by rivers, instead receiving all its water via precipitation. (Photo: Wollertz/Shutterstock)

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.

Long Valley Caldera, California

Long Valley Caldera, California Mule deer congregate at California's Long Valley Caldera. (Photo: Sarah Purcell/U.S. Bureau of Land Management)

About 760,000 years ago, California's Long Valley Caldera was formed by a "supereruption" that expelled roughly 1,400 times more lava, gas and ash than Mount St. Helens in 1980. It hasn't erupted for tens of thousands of years, although the USGS notes it "remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity and other unrest in recent years."

In 2018, researchers reported evidence of a large magma reservoir beneath Long Valley, holding an estimated 240 cubic miles (1,000 cubic km) of molten rock. "We estimate the reservoir currently contains enough melt to support another supereruption comparable in size to the caldera-forming eruption" 760,000 years ago, the researchers wrote in the journal Geology. And while it's impossible to know when that will happen, study co-author and USGS scientist Ashton Flinders tells LiveScience that "an eruption from Long Valley in our lifetimes is extremely unlikely."

Yellowstone Caldera, Wyoming

Excelsior Geyser, Yellowstone National Park 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.

Which U.S. volcanoes are most dangerous?
With more than 160 active volcanoes, the United States is at greater risk from eruptions than many people realize.