10. Crater Lake Volcano, Oregon
Photograph by David McLain, National Geographic
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."
9. Redoubt Volcano, Alaska
Photograph courtesy R. Clucas, Alaska Volcano Observatory/USGS
Alaska's Redoubt Volcano, which last erupted in 2009 (pictures), presents a high risk because of its proximity to the city of Anchorage, its international airport, and the flight paths overhead.
Pictured on April 21, 1990, erupting Redoubt Volcano in 1989-90 sparked volcanic mudflows, or lahars, which swept into an inlet some 13.5 miles (35 kilometers) away.
Ash from the volcano temporarily shut down a 747's engines in December 1989. Though the crew restarted the engines in midair, the incident's aftershocks can still be felt in recent on-again, off-again airport shutdowns related to Icelandic volcanic ash.
8. Mauna Loa Volcano, Hawaii
Photograph by Bill Curtsinger, National Geographic
The largest volcano on Earth, Hawaii's Mauna Loa has erupted 33 times since its first documented eruption in 1843, but has been relatively quiet since 1984. (Video: Hawaii's Mauna Loa.)
Mauna Loa's general ooziness is due largely to the fact that "the Pacific crust is sinking down below the continents," the USGS's Eichelberger said. "That introduces a lot of water into the hot area of the mantle, which causes melting—and then you get volcanoes."
The molten rock is less dense than solid rock, so it tends to rise to the surface, as at Mauna Loa, where lava constantly flows into a beleaguered nearby community.
7. Lassen Volcanic Center, California 
Photograph by Priit Vesilind, National Geographic
It might be slightly unfair to single out Lassen Peak (pictured), since the Redding, California-area volcano (map) is simply one among several volcanoes in a volatile cluster.
"The next eruption might not be on Lassen Peak," said the Cascades Volcano Observatory's Scott. The blast could take place at a neighboring volcano—or create a new one altogether.
Lassen Peak last erupted during between 1915 and 1917. Like Mount St. Helens 30 years ago, the California volcano blew down a patch of forest, but on a much smaller scale. The previous eruption in the area—called the Lassen Volcanic Center—in the mid-17th century formed a new volcanic cone about 10 miles (16 kilometers) east of Lassen Peak.
6. South Sister Volcano, Oregon
Photograph from Panoramic Stock/National Geographic
With South Sister (pictured) ranked by the USGS as a "very high threat" volcano, the Three Sisters area is a volcanic hotbed spanning about 115 square miles (300 square kilometers) just west of Bend, Oregon.
The next major activity in the area might not be an eruption of one of the three volcanic peaks—Middle, South, and North Sister—but the start of a new volcano altogether, the Cascades Volcano Observatory's Scott said. "It could really occur almost anywhere in that broad area."
In the late 1990s and early 2000s, an area of ground west of the Middle and South Sister started to deform. Volcanologists closely monitored the 9-mile-wide (15-kilometer-wide) area they dubbed "the Bulge," since ground deformation can indicate magma moving and accumulating underground.
The Bulge, though, is now deflating. "In the end, it didn't result in an eruption," Scott said. "But it may be evidence of a process that may eventually produce one."
5. Mount Shasta Volcano, California
Photograph by James P. Blair, National Geographic
Flanked by an interstate, California's Mount Shasta volcano looms over thousands of homes, a key to its USGS "very high threat" rating (more California volcano pictures).
Around Mount Shasta an eruption's pyroclastic flow—rapid currents of superheated gas, ash, and rock caused by a volcanic explosion—as well as ash-infused mudflows could put towns and infrastructure in harm's way.
The last reported eruption was seen from the Pacific Ocean in 1786 and may not have "been such a big deal," the Cascades Volcano Observatory's Scott. "We haven't had [an eruption] since settlement by European settlers, but in the geologic sense the volcano has been quite frequently active."
4. Mount Hood Volcano, Oregon
Photograph courtesy Sam Abell, National Geographic
Climbers cross a ridge on Oregon's most dangerous volcano, Mount Hood. "A key issue with Mount Hood is that people live on the flank of the volcano, state highways cross its flanks—so there's a lot of stuff up close," the Cascade Volcano Observatory's Scott said.
Mount Hood was very active at the end of the last ice age, and there have been two significant eruptions in the last 1,500 years—the last of which happened just before Lewis and Clark came through in the early 1800s.
One interpretation of Mount Hood's volcanic activity is that the mountain has recently ended a long period of dormancy and "in the future it might be erupting on the order of every few centuries," Scott said.
3. Mount Rainier Volcano, Washington 
Photograph by Barbra Kates, Your Shot
The danger with Washington State's Mount Rainier is that it's covered by more snow and ice than all the other Cascade Range volcanoes combined, so it presents a high risk of lahars, or volcanic mudflows. "A lahar is like concrete flowing down the chute of cement mixer," said the Cascades Volcano Observatory's Scott.
In the past Mount Rainier's lahars have run down the river valley all the way to Puget Sound, a distance of more than 62 miles (100 kilometers). "That's where a lot of the [urban] development is. so that's a key hazard," Scott said.
Mount Rainier undergoes significant volcanic activity every 500 to 1,000 years, Scott said—and the volcano's last big explosion was about 500 years ago. "But right now we know the volcano is at rest."
2. Mount St. Helens Volcano, Washington
The eruption of Mount St. Helens 30 years ago on May 18, 1980 (pictured), is the most devastating and most studied volcanic explosion in U.S. history. The blast killed 57 people and spewed 520 million tons of volcanic ash, darkening the skies of Spokane, Washington, more than 250 miles (400 kilometers) away.
Over the last 10,000 years, Mount St. Helens has erupted more frequently than any other volcano in the Cascade Range, and has continued erupting, albeit gently, in recent years.
1. Kilauea Volcano, Hawaii
Photograph courtesy Frans Lanting, National Geographic
A photographer slinks into the Pu'u 'O'o crater of Hawaii's Kilauea volcano, which has been active since 1983.
"A lot of the time the lava's flowing into the sea, and you can walk right up to it," said the USGS's Eichelberger.
One of Kilauea's hazards includes the potential for explosive eruptions. "That's when the magma comes flying out as blocks or particles of ash instead of flowing out intact as lava," Eichelberger explained.
What volcanologists call explosive activity varies. "Sometimes they're discrete explosions ... and things go flying all over the place. Other times it's more like a spray from a fire hose," he said.
As a general rule, "it's wise not to be close to an explosive eruption."
Source: National Geography
