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There are 65 volcanoes in the United States and its territories that scientists consider active, including Mount St. Helens. Of those volcanoes, 12 are on alert, which means they are on heightened watch for eruptive activity, and two are erupting right now or expected to erupt shortly.
Here are the seven most dangerous volcanic areas in the U.S., according to the USGS Volcano Hazards Program's Volcano Alerts watch list:
1. Kilauea Volcano
A currently erupting volcano, Kilauea means "spewing" or "much spreading" in Hawaiian and experienced frequent volcanic activity during the 19th century. Kilauea has been continually erupting since January 3, 1983. Lava flows from one of its vents and pours into the ocean.
Presently, the USGS has listed Kilauea as an orange level alert, because of elevated sulfur dioxide emissions from two of its vents and lava that is visible in the summit vent. (There are four warning levels - green, yellow, orange and red; red is the highest.)
One of the five volcanoes that form the Island of Hawaii, Kilauea is believed to be the home of Hawaii's mythical goddess of volcanoes and fire, Pele. Hawaiian tribal lore tells that Kilauea's eruptions are Pele's way of giving a gift.
2. Pagan Volcano
One of the two volcanoes that make up Pagan Island in the western Pacific, Mount Pagan is possibly the most dangerous volcano in the U.S. commonwealth of the Northern Mariana Islands because of its history of large eruptions.
The volcano's most recent massive eruption occurred in 1981, when debris from Mount Pagan mixed with heavy rainfall and buried a large area of the island, including a village containing a school and a church. The eruption forced residents to evacuate the island, and it has remained unpopulated ever since, with geologists surveying the volcano's activity via satellite imaging.
After the 1981 eruption, the volcano was intermittently active through 1996, mostly producing light ashfall. Currently, Mount Pagan is on yellow-level alert because of its gas emissions and the gas plumes it is producing on the island.
3. Anatahan Volcano
When Anatahan first erupted in 2003, the island's 23 residents were already long gone - they had been evacuated during seismic activity in 1990. Anatahan's most recent eruption lasted from 2007 to 2008, and the volcano remains on green-level alert.
The tiny volcanic island is located in the Northern Mariana Islands, and is about 5.6 miles (9 kilometers) long and 2 miles (3 kilometers) wide. It became well known after a group of Japanese World War II soldiers hid out on the island for years because they were unaware that the war had ended. They finally surrendered in 1951, when had already been over for six years.
4. Yellowstone Supervolcano
What makes the Yellowstone Volcano so unique is that it is the site of multiple calderas. These basin-shaped features usually form when a large volume of magma is removed from beneath a volcano, causing the ground to collapse into the emptied space and form huge depressions.
Situated in Yellowstone National Park in Wyoming, the Yellowstone caldera's floor moved upward almost 3 inches (7.6 cm) each year between 2004 and 2008. Since 2009, the uplift has significantly slowed, but the movement of the caldera's floor is considered to be unpredictable. During the month of April 2010, the Yellowstone region experienced 117 minor earthquakes, which are common volcanic activities.
5. Long Valley Volcanic Center
Since April 23, 2010, at least ten minor earthquakes have occurred in or near the calderas of the Long Valley region, which lies east of the central Sierra Nevada Range in California. The Long Valley Caldera contains an active hydrothermal system that includes hot springs and steam vents. Some of the hot water is released into the Hot Creek Gorge, a popular year-round swimming hole.
6. Cascade Range
The Cascade Range stretches across the northwestern U.S. and into Canada, and has both active and non-active volcanoes, including Mount St. Helens in Washington.
In 1980, Mount St. Helens erupted, killing 57 people as well as thousands of birds and mammals. The explosive eruption created a volcanic ash cloud that drifted east across the U.S. in three days and circled Earth over the course of 15 days. Having caused an estimated $1 billion in damage, the Mount St. Helens eruption was the most economically destructive volcanic event in US history.
Minor eruptions at Mount St. Helens have occurred since then. The most recent lasted for three and a half years and ended in 2008.
The Cascade Range is a part of the Ring of Fire, an area containing half of the world's active volcanoes around the Pacific Ocean.
7. Redoubt Volcano
Alaska's Redoubt volcano erupted in December 2009 and then experienced a series of small repetitive earthquakes near its summit from April 17 to April 20, 2010. Located in the Chigmit Mountains of Alaska, Redoubt even has its own iPhone application for those interested in tracking its volcano alert status.
During its 1989-90 and 2009 eruptions, Redoubt produced ash plumes that significantly disrupted air traffic. One plane, KLM Flight 867, was nearly grounded after it flew through an ash cloud in 1989. All four of the aircraft's engines failed within 59 seconds of encountering the ash cloud, but then restarted with reduced performance as the aircraft approached to Anchorage. The eruption deposited fine gray dust in the city of Anchorage and other communities Alaska.