Mount St. Helens is a part of a volcanic "ring of fire" so vast that it encircles the Pacific Ocean.
Indeed, the other 12 major volcanoes in the Cascade Range of northern California, Oregon and Washington state lie within this geological phenomenon as well.
The entire ring -- from the tip of South America up through Alaska, Japan and the Kamchatka Peninsula of Russia, down through the Philippines and Indonesia into New Zealand -- includes about three-fourths of the world's active and dormant volcanoes, scientists say.
Another one in the ring, a 12,533-foot volcano on Mexico's west coast, belched plumes of smoke and fired hot rocks down its slopes this week. The explosions were provoked by the collapse of a dome that had formed recently in the crater of the volcano, located in the Pacific coast state of Colima.
Most of the activity on the 8,364-foot Mount St. Helens is related to shifting in the vast sections of the Earth's surface known as tectonic plates, continent-size chunks of crust that float atop the planet's molten core.
Mount St. Helens and the Cascades lie near the edge of the Juan de Fuca plate, which is diving under the North American plate to create a 700-mile long "subduction zone" along the ocean floor that triggers earthquakes and pushes molten rock upwards.
Called magma underground and lava when it surfaces, the molten rock is forced up through fissures and weak spots in the crust.
St. Helens most active in Northwest
Mount St. Helens lies along a particularly weak area of the crust, causing it to be the most active volcano in the Northwest over the centuries, said Jon Major, a U.S. Geological Survey researcher in Vancouver, Wash. Its most spectacular showing was in May 1980, with an eruption that blew the top 1,400 feet off the mountain.
"It sits near the St. Helens seismic zone, an area where the crust is pulled apart a little bit," Major said. "That lets magma push up and explains why it's so active and others are not so active."
For example, Mount Adams lies only about 50 miles east of Mount St. Helens but has not erupted in thousands of years, Major said.
Mount Jefferson, which lies between Mount Hood and the Three Sisters in the Oregon stretch of the Cascades, appears to have been dormant since the last Ice Age despite relatively recent eruptions on neighboring peaks, he said.
In the rest of the Cascade Range, which stretches from Canada to Northern California, two of the tallest peaks -- Mount Rainier in Washington state and Mount Shasta in California -- both have erupted at least once in the past 200 years and have had several more over the last 2,000 years. Most were considered minor, according to USGS figures.
The Northwest, in turn, has been relatively quiet compared to other areas of the ring, according to Jim Luhr, director of the global volcanism program at the Smithsonian Institution in Washington, D.C.
The Aleutian Island chain in Alaska, Central America, Japan and Indonesia have all been more active recently, Luhr said.
"The Aleutians are one of the most vigorous volcanic parts of North America," he said.
But he noted that other parts of the world have plenty of dormant volcanoes, including France and Germany.
Luhr recently returned from a trip to Armenia where ancient petroglyphs show evidence of eruptions.
"There are relatively young volcanoes all over Armenia," he said. "None have erupted in the last 4,000 years, but clearly ancient peoples have seen them."
There is a chance that other Northwest volcanoes could erupt. But like Mount St. Helens, it will probably be mostly rock and ash that spew forth, not the dramatic, fiery rivers of lava that accompany eruptions in Hawaii, scientists say.
The Mount St. Helens eruption in 1980 killed 57 people, but other volcanoes have taken a deadlier toll.
In January 2002, lava rolled down the slopes of the African volcano Mount Nyiragongo and flooded the streets of Goma, Congo, killing at least 75 people.