for National Geographic News
Mount St. Helens blew a cloud of steam and gray ash into the skies over Washington State earlier today. The small blast occurred shortly after noon.
Observers reported the eruption lasted 20 minutes, after which the cloud rising over the volcano began to dissipate.
Earlier this week, scientists said the volcano was primed to erupt any day, predicting that the blast would be far less powerful than the May 18, 1980, explosion that killed 57 people.
"There's been so much activity [that] we're definitely expecting something to happen," Tom Pierson, a research scientist with the U.S. Geological Survey at the Cascade Volcano Observatory in Vancouver, Washington, said on Thursday.
When Mount St. Helens began stirring last week, scientists first assumed the tiny earthquakes underneath the volcano were being caused by steam from water seeping into the mountain.
This usually happens in the fall. As rainwater encounters hot rock that has had the summer to dry out, the resulting steam pressures the system underneath the volcano and causes the rock to fracture.
Only, this time, the earthquakes grew stronger and became increasingly frequent. That led scientists to believe that something else was moving and pushing up from below: magma.
Magma, or molten rock, is formed when a part of the Earth's lower crust or upper mantle melts. The movement of magma causes volcanic activity. As rock inside the Earth melts, it can rise toward the surface and may erupt in a volcano.
On May 18, 1980, that's what happened at Mount St. Helens, which is located 100 miles (160 kilometers) south of Seattle. The blast blew off the top of the mountain, killing 57 people and showering towns as far as 250 miles (400 kilometers) away with volcanic ash.
On Thursday, the flurry of earthquakes had intensified, striking four times per minute, prompting scientists to warn that a small or moderate blast was imminent.
Around the Pacific
There are about 1,900 volcanoes around the world that are active today or have been active historically. Almost 90 percent of them fall along the so-called Ring of Fire, a band of volcanoes circling the Pacific Ocean.
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