Here's a look at Mount St. Helens, the most active volcano in the Cascade Mountains in Washington.
Facts: Mount St. Helens is a volcano in the Cascade Mountains, in the area called the Mt. St. Helens National Volcanic Monument. It is administered by the National Forest Service, not the National Park Service.
It is located in Washington, about 55 miles northeast of Portland, Oregon, and 95 miles south of Seattle.
Over the last 500 years, Mount St. Helens has had at least four major explosive eruptions and many minor eruptions.
Mount St. Helens was named by Commander George Vancouver for British diplomat Alleyne Fitzherbert, whose title was Baron St. Helens.
Timeline: 1792-1794 - The mountain is named while Commander Vancouver is surveying the northern Pacific coast.
1835 - Is first recognized as a volcano.
1857-1980 - Is inactive.
May 18, 1980 - Erupts.
Fifty-seven people are killed. Damage caused by the blast costs $1.1 billion (USGS).
Hot ash causes forest fires, and snow melt from the top of the mountain causes floods.
Volcanic ash spreads across the Northwest. More than 900,000 tons of ash are cleaned up from areas around Washington.
The 1980 eruption is the first eruption in the continental United States outside of Alaska since 1917.
1980-1986 - Many small eruptions occur.
September 2004 - Several days of unusual seismic activity lead seismologists to believe that an explosion resulting from steam buildup is likely to occur.
October 2004 - Several small steam and ash explosions occur.
March 8, 2005 - Releases a column of smoke and ash nearly six miles high, leaving a plume visible for more than 50 miles.
January 2008 - Eruptions that have occured continously since October 2004 cease.
July 2014 - Final preparations begin for what geophysicists call the "equivalent of a combined ultrasound and CAT scan" of the inside of the volcano. The joint project by scientists at Rice University, the University of Washington, the University of Texas at El Paso and others involves placing 3,500 seismic sensors around the volcano. The project will take four years and aims to improve volcanic monitoring and advance warning systems.
November 3, 2015 - Scientists investigating the interior of the volcano present the first results at the Geological Society of America convention, saying there is not only a magma chamber directly below Mount St. Helens, there are other chambers east of the mountain that appear to be connected, with magma flowing between them.
Spring 2016 - Over the course of eight weeks, more than 130 small earthquakes occur beneath the surface of Mount St. Helens.
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