Mount St. Helens erupted on May 18, 1980, after two months of increasing volcanic activity.
Since its most recent eruption in 2008, there has been a swarm of earthquakes, which are thought to be a result of the magmatic system's "recharging," according to the Pacific Northwest Seismic Network.
Similar seismic swarms were detected during recharging periods before a small eruption in 2004 and through a period of volcanic activity that ended in 2008.
In March through May of this year, swarms of deep earthquakes, not even felt on the surface, have been detected.
Seismic swarms do not directly indicate that an eruption is imminent, because volcanic forecasting is difficult, according to the U.S. Geological Survey.
The 1980 eruption is widely considered the most disastrous volcanic eruption in U.S. history. It killed 57 people and destroyed hundreds of homes, 57 bridges and some 200 miles of roads, in addition to leveling tens of thousands of acres of forest.
The eruption sent an ash cloud more than 12 miles into the atmosphere in just 10 minutes. Fine ash reached the Northeast two days later and circled the earth within 15 days, according to the U.S. Geological Survey. The volcano had been dormant for more than 100 years until seismic activity started to increase in March 1980.
A series of earthquakes caused cracks in the snow and ice at the top of the mountain. On March 27, 1980, ash began to spray from the mountain's peak.
What happened next caught many scientists by surprise. At 8:32 a.m. on the day of the big eruption, a 5.1 magnitude earthquake shook the area, and the mountain's summit and much of its northern flank collapsed, sending a huge explosion out from the north side instead of a typical eruption from the top.
Some 3.2 billion tons of ash spewed into the surrounding area, according to the United States Geological Survey. Streets and buildings were covered, and the eruption caused an estimated $1 billion in damage.
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