On Sunday morning, May 18, 1980, an earthquake that measured magnitude 5.1 triggered an eruption of Mount Saint Helens in Washington state that did not fully cease until 1986.
The force of the eruption destroyed more than 200 homes and more than 185 miles of roads, according to the United States Geological Survey (USGS). The lava flow from the volcano scorched 230 square miles of forest. By the time the eruption ended, 57 people had been killed.
"A volcano can go from nothing to a very large eruption in a very short time," said USGS Hydrologist and Outreach Coordinator Carolyn Driedger. "Mount Saint Helens only took one week to go from nothing to a full eruption."
Since that day in May 1980, scientists at the USGS have been monitoring conditions at Mount Saint Helens for signs of a future eruption.
"We learned in 1980 with Mount Saint Helens and we changed how we do business and how we monitor these explosive volcanoes," Driedger said.
Teams of scientists use seismographs to monitor the movement of magma below the surface of the volcano. They use GPS units to measure the movement of rocks and changes in the volcano at ground level and they measure the gases that are coming from the volcano to determine how close the magma is to the surface, said Driedger.
"Gases measured from the crater help predict the force of future eruptions." Driedger said the more gas that is trapped in the silicate rocks during an eruption the larger the explosion will be.
Mount Saint Helens erupted again during the years of 1989 through 2001 and 2004 through 2008. Those eruptions were not anywhere near the force of the eruption of 1980.
"The first eruption was very explosive," said Driedger. "The second eruption was relatively calm, like toothpaste squeezing out of a tube."
Driedger said volcanoes can erupt in a variety of ways. Driedger believes future eruptions will be less explosive than the one in 1980. "The eruption in 1980 collapsed the northern side of the volcano, so a future eruption won't be able to cause as large a landslide."
The landslide caused by the collapse of the northern side of Mount Saint Helens in 1980 was the largest debris avalanche on Earth in recorded history, according to the USGS.
What does the future hold for Mount Saint Helens?
"We know magma is forming, we see little earthquakes, and we know it is reloading. We know it will erupt again and we know we may have as little as a week's notice. We have to be ready," Driedger said.