"If a really big batch of magma were involved, you would expect to see more of the surface of the ground, including the outer flanks of the volcano, start to move," said Tom Pierson, a research scientist with the U.S. Geological Survey at the Cascade Volcano Observatory in Vancouver, Washington.
Mount St. Helens is a very young volcano. Until recently, scientists believed it to be only 40,000 years old, though new geological evidence suggests it may be a few hundred thousands years old. Other volcanoes in the Cascades are one million or several million years old.
Mount St. Helens "has a history of large and very explosive eruptions, which makes it a pretty dangerous volcano," Pierson said.
Its volcanic activity is closely linked with plate tectonics. As huge slabs of rock collide in the Earth's outer shell, they can produce magma and trigger a chain of events that may end with explosions.
Before 1980 the volcano had been silent since 1857, when a 57-year period of intermittent steam bursts and eruptions of ash and lava ended.
But two months before Mount St. Helens's May 18, 1980, eruption, earthquakes began. For weeks the volcano vented plumes of steam and ash. At times it seemed to doze off, prompting geologists to describe its eruption level as "low-energy mode."
Then, on May 10, 1980, the volcano roared back. Some of the earthquakes were close to 5 in magnitude. The mountain's north face swelled at a rate of 5 feet (1.5 meters) per day. It became clear the volcano would not remain on hold much longer.
On the morning of May 18, David Johnston, a U.S. Geological Survey geologist, was monitoring the eruptions from a camp called Coldwater II, six miles (ten kilometers) from the mountaintop. He radioed the volcanic observatory. "Vancouver! Vancouver! This is it " he said.
A 5.1 earthquake triggered the powerful blast. It obliterated the top 1,300 feet (396 meters) of the volcano. But the explosion was also unexpectedly sideways, causing the collapse of the fractured north side of the volcano.
Fanning out northwest and northeast, huge waves of fire-hot debris traveled at 200 miles an hour (320 kilometers an hour), snapping 150-foot (46-meter) trees like brittle straws and clear-cutting 220 square miles (570 square kilometers) of forest.
A photograph from the scene by Robert Madden, a former National Geographic photographer who covered the aftermath of the eruption, shows what looks like hairs on a dog. On closer inspection, the image shows 6,000 trees blown over in one direction.
"It was impossible to capture or digest the damage," Madden said. "You would fly over areas of complete devastation for 20 minutes in a row."
The blast quickly melted the ice on top of the mountain, sending an estimated 46 billion gallons (174 billion liters) of water down its side in a combination of mudflow and flooding.
The ash was ubiquitous. Within 15 minutes an ash cloud had soared 80,000 feet (24,000 meters). The ash turned the morning into darkest nightthe day later became known as Black Sunday. The ash blanketed 22,000 square miles (57,000 square kilometers) in eastern Washington, northern Idaho, and western Montana.
Fifty-seven people were killed, including David Johnston, the USGS geologist who first announced the eruption.
Perhaps the most famous casualty was 84-year-old Harry Truman, who had lived on the shores of Spirit Lake on the slopes of Mount St. Helens for more than half a century. He had refused to flee his home when the rumblings began.
"The mountain is part of me," said Truman, who became a folk hero to some.
His lodge was buried under hundreds of feet of ash and debris and the raised waters of Spirit Lake.
There were also seemingly miraculous stories of survival.
On the morning of May 18 videographer David Crockett of Seattle's KOMO-TV was following logging roads upstream for a closer look at the mountain when it erupted.
He quickly turned his station wagon around, but debris flows washed out his escape route and the black cloud overtook him.
He started his camera and recorded a narration of his extraordinary attempt to walk out. Somehow he survived, along with six minutes of tape that showed only blackness.
By May 21 the mountain had closed down to sporadic ventings.
Later the federal government turned the area into the 110,000-acre (44,500-hectare) national volcanic monument. Scientists quadrupled their knowledge of these types of volcanic eruptions.
"People knew you could have landslides on volcanoes, but nobody realized they could be quite that big," said Pierson, the USGS geologist. "We also had virtually no experience with lateral blasts prior to that. A few geologists said it could go sideways, but nobody knew it could go really sideways."
To those who witnessed it first hand, the eruption underscored nature's awesome power. "What impressed me the most is how much energy can be stored up in these things," said Nieland, the Forest Service official.
Rowe Findley, a National Geographic writer who witnessed the eruption and covered both its buildup and aftermath in an article for the magazine, wrote:
"More than fear for personal safety, I felt a growing apprehension for all of us living on a planetary crust so fragilely afloat atop such terrible heats and pressures. Never again, it came to me then and remains with me to this day, would I regain my former complacency about this world we live on."
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