Mount St. Helens in Washington State is celebrating a dubious anniversary. On May 18,1980, it made history with a massive and destructive volcanic eruption. Since then, this active volcano has been closely studied—and watched. What do we know about Mount St. Helens, and what does its future hold?

The eruption of Mount St. Helens 20 years ago did not occur without warning. The mountain, almost two miles high, had long been known as an active volcano, and it sat amid a chain of active volcanoes in the Cascade Range of the northwestern United States.

In the two months preceding the historic explosion on May 18, 1980, more than 10,000 mostly small earthquakes had rumbled beneath the mountain. Such seismic activity usually precedes a volcanic eruption. But even the experts did not predict that a final large earthquake, measuring 5.1 on the Richter scale, would so suddenly set off the largest volcanic eruption in the continental United States in more than a century.

"The scenarios of what was going to happen were all over the map," says Steve Malone, a geophysicist for the Pacific Northwest Seismic Network at the University of Washington. "It varied all the way from 'It could all die out and nothing would occur' to 'It would be a massive, explosive eruption' to 'It could be a landslide and a small blast.' Of course on May 18, the worst-case scenarios that had been developed up to that point were exceeded."

The volcanic blast—which traveled at over 300 miles per hour—and the mudslides that followed, spread so quickly that 57 people who were miles away were killed before they could escape. What impressed scientists more than the suddenness of this event, though, was its sheer destructiveness.

"The whole north flank of Mount St. Helens collapsed down into the valley below it," recalls Ed Klimasauskas, a geologist and public information officer for the U.S. Geological Survey (USGS). He stands on Johnston Ridge, named for a USGS scientist killed in the eruption.

"As the landslide collapsed into the valley, it released all its pent up energy," continues Klimasauskas. "And all of these gases exploded outward in a tremendous blast that covered a 230-square-mile area in a matter of minutes."

Some area residents survived by driving their cars at speeds of 100 miles per hour just to escape. Besides the human fatalities, the blast killed an estimated 7,000 deer, elk, and bear, as well as all birds and small mammals in the area. Trees in the surrounding forests were flattened. They contained enough wood to build an estimated 300,000 homes.

What followed, according to the USGS, was a series of other devastating natural events:

Eruption column and cloud: Within 15 minutes, a cloud of ash and gas rose 15 miles. During the day, the winds blew 520 million tons of ash eastward across the United States. The ash cloud disrupted airplane routes and even caused complete darkness 250 miles away in Spokane, Washington.

Mudflows: The hot rocks and gas from the eruption almost immediately caused the snow and ice at the top of the volcano to melt. The surging water that emerged joined with the debris from the landslide to form the massive mudflows, which extended for more than 50 miles.

Pyroclastic flows: Several hours later, a combination of ash, pumice, and gas at least 1,300°F (700°C) poured from the crater atop the fractured mountain. The flow traveled up to 80 miles per hour, scorching everything in its path.

Mount St. Helens has been an active volcano for much of its more than 40,000 years of existence. Native Americans living nearby named it Louwala-Clough, which translates into "smoking mountain." Mount St. Helens received its present name in 1792 from Captain George Vancouver, who was exploring the Pacific Northwest for the British.

Geologists have identified at least 3 periods lasting between 5,000 to 10,000 years when the volcano lay dormant. They have also found evidence of major eruptions approximately in the years 1480 (12 years before Columbus sailed for America) and 1800.

Mount St. Helens was not done after its 1980 explosion. Over the next six years, the site remained volcanically active through a sequence of "dome-building episodes." Seismic activity continued as molten magma continued to flow underground. When the magma broke through the surface of the volcano's crater, it built-up layer upon layer of rock, forming a "lava dome."

Think of a lava dome as a cork in a champagne bottle: It provides the cap to an active volcano. When the pressure beneath grows too great, it pops! In 1982, it looked as if the dome building activity was going to slow down, but a sizable eruption and a large outpouring of lava destroyed the lava dome and surprised scientists. In fact, the eruption destroyed their scientific measuring equipment. And while there were several eruptions in 1982, this was by far the largest.

Mount St. Helens probably is not done yet. The volcano has been mostly quiet for the past 15 years. But USGS scientists at the Cascades Volcano Observatory have been closely monitoring Mount St. Helens and its volcanic neighbors. The observatory was created in the aftermath of the Mount St. Helens' eruption in 1980. Geologist Ed Klimasauskas is stationed there.

"Since the last eruption in 1986," he notes, "Mount St. Helens has had a few bouts of explosions from the dome and also some intermittent earthquake activity. And this tells us that this is a very active, dynamic place." But, he adds, another explosion to rival the one 20 years ago may not come for centuries.

The USGS estimates that the Pacific Northwest has at least 12 volcanoes that could become active. These include Mt. Rainier, outside of Washington; Mt. Hood, near the Columbia River in Oregon; and Mount Shasta, in northern California.

The monitoring efforts are taking on even greater importance as the population in this part of the country continues to increase.

More Links

• Visit the Cascades Volcano Observatory of the U.S. Geological Survey.

• Get a USGS Fact Sheet about Mount St. Helens.
  

• Check out an Eruption Summary of the historic volcanic eruption on May 18, 1980.

Related Resources

• Get Mount St, Helens: The Eruption and Recovery of a Volcano by Rob Carson.