Can they know when Mount St. Helens will blow?

Answers to your questions about the news.
Oct. 5 2004 1:15 PM

How To Predict a Volcanic Eruption

Can they know when Mount St. Helens will blow?

Mount St. Helens: Clear your calendar
Mount St. Helens: Clear your calendar

Scientists monitoring Mount St. Helens said last week that there was a 70 percent chance the mountain would erupt within the month. This weekend, serious tremors followed a small explosion Friday that sent steam 10,000 feet into the air, and the team at the Cascades Volcano Observatory now says it expects a significant eruption. How do scientists predict volcanic eruptions? And how did they calculate that 70 percent figure?

Scientists with the U.S. Geological Survey consider a volcano's geological profile and history, data from tests monitoring seismic activity, whether it is releasing gas, and any deformations of the mountain's surface. They're concerned with the frequency, location, and size of earthquakes; whether the earthquakes suggest that magma is moving underneath the mountain; whether the magma involved is old or fresh; whether the volcano is emitting gas, suggesting magma rising to the surface; whether the ground is swelling or compacting ("deforming," if you're a geologist), and so forth. Basically, the more the mountain shakes, the walls warp, and gas rises, the more likely the mountain will erupt.

Advertisement

How did the USGS arrive at that 70 percent figure, then? Scientists gathered as much data as possible to create the most accurate picture of what they believed was happening. They then compared it with a database of past volcanic activity around the world. In 70 percent of cases in which a volcano showed the same kind of activity that Mount St. Helens was displaying last week, it erupted.

The degree of confidence with which scientists make their probability estimates can vary, depending on the intensity and reliability of the data. In the case of Mount St. Helens, they felt that thousands of small earthquakes at a rate of four or five an hour was a very strong indicator of a shift in the volcano's activity, whereas GPS tests showed no significant swelling of the mountain's flanks. "Sometimes these are seat of the pants estimates," said Steve McNutt, a volcano seismologist at the Geophysical Institute of the University of Alaska Fairbanks. The system works, though: The USGS has accurately predicted every eruption of Mount St. Helens since 1980. (This does not, however, mean they can predict exactly when it will explode—though there were clear signs of impending eruption for months in 1980, the explosion came after a brief calm and caught many by surprise, killing 57.)

Now, the data indicates an imminent eruption. After the small eruption on Friday—Mount St. Helens' first in 18 years—the seismic activity has ramped up, with one tremor lasting 50 minutes. This and new test results have prompted the team at the Cascades Volcano Observatory to raise the alert at Mount St. Helens to Level 3—the most severe warning of danger. That hasn't scared away the voyeurs; thousands have flocked to the mountain, hoping to see the show.

Explainer thanks Cynthia Gardner and Jacob Lowenstern  of the Cascades Volcano Observatory and Steve McNutt of the Alaska Volcano Observatory.

Louisa Thomas is on the editorial staff of The New Yorker.

TODAY IN SLATE

Justice Ginsburg’s Crucial Dissent in the Texas Voter ID Case

The Jarring Experience of Watching White Americans Speak Frankly About Race

Here’s Just How Far a Southern Woman May Have to Drive to Get an Abortion

The Most Ingenious Teaching Device Ever Invented

Marvel’s Civil War Is a Far-Right Paranoid Fantasy

It’s also a mess. Can the movies do better?

Behold

Sprawl, Decadence, and Environmental Ruin in Nevada

Space: The Next Generation

An All-Female Mission to Mars

As a NASA guinea pig, I verified that women would be cheaper to launch than men.

Watching Netflix in Bed. Hanging Bananas. Is There Anything These Hooks Can’t Solve?

The 2014 Kansas City Royals Show the Value of Building a Mediocre Baseball Team

  News & Politics
The World
Oct. 20 2014 1:50 PM Why We Shouldn’t be Too Sure About the Supposed Deal to Return the Abducted Nigerian Schoolgirls
  Business
Moneybox
Oct. 20 2014 2:16 PM Even When They Go to College, The Poor Sometimes Stay Poor
  Life
Outward
Oct. 20 2014 2:19 PM A Procedural Rule Could Keep Gay Marriage From Ever Reaching SCOTUS Again
  Double X
The XX Factor
Oct. 20 2014 1:10 PM Women Are Still Losing Jobs for Getting Pregnant
  Slate Plus
Tv Club
Oct. 20 2014 7:15 AM The Slate Doctor Who Podcast: Episode 9 A spoiler-filled discussion of "Flatline."
  Arts
Brow Beat
Oct. 20 2014 2:39 PM Gwen Stefani Does Her Best Rihanna Impression on New Song
  Technology
Future Tense
Oct. 20 2014 1:51 PM Will Amazon Lead Us to the Golden Age of Books? A Future Tense Event.
  Health & Science
Medical Examiner
Oct. 20 2014 11:46 AM Is Anybody Watching My Do-Gooding? The difference between being a hero and being an altruist.
  Sports
Sports Nut
Oct. 20 2014 10:23 AM Where I Was Wrong About the Royals I underestimated the value of building a team that’s just barely better than mediocre.