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Predicting Volcanic Eruptions

The signs scientists use to predict volcanic eruptions.

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Predicting Volcanic Eruptions

Can she see your future?

No one can predict exactly when a volcanic eruption will take place. Scientists do a bit better forecasting volcanic eruptions than earthquakes. Still, volcanologists have a high fatality rate because forecasting eruptions is so difficult.

Predicting Volcanic Eruptions

Many pieces of evidence can mean that a volcano is about to erupt, but the time and magnitude of the eruption are difficult to pin down. This evidence includes the history of previous volcanic activity, earthquakes, slope deformation, and gas emissions.

History of Volcanic Activity

A volcano’s history — how long since its last eruption and the time span between its previous eruptions — is a good first step to predicting eruptions. Active and dormant volcanoes are heavily monitored, especially in populated areas.


Moving magma shakes the ground, so the number and size of earthquakes increases before an eruption. A volcano that is about to erupt may produce a sequence of earthquakes. Scientists use seismographs that record the length and strength of each earthquake to try to determine if an eruption is imminent.

Slope Deformation

Magma and gas can push the volcano’s slope upward. Most ground deformation is subtle and can only be detected by tiltmeters, which are instruments that measure the angle of the slope of a volcano. But ground swelling may sometimes create huge changes in the shape of a volcano. Mount St. Helens grew a bulge on its north side before its 1980 eruption. Ground swelling may also increase rock falls and landslides.

Gas Emissions

Gases may be able to escape a volcano before magma reaches the surface. Scientists measure gas emissions in vents on or around the volcano. Gases, such as sulfur dioxide (SO2), carbon dioxide (CO2), hydrochloric acid (HCl), and even water vapor can be measured at the site (Figure below) or, in some cases, from a distance using satellites. The amounts of gases and their ratios are calculated to help predict eruptions.

Scientists monitoring gas emissions at Mount St. Helens

Scientists monitoring gas emissions at Mount St. Helens.

Remote Monitoring

Some gases can be monitored using satellite technology (Figure below). Satellites also monitor temperature readings and deformation. As technology improves, scientists are better able to detect changes in a volcano accurately and safely.

Picture of an Earth observation satellite

An earth observation satellite.


Since volcanologists are usually uncertain about an eruption, officials may not know whether to require an evacuation. If people are evacuated and the eruption doesn’t happen, the people will be displeased and less likely to evacuate the next time there is a threat of an eruption. The costs of disrupting business are great. However, scientists continue to work to improve the accuracy of their predictions.


  • Volcanologists use several lines of evidence to try to forecast volcanic eruptions.
  • Magma moving beneath a volcano will cause earthquakes and slope deformation. Gases may be released from the magma out of the volcano vent.
  • Deciding whether to call for an evacuation is very tricky.


  1. What are the detectable signs that magma is moving beneath a volcano?
  2. What are the consequences of incorrectly predicting a volcanic eruption?
  3. How would a successful prediction of a volcanic eruption resemble a successful prediction of an earthquake?

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