Predicting Volcanic Eruptions     Volcanologists attempt to forecast volcanic eruptions, but this has proven to be nearly as difficult as predicting earthquakes. Many pieces of evidence can mean that a volcano is about to erupt, although the time and magnitude of the eruption are difficult to predict. Signs an eruption is likely may come from a history of previous volcanic activity, the occurrence of earthquakes, slope deformation, and gas emissions.     A volcano’s history—how long since its last eruption and the time span between its previous eruptions—is a good first step in predicting... Show more

Predicting Volcanic Eruptions    
Volcanologists attempt to forecast volcanic eruptions, but this has proven to be nearly as difficult as predicting earthquakes. Many pieces of evidence can mean that a volcano is about to erupt, although the time and magnitude of the eruption are difficult to predict. Signs an eruption is likely may come from a history of previous volcanic activity, the occurrence of earthquakes, slope deformation, and gas emissions.    
A volcano’s history—how long since its last eruption and the time span between its previous eruptions—is a good first step in predicting eruptions. Currently erupting volcanoes (active volcanoes) and those that have erupted recently (dormant volcanoes) are heavily monitored because they may erupt again.    
Moving magma shakes the ground, so the number and size of earthquakes may increase before an eruption. A volcano that is about to erupt may produce a sequence of earthquakes. Scientists use seismographs to record the length and strength of earthquakes to help them predict whether an eruption is imminent.    
Magma and gas can push a volcano’s slope upward, causing deformation in the ground. The changes may be subtle and only detectable by tiltmeters. These are instruments that measure the angle of a slope. In other cases, the changes may be very obvious.

For example, Mount St. Helens grew a huge bulge on its north side before its 1980 eruption.    
Ground swelling may also cause rock falls and landslides.    
Gases may escape from a volcano before magma reaches the surface.

Scientists measure gas emissions in vents on or around volcanoes or from a distance using satellites. The gases measured may include sulfur dioxide, carbon dioxide, hydrochloric acid, and water vapor. The amounts of gases and their ratios are calculated to help predict eruptions.    

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