Scientists cannot exactly predict eruptions, but they have made significant advances in forecasting them.  One way the character of an eruption can be forecast is based on the prehistoric and historic record of the volcano in question and its volcanic products. For example, a violently erupting volcano that has produced ash fall, ash flow and volcanic mudflows (or lahars) is likely to do the same in the future. In addition, a number of tools can be used to record warning signs. Seismographs can detect earthquakes, while tiltmeters and geodimeters can measure the subtle swelling of a volcano, as can satellites. Correlation spectrometers (COSPECS) can measure amounts of sulfur dioxide--a telltale gas that is released in increasing quantities before an eruption.

Before the eruption of a land volcano, magma moves into the area beneath the volcano and collects in a magma chamber, or reservoir. As it comes closer to the surface, the magma releases gases. These events are related to three warning signs about the likelihood of an eruption. 

The first major warning signs are volcanic earthquakes.  These take three forms: short-period earthquakes, long-period earthquakes, and harmonic tremor. Short-period earthquakes are like normal fault-related earthquakes. They are related to the fracturing of brittle rock as the magma forces its way upward. These short-period earthquakes signify the growth of a magma body near the surface. Long-period earthquakes are believed to indicate increased gas pressure in a volcano's "plumbing system." Harmonic tremor occurs when there is sustained movement of magma below the surface.  Eruptions are likely especially if long-period events become dominant and episodes of harmonic tremor appear.

A second warning sign is sulfur dioxide, one of the main components of volcanic gases. Increasing amounts of it herald the arrival of more and more magma near the surface. On many occasions sulfur dioxide emissions increase and then drop to low levels prior to eruptions.  Most scientists believe that this drop in gas level is caused by the sealing of gas passages by hardened magma. Such an event leads to increased pressure in the volcano's plumbing system and an increased chance of an explosive eruption. 

The third warning sign is swelling of the volcano that signals that magma has accumulated near the surface. Scientists monitoring an active volcano will often measure the tilt of the slope and track changes in the rate of swelling. An increased rate of swelling--especially if accompanied by an increase in sulfur dioxide emissions and harmonic tremors--is a sure sign of an impending event. 

An excellent example of successful forecasting occurred in 1991. Volcanologists from the U.S. Geological Survey accurately predicted the June 15 eruption of the Pinatubo Volcano in the Philippines, allowing for the timely evacuation of the Clark Air Base and saving thousands of lives.

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Image: Volcano World

LAVA TEMPERATURE is one of several indicators that volcanologists regularly monitor in hopes of forecasting major eruptions.

Answer posted November 29, 1999


 
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