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Let's take a closer look at how volcanologists use precursory phenomena such as seismicity, gas emissions, and ground deformation to forecast eruptions.

Seismicity
Seismic activity (small earthquakes and tremors) always occurs as volcanoes awaken and prepare to erupt. Some volcanoes normally have continuing low-level seismic activity, but an increase can signify an eruption. The types of earthquakes that occur and where they start and end are also key signs.

 Scientists monitor activity at Mt. Pinatubo.

Volcanic seismicity has three major 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." They are similar to the clanging sometimes heard in your home's plumbing system. Harmonic tremor occurs when there is sustained movement of magma below the surface.

Patterns of seismicity are complex and often difficult to interpret. However, increasing activity is very worrisome, especially if long-period events become dominant and episodes of harmonic tremor appear.

Gas Emissions
As magma nears the surface and its pressure decreases, gases escape. This process is much like what happens when you open a bottle of soda and carbon dioxide escapes. Sulfur dioxide is one of the main volcanic gases, and increasing amounts of it usually herald the arrival of more and more magma near the surface. For example, on May 13, 1991, 500 tons of sulfur dioxide were released from Mt. Pinatubo in the Philippines. On May 28--just two weeks later--sulfur dioxide emissions had increased to 5,000 tons, ten times the earlier amount. Mt. Pinatubo erupted on June 12, 1991.

On several occasions, such as before the Mt. Pinatubo eruption, sulfur dioxide emissions have dropped to low levels prior to eruptions. Most scientists believe that this drop in gas levels 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.

Ground Deformation
Swelling of the volcano 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 almost a sure sign of an impending event.

"Volcanoes" is inspired by programs from Earth Revealed.