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Most triggering events do not lead to eruptions.

Triggering events are critical for forecasting and predicting eruptions, however most triggers fail to cause an eruption.  A volcano spends the vast majority of its time in a stable state where it is regularly bombarded with earthquakes, landslides, and heavy rainfall but does not erupt.  The eruption of a volcano begins deep within the earth and a long chain of events is often required for eruption to occur.  The greatest factor affecting the activity of a volcano is the pressure built up by magma in the magma chamber.  If overpressure is insufficient, there will be no eruption.

It is important to discern that triggers are not the actual cause of the eruption, but instead may be the catalyst for an eruption.  They are often an  event unrelated to the volcano which occurs when the volcano is ready to erupt.  This was the case at Mt. Saint Helens on May 18, 1980 where instability was created by a large magma bulge and an internal earthquake caused the slope to fail leading to eruption. It is possible that the slope failure could have occurred without the earthquake and simply due to the increasing instability of the bulge which was magnified by glacial melt.

One of the most common triggers of volcanic eruptions is nearby or internal earthquakes which may build magma pressure by stirring and mobilizing the magma or break the crust releasing pressure of the confining crust. There are, however, far more earthquakes than volcanic eruptions each year, so many do not cause any eruptive activity. This does not mean that earthquakes should not be monitored.  Earthquake cycles within a volcano can be monitored to give accurate predictions of eruptions at some volcanoes.  This is the case at Soufriere Hills, Montserrat volcano which can be read about Here.  A regular cycle of seismicity and lava dome building followed by eruptions has been recorded here and allows for accurate eruption prediction.  Because of the inconsistency between volcanoes, each volcano requires close monitoring to determine if internal pressure is great enough to destabilize the volcano. Unfortunately, resources are often insufficient to devote this amount of time to every active volcano.

Soufriere Hills Volcano lava dome prior to eruption in 2006. Source:

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