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4.4

Most volcanoes are susceptible to multiple triggering events.

Volcanoes are often subject to multiple factors which weaken their integrity and may cause eruption.  It is critical to understand which triggers a volcano is susceptible to in order to better predict when an eruption will occur.  A volcano can have many possible triggers such as tall, steep, high latitude volcanoes which are susceptible to nearly all triggering events.  If internal pressure is great enough, a combination of events, any number of single events, or no event at all may trigger an eruption.

This complexity can make determining the exact cause of an eruption difficult as in the case with the Mt. Saint Helens 1980 eruption.  As the event developed, a broad range of triggering events developed which required examination.  Magma mixing occurred and magma rose, signaled by an earthquake beneath the volcano.  As internal pressure grew, the magma melted glaciers atop the volcano and melt water contacted the groundwater table resulting in small phreatic eruptions.  The magma continued to rise and increasing pressure was reduced through the creation of a large bulge on the flank of the volcano.  This consequently destabilized the volcano.  This instability was magnified by the increased pore water pressure  and increased load of water in the topsoil from the melted glacier and snow cover.  A month after this activity began, a large earthquake occurred inside the volcano and caused the complete destabilization of the system. A large flank collapse was the final event which released nearly all confining pressure. Without this great mass of earth preventing the magmas escape, a violent eruption occurred.  Scientists and government officials were unable to maintain a full evacuation for the nearly two months of activity identifying the impending eruption and as a result 57 people lost their lives.

The case of Mt. Saint Helens is an unfortunate one but it highlights the importance of understanding volcanic triggers.  Without a knowledge of potential triggers and close monitoring by volcanologist, it  is likely that many hundreds more people may have died.  Many of those that died were the overly curious or stubborn and not the unsuspecting residents of the area who heeded the warning of scientists.  For complete notes of the events leading up to the eruption click Here.

The large bulge can easily be seen on the right side of this image of Mt Saint Helens

Eruptions have also occurred in recent history which have had no trigger or precursory activity.  Mount Spurr in Alaska erupted in  periodically from June 1991 to September 1992 with little to no indication of its eruptive potential.  These cases represent the high variability between volcanoes around the world.  This highlights the importance of educating those who are susceptible to them.

Eruption of Spurr in August 1992

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