What type of boundary is Mount Saint Helens located on? What type of volcano is Mount Saint Helens? What caused the eruption on May 18 1980? What were the effects of the pyroclastic flow?

rocky Jane

Hi Jane,

Mt. St. Helens is on an ocean-continent subduction boundary (the Juan de Fuca plate is subducting under the N. American plate). Mt. St. Helens is an active strato volcano. The easiest answer to what caused the eruption is that a body of magma moved into the shallow part of the volcano between the initial unrest (in March) and by May 18th had become so pressurized that the volcano failed and this magma was able to erupt. There was also an earthquake at this time which probably helped trigger the event. There is a big long section in VolcanoWorld on Mt. St. Helens, and it has a sub-section that deals specifically with the effects of the pyroclastic flows. You can find it at: http://volcano.und.edu/vwdocs/msh/msh.html

Sincerely,
Scott Rowland 

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