What made Mount St. Helens erupt?

Reader

The eruption of May 18, 1980, was caused by a sudden collapse and sliding of the north slope of the volcano. This released the pressure on the magma under Mount St. Helens. The sudden release of pressure caused gas to come out of the magma. It is the gas that propelled the eruption. (In a similar manner, when you shake a can of soda, gas bubbles appear. If you open the can, the soda gushes out, propelled by the gas bubbles.)

Sincerely,
John Dvorak

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