Causes of Mt St Helens

Answer 1

I'm sure someone can provide a better answer, but pressure built up in the magma under the volcano. When the pressure gets too great: eruption.

Answer 2

The eruption of Mt. St. Helens in 1980 was primarily caused by the accumulation of magma beneath the volcano, causing increased pressure that eventually led to a violent explosion. The eruption was also influenced by a series of earthquakes in the preceding weeks, which indicated the movement of magma and further destabilized the volcano. Additionally, the collapse of the volcano's north flank triggered the lateral blast that devastated the surrounding area.