Why did Mt. Saint Helen almost self-destruct instead of erupting like we know most volcanoes to?

Question

I didn’t know about Mt. Saint Helen today, and watching the videos, it feels almost unreal. What’s the science behind it?

In: Geology

Answer 1

Many volcanos behave like St Helens. Or rather, St helens behaves like many volcanos: The hole at the top gets blocked. Gasses dissociating from magma result in a buildup of pressure. Eventually, that pressure is sufficient to shatter the blockage and erupt out the top. In Helens’ case, it got stuck in a tube to the side of the cone instead of at the top of it, and the explosion blew the side of the mountain off instead of the top of it. [Here](https://youtu.be/AYla6q3is6w)’s a video that shows the lop-sided nature of the magma build-up. Then, a large earthquake caused the side of the mountain to collapse, releasing all of the trapped gas all at once instead of gradually over time: as if you were to shake up a can of coke and then cut the top off with a sword, instead of just popping the cap.