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

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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

There was a huge amount of carbon dioxide and sulfur dioxide gasses under a huge amount of compression in the lava before it erupted. When it erupted the gasses fizzed the lava like a shook up soda bottle. Those gasses when they came out of compression exploded the molten lava, which resulting in all the ash.
This type of volcano is called a composite volcano.

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.

You’re probably thinking that Hawaiian-style volcanoes, with flowing lava, are the most common form of volcanoes, and they’re not. The pyroclastic granite type like Mt. Saint Helens, the ones that go BOOM, are far more common. The difference is primarily the chemical nature of the rock/magma involved.