Question: What causes volcanos to errupt

Ciarán O'Brien answered on 7 Nov 2014:

It’s mostly pressure from all the molten magma inside the earth. It’s insanely hot molten rock, and it churns about and has areas of high and low pressure, much like the weather, only it’s much hotter and thicker. Anyway, Volcanoes are usually found where this magma bubbles close to the surface, where the earth’s crust is thinner, or there are fault lines where the crust is actually cracked. A change in the pressure of the magma can build up as it churns about, or it could suddenly spike from an earthquake shifting the earth’s crust suddenly. Either way, the pressure of the magma gets too much for the earth’s crust to hold it in and it explodes to the surface in an eruption.