It's just the weight of literally everything on the planet pushing down, as well as miles of rock :)
Indirectly. The actual spike of pressure that ejects the magma comes from the gasses dissolved therein. When the magma moves up, the pressure drops and the gasses become oversaturated and thus prone to violent release.
“just” is doing a lot of heavy lifting there ;)
alt Hacker News
Indirectly. The actual spike of pressure that ejects the magma comes from the gasses dissolved therein. When the magma moves up, the pressure drops and the gasses become oversaturated and thus prone to violent release.