Space elevators involve climbing up a 30,000 mile long tether with electric motors, not being fired out of a gun. Volcanic gas eruption velocities are much slower than what it takes to go into orbit. The fastest large-payload HARP shot was 3.6 km/s, but it takes more than 9 km/s to launch to low earth orbit. If we could build a gun barrel out of magic handwavium it might work, but unfortunately we only have steel, titanium and cobalt to work with.
