The thing is, think about what are you relieving: magma. It's not gas. It's literally liquid rock being squeezing up a channel pushing against the weight of the

The thing is, think about what are you relieving: magma. It's not gas. It's literally liquid rock being squeezing up a channel pushing against the weight of the earth above it. If the pressure of the earth is greater than the magma it can start to spread horizontally and forms a chamber like a bladder. Then it keeps building pressure lifting the earth until it either bursts or maybe the earth shifts and pressure is relieved or moved elsewhere, or or equalizes and just hangs around. It's like a pimple really. Don't pop it unless you want to clean up after it.