What causes the force that pushes magma from the magma chamber through the pipe and vent of a volcano?

expanding gases

Explanation:

Under the Earth's crust the rocks are held in liquid form under high pressure and temperature. As the magma rises the dissolved gases in the magma expands and the magma forces out of the magma chamber through a pipe and out the vent of a volcano as lava.

i really hope this is right :/