As the volume increases, there is more space so the average distance between the gas molecules also increases.
The rolling magma causes uneven currents in underneath the earths crust. Sort of similar to wind patterns in the sky but think of them as underground and in a much thicker fluid. Ocean currents might be a better analogy. Because of it being a much more viscous fluid, there is greater pressure buildup which can cause torsion, compression, expansion etc. when you bake a cake look how the dough underneath expands & the top crust cracks and stretches. Hope that helps.
