Jul 10, 2011 · The key difference between lava and magma is that lava is the hot mixture of gases and molten rocks that come out of the volcano whereas magma is the molten rock material deep inside earth’s crust. Hence, the difference between lava and magma pertains to their location. takes values between 0 and 1 for the cells containing the interface; otherwise either the value 0 or 1: (1) For incompressible flows, the movement of the interface is tracked by solving the following equation (2) where is the local velocity vector. This equation reflects the fact that the VoF function f is advected with the local fluid velocity.