The mantle of the earth crust melts when the oceanic plate descends into the mantle. The density of this mantle is quite lower than the surrounding rocks and thus it rises upto the surface of earth to form a volcano. However, the pressure beneath the earth crust enables magma to break the rock and gush out of the cracks
I'm not exactly sure what specifically you are asking for in relationship to balancing equations, but I have some examples with working out so you can examine them and try to go through the steps with them.