Answer:
Magma is created when rocks are at the correct temperature and pressure to cause melting also know as passing the Liquidus (aka melting point with respect to pressure and temperature).
Magma is created from the subduction of cold, dense lithospheric plates into the hot rock of the mantle. The water in the subducting slab lubricates the rock and lowers (with respect to temperature) the liquidus causes the melting of the rocks, creating rising plumes of magma into the crust.
Magma is also created in massive quantities at divergent plate boundaries, know as mid-ocean ridges, where decompression melting of mantle rock occurs where the crust is being removed from the mantle. The magma and, to a much smaller extent, the mantle then rises to fill in the gap between the spreading ocean plates.
Additionally, magma is also created at mantle plumes, where there are stationary locations of rising heat from deep in the Earth's interior. As the hot rock rises, it experiences a decrease in pressure allowing it to pass the liquidus and undergoes melting.