Question

Why is expansion of rock important for magma to form?

Answer 1

The melting temperature of materials is affected by the pressure they are under. So when "rock" in the Earth's mantle experiences a decrease in confining pressure, not only does it expand, it's melting temperature drops. If the melting temperature of the material drops below the background (also known as the
in-situ
) temperature, then melting will occur and in this case magma will form.