Between the two temperatures a rock is partially melted. The differences between liquids and solidus temperatures can be 100s of degrees and is different for different composition magmas. Different magmas produce different minerals, and different minerals crystallize at different temperatures. Igneous rocks form when magma, or melted rock from inside the Earth, cools and hardens. They can form above ground when magma erupts from a volcano as lava. They can also form deep inside the Earth's crust and then be exposed at the surface by the overlying rocks wearing away.