Igneous rocks may form beneath Earth's surface or on Earth's surface. Magma that cools deep inside the crust forms intrusive igneous rock. The magma that forms these rocks intrudes, or enters, into other rock masses beneath Earth's surface. The magma then slowly cools and hardens.Lava that cools at Earth's surface forms00131extrusive igneous rock. Intrusive and extrusive igneous rocks differ from each other not only in where they form but also in the size of their crystals or grains. The texture of igneous rock is determined by the size of the crystals in the rock. The size of the crystals is determined mainly by the cooling rate of the magma.